EXPERIMENTAL

PARASITOLOGY

39,

150-169

( 19%)

PARASITOLOGICAL Intermediate

Host

Specificity PAUL F.

Department

REVIEW in Schisfosoma

mansoni

BASCH

of Family, Community and Preventive Stanford University School of Medicine, Stanford, California 94305

Medicine,

BASCH, P. F. 1976. Intermediate host specificity in Schistosoma munsoni. Experimental Parasitology 39, 150-169. Miracidia of Schistosoma mansoni penetrate into many kinds of snails, but development of normal sporocysts takes place only in certain species of Biomphakwia. Different populations of this snail vary greatly in laboratory infection rates with S. mansoni originating from diverse geographic localities. Cross-exposure experiments show that compatibility factors exist in both snails and parasites. Susceptibihty of stocks of Biomphalaria to particular strains of S. mansoni is genetically determined and may be modified by selection in the laboratory. In a compatible snail, the sporocyst develops without host tissue reaction; in incompatible snails the early larvae are rapidly surrounded by amebocytes and fibroblasts, and destroyed. This reaction resembles the generalized host cellular response elicited by any foreign body. An individual snail exposed to many miracidia may have both developing and encapsulated sporocysts side by side within its tissues. The weight of current evidence suggests that elicitation or absence of this cellular response resides in the recognition or nonrecognition of the sporocyst as a foreign body. The sporocyst tegument surface, which forms within a few hours after miracidial penetration, may have a molecular conformation identical with that of the snail, or may be able to bind specific host molecules, so that detection and subsequent encapsulation by host cells are averted. Presuming genetic determination of the sporocyst surface structure and of the host cell detection capability, differing infection rates would result from the particular frequencies of relevant genes in the populations concerned. INDEX DESCRIPTORS: Parasitological reviews; Mollusca; Snails; Trematodes; Schistosomes; Schistosoma mansoni; Biomphalaria; Miracidia; Host-parasite relations; Host susceptibility; Host specificity; Genetics of susceptibility; Invertebrate immunity; Invertebrate amebocytes; Recognition of foreignness.

I. Introduction II. Experimental exposures of Biomphalaria snails to allopatric S. mansoni III. Factors controlling infection A. Specificity of miracidial attraction B. Fate of miracidia in “insusceptible” snails C. Genetic factors IV. Immunity in Biomphalariu A. Multiple miracidial penetrations B. Experimental implantations C. Recognition of foreignness V. Recognition of sporocyst in host A. Early surface changes in the miracidium-sporocyst transitions B. The parasite surface and host cells C. Commonality of antigenic groupings VI. Conclusions Acknowledgments References 150 Copyright All rights

1976 by Academic Press, Inc. oP reproduction in any form reserved.

1.51 151 159 159 159 160 162 162 162 163 164 164 164 165 165 166 166

INTERMEDIATE

HOST SPECXFICITY IN SCHISTOSOMES

I. INTRODUCTION Since the late 1880’s when the general pattern of trematode life cycles was first described, thousands of published studies have dealt with these organisms and their snail hosts. No aspect of this relationship has been more bewildering than the problem of susceptibility of snails to infection by certain trematodes and their suitability as hosts for continued development. The intramolluscan stages of trematodes are generally more fastidious with respect to their hosts than are metacercariae (where present ) or adults. Schistosoma munsoni utilizes only snails of the genus BiomphaMaria (as presently construed) for intermediate hosts. Wright (1973) has made an exhaustive compilation of recorded localities and molluscan hosts of S. mansoni throughout its range in Africa and southwest Asia, the Caribbean region, and the South American mainland. Among and within species of BiomphaZaria, populations vary greatly in their suitability as carriers for different strains of S. mansoni. A consideration of the total relationships of these two groups of organisms is beyond the scope of the present limited review, which is intended to summarize current knowledge concerning the biological basis of specificity between this important parasite and its various intermediate hosts. II. EXPERIMENTALEXPOSURESOF Biom.phalaria SNAILS TO ALLOPATRICS. mansoni The discovery of potential snail hosts for S. munsoni in the United States (Cram, Jones, and Wright 1945) prompted a series of early susceptibility trials (Table I, Notes 2, 3, 4, 7, 10, 11, 12, 13) involving North American mollusks. In the ensuing decades numerous stocks of Biomphalaria snails have been tested against geographic strains of S. munsoni from the entire distribution range of both organisms. Some exposure trials have been made to estimate the “vec-

151

tor potential” of Biomphalaria snails from countries where S. mansoni does not occur (Table I, Notes 17, 18, 19, 27, 34, 36, 37) or from schistosome-free areas of countries, such as Brazil, where a strong potential for introduction exists (Table I, Note 33). Other infection trials were intended to study biological parameters, such as changes in population infection rates over time (Table I, Note 30) or for a variety of purposes. The results of some of these experiments have led to hypotheses concerning origin and distribution of the organisms. Files and Cram (1949) concluded that the New World S. mansoni is more closely related to West African than to Egyptian strains, and probably had been introduced by the slave trade. As a general rule, there is no correlation between the distance separating snail and parasite populations and experimental infection rates between them (Paraense and Corr&a 1963a; Barbosa and Figueiredo 1970). Certain populations of otherwise susceptible species are unusually refractoly, such as the Salvador strain of B. glal~rata (Table I, Note 9) or the Lake Navaisha strain of B. sudunica (Table I, Note 35a ) . Particular combinations of snail and trematode repeatedly yield little or no infection, e.g., Egyptian snails and Puerto Rican S. munsoni (Table I, Notes 4, 8, 29)) although the reverse combination may be compatible (Table I, Notes 10, 29). It seemscommonly to be believed (e.g., Files 1951) that a local stock of Biomphalaria snails and the strain of S. mansoni carried by it must be “‘well adapted” to each other. However, in many areas endemic for schistosomiasis, natural snail infection rates are consistently low (Paraense and Corr&a 1963a; Hairston 1973). Low rates may reflect a small degree of pollution but may also indicate a small proportion of susceptible individuals in a population exposed to heavy pollution (Paraense and Corr&a 196313). Moreover, some populations of Biomphalaria yield higher

152

PAUL

F.

BASCH

TABLE Selected

Experimental

Exposures Snail locality

Snail

PI. pfeiferi PE. pfeifferi Pl. guadazoupensis Pl, yuadalmrpensis T . havanensis T . hauanensis PZ. boisayi T . hauanensis T . hauanensis T . obstructus T . o. donbilli A. olabmtus A. globmtus A. glabratus A. glabrotus Pl. boissyi A. glabratus PI. boisayi Planorbis A. glabratus A. glabratus A. glabratus A. glabratus A. glabratus A. olabratus A. glabratus A. glnbratus A. #lab&us A. glabratus A. glabratus A. glabratus B. pfeifl’eri B. pfeifleri B. pfeiffwi B. boissyi B. boisrgi B. boissyi B. boissyi B. boissyi B. boisryi B. boisayi B. boissvi A. glabratus A. glabratus A. ylabratus A. glabratus A. glabratus A. glabratus A. glabratus A. ylabratus A. fllabratus A. dabratus A. Qtabratus A. glabtotus A. dubratus A. glabratus B. pfei#eri B. pfeiileri B. pfeijeri T . hauanensis T . hauanensis T . albieans

of Biomphalavie

West Africa West Africa Dutch Guiana Dutch Guiana Baton Rouge, Baton Rouge Egypt Louisiana. Baton Rouge Texas Texas Puerto Rico Puerto Rico Puerto Rico Puerto Rico

s. mansoni locality

La.

Egypt unknown, S. Amer. or W. Indies J%wt Egwt Puerto Rico (San Juan)

Venezuela (Caracas and L. Valencia) Brazil (Recife)

Liberia (Monrovia) Egypt (Cairo)

Egypt Egypt Egypt Egypt Puerto Rico Puerto Rico Puerto Rico Domin. Rep. Domin. Rep. Surinam Surinam Surinam Surinam Salvador Salvador Salvador Salvador Salvador Liberia Liberia Liberia Louisiana El Pompon, Cuba Puerto Rico

Snails

I to Miracidia No. of snails

Brazil French Guinea Brazil Fr. Guinea nd nd Puerto Rico Puerto Rico Puerto Rico Puerto Rico Puerto Rico Puerto Rico Puerto Rico Puerto Rico Puerto Rico Puerto Rico Uganda

30 nd 121 30 6 6 hundreds hundreds 1001 461 483 22 24 78 63 271 nd

Uganda Liberia P. R. (anim.) P. R. (hum.) VeIleZU..?la Egypt P. R. (anim.) P. R. (hum.) Veneauela Egypt P. R. (an&.) P. R. (hum.) Venezuela Egypt P. R. (anim.) V~lX!ZUL?lZi Egypt P. R. (anim.) P. R. (hum.) Venezuela En-pt Puerto Rico Egypt Egypt Egypt Surinam (anim.) Salvador Egypt Velle2uela Egypt P. R. (a&n.) P. R. (hum.) Surinsm (anim.) Egypt P. R. (anim.) P. R. (hum.) Surinem (anim.) Surinam (hum.) Egypt P. R. (anim.) Surinam (anim.) Egypt Egypt Egypt Egypt

nd nd 244 114 158 134 143 25 122 96 144 92 125 113 70 72 32 60 39 70 122 254 44 100 120 nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd 586 (14) 1114 (20) 853 (12)

of ALlopatric No. of miracidia each 1 1 1 1 nd nd nd nd varied nd nd nd nd nd nd nd nd nd nd

Individ. 3-6 or nxiS8 wJO/20 snails) for all trials

4-6

for all trials

3-6, or 10 individ. or

mostly

5-7

Schistosoma Results

13+/22s (59%) 31%b+ 16+/85S (18.8%) ot If 2+ o+ Of 9+ Of o+ (95%) 19+ (79%) 52+ (67%) 43+ (68% ) o+ 1+

21-t

1+ could not be infected 113-t/2075 43+/94s 68+ 1275 13+/108s 47+/1125 4+/2OS 28+/87s 7+ 187s O/1325 S+/SSS 0/115s o/1075 27+/55S 47f/48S 6+/15S 0/43s o/305 0/5os 36+ /948 195s. o+ 34S, 20.4%+ 79S, 26.0%+ 96S, 5.8%+ 19S, 36%+ 5W 6%+ 1345, 12%+ 228, 27%-t 12s, o+ 395, 82%+ 37s. 30%0+ 82S, 46% + 428. 14%+ 32S, 0+ 30s. o+ 67S, 0+ loos, o+ 768, 0+ 1218. 76%+ 748, 66%f 358, 3O%b+ 2545. 1.57%+ 612S, 0+ 3423, Of

munsoni* Note

1 1 1. a 1 a 2 3, a 3, a, b 4, a 4 4 4, b 4, c 4, d 4, e 4 5

2,

5 6 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 8 8, 8 8, 9 9 9 9 9 9 9 9 9 9, 9. 9, 9, 9, 9 9 9 10 10 10

a b

a a a a a

INTERMEDIATE

HOST

SPECIFICITY

TABLE Snail locality

finail

T. T. A. A. B. T. T. T.

pallidus obstructus glabratus glubratus boissyi hauanensia havanensis havanensia

Venezuela Guatemala VWX?ZU& Puerto Rico Egwt New Orleans New Orleans New Orleans

T. T. A. A. T. A. A. A. T. T. A. T. B. B. B. B. B. B.

hauanen?ia havnnensis globratus glnbratus ) hauonensis glabratus glabmtus glabratus havanensi.s havanensw glabratus huonnensis glabrata olabratn ylabratu glabrata ulubrata choanomphala

Baton IIatou

B. sudanica sudarkz B. sudanicu tanganikarm B. sudanica tanganikana B. riipellii B. rzipellii B. riipellii B. tip&i B. elegans B. smithi B. smithi B. smithi

Rouge Rouge nd nd 13at.m Rouge Puerto Rico Puerto Rico Puerto Rico Raton Rouge naton Rouge Puerto Rico Baton Rouge Domin. Rep. Salvador, Br. Puerto Rico Venezuela (1946) Venezuela. (1952) I,. T‘ictoria Napoleon Gulf Uganda Nile *w Owen Falls Uganda Napohm Gnlf and Nile L. Victoria IX. Nzoia Riv.. KFXlya Jarvis Dam Uganda Kibimlm Riv. Uganda Jarvis Dam Kibimha River > Kihimhs Riv. Kzoia Riv. Nyenga, Uganda Kiliimha Nyenga Kihirnba > Nyenga Bujagali Uganda Bujagali Arua, Uganda Arm, TV. Nile Arua, W. Nile Arua, W. Nile 1,ake Alhert Lake Edward, Uganda J,ake Edward Lake Edward

153

SCHISTOSOMES

I-(Continued)

s. mansoni locality

QYrjt EgYPt Egypt Egypt Egypt Puerto Rico Puerto Rico unknown (baboon, N. 0. zoo) Puerto Rico Puerto Rico Puerto Rico and hahoons 1 Puerto Rico Puerto Rico Puerto Rico Puerto Rico Puerto Rico Puerto Rico Puerto Rico Puerto Rico Puerto Rico Puerto Rico Puerto Rico Puerto Rico Puerto Rico West Nile

West

IN

Nile

No. of snails

307 (7) 412 (13) 1181 (19) 1018 (1.5) 874 (21) 187 186 143

No. of miracidia each

5 mass exp maSS exp

i6 131 nd nd 495 90 2 5 6 9 20 38 nd nd nd nd rrd 55

5 mass exp 5 or more > 20 5 10 20 10 20 100 100 5 5 5 5 5 15

45

10

10 10, 10, 10, 10, 11 11 11

619, Of 13+/131s + + 16.&V%+ + See notes

142S, 48S, 765 825: am, lo+,

46%+ o+ 43% + 27%+ 68% + (18%)

11 11, 11 11 12, 12 13, 13, 13, 13, 13. 13, 14 14 14 14 14 15

11+

(24%)

15

400

30

10

22+

h‘yanza

Prov.

100

15

94+/94S

Nyarrsa

Prov.

300

Nyanaa Prov. West Nile West Nile West Nile Nyanza I’rov.

25 25 50 60

200

West Nile Nyanza Prov.

25 53

Nyanaa

Prov.

West

ma.w nd nd 15 10 nmS8 10 15

400

Nile nd Nyansa Prov. Nyanza Prov. Nyanza Prov. West Nile

100 nd 300 25 30 20

Nyanza Prov. Lake Prov. Tanganyika

30 50

mass 15

Note

1925, Of 2968 12465. 31.7%+ 727s. 33.4% + 6085, 43.6% + 164S, Of 1608. 0+ 78s. o+

Nyarma Prov. Xeilya Nyanza Prov. nr. Nzoia Riv., Kenya West Nile

25 50

mass 3 times nd nd

Results

236+

(59%)

7f 42+/42s

lY7+

15

(100%) (73%)

15 15 15

(lOO%, (60%)

15 15

100% 100% 4f (8%) 14+ (23%) 25+ (12%)

15 15 15 15 15

3+ 26f

15 15

111+

(49%) (28%)

15

10 10

loot (100%) nd 104+/172S (60%) 17f o+ 14+ (70%)

15 15 15 15 15 16

10 10

26+ 41f

16 16

IId nmSS nd

(86%) (82%)

a 1) c d

a

a a b c d e f

154

PAUL

TABLE Snail locality

Snail

B. stanzeyi B. stanzeyi I?. slmleyi B. elegans B. eleaans B. pfeifleri (subsp?) B. pf&J’& B. admensis nairobiensis B. adowensis nairobiensis T . philippianus T . chilensis PZ. metidjemia A. glabmtus A. glabratus T . obstructus A. glabratzla A. stramineus A. tenagophilus A. tenagophilus A. tenagophilus A. tenagophilus A. tenaaophilus A. tenagophilus A. tenagophilus A. tenagophilus A. tenaaophilus A. tenagophilus A. tenagophilus A. tenaaophilus A. tenagophilus A. tenagophilus A. tenagophilus A. tenagophilus

s . monsoni locality

(Continued) No. ot snails

No. of miracidia each

30

10

18+

(60%)

16

Nile

50

10

44+

(88%)

16

Musoms, Tang. Nyansa Prov. West Nile

50 30 50

10 10 10

50-k (loo%t 2+ (7%) 10+ (20%)

16 16 16

Lake

Prov.

50

10

50-t

Us%)

16

Note

West Lake

Nile Prov.

50 50

10 10

48f 47-t

(96%) (94%)

16 16

Nairobi

West

Nile

50

10

39+

(78%)

16

20

Riv.

Ecuador

B. A. A. A. A. A. A. A. A. A. A. A. A. A. A. A. A. A. A.

Gambia Brazil Puerto Rico St. Kitts Island ChRUXS Capanema CUrUrUpU SBo Luis Estremos Pspari Joa0 Pessoa Paulista Peixinhos Recife Salvador I Salvador II Salvador III Salvador III Salvador III

West

Prov.

Results

Nyenza

B. sudunica pfeifferi glabratus alabratus glabratus alakatus ala3ratus glabratus alabratus alabratus glabmtus glafmtus alabratus glabratus alabratss alabratus alabratus glabratus glabratus glabratus

I-

BASCH

Ndsga Bay, S. Lake Albert Buhuka, Lake Albert Buhuka Buhuka Butiaba. Lake Albert Mwansa Tanganyika MWanZa Nairobi Riv., Kenya

Santiago, Chile Fr. Morocco Salvador Pernambuco Dade Co., Florida Be10 Horiaonte Pernambuco s&l Paul0 santos St. Andre SE.0 Bernardo Pindamonhangsba Aparecida Taubate SlO Paul0 Guanabara Jaoarepagu& AC&rat Rio de Janeiro Niterof sso Qon$alo Cabo Frio ItajubB, Minas Gerais Guaiba, Rio Grande do Sul Sudan

A. tenagophilus

F.

Be10 Horisonte, Brazil Brazil Brazil Pernambuco Pernambuco P. R.? Be10 Horisonte

17 60 12 440 120 1101 195 5a 100 200 100 200 100 100 100

TraIlSVrtal (Komatipoort) Trmsvaal Transvaal Be10 Horizonte

1+/14s

17

variable 20 10 10 10-50 lo-20 nd

3+/443 (6.8%) 1+/1s 4+/233S (1.7%) SS+/SlS (89%) no infections 1555, SO%+ 368, 5.5%+

18 19 20 20 21 22 22

10-20 for all trials

3+/853 1+/164S O/785 o/1345 0/63S 0/34s O/865

22 22 22 22 22 22 22

(3.5%) (0.6%)

100 90

0/53s 0/59s

22 22

68 50 100 25

1+/33s 0/46S O/785 0/21s

100

O/855

22

(3%)

22 22 22 22

nd

nd

no infection

23

nd nd 23 11 37 31 25 33 24 30 50 47 97 52 103 98 102 71 109

nd nd

some infection some infection 91.3%+ 90.0%+ 92.0%+ 96.8%+ 72.0%+ 69.7%+ 95.8%+ s&s%+ 48.0%+ 97.9%'0+ loo.o%+ 96.1%‘0+ o+ O-t O-t o+ Of

23 23 24 24 24 24 24 24 24 24 24 24 24 24 24 24 24 24 24

10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 50 100

INTJXRMEBL4TE

HOST

SPECIFICITY

TABLE Snail

A. A. A. A. A. A. A. A. A. A.

glabmtus glabratus glabratus glabmtus glabratus qlabratus qlabratus qlabratus glabratus tenaqophilus

A. A. A. A. A. A. A. A.

tenaqophilus tenaqophilus tenaoophilus tenaqophilus qlabmtus glabratus glubratus teaaoophilus

A. A. T. T. T. A. A.

qlubratus olubratus riisei albicans obstructus tenngophilus tenngophilus

B. A. A. A. A. A. A. A. A. A. A. A. B.

Alemndrina glabratus olabratus g1abmtu.s ylabratus glabratus qlabratus qlabmtus ulabratus olabratus qlabratus glubratus alezandrino aleznndrina B. alexandrina alesandTi?La B. olezandrinn alezandCm B. alezandrina alezandrina B. sudanica tanyanyicensis B. sudonica tanqanyicensis B. sudanica tanganyicensis B. sudanica tanoanyicensis B. alezandrina alezandrina B. aleznndrinn alezandrina 9. olabratus .4. glabratus .4. glabratus A. olabratus

Snail locality

Salvador III Lagos. Feia Mascarenhas AI&K& Santa Luaia Relo Horizon& Alvin6polis Jacarezinho Curitiha Sfto Jose dos Campos (SacI Paula)

Belo

Horiz.

Fazenda Exper., Slo Paul0 Santa Lucia Puerto Rico Puerto Rico Puerto Rico Puerto Rico Peru Peru Egypt Brazil Brazil Puerto Brazil Brazil Brazil Brazil Puerto Puerto Puerto Puerto EmM

Rico

Rico Rico Rico Rico

Egmt

I-

s. mansoni locality

S.J.C.

Belo Be10 S.J.C. S.J.C. Belo Belo

Ho&. Horiz.

Ho&. Horiz.

Belo Hark. Puerto Rico Puerto Rico Puerto Rico Puerto Rico Be10 Horiz. SLo Jose dos Campos Aden Aden St. Lucia St. Lucia Egypt (W) Puerto Rico

IN

SCHISTOSOMES

(Continued) No. of snails

No. of miracidia each

96 29 101 33 113 102 25 27 26 85

1000 10 10 10 10 10 10 10 10 10

10 20 100 20 45 15 54 125

50 100 20 100 5 100 5 50

20 169 642 172 264 60 55

5 In most ahout

25 25 25 25

200

20 20 10 10 10 10 4-6 in all trials

19.8% + 24.1%+ l.o%+ 84.8Yo + 94.?Y*o+ 95.l%b+ loo.o%+ 85.2%+ 65.4%+ 54+/82S 9+/9s 15+/16S O/QBS o/zos 0/45s 0/15s 47+/503 O/1255

BXPOB. 8

(94%)

25 25 25 25 25 25 25 25

9+/25s o+ 19+/23S Of 15k3+/1953 (81%) 36+/96S (37.5%) 2+/77S (2.6%) 13+58S (22.4%) 40+/119S (33.6%) 96+/1OOS (96%) 15+/1oos (15%) 19+/8OS (23.8%) 3+/107s (2.8%)

28 28, 28, 28. 29, 29 29, 29 29 29 29 29 29 29

Puerto

100

O/938

160

78+/151S

Egypt

(100%) (93.7%)

25 26 26 26 26 27, a 27, b

100 80 60 120 100 100 80 120

EICYPt

(65.8%)

24 24 24 24 24 24 24 24 24 25

1s+/2os (90%) 72+ (43%) 18+ (3%) 4+ (2%) o+ 43.3%+ 72.7%+

Egypt hrwanza Egypt W) Puerto Rico JkYPt Mwanza Egypt W) Rico

Note

Results

60

2+/48S

(0%) (51.7%) (4.2%)

29 29

nraarraa

Egypt

(WI

60

17+/57s

(33.5%)

29

Jlwanza

Puerto

Rico

80

19+/778

(24.7%)

29

80

1+/75s

Egypt

93+/151S

(61.6%)

29 29

25

2+/22s

Egypt

25

14+/248

(58.3%)

29

Egypt (WI Egypt P. R. (hum.) P. R. (hum.)

25 25 25 25

21+/23S 2+/21s 9+/25S 23+/25S

(95.5%) (9.5%) (36%) (92%)

29 29 29 29

Egypt

Egypt

&YPt Brazil Brazil Brazil Puerto

Rico

(1.3%)

W)

(9.1%)

29

a a a a h

156

PAUL

TABLE Snail

Snail locality

s.

7nanSonl

locality

F.

BASCH

I- (Continued) No. 01 snails

No. of miracidia each

Results

B. alezandrina alexandrzna B. sudanzca tanganyicensis A. glabratus A. glabratus A. glabratus A. glabratus A. glabratus A. glabratus A. glabmtus A. glabratus A. glabmtus A. glabratus A. glabratus A. slab&us A. glabratus B. pfeifleri B. alezandrina B. alezandrina B. alexandrina B. alezandrina B. alezandrina B. alexandrina B. alerandkm B. alexandrina B. alesandrina B. alezundrina B. alezandrina B. alexandrina B. alezandrina B. alexandrina B. pfeiffwi B. alezandrina B. alexandrinu B. alezandrim B. alezandm’na B. alezand~ina B. alezandrina B. pfeifferi B. alezand~ina B. alezandrina B. alezandrina B. alemndrina B. alezandrina B. alezandrina B. alezandn’na B. alezandrina B. alezandrina B. alezandrina B. alezandrina B. alezandrina B. sudanica tanganyicensis B. alexandrina B. sudanica tanganyicensis B. alezandrina B. alexandrina B. alezandrina B. alezandrina

Egypt

P. R. (hum.)

25

5

o/m

Mwansa

P. R.

25

5

5+/238

Puerto Rico Puerto Rico Puerto Rico Be10 Horis. Relo Horis. Be10 Horis. Be10 Horis. Be10 Ho& Puerto Rico Be10 Horis. Be10 Ho&. Be10 Horia. Puerto Rico Rhodesia Qalyub N&W& Sues Ismailiya Alexandria Egypt Egypt Egypt Cairo Qalyub N&W& SW2 Ismailiya Alexandria Rhodesia Cairo Qalyub N&W& SUtU Ismailiya Alexandria Rhodesia Khartoum Abu Rawaash Alexandria SUtZZ Ismailiya Qalyub Abu Rawaash Alexandria Sues Ismailiya Qalyub Abu Rawaash Mwansa

Puerto Rico Puerto Rico Puerto Rico Puerto Rico Puerto Rico Puerto Rico Brazil Brazil Brazil Brazil Brazil Brazil Brazil Cairo Cairo Cairo Cairo Cairo Cairo Liberia Tanzania West Indies Cairo Cairo Cairo Cairo Cairo Cairo Cairo Alexandria Alexandria Alexandria Alexandria Alexandria Alexandria Alexandria Tanzania Tanzania Tanzania Tanzania Tanzania Tanzania Tanzania Tanzania Tanzania Tanzania Tanzania Tanzania Tanzania

B. alexandrina

Khartoum

Congo

B. alezandrina

Khartoum

Egypt

(rib.) @lb.)

@lb.) @lb.) (alb.)

(hum.)

2962 5075 4075 869 1957 1940 2109 1157 2663 1475 1820 3060 1360 50 50 50 50 50 50 150 100 nd 75 250 250 250 250 250 nd 75 50 50 50 50 50 nd 250 250 125 125 125 250 25 25 25 25 25 25 25

6-12 in all trials

ca. 6 in all trials

nd nd nd 6 in all trials

6 in all trials

(0%)

Note

29

(21.7%)

29

722+/1982S (36.4%) 2336+/3785S (61.7%) 1999+/3113S (64.5%) 40+/631S (6.3%) 187+/15268 (12.3%) 329+/15463 (21.3%) 940+/15648 (60.1%) 552+/931s (59.3%) 873$/1774S (49.2%) 279+/1099S (25.4%) 673+/13738 (49%) 1373+/2564 (53.5%) s53+/1054s (80.9%) 42+/468 (91.3%) 32+/488 (66.7%) 19+/48S (39.6%) 20+/43S (46.5%) 9+/47s (19.1%) 0/5os (0%) 1+/135s (0.71% 1+/1OOs o+ 66fi68S (97%) 122+/211s (57.8%) 45+/2308 (19.6%) 74+/22OS (33.6%) 23+/233S (9.9%) 21+/238S (8.8%) nd (95.8%) 65+/71S (91.5%) 28+/458 (62.8%) 11+/5os (22.0%) 14+/46S (30.4%) 5+/47S (10.6%) 3+/483 (6.3%) nd (91.3%) 92+/223s (41.3%) 188+/244S (77.1%) 37+/107S (34.61% 20+/93S (21.6%) 3+/76S (3.9%) 3+/157s (1.9%) 20+/25S 030%) 5+/22S (22.7%) 3+/16S (18.8%) 1+/16S (6.3%) 0/16S (0%) nd (25%) nd (55%)

30, 8. 30,b 30, c 30. d 30. e 30, f 30, g 30, h 30, i 30, j 30. k, n 30.1, n 30, In. n 31. * 31. & 31, & 31, & 31, 8. 31, a 31 31 31, b 31. c 31, c 31, c 31, c 31, c 31, c 31, c 31. c 31, c 31, c 31, c 31, c 31, c 31, c 32 32 32 32 32 32 32 32 32 32 32 32 32

nd nd

32 32

1 Abu Rawaash Mwansa

Egypt

6 locations

Tanzania. Tanzania Liberia Congo St. Lucia Madagrtsoar

25 25 3 batches of 25 for ea. Sm. strain 4 expo8. of 25 ea. nd

nd nd nd

0 0 0 0

nd

7.3%

nd

40%

(48%) (60%)

32 32 32 32 (mean

rate)

32 32

INTERMEDIATE

HOST

SPECIFICITY

TABLE Hn:tiI

Snail locality

I,spa. Paran& Imperial Co., Imperial Co. Imperial Co.

Urcrzil Calif.

No. of snails

West Indies I’ernambuco Pernambuco I’er11ambuco Pernambuco I’ermunlnK!o I’ernaml~ueo RQ N I’WXUd,UW .\lagoas I’orrmnbrK!o RGN 1 l’errrambrwo RCN 2 l’ernambnc” I’araiba 1

No. of miracidia each

nd 8GlOO 3&201 lo-138 s2.30 14-310 25-198 42 90 63 47 98 96 94 82 62 66 42 138 100 84 20 20 14 30 30 29 nd 32 35 26 34 30 173 21 26

I'O~KUIhlCO

Parafba

157

SCHISTOSOMES

I-(Continued)

s. munuo71i locality

Khartoum N. E. Brazil COlL!G RGN Paralh I’ernamlrnco .\h%gOM RCN RGN .~lagow Magoaa RGN 1 RON 1 RGN 2 RGN 2 I’arailm 1 I’ilrail~:l 1 I’nrailm 2 I’nrail,a 2 .\lagoas .\lagoss I+. Guiana Fr. Guiana I+. Guiana Ilominica Puerto Rico Domill. Rep. St. 1,nria Trinidnd Puerto Rico lil3lly:L JienyN (:ambiu Grenads Chillognll”, Ew. Chillogall”

IN

2

Pemambuco

Alagoas I’ernambwzo St. Lucia St. Lucia St. Iaciit St. I.ucia St. ljucia St. Lucia St. Lucie St. I,ucia Puerto Rico Puerto Rico Puerto Rico Puerto Rico Puerto Rico? I~olo Horiz. 8po Jos6 dos Campos? nolo Horiz. Puerto Rico Yemen (.Idon) Yemen

nd 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 5 in all trials

10 in all trials

nd 10 10

20 165 162 10

20 Ild nd Id

0 0 to 0 to 0 to 0 to 0 to 0 to 100% 70% 60.3% 0% 0% 1% 6.4% 4.9%

0% 0% 0%

OYO 0%

1.2% 0/19s 0/19s 1+/3s 15+/29s 10+/3os 5+/253 o/255 30+/328 H/265 23+/28S 15f288 12+/1735 7+/21s -if/268

TO, TABLE

1. (Vogel lY41) a. Only 106 mirwidia penetrated. 2. (Cram, Jones, and Wright 19453 a. Juveniles, 3 m m diam., exposed three times. 3. (Htunkard 1946) a. Miracidia penetrated and transformed to primary sporocysts, then resorbed; b. 22 species of snails from eastern U. S. also exposed with some early development in ifeliwma, Physa, ~‘ivipal-tls. 4. (Cram, Files. and Jones 1947) Summary of many trials, I,“th wild and lab-raised snails, various exposure methods; a. 22 other species of planorbid and four nonplanorbid snails also tested, all refractory; b. 1943; c. 1944; d. 1945; e. 1946. 5. (Cowper 1947.) 6. (Kikuth and GGnnert 1948.) 7. (Files and Cram 1949.) R. (At&l-M.?lck lY50) :L. Reex~m~~wea from the grou,) of 2.54 unifecled l,y 1’1~erl.o Iliarn S. niwrae,ri: Ii. sonils dwarkal I,y crowding.

(33%) (55%) (33%) (20%)

(93.8%) (11.5%) (76.8%) (83.4%) (7%)

35 35, a 35 35 3ti, a 37 37

'J+/2os O/llOS o/Yes

* Biumpbaloriu is hero convtrued 88 the genus described in Opinion 735 (lY65) of the International IGomenelature. of which Trupicurbis and Au.druEmbis are junior synonyms. + = infected; S = Sllrriring; 1x1 = no data; .I = Aiustrulorbi.s; 13 = Biompkrluria; 1’1 = Plumrbis;

NOTICS

100% 28.8% 16.7% 34.5% 17.1% 35.3%

3i 38 38 38 Commission

on

Zoological

T = Trupicorbia.

I

9. (Files 1951) Some data from Files and Cram (lY4Y) illcorporated in publication not repeated here; a. the refractory stock of B. glabruta used by Newton (1952, 1953, lY54) aud others. 10. (Kunts 1052) Numbers in parentheses = number of lots exposed; a. one poorly developed sporocyst in each of two snails; b. 4.9 to 83.4% in lota; e. 0 to 72.1% in lots; d. 0 to 90.5% in lots. 11. (McQuay 1952) Some T . havanerwia were exposed several times; a. progeny from two susceptible snails. 12. 0lcQuay 1953) 8. All snails clone of progeny from single susceptible snail, had average of 1.6 exposures. 13. (Brooks 1953) 8. Sectioned after 12 hr, three sporocysts in one, four in the other; II. sectioned ut 12 hr, average 5.4 ~1mr0eynt.n : r. w.vl ioned :I t. 12 hr. arrrxgr 3.8 ~poroc~yatii; (1. nwtioued at 12 hr. averuge 8.8 nporovyatrr; e. swiioned at, 1. 2. 3, 4, 6 daya, average 28.2 to 33.5 sporocyats; f. sectioned

158

PAUL

F.

TABLE NOTES

TO

ITABLE

at intervals 0.5 to 20 days, number of sporocysts declined from 25 to 30 first few days to 0 at 20 days. 14. (Dewitt 1955) Exposures at 25 C only listed-rates were higher at 35 C but no Salvador snails infected at any temperature. 15. (Cridland 1955). 16. (Cridland 1957.) 17. (Barbosa, Barb-a, and Rodriguez 1958.) 18. (Barbosa. and Barbosa 1958.) 19. (Barbosa, Barbosa, and Rego 1959.) 20. (Barboss and Barr&o lY60.) 21. (Leigh 1961.) 22. (C&ho 1962.) 23.

(Wright 1962.) (Parrtense and CorrBa 1963a). (Paraense and Cow% 1963b). 26. (Richards 1963) 22 species of freshwater exposed and miracidia penetrated into seven. 27. (Paraense, Ibafiez. and Miranda 1964) only, no cerariae; b. cercarial shed, 28. (Saoud 1964) a. Stock known susceptible from Egypt and Brazil.

24. 25.

snail8 a.

were

Sporocyst

to S. mans&

infection rates with S. munsoni from distant localities than with the strain they theyselves transmit (Cridland 1970). It should be pointed out that S. munsoni and Biom~haZuria are by no means unique in the variety of relationships between different geographic strains of both organisms. Both S. huemutobium (Paperna 1968; Berrie 1970; Lo 1972) and S. jupofiicum (W. H. Wright 1973) show similar relationships. Nonschistosomes such as Fusciolu, with their Lymnaeid hosts, also reveal a great variety of compatibility combinations (Boray 1966; Kendall 1970) and it is likely that the same is true for all trematodes and their molluscan hosts. Identification of snail species is a continuing problem. Infra- and supraspecific categories, proposed by various authors, are beyond the scope of the present review. General publications dealing with this problem (Mandahl-Barth 1958; C. A. Wright 1962, 1971; Berrie 1970; Pan American Health Organization 1968) discuss taxonomic problems in African or American B~omphuZuria. Kendall ( 1964) has stated with respect to molluscan taxonomic studies: “None of this work seems however, to be of great assistance at the level of speciation at which host specificity may

BASCH

(Continued) I-(Continued) 29.

(Saoud 1965) a. Egypt (W) = Wellcome strain pnssaged in B. galbmta for 15 yr; b. Kalubiya Province, wild strain. 30. (Kagan and Geiger 1965) a. 1962; b. 1963; c. 1964; d. 1962; e. 1963; f. 1964; g. 1962; h. 1963; i. 1962; j. 1963; k. 1963; 1. 1964; m. 1964; n. S. wxznsoni strain adapted to albino snails of Belo Horiaonte origin. 31. (Cridland 1968) a. Group exposures of 50 snails each; b. “could not be infected;” c. individual exposures. 32. (Cridland 1970). 33. (Barbosa and Figueiredo 1970) Sxmils tested in butches; a. 16 populations, states of Rio Grande do Norte Parafba, Pernambuco, Alagoas; b. 64 populations tested; e. 30 populations from Rio Grande do Norte; d. 38 populations tested; e. 70 populations tested; f. 21 populations tested. 34. (Sturrock and Sturrock 1970) a. Sympatric snail stock, as control, became 50, 67, 94, and 100% infected. 35. (Taylor 1970) a. B. sudanica from Lake Naivashn, Kenya, could not be infected with their local strain of S. mamoni. 36. (Richards lY73a) Juvenile snails 1 to 3 m m diam., some of the infected snails eventually shed cercariae. 37. (Paraense and CorrBa 1973). 38. (Basch, Grodhaus, and DiConaa lY75).

be evident and it is probably true to say that the parasite represents a speciessplitter of greater subtlety than any systematist.” W. H. Wright (1973) provides a good recent overview. Table I represents an edited compilation of reports of infection trials with allopatric combinations of S. munsoni and Biomphduriu. Species names are those of the authors; the publications mentioned above may be consulted to trace synonymies. In no case did an author mention deposit of voucher specimens in a museum for later verification of identity; this procedure, if consistently followed, would add greatly to the value of such studies. Omitted from Table I are reports dealing exclusively with sympatric combinations (i.e., a Biomphulariu stock and the S. munsoni strain normally transmitted by it), and data on laboratory-produced hybrids of either organism. A few published records considered unclear have been intentionally omitted, and others may have been overlooked. The definition of “an infection” varied greatly among authors; some counted any development of sporocysts, some only cercarial production. In some trials all snails dying before patency were dissected and tallied; others included only snails surviv-

INTERMEDIATE

HOST SPECIFICITY IN SCHISTOSOMES

ing for a predetermined period. Some authors have described differences between populations of Biomphaluria in the time interval until cercarial appearance, number of cercariae shed, or other characteristics of the infection. Authors employed a variety of sources for miracidia: human cases, or the feces or livers of experimental animals. Exposures were made and snails maintained under differing conditions, often uncontrolled or undescribed. It is impossible to account for all these variables in Table I, and original sources must be consulted for specific details. Infection rates listed should be interpreted with caution. Repeated trials of some combinations have yielded widely varying results (e.g., Notes lob, c, d). The rates compiled in Table I should be viewed as general indications of probabilities of infection in those combinations. III.

FACTOHS CONTROLLING

INFECTION

The success or failure of any encounter between snail and miracidium must be determined in part by hereditary and in part by environmental factors. Most studies of conditions for successful development have dealt with the latter. Environmental parameters such as temperature are readily quantifiable and simple manipulations lead to replicable changes in infection rates (Standen 1952; Dewitt 1955; Purnell 1966). Modifications of such factors, while epidemiologically important (Webbe 1967), are unlikely to provide an explanation for the basic biological question: Why can a miracidium develop to produce a normal infection (i.e., sporocysts and cercariae) in one snail and not in another? A. Specificity of Miracidial Attraction Many studies have dealt with host-finding by miracidia of S. mansoni, in a search for specific attractant substances produced by certain snails. Barbosa and Carneiro ( 1965) summarized earlier publications of authors favoring or opposing specific at-

159

traction. They themselves believed any attraction to be generalized, pointing out that miracidia of S. mansoni may even burrow through the skin of tadpoles. Stunkard (1946) had earlier reported penetration into snails of the genera Helisoma, Gyraulus, Menetus, Physa, and Viviparus by miracidia of this species. Cram, Jones, and Wright ( 1945) and Cram, Files, and Jones (1947) found insusceptible snails to be attractive to and penetrated by S. mansoni miracidia, a finding confirmed by Barbosa and Barreto (1960), Sudds ( 1960), LeRoux ( 1961), and Basch, Grodhaus, and DiConza ( 1974). Richards ( 1963), in Puerto Rico, observed that penetration occurrcd more frequently and with greater facility in Biomphalaria obstructa, in which cercariae were never produced, than in the normal host, B. glabrata. Miracidium-snail relationships have been reviewed recently by Cheng ( 1968), Berrie (1970), and Chernin (1974), who concluded that characteristic prepenetration behavioral changes are induced in miracidia of S. mansoni by substances emitted by a variety of snails. MacInnis, Bethel, and Cornford (1974) have shown that mixtures of amino acids similar to those “leaked” by snails can elicit characteristic responses in these miracidia. On the basis of published studies, it may be concluded that intermediate host specificity in S. mansoni is not determined in the water by any substances produced by snails. B. Fate of Miracidia in “Insusceptible” Snails In 1952, Newton obtained two stocks of B. glabrata snails. One, from Puerto Rico, was susceptible to S. munsoni from the same locality, but the other, from Salvador, Brazil, yielded no infections with a variety of strains of the parasite (Table I, Note 9a). Newton exposed snails of both stocks to numerous mirncidia, then killed, fixed and sectioned them at intervals to follow

160

PAUL

the fate of the miracidia which had penetrated. Each of the Puerto Rican snails contained, in the subcutaneous tissues, several parasites developing normally without any reaction on the part of the snail. But in Salvador snails fixed 15-18 hr after infection, with one exception, no recognizable schistosome material was to be found. There were, however, in each snail, several areas of cellular reaction to unrecognizable stimuli. These areas were characterized by variably, but often rather concentrically, arranged layers of cells about a core. In some instances there appeared to be a fibrous type of tissue interwoven among these cells. (Newton 1952).

Soon after, Brooks ( 1953), studying B. obstructa from Baton Rouge, La., found an identical picture, noting, however, that after exposure of many snails to large numbers of miracidia ( 100 per snail), one or a few normally developing mother sporocysts were seen in some. Similar observations were soon reported by other workers using a variety of species of Biomphalaria and strains of S. mansoni (Barbosa and Coelho 1956; Coelho and Barbosa 1956; Coelho 1957, 1962; Barbosa and Barreto 1960). The B. glabruta stock from Salvador which Newton had found refractory to Puerto Rican S. mansoni was reexposed using a Brazilian parasite, with the same results: although the Salvador snails were morphologically identical to a susceptible stock, they were almost entirely uninfectable (Barbosa and Barreto 1960). In trials with other combinations a few developing mother sporocysts were sometimes found in sections amidst a large number of encapsulated and dead ones (Barbosa, Carneiro, and Barbosa 1961). The nature of the encapsulating tissue reaction was closely studied by Pan ( 1963) who designated it as a Type I response, minimal and focal in nature, as opposed to the Type II responses, or generalized proliferative tissue reactions occurring in chronically infected snails. Type I responses were associated with degenerating mother sporocysts within 48 hr of infection.

F.

BASCH

They were found very rarely in susceptible Puerto Rican B. glubrata exposed to large numbers ‘of miracidia of a compatible Puerto Rican strain. Histologically, Type I responses are characterized by degenerating organisms surrounded by accumulations of fibroblasts, with a few small scattered amebocytes. “Since hyperactivity of connective tissue around the granulomas in Type I changes is not present, and since amebocytes transform readily into fibroblasts, it is likely that fibroblasts in the snail granulomas derived from the small amebocytes normally circulating in A. glabratus.” (Pan 1963). It appears, therefore, that upon massive exposures a few mother sporocysts may develop successfully in resistant snails, and a few may be encapsulated and killed in susceptible snails. C. Genetic Factors Stunkard (1946) observed that “different genetic strains of a single species from the same locality may manifest varying degrees of resistance,” and “even among susceptible stocks, certain individuals are more resistant than others.” More recent work, particularly that of Richards, has shed light upon the distribution and inheritance of susceptibility factors in B. glabrata. Genetic crossing studies are time-consuming, and difficult to do in Biomphaluria because the snails are hermaphroditic. Each individual contains complete and functional male and female reproductive systems and is capable of self-fertilization so that a population may be started by a single individual isolated from birth. In matings between conspecific allopatric individuals, cross-fertilization does not necessarily completely replace self-fertilization, and eggs not fertilized by the partner may be self-fertilized (Paraense 1956; Richards 1970). Certain markers are therefore needed to identify hybrid individuals.

INTERMEDIATE

HOST

SPECIFICITY

Newton (1953, 1954) discovered a single albino mutant snail in a stock colony of the refractory Salvador strain of B. glabrata. It bred true, with albinism a simple recessive to the wild-type pigmented condition. By placing albinos together with pigmented snails, then separating them and permitting them to deposit eggs, F1 hybrids were readily identifiable as pigmented offspring of albino parents. As F1 snails became available, some were isolated for self-fertilization and some exposed to 10 miracidia of the Puerto Rican strain of S. mansoni, to which the pigmented parent was highly susceptible. Of 124 F1 snails surviving the incubation period of S. mansoni, 15 shed cercariae. Fa populations derived from isolated “selfed” nonsusceptible F1 snails had infection rates varying from 0 to 82%. It is especially interesting that particular infection rates were replicated in several of the populations. Of 24 Fz snails fixed and sectioned 54 hr after exposure to 20 miracidia each, 14 showed the Salvador response (encapsulation of all sporocysts), and five showed the Puerto Rico response (all sporocysts developing well). Five snails showed both types of response, having normally developing mother sporocysts side by side with others undergoing destruction by intense cellular reaction. Similar observations have been discussed above in snail populations where almost all sporocysts are killed, or ahnost all develop normally. McQuay ( 1953) studied the inheritance of susceptibility in a clone of “Tropicorbis havanensis” (possibly introduced B. straminea, C. S. Richards, personal communication) from the Baton Rouge site (Table I, Note 12a). He derived 495 descendants by repeated self-fertilization in five to eight generations from the single susceptible snail. McQuay found that on exposure to 20 miracidia each there was no appreciable difference in infection rates in offspring of susceptible or of refractory parents. Se-

IN

SCHISTOSOMES

161

lection for susceptibility was apparently ineffective in this case. In a restudy of the refractory Salvador parental population, Newton exposed baby snails as young as 1 or 2 days old, obtaining some infection and showing an agesusceptibility factor. Richards ( 1970, 1973a) has made extensive studies of the genetics of susceptibility of 23. glabrata to S. munsoni, identifying three types of patterns: susceptible, refractory, or juvenile susceptible and adult refractory. The identification of genetically determined agerelated susceptibility patterns may help resolve the discrepant results reported by various authors with respect to relative susceptibility of different sized snails (review in Lim and Heyneman 1972). It has been shown (Wright and Ross 1963) that a marked increase in hemoglobin may occur with growth. Richards (1970) has described clones of B. glabrata showing variation in the degree of hemolymph pigment and in the time of expression of such variation. Using stocks of B. glabrata that Newton developed 20 years ago, Richards and Merritt (1972) have described individual exposures of more than 6000 snails, concluding that juvenile susceptibility is genetically controlled by a complex of four or more factors, that factors for insusceptibility may be present in susceptible snails, and vice versa. Interbreeding snail populations may maintain a stable proportion of susceptibles over a long period of time, but an abrupt change can occur in susceptibility frequency, This interesting comment is made: Most studies on susceptibility have deaft with interbreeding populations of snails, either laboratory or field populations. When tests on such populations yield intermediate infection frequencies (such as 30%) this should not be interpreted as meaning that each snail is 30% susceptible, but rather that 30% of the snails exposed were SUSceptible and ‘70% refractory, reflecting gene frequencies in the population and a variety of gene combinations. ( Richards and Merritt 1972 ).

162

PAUL

F. BASCH

The genetics of S. mansoni may also be investigated by means of crossing male and female worms from different strains. Unimiracidial infections in snails yield cercariae of a single sex; by mixing maleproducing cercariae of one strain with female-producing cercariae of another, hybrids may be produced in experimental animals. Seven cross-strains of S. mansoni were produced by Files ( 1951), yielding varying infection rates in tested snail stocks. More recently, Taylor (1970) has produced a number of “interspecific” crosses in African mansoni-group and haematobium-group schistosomes. The results of these few experiments in schistosome hybridization do not yet lend themselves to summarization. Kagan and Geiger (1965) felt that the infection rate in albino B. glubrata was enhanced by selection of infective miracidia, and concluded that the snail host is potentially susceptible, with the genetics of the miracidium the deciding variable in the relationship. Saoud (1965) found that a strain of S. mansoni originahy isolated from B. alezandrina alexandrina in Egypt, and passaged for years in a laboratory colony of B. glabrata, had almost entirely lost its compatibility with the original Egyptian host species (Table I, Note 29a). The effect of genetic selection may be presumed in this case. Perhaps the clearest example of the presence of compatibility factors in both snail and schistosome is provided by Paraense and Correa ( 1963b), who show that a S. mansoni strain adapted to B. tenagophila will not infect B. glaabrata, and vice versa (Table I, Note 25). It may be concluded from the discussion above that “susceptibility” to S. mansoni is not a characteristic of a snail population, nor even of an individual, any more than “infectivity” is an inherent characteristic of miracidia. The compatibility of snail and parasite are tested with each individual miracidial penetration. A snail may be com-

patible with one miracidium but not with another from the same population, IV. A. Multiple

IMMUNITY

IN

Miracidial

Biomphahia Penetrations

Although two or more mother sporocysts can develop normally at the same time, would there be any tissue response to a second miracidium penetrating some time after one sporocyst has become established? Schistosomes can provide a direct answer because the adults are dioecious, and a snail infected by one miracidium will later release larvae of the same sex. If the same snail, following a second exposure to miracidia, yields cercariae of both sexes, there is no absolute barrier towards development of subsequent sporocysts. If on the other hand bisexual infections do not ensue, an “immunity” may be inferred. This hypothesis has been’ tested several times (Stirewalt 1951; Kagan and Geiger 1964; File 1971) and it is agreed that bisexual infections are readily produced. Individual snails collected from the field may shed cercariae of both sexes, indicating that multiple infections occur in nature also. Maldonado and Velez-Herrera (1949) exposed 93 guinea pigs separately to cercariae from 93 naturally infected field-collected snails, finding 64% with bisexual infections. There is, furthermore, no barrier to entrance of miracidia of several different species of trematodes belonging to a variety of groups so long as each separately is compatible with the host snail. Lim and Heyneman (1972) have summarized the results of a decade of studies in which individual snails of many species have been infected with two or more trematodes. Barbosa and Coelho (1956) have, however, described tissue reactions around young developing mother sporocysts in reinfected snails in a much-quoted example of “acquired immunity” in snails. B. Experimental

Implantations

The fate of foreign objects introduced into B. glabrata was investigated by Tripp

INTERMEDIATE

HOST

SPECIFICITY

(1961) who found that small particles were taken up by phagocytic amebocytes, whereas larger objects like pollen grains or polystyrene spheres elicited an initial amebocytic response later reinforced by fibroblastic encapsulation and nodule formation. This description appears identical to Pan’s Type I focal response to certain sporocysts of S. munsoni. Tripp also implanted pieces of fresh or formalin-fixed B. glubrata tissue from one snail to another, finding that the fresh tissue elicited no response and became fused with host tissue, whereas the fixed tissue was encapsulated like other large foreign bodies. An implantation of fresh tissue from a related, but different species of planorbid snail stimulated a strong reaction and the donor tissue was destroyed. The B. glabrata snails are therefore capable of detecting and reacting on a cellular level to foreign bodies, and can distinguish foreign snail tissue from homologous tissue. This latter distinction was investigated by Cheng and Galloway (1970), using a different species of planorbid snail (Helisoma duryi normale) as recipient. They found strong reactions against xenografts from three other species of snails, whereas allografts were reacted to in a milder and slower manner. Host reaction to artificially transplanted schistosome sporocysts in B. glabrata and other snails was studied by Chernin ( 1966). Transplantation into susceptible B. glabrata of pieces of digestive gland containing active sporocysts, or of intact mother or daughter sporocysts resulted in normal development in the recipient snail, followed by cercarial shed. Stocks of 13. glahata resistant to infection via the normal miracidial route did not take the grafts and cercariae were never shed. Refractory snails are, therefore, refractory by implantation as well as by miracidial penetration. DiConza and Basch (1974) have shown that daughter sporocysts of S. mansoni cultured in an axenic medium can develop when implanted into appropriate snails, and Basch and DiConza (1974) have SUC-

IN

SCHISTOSOMES

cessfully implanted mother grown in vitro from miracidia. C. Recognition

163 sporocysts

of Foreignness

The general subject of molluscan immunity to metazoan parasites has been reviewed recently by Brooks ( 1969), Cheng ( 1970)) and Tripp ( 1970a, 1970b). The acceptance by B. glabrata of isografts and transplanted sporocysts, and the tissue reactions developing around foreign bodies clearly suggest a mechanism on the part of snail cells for recognition of “self” and “nonself.” The sequence of events following detection of “nonself” in B. gZubrata appears to be: ( 1) mobilization of amebocytes; (2) phagocytosis if the foreign body is small; or (3) surface covering if large, followed by a transformation of amebocytes to fibroblasts; and (4) lysis of the “nonself” substance if it is of a suitable composition. In this way the granulomalike Type I focal reactions around incompatible sporocysts eventually disappear, whereas those surrounding polystyrene spheres are long-lasting. The similarity of these reactions in B. glabrata and in oysters, two mollusks of vastly different life modes, is striking. Equally striking is the destruction in B. glabrata of certain sporocysts while adjacent ones develop without hindrance. The cellular defense reactions (phagocytosis, pinocytosis, and encapsulation) have been reviewed in a recent brief survey by Bang (1973) who concludes, inter alia, that “recognition of foreignness is SO common among invertebrates that various parasites have evolved mechanisms to protect themselves against the host’s reaction.” The existence of possible immune processes in invertebrates was considered by Burnet (1970) in his survey of “Immunological Burnet has suggested that Surveillance.” nonrecognition of parasites is most likely due to the elimination in evolution of surface groupings on the parasite, analogous to antigenic determinants, which are recognizable as foreign by host hemocytes. The

164

PAUL F. BASCH

converse possibility was also mentioned, that all host cell surfaces have a recognizable self character and that defense reactions are initiated when contact does not result in positive recognition. More recently, Burnet (1974) has reviewed invertebrate precursors to immune responses with particular reference to the self-recognition capability of various types of invertebrate cells. Both Bang (1973) and Burnet (1970, 1974) have relied heavily upon the work of Salt, whose many studies of cellular defense reactions in insects have been summarized in a comprehensive monograph (Salt 1970). Although mollusks and trematodes are not mentioned, the similarities between insect and snail defense systems are remarkable with respect to foreign body response, reactions to allo- and xenografts, and tolerance of specific parasites. Salt argues convincingly for the concept of “molecular mimicry,” where the successful parasite is not recognized as foreign by host amebocytes. Nappi (1974) has discussed the recognition of foreignness by insect hemocytes, viewing in particular the possible role of hormones in hemocytic changes leading to encapsulation and melanization of parasites. Similar studies on molluscan hemocytes would be of great value. V. RECOGNITION

OF SPOROCYST IN HOST

A. Early Surface Changes in the Miracidium-Sporocysf Transition In a detailed ultrastructural study, Southgate (1970) has clarified the early events associated with the miracidium-sporocyst transition in Fasciola hepatica. Southgate described the formation of a new tegument within a few hours after the loss of the ciliated epidermal cells of the miracidium. A granular material, mobilized from its place of storage in the subepidermal layer, was shown to flow upwards and cover the metamorphosing organism. By forming a

fine matrix covering the newly produced tegument “. . . the content of the granules may be an important factor in the nonrecognition of the parasite by the snail host . . .” ( Southgate 1970). Although C. A. Wright ( 1971), citing unpublished electron microscope studies by Brooker, has stated that the ciliated epidermal cells of S. mansoni cannot be cast off, this has been shown to occur readily in vitro by Voge and Seidel (1972) and Basch and DiConza ( 1974). Ultrastrnctural study (Basch and DiConza 1974) shows that early surface changes of cultured sporocysts of S. mansoni closely resemble those described in F. hepatica. B. The Parasite Surface and Host Cells Considering specificity in snail-trematode systems, Wright (1971) has suggested various ways in which nonrecognition might come about: by a miracidial “tegumentary constitution identical with that of the host, by absorbing host ‘antigen’ on its surface, or by synthesizing appropriate snail antigens.” In addition the parasite might actively inhibit host cellular response. The discussion of these alternatives by Wright ( 1971, p. 125 ff.) should be read carefully by those interested in problems of snail-trematode specificity. Heyneman, Faulk, and Fudenberg (1971) have proposed that successful trematode larvae are invested in a protective protein coating derived from the host which serves to prevent detection by amebocytes. Faulk et al. (1973) report a dose-dependent effect of irradiation of snails from 13Cs source in increasing infection rates of B. glabrata subsequently exposed to miracidia of Echinostoma lindoense, suggesting that a radiosensitive cell type is important in snail resistance. Recognition of foreignness by snail hemocytes appears to be based upon surface characteristics. When the sporocyst surface is broken, for instance by interspecific predation within the host, and the

INTERMEDIATE

HOST

SPECIFICITY

contents leak out, the damaged sporocyst is rapidly surrounded by snail amebocytes (Lim 1970). An identical circumstance in insects is described in Salt ( 1970). C. Commonality

of Antigenic

Groupings

The evidence for common antigens between snails and their trematode parasites has been summarized by Kemp, Greene, and Damian (1974), who show that serum derived from rabbits immunized with uninfected B. pfeifferi hepatopancreas will produce the cercarienhiillen reaktion in cercariae of S. mansoni. The occurrence of common antigens between snails and cercariae derived (or rediae removed) from them is not unexpected. In order to determine whether larval trematodes independently possess antigens identical with their particular snail hosts, a culture method must be employed free from snail-derived molecules. With the recent availability of such methods (Voge and Seidel 1972; Basch and DiConza 1974), definitive answers may be anticipated. Recent reviews by Smithers (1972) and Clegg (1972) describe their work demonstrating the attachment of mouse erythrocyte antigens to the surface of young schistosomula of S. mansoni reared in mice, or in zjitro. This incorporation of host antigen has been confirmed by electron microscopy of worms treated with ferritinconjugated antiserum, and the specific substance acquired appears to be a glycolipid originating in the host red cell membrane (Clegg 1972). Dean ( 1974) has shown that selective antigen adsorption by schistosomula in culture appears to be a passive process, occurring even in 0.2% formalinfixed organisms. The role of host antigen adsorption in protection against lethal antibodies has been described by Clegg and Smithers ( 1972) ; it remains to be seen whether an analogous method of protection by antigen acquisition exists in the sporocysts, against snail amebocytes.

IN

SCHISTOSOMES

VI.

165

CONCLUSIONS

The amebocyte recognition mechanism functions to detect foreignness, i.e., chemical structural groupings not found in the host. Since protein structure is genetically determined, the range of detection capability of amebocytes is presumably subject to genetic control. The basis of compatibility may reside, therefore, in a concordance of genetically determined phenotypes in snail and schistosome, each of which is polymorphic with respect to the relevant traits (Basch 1975). Ewers and Rose (1966) have described differential infection rates with several trematodes (notably heterophyids) correlated with genetically determined differences in shell banding patterns in the gastropod Velacumantus australis. Wium-Andersen (1973) has found esterases to vary from one population to another of Biomphalaria alerandrina, whereas these enzymes were constant in populations of B. pfeiferi. Such a pattern parallels the highly variable infection rates for S. mansoni found in B. alexandrina and the uniform susceptibility in B. pfeiferi (Table I, Note 31) and is an indication of genetic diversity in populations of the Egyptian snail. Cheng (1968) proposed that the absence of growth and development stimulating substances, or the presence of inhibitors, as modified by host nutritional status and cellular defenses, may be responsible for incompatibilities between snails and trematodes. It is clear that hormonal and nutritional factors in the host’s internal environment, together with external conditions (e.g., temperature, water volume, current), and the detection capability of miracidia, will affect the establishment and course of infection in any potential snailmiracidium encounter. However, the weight of present evidence concerning the basic underlying mechanism of specificity points to the inherent genetically-based ability of snail amebocytes to recognize foreignness, coupled with the degree of

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166

foreignness exhibited by the surface of the larval trematode. Compatibility would thereby arise from host unresponsiveness to the presence of those sporocysts that are specifically “preadapted” to that host. Different frequencies of relevant, alleles in populations of host and parasite can provide the probabilistic foundation for the variety of observed infection rates. ACKNOWLEDGMENTS This work was from the National fectious Diseases, Bethesda, Maryland.

supported Institute National

by Grant AI-10271 of Allergy and InInstitutes of Health,

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