Refer to: Basch PF, Grodhaus G, DiConza JJ: Introduced schistosomiasis-Evaluation as a Public health hazard. West J Med 122:127-129, Feb 1975
Introduced Schistosomiasis Evaluation as a Public Health Hazard PAUL F. BASCH, PhD, MPH, Stanford, GAIL GRODHAUS, MA, Berkeley, and JOSEPH J. DiCONZA, PhD, Stanford
Schistosomiasis was found in many agricultural workers from Yemen who are residing in the San Joaquin Valley, and a study was done to evaluate the public health hazard of this imported disease. If the necessary intermediate hosts are present, a local focus of infection could be established. Numerous Biomphalaria obstructa snails collected in the vicinity of the Avenue 82 irrigation canal near Oasis, Riverside County, showed no evidence of natural schistosome infection. Laboratory-reared offspring of these snails were exposed to miracidia of Schistosoma mansoni originating from Yemen immigrants and to miracidia from a standard laboratory strain of Puerto Rican origin obtained from the National Institutes of Health. All exposures of California B. obstructa were made with a laboratory stock of highly susceptible B. glabrata snails also obtained from NIH as controls. Although miracidia penetrated California snails no evidence of infection was detected whereas the B. glabrata controls showed normal, mature infections with numerous cercariae. There appears little likelihood that B. obstructa can serve as intermediate host for schistosomiasis.
THE DISCOVERY OF A LARGE NUMBER of persons in California infected with schistosomiasis has
raised the question of possible establishment of this disease within the state. The infected population consists of agricultural workers originating from Yemen and now resident mainly in the San Joaquin Valley. Their number has been estimated From the Department of Family, Community and Preventive Medicine, Stanford University School of Medicine (Drs. Basch and DiConza); and the Vector Control Section, California Department of Health (Mr. Grodhaus). Submitted May 1, 1974. Supported by NIH Grant AI-10271. Reprint requests to: P. F. Basch, PhD, Department of Family, Community and Preventive Medicine, Stanford University School of Medicine, Stanford, CA 94305.
at about 2,000, about half of whom show evidence of infection.' The present study was undertaken to evaluate the hazard of this imported disease to the general population. Schistosomiasis is caused by trematode worms which inhabit the mesenteric or vesicular veins of man. Eggs produced by the female worms leave the body and if they should fall into fresh water, release a ciliated swimming larva. This organism, a miracidium, must enter a snail of a certain type in which it undergoes necessary developmental processes that result in the release of large numbers of cercariae. These exit from the snail and THE WESTERN JOURNAL OF MEDICINE
127
INTRODUCED SCHISTOSOMIASIS
swim in the water until they encounter an appropriate mammal which they enter by penetration of unbroken skin. There they change to adult forms and repeat the cycle. The establishment of this parasite in any region depends therefore upon the presence of (1) a source of schistosome ova, (2) contamination of freshwater by human dejecta, (3) appropriate susceptible snails, and (4) human contact with this water by swimming or wading. In California, conditions 1 and 4 are met, and 2 may be presumed to occur to some extent. The presence of the proper snail hosts may therefore result in the establishment of a local focus of infection. In the Yemeni workers two species of schistosomes are present. One, Schistosoma haematobium, requires a snail host of a group not known to occur in California, but the other, S. mansoni, passes through snails of the genus Biomphalaria which is found in small numbers in the state. In March 1972 one of us (G.G.) collected numerous specimens of small Biomphalaria snails from the vicinity of the Avenue 82 irrigation drainage canal near Oasis, Riverside County, adjacent to the Salton Sea. The snails were found mainly on surface vegetation, especially dead saltgrass, and mostly in less than 3 cm of water. It is not known if these snails occur in other parts of California. No evidence of natural schistosome infection was found in more than 100 snails brought to the laboratory. Their offspring have formed the population exposed to schistosome miracidia in the laboratory.
Experimental Procedure Laboratory-raised snails were exposed to miracidia of S. mansoni both individually and in groups (mass exposures). All ages from newly hatched to mature adult snails were employed. Three strains of S. mansoni were tested: a standard laboratory strain of Puerto Rican origin obtained from the National Institutes of Health, Bethesda, Maryland; a strain from Yemen, isolated in our laboratory from an adult male agricultural worker who was admitted to hospital in Santa Cruz for scrofula; and a strain from the region of Aden, southern Yemen, isolated by us from an adult male immigrant, resident in San Jose. The parasites were maintained in mice and hamsters and in a laboratory stock of a highly susceptible snail, Biomphalaria glabrata, albino stock PR-B, derived from NIH. All exposures of California Biomphalaria snails were made with B. 128
FEBRUARY 1975 * 122 * 2
TABLE
1.-Infection
of Biomphalaria Snails with Schistosoma mansoni, 1972-1974
Strain of S. mansons
B. glabarata*
Puerto Rico* Number exposed ......... Number surviving ........ Number infected ......... >80% Southern Yemen Number exposed ......... 121 Number surviving ........ 85 Number infected ......... 39
Yement Number exposed ......... Number surviving ........ Number infected .........
18 17 7
B. obstructa
165 110 0 162 90
0 10 9
0
*Our routine laboratory combination in use for several years; thousands of snails have been infected. tThis strain did not grow well in mice and was lost on the second passage.
glabrata as controls. Exposed snails were checked at intervals and those not found shedding cercariae after four to six weeks were dissected and examined for signs of schistosome infection.
Results Table 1 shows the number of snails exposed to each strain of S. mansoni, and the results. Although miracidia were seen to penetrate into the California snails, cercariae were never shed and no evidence of infection was detected upon dissection. The B. glabrata controls had normal, mature infections with numerous cercariae.
Discussion The snails collected near Oasis have been identified by one of us (P.F.B.) as Biomphalaria obstructa (Morelet) (Figure 1), known from the southern United States, Mexico, Puerto Rico and Cuba.2 Previous collections of snails of this type have been made from the Salton Sea area, as in the present case, from the vicinity of tropical fish farms. The classification and nomenclature of these small planorbid snails is chaotic and unsettled; therefore it is difficult to trace records concerning their susceptibility to schistosome infection. However, it seems unlikely that other California planorbid snails could serve as intermediate hosts. About 30 years ago Cram and coworkers3'4 found similar small snails from Baton Rouge, Louisiana, susceptible to a strain of Puerto Rican S. mansoni, achieving a 9 percent infection rate in their exposures. Subsequent studies by McQuay5'6 confirmed susceptibility in the Baton Rouge population but found New Orleans snails
INTRODUCED SCHISTOSOMIASIS
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tively identified by Leigh9 as B. obstructa. The presence of many infected Puerto Rican farm workers in that area suggested a study similar to the pne reported here, and no infections could be induced in the Florida snails. We do not know whether B. obstructa is native to California or introduced, perhaps with exotic fish or aquarium plants. Its present known distribution is limited and although it may extend its range in the state- there appears little likelihood that it can serve as an intermediate host for schistosomiasis.
k
REFERENCES
N.
'l-
1.:
Figure 1.-The shell of Biomphalaria obstructa. (Reproduced by permission from Pan American Health Organization, Scientific Publication No. 168.)
of the same species entirely refractory. Many collections of similar snails from various states al yielded negative results with S. mansoni.7 The Louisiana snails were identified as Tropicorbis havanensis, which is the same species complex as B. obstructa8 and is closely related to or perhaps identical with the California form. In south Florida, drainage canals teem with small snails tenta-
1. California Morbidity No. 23, 1972 2. Pan American Health Organization-A Guide for the Identification of the Snail Intermediate Hosts of Schistosomiasis in the Americas. Sci Pub No. 168, 1968 3. Cram EB, Jones M, Wright WH: A potential intermediate host of Schistosoma mansoni. Science 101:302, 1945 4. Cram EB, Files VS: Laboratory studies on the snail host of Schistosoma mansoni. Am J Trop Med 26:715-720, 1946 5. McQuay RM Jr: Susceptibility of a Louisiana species of Tropicorbis to infection with Schistosoma mansoni. Exper Parasitol 1: 184-188, 1952 6. McQuay RM Jr: Studies of variability in the susceptibility of a North American snail, Tropicorbis havanensis to infection with the Puerto Rican strain of Schistosoma mansoni. Trans Roy Soc Trop Med Hyg 47:56-61, 1953 7. Cram EB, Files VS, Jones MF: Experimental molluscan infection with Schistosoma mansoni and Schistosoma haematobium. Nat Inst Hlth Bull No. 189:81-94, 1947 8. Harry HW: A critical catalogue of the nominal genera and species of neotropical Planorbidae. Malacologia 1:33-53, 1962 9. Leigh WH: Exposure of South Florida snails of the genus Tropicorbis to miracidia of Schistosoma mansoni. I Parasitol 47:
50-51, 1961
THE WESTERN JOURNAL OF MEDICINE
129
Introduced Schistosomiasis Evaluation as a Public Health Hazard PAUL F. BASCH, PhD, MPH, Stanford, GAIL GRODHAUS, MA, Berkeley, and JOSEPH J. DiCONZA, PhD, Stanford
Schistosomiasis was found in many agricultural workers from Yemen who are residing in the San Joaquin Valley, and a study was done to evaluate the public health hazard of this imported disease. If the necessary intermediate hosts are present, a local focus of infection could be established. Numerous Biomphalaria obstructa snails collected in the vicinity of the Avenue 82 irrigation canal near Oasis, Riverside County, showed no evidence of natural schistosome infection. Laboratory-reared offspring of these snails were exposed to miracidia of Schistosoma mansoni originating from Yemen immigrants and to miracidia from a standard laboratory strain of Puerto Rican origin obtained from the National Institutes of Health. All exposures of California B. obstructa were made with a laboratory stock of highly susceptible B. glabrata snails also obtained from NIH as controls. Although miracidia penetrated California snails no evidence of infection was detected whereas the B. glabrata controls showed normal, mature infections with numerous cercariae. There appears little likelihood that B. obstructa can serve as intermediate host for schistosomiasis.
THE DISCOVERY OF A LARGE NUMBER of persons in California infected with schistosomiasis has
raised the question of possible establishment of this disease within the state. The infected population consists of agricultural workers originating from Yemen and now resident mainly in the San Joaquin Valley. Their number has been estimated From the Department of Family, Community and Preventive Medicine, Stanford University School of Medicine (Drs. Basch and DiConza); and the Vector Control Section, California Department of Health (Mr. Grodhaus). Submitted May 1, 1974. Supported by NIH Grant AI-10271. Reprint requests to: P. F. Basch, PhD, Department of Family, Community and Preventive Medicine, Stanford University School of Medicine, Stanford, CA 94305.
at about 2,000, about half of whom show evidence of infection.' The present study was undertaken to evaluate the hazard of this imported disease to the general population. Schistosomiasis is caused by trematode worms which inhabit the mesenteric or vesicular veins of man. Eggs produced by the female worms leave the body and if they should fall into fresh water, release a ciliated swimming larva. This organism, a miracidium, must enter a snail of a certain type in which it undergoes necessary developmental processes that result in the release of large numbers of cercariae. These exit from the snail and THE WESTERN JOURNAL OF MEDICINE
127
INTRODUCED SCHISTOSOMIASIS
swim in the water until they encounter an appropriate mammal which they enter by penetration of unbroken skin. There they change to adult forms and repeat the cycle. The establishment of this parasite in any region depends therefore upon the presence of (1) a source of schistosome ova, (2) contamination of freshwater by human dejecta, (3) appropriate susceptible snails, and (4) human contact with this water by swimming or wading. In California, conditions 1 and 4 are met, and 2 may be presumed to occur to some extent. The presence of the proper snail hosts may therefore result in the establishment of a local focus of infection. In the Yemeni workers two species of schistosomes are present. One, Schistosoma haematobium, requires a snail host of a group not known to occur in California, but the other, S. mansoni, passes through snails of the genus Biomphalaria which is found in small numbers in the state. In March 1972 one of us (G.G.) collected numerous specimens of small Biomphalaria snails from the vicinity of the Avenue 82 irrigation drainage canal near Oasis, Riverside County, adjacent to the Salton Sea. The snails were found mainly on surface vegetation, especially dead saltgrass, and mostly in less than 3 cm of water. It is not known if these snails occur in other parts of California. No evidence of natural schistosome infection was found in more than 100 snails brought to the laboratory. Their offspring have formed the population exposed to schistosome miracidia in the laboratory.
Experimental Procedure Laboratory-raised snails were exposed to miracidia of S. mansoni both individually and in groups (mass exposures). All ages from newly hatched to mature adult snails were employed. Three strains of S. mansoni were tested: a standard laboratory strain of Puerto Rican origin obtained from the National Institutes of Health, Bethesda, Maryland; a strain from Yemen, isolated in our laboratory from an adult male agricultural worker who was admitted to hospital in Santa Cruz for scrofula; and a strain from the region of Aden, southern Yemen, isolated by us from an adult male immigrant, resident in San Jose. The parasites were maintained in mice and hamsters and in a laboratory stock of a highly susceptible snail, Biomphalaria glabrata, albino stock PR-B, derived from NIH. All exposures of California Biomphalaria snails were made with B. 128
FEBRUARY 1975 * 122 * 2
TABLE
1.-Infection
of Biomphalaria Snails with Schistosoma mansoni, 1972-1974
Strain of S. mansons
B. glabarata*
Puerto Rico* Number exposed ......... Number surviving ........ Number infected ......... >80% Southern Yemen Number exposed ......... 121 Number surviving ........ 85 Number infected ......... 39
Yement Number exposed ......... Number surviving ........ Number infected .........
18 17 7
B. obstructa
165 110 0 162 90
0 10 9
0
*Our routine laboratory combination in use for several years; thousands of snails have been infected. tThis strain did not grow well in mice and was lost on the second passage.
glabrata as controls. Exposed snails were checked at intervals and those not found shedding cercariae after four to six weeks were dissected and examined for signs of schistosome infection.
Results Table 1 shows the number of snails exposed to each strain of S. mansoni, and the results. Although miracidia were seen to penetrate into the California snails, cercariae were never shed and no evidence of infection was detected upon dissection. The B. glabrata controls had normal, mature infections with numerous cercariae.
Discussion The snails collected near Oasis have been identified by one of us (P.F.B.) as Biomphalaria obstructa (Morelet) (Figure 1), known from the southern United States, Mexico, Puerto Rico and Cuba.2 Previous collections of snails of this type have been made from the Salton Sea area, as in the present case, from the vicinity of tropical fish farms. The classification and nomenclature of these small planorbid snails is chaotic and unsettled; therefore it is difficult to trace records concerning their susceptibility to schistosome infection. However, it seems unlikely that other California planorbid snails could serve as intermediate hosts. About 30 years ago Cram and coworkers3'4 found similar small snails from Baton Rouge, Louisiana, susceptible to a strain of Puerto Rican S. mansoni, achieving a 9 percent infection rate in their exposures. Subsequent studies by McQuay5'6 confirmed susceptibility in the Baton Rouge population but found New Orleans snails
INTRODUCED SCHISTOSOMIASIS
$
.
/
I.
I.. .. "
I-
.yI
-4-1
j. A
i .v t -.
i frJ
tively identified by Leigh9 as B. obstructa. The presence of many infected Puerto Rican farm workers in that area suggested a study similar to the pne reported here, and no infections could be induced in the Florida snails. We do not know whether B. obstructa is native to California or introduced, perhaps with exotic fish or aquarium plants. Its present known distribution is limited and although it may extend its range in the state- there appears little likelihood that it can serve as an intermediate host for schistosomiasis.
k
REFERENCES
N.
'l-
1.:
Figure 1.-The shell of Biomphalaria obstructa. (Reproduced by permission from Pan American Health Organization, Scientific Publication No. 168.)
of the same species entirely refractory. Many collections of similar snails from various states al yielded negative results with S. mansoni.7 The Louisiana snails were identified as Tropicorbis havanensis, which is the same species complex as B. obstructa8 and is closely related to or perhaps identical with the California form. In south Florida, drainage canals teem with small snails tenta-
1. California Morbidity No. 23, 1972 2. Pan American Health Organization-A Guide for the Identification of the Snail Intermediate Hosts of Schistosomiasis in the Americas. Sci Pub No. 168, 1968 3. Cram EB, Jones M, Wright WH: A potential intermediate host of Schistosoma mansoni. Science 101:302, 1945 4. Cram EB, Files VS: Laboratory studies on the snail host of Schistosoma mansoni. Am J Trop Med 26:715-720, 1946 5. McQuay RM Jr: Susceptibility of a Louisiana species of Tropicorbis to infection with Schistosoma mansoni. Exper Parasitol 1: 184-188, 1952 6. McQuay RM Jr: Studies of variability in the susceptibility of a North American snail, Tropicorbis havanensis to infection with the Puerto Rican strain of Schistosoma mansoni. Trans Roy Soc Trop Med Hyg 47:56-61, 1953 7. Cram EB, Files VS, Jones MF: Experimental molluscan infection with Schistosoma mansoni and Schistosoma haematobium. Nat Inst Hlth Bull No. 189:81-94, 1947 8. Harry HW: A critical catalogue of the nominal genera and species of neotropical Planorbidae. Malacologia 1:33-53, 1962 9. Leigh WH: Exposure of South Florida snails of the genus Tropicorbis to miracidia of Schistosoma mansoni. I Parasitol 47:
50-51, 1961
THE WESTERN JOURNAL OF MEDICINE
129
