J. COMP. PATH. 1976. VOL. 86.
73
OBSERVATIONS ON THE RESUMED DEVELOPMENT ARRESTED OSTER.TAGIA OSTERTAGI IN NATURALLY INFECTED YEARLING CATTLE
OF
BY
J. F.
MICHEL,
M. B.
LANCASTER
Veterinav
Laboratory,
Central
and C.
HONG
Weybridge
INTRODUCTION
For a number of years it has been known that during the winter following their first grazing season,many yearling cattle harbour very large burdens of Ostertagia ostertagi arrested in their development at the early fourth stage. Anderson, Armour, Jennings and Ritchie (1966) observed that the presence of these burdens was associatedwith the grazing of infested pasture during the autumn and that it occurred equally in susceptibleand resistant cattle (Anderson, Armour, Jennings, Ritchie and Urquhart, 1965). Subsequently Armour, Jennings and Urquhart (1969) showed that early parasitic development was arrested when the free-living stageshad been exposed to appropriate environmental conditions. The view has therefore come to be accepted that, in Britain at least, the arrested development of 0. ostertagi is a part of what is termed ‘the strategy of overwinter survival’. This is in line with the opinion of Taylor and Michel (1953) that arrested development might be seen as an adaptation enabling the parasite to survive a period of unfavourable conditions. Certainly, a severe selection operates against 0. ostertagi which become adult in late autumn or winter. Becausethe life of adult worms of this speciesis generally short, and becauseeggsreaching the pasture after September have a negligible chance of survival, such worms leave no progeny. Little is known about the resumed development of arrested 0. ostertagi. Surveys of the kind undertaken by Ross (1965) showed that in the spring the numbers of early 4th stage worms declined while numbers of adults tended to increase. It is reasonable to conclude that development was occurring. No direct observations have been reported and accordingly, sometrials of a very simple form were undertaken and are reported here. In 3 successive years, groups of calves were used to contaminate a pasture during June and July. They returned to this pasture in September and remained on it until the end of December so that they might acquire large burdens of arrested 0. ostertagi. From the beginning of January the cattle were housed in conditions calculated to prevent further nematode infection and thereafter, and until early summer, animals were killed at intervals and their worm burdens examined. MATERIALS
AND
METHODS
Worms. Ostertagia ostertagi originating from the “Glasgow field strain”, made available in 1970 by Dr J. Armour, were used. Cattle. In 1972-73 Jersey steers were used which had been purchased from local breeders when 5 days old and reared at the laboratory. In the 2 following years some of the calves were of this description, but the remainder were home bred Hereford cross calves. All were reared in conditions which have been found effective in preventing infection with trichostrongylid nematodes.
74
J. F. MICHEL
et al.
Infection of calves and contamination of pasture. The calves were initially infected by allowing them to graze in the spring on paddocks carrying an overwintered infection of 0. ostertagi. They were moved in June to the pasture on which subsequently they were to acquire the experimental infection. Both faecal egg output and the resulting infestation on the herbage were monitored by standard techniques. The process of contamination was allowed to continue until dangerous levels of infestation were reached in late July or early August when the cattle were either housed or moved to other pastures. In late September they were brought back to the contaminated pasture and remained on it until the end of December, supplementary feed being given when it became necessary. Housing and necropsy. In the 1st and 3rd year the cattle were loose-housed in a covered yard; in the 2nd year they were tied by the head. In the second and third years the cattle were weighed weekly from the beginning of January until they were killed. One or two cattle were killed when the group was housed; thereafter, animals were killed at intervals and worm counts made at necropsy. RESULTS
Period 1972-73 Worm counts and some other details of all the calves used in these observations are shown in Table 1. In the first year, large burdens of around 300 000 arrested early 4th stage 0. ostertagi were established. Individual variation in these burdens was considerable and tended to obscure changes that may have occurred until the beginning of April. As will be seen from Fig. 1 numbers of late 4th stage and 5th stage worms remained at a low level until the end of February. In March the number of late 4th stage larvae rose sharply and this I
I
1
I
Early 4th stage fl
L
I
I dun Fig.
1. Worm
I Fob
counts
at necropsy
I MW
in cattle
I APr
I May
used in 1972-73.
was accompanied by an increase in the number of adult worms. At the beginning of April the number of adults had reached a peak and the number of late 4th stage larvae, though still high, had apparently begun to decline; thereafter the number of worms of all stagesdecreased rapidly. If it is assumed that the time taken for development through the late 4th stage does not vary
RESUMED
DEVELOPMENT
OF
0.
ARRESTED
75
OSt&Ugi
greatly, then the number of worms at this stage during March could indicate that up to 450 000 larvae developed in that month. This could account for the disappearance of the entire burden of arrested larvae; it is not necessary to postulate a loss by any other route. TABLE DETAILS
Date killed
Year 28.12.72 18. 8. 1. 29. 5. 24. 26. 21. 2.
Yearling NO.
D D
CATTLE
Age on 2nd Jan Cd.)
USED
wt on 2nd Jan (kg. )
AND
1
OF THEIR
Wt change in March (kg/week)
WORM
COUNTS
AT NECROPSY
,vo. of 0.
ostertagi
recouere~I
5th stage
late 4th stage
Early 4th stage
15 500
4100 1800 2100 5200 11 900 61 300 36 500 3400 10 000 700 400
449 100 357 000 154 500 397 000 254 400 117400 334 000 27 000 72 300 24 200 1200
Total
1 456 459 462 459 455 455 455 455 455 459 455
1.73 2.73 3.73 3.73 4.73 4.73 4.73 5.73 6.73
Year 2 13.12.73 2. 1.74 15. 1.74 29. 1.74 12. 2.74 26. 2.74 6. 3.74 12. 3.74 18. 3.74 21. 3.74 25. 3.74 28. 3.74 2. 4.75 9. 4.74 16. 4.74 23. 4.74 7. 5.74 21. 5.74
D D
OF THE
Year 2. 14. 14. 16. 6. 9. 13. 13. 27. 10. 20. 1. 24. 1. 13.
3 1.75 1.75 1.75 1.75 2.75 2.75 2.75 2.75 2.75 3.75 3.75 4.75 4.75 5.75 5.75
D =
acute
313 326 337 375 313 354 292 338 320 318 295 315 347 314 339 413 609
110 241 130 232 152 160 111 200 132 150 112 172 168 205 177 225
360 350 364 424 356 412 392 426 372 432 474 428 358 393 436
101 104 233 140 152 142 230 154 266 130 236 119 144 169
329
ostertagiasis,
8300 8300 9800 26 100 58 200 87 000 17 500 13 100 600 1100
killed
on humanitarian
+ 10.5 +11.7 + 8.5 + 7.7 + 7.8 + 4.8 + 6.8 + 6.0 + 5.8 + 6.2 + 4-2
+ + +
5.5 2.3 4.0 l-4 3-o
grounds.
7500 15 400 20 250 4900 5300 6700 27 300 13 100 12 800 5000 4500 5300 12 400 16 400 2300 8300 4300 0
3400 47 100 39 000 30 200 11000 95 200 89 300 1800 81 500 44 600 199 100 119500 4800 2000 100
300 1200 4250 800 1100 2900 5300 3700 3400 1000 1500 2z 8700 300 2700 800 0
3300 62 000 126 000 7600 3000 27 100 71 900 1200 25 000 22 800 61 300 37 000
468 367 164 412 292 236 457 47 95 25
700 100 900 000 400 900 500 900 400 500 2700
100 98 75 10 16 37 70 24 55
700 100 750 000 000 900 400 600 200 2400 6000 3400 12 600 48 600 1100 10 000 1600 300
IO8 500 114 700 100 250 15 700 22 400 47 500 103 000 41 400 71400 8400 12 000 9500 27 300 73 700 3700 21000 6700 300
122 800 237 500 231 500 202 000 140 700 131 600 140 700 99 750 305 000 106 900 221 100 44 300 10 200 189M) 1200
129 500 346 600 396 500 239 800 154 700 253 900 301900 102 750 411 500 174 300 481 500 200 800 17 200 21900 1400
76
J. F. MICHEL
et a/.
Period 19 7% 74 In the second year, the herbage larvae were very much smaller and Fig. 2, numbers of adult constant throughout the period stage larvae declined steadily. indicative of the development
infestation and the resulting burdens of arrested than in the first. As may be seen from Table 1 and late 4th stage worms remained roughly of observation while the number of early 4th The number of worms in the late 4th stage is of approximately 50 000 larvae during the
I
I D0c
Fig. 2. Worm
I JOI!
counts
I Feb
at necropsy
I M0r
in cattle
Fig. 3. 1974-75. Changes in mean live weight of the 4 yearlings ( - - - - ) and of the rest of the group (---).
I APT
MOY
used in 1973-74.
which
developed
acute
ostertagiasis
RESUMED
DEVELOPMENT
OF ARRESTED
0.
77
o.&?rta@
period of observation and is therefore sufficient to account for the loss of the entire burden of arrested early 4th stage larvae. There is no clear indication of an increase in the number developing in March, but the population of arrested larvae was already very small at this time.
2 I 0 I 2
2 I 0 I
2 t 3
I
1 ’ Fig. 4. Worm above,
I
Jan
I
I
fib
counts at necropsy in cattle the remainder below.
I
Mar
used in 1974-75.
I
I
APT
The 4 casts ofwinter
I
0
ostertagiasis
are shown
’ 4
250
c8 E -0 Y
0 0
I50
8
0
. 0
0 2loo
0
1
I ’
I
350
400
I 450
I
Aw
Fig. 5. Relationship of weight (kg.) to age (days), animals which developed acute ostertagiasis rest of the group by ( 0).
on 2nd January, of cattle in January and February
used in 1974-75. The 4 are shown by ( l ), the
78
J. F. MICHEL
et
al.
Period 1974-75 In the third year heavy herbage infestations were again achieved and not only did the cattle acquire large burdens of arrested Ostertagia, but their growth was adversely affected. When they were housed, most of the animals were holding their own, but 4 clearly differed from the remainder of the group in that they were in markedly poorer condition. Their liveweight, shown in Fig. 3, continued to decline and it was necessary to kill 2 in mid-January and 2 in February. In all 4 cases the number of developing and adult worms present was large (see Fig. 4) and it was clear that a large part of the burden of arrested worms had resumed its development. In the other cattle the number of late 4th stage and 5th stage worms remained low until the end of February. Both developing and adult worms greatly increased in number in March and the burden of early 4th stage larvae diminished. By May, few worms of any stage remained. It must be concluded that as in the first year of these observations, most of the arrested larvae resumed their development in March and that the resulting adult worms did not persist for very long. Although there was no change in feeding or in the conditions in which the cattle were maintained, their rate of live weight increase, shown in Fig. 3, was reduced during March and early April. DISCUSSION
While it is evident that arrested larvae can and do resume their development at other times, the present work reveals a marked tendency for nearly all those present in March to develop by the beginning of April. The adult worms resulting from this development are lost fairly quickly and by the end of May few worms of any stage remain. Resumption of development at a particular time of year is a necessary sequel to a seasonally induced arrest if this is to enable worms to postpone their adult life until conditions again favour the survival of their progeny. The mechanisms by which this seasonally synchronized development is brought about remain to be elucidated. Armour and Bruce (1974) suggested that arrested larvae develop spontaneously 16 to 18 weeks after their entry into the host. Equally, and perhaps more probably, their results might indicate spontaneous development 2 1 to 23 weeks after the start of the conditioning treatment to which the infective larvae were subjected. It may be relevant that the deconditioning of larvae, whereby they lose the ability to become arrested if ingested by the host, also occurs after about this period of time (Michel, Lancaster and Hong 1970, 1974). Spontaneous development after a fixed lapse of time need not be the only factor involved. Duplication of mechanism, discussed in another context by Barcroft (1934), is likely, and the possibility cannot be discounted that arrested larvae are sensitive to endocrine signals transmitted by the host. In all three years of these observations, developing, i.e. late 4th stage, worms were constantly present in the cattle from January until May. Since new infection was prevented, these must have been derived from arrested larvae. Their numbers indicate a loss from the reservoir of arrested worms of the order of 65 000 in 1973,50 000 in 1974 and 75 000 in 1975. This steady loss was most
RESUMED
DEVELOPMENT
OF
ARRESTED
0. ostertagi
79
plainly seen in 1974 when the initial level was low and the decrease in the number of early 4th stage larvae was apparently linear. Michel (1970)) working with a system in which constant recruitment to the population was provided by the daily administration of infective larvae, concluded that the mean life span of adult 0. ostertagi was about 25 days. A turnover at this rate implies a population structure with between four and five times as many adults as late 4th stage larvae and this ratio is seen in the present results except where, as in March 1973 and March 1975 or in the four acute cases in 1975, the numbers resuming their development were greatly increased. The size of a population of adult worms depends on their life span and on the rate of recruitment which, in the present case, is the rate at which arrested larvae resume their development. How this rate is determined is still obscure. Neither unpublished work by Michel (1967) nor the present data suggest that there is any simple relationship between the number of early 4th stage larvae and the number re-embarking on their development per unit time. Meanwhile, the results of Michel (1963, 1971) provide evidence that the removal of adult worms can lead to the development of arrested worms though probably only in numbers sufficient to replace the adults removed. The somewhat circular relationship implied by these findings, namely that the number of adult worms is determined by the rate at which arrested worms resume their development which in turn, depends on the number of adults, must present conceptual difficulties. The simultaneous development of large numbers of arrested larvae can also occur at other times than in March. Outbreaks of clinical ostertagiasis referable to the development of arrested larvae occur in the field from January onwards. In the third year of the present observations, four animals reacted differently from the remainder of the group. Their weight decreased sharply, they showed symptoms of acute ostertagiasis, two had to be killed in January and two in February, and it was plain from their worm burdens that large numbers of arrested worms had resumed their development. These four animals were among the youngest and the smallest of the group and they were light for their age (Fig. 5). These circumstances suggest that development was prompted by some failure of host resistance, using the term in its widest sense. It is significant in this connection that host resistance has been shown by Michel, Lancaster and Hong (1973) to be a possible cause of arrested development and unpublished work by the same authors (1975) shows that it can be as effective a cause as environmental conditioning of the free-living stages. It is clear that just as arrested development of 0. ostertagi may be due to more than one cause so development may be induced to recommence in at least three circumstances: ( a ) a small constant number resumes its development every day, (b) the rate is very greatly increased during March (c) this increase may also occur earlier, apparently through a breakdown of host resistance. The question arises whether distinct mechanisms are involved and whether worms arrested on account of one cause will only recommence their development in response to the corresponding stimulus. The present observations indicate that this is not so.
80
J. F. MICHEL
et
Uf.
SUMMARY
In three successive years, groups of yearling cattle were exposed in autumn to grazing infested with Ostertagia ostertagi. They were housed at the end of December and the fate of the burdens of arrested early fourth stage Ostertagia larvae which they had acquired was studied by means of post mortem worm counts on animals killed at intervals until early summer. Arrested worms resumed their development at a steady rate of approximately 500 per day. Nearly all the arrested worms remaining in March developed during that month. In a small number of animals whose history and condition suggested an impairment of resistance, the development of very large numbers of arrested worms in January and February led to fatal disease. REFERENCES
Anderson, N., Armour, J., Jennings, F. W., and Ritchie, D. S. (1966). Clinical bovine ostertagiasis. Proceedings, First International Congress of Parasitology, (1964) II, 549. Rome. Anderson, N., Armour, J., Jennings, F. W., Ritchie, J. D., and Urquhart, G. M. (1965). Inhibited development of Ostertagia ostertagi. Veterinary Record, 77, 146147. Armour, J., and Bruce, R. G. (1974). Inhibited development in Ostertagia ostertagi infections-a diapause phenomenon in a nematode. Parasitology, 69, 161-174. Armour, J., Jennings, F. W., and Urquhart, G. M. (1969). Inhibition of Ostertagia ostertagi at the early fourth larval stage. II. The influence of environment on host or parasite. Research in Veterinary Science, 10, 238-244. Barcroft, J. (1934). Features in the Architecture of Physiological Function. Cambridge University Press. Michel, J. F. (1963). The phenomena of host resistance and the course of infection of Ostertagia ostertagi in calves. Parasitology, 53, 63-84. Michel, J. F. ( 1970). The regulation of populations of Ostertagia ostertagi. Parasitology, 61,435-447. Michel, J. F. (1971). Adult worms as a factor in the inhibition of development of Ostertagia ostertagi. International Journal for Parasitology, 1, 3l-36. Michel, J. F., Lancaster, M. B., and Hong, C. (1970). Observations on the inhibition of Cooperia oncophora in calves. British Veterinary Journal, 126, xxxv-xxxvii. Michel, J. F., Lancaster, M. B., and Hong, C. (1973). Ostertagia ostertagi: Protective immunity in calves. The development in calves of a protective immunity to infection with Ostertagia ostertagi. Experimental Parasitology, 33, 179-l 86. Michel, J. F., Lancaster, M. B., and Hong, C. (1974). Studies on arrested development of Ostertagia ostertagi and Cooperia oncophora. Journal of Comparative Pathology, 84, 539-554. Ross, J. G. (1965). The seasonal incidence of ostertagiasis in cattle in Northern Ireland. Veterinary Record, 77, 16-19. J. F. (1953). The parasitological and pathological Taylor, E. L., and Michel, significance of arrested development in nematodes. Journal of IIelmintholopy, 27, 199-205. [Received for publication,
June 16th, 19751
73
OBSERVATIONS ON THE RESUMED DEVELOPMENT ARRESTED OSTER.TAGIA OSTERTAGI IN NATURALLY INFECTED YEARLING CATTLE
OF
BY
J. F.
MICHEL,
M. B.
LANCASTER
Veterinav
Laboratory,
Central
and C.
HONG
Weybridge
INTRODUCTION
For a number of years it has been known that during the winter following their first grazing season,many yearling cattle harbour very large burdens of Ostertagia ostertagi arrested in their development at the early fourth stage. Anderson, Armour, Jennings and Ritchie (1966) observed that the presence of these burdens was associatedwith the grazing of infested pasture during the autumn and that it occurred equally in susceptibleand resistant cattle (Anderson, Armour, Jennings, Ritchie and Urquhart, 1965). Subsequently Armour, Jennings and Urquhart (1969) showed that early parasitic development was arrested when the free-living stageshad been exposed to appropriate environmental conditions. The view has therefore come to be accepted that, in Britain at least, the arrested development of 0. ostertagi is a part of what is termed ‘the strategy of overwinter survival’. This is in line with the opinion of Taylor and Michel (1953) that arrested development might be seen as an adaptation enabling the parasite to survive a period of unfavourable conditions. Certainly, a severe selection operates against 0. ostertagi which become adult in late autumn or winter. Becausethe life of adult worms of this speciesis generally short, and becauseeggsreaching the pasture after September have a negligible chance of survival, such worms leave no progeny. Little is known about the resumed development of arrested 0. ostertagi. Surveys of the kind undertaken by Ross (1965) showed that in the spring the numbers of early 4th stage worms declined while numbers of adults tended to increase. It is reasonable to conclude that development was occurring. No direct observations have been reported and accordingly, sometrials of a very simple form were undertaken and are reported here. In 3 successive years, groups of calves were used to contaminate a pasture during June and July. They returned to this pasture in September and remained on it until the end of December so that they might acquire large burdens of arrested 0. ostertagi. From the beginning of January the cattle were housed in conditions calculated to prevent further nematode infection and thereafter, and until early summer, animals were killed at intervals and their worm burdens examined. MATERIALS
AND
METHODS
Worms. Ostertagia ostertagi originating from the “Glasgow field strain”, made available in 1970 by Dr J. Armour, were used. Cattle. In 1972-73 Jersey steers were used which had been purchased from local breeders when 5 days old and reared at the laboratory. In the 2 following years some of the calves were of this description, but the remainder were home bred Hereford cross calves. All were reared in conditions which have been found effective in preventing infection with trichostrongylid nematodes.
74
J. F. MICHEL
et al.
Infection of calves and contamination of pasture. The calves were initially infected by allowing them to graze in the spring on paddocks carrying an overwintered infection of 0. ostertagi. They were moved in June to the pasture on which subsequently they were to acquire the experimental infection. Both faecal egg output and the resulting infestation on the herbage were monitored by standard techniques. The process of contamination was allowed to continue until dangerous levels of infestation were reached in late July or early August when the cattle were either housed or moved to other pastures. In late September they were brought back to the contaminated pasture and remained on it until the end of December, supplementary feed being given when it became necessary. Housing and necropsy. In the 1st and 3rd year the cattle were loose-housed in a covered yard; in the 2nd year they were tied by the head. In the second and third years the cattle were weighed weekly from the beginning of January until they were killed. One or two cattle were killed when the group was housed; thereafter, animals were killed at intervals and worm counts made at necropsy. RESULTS
Period 1972-73 Worm counts and some other details of all the calves used in these observations are shown in Table 1. In the first year, large burdens of around 300 000 arrested early 4th stage 0. ostertagi were established. Individual variation in these burdens was considerable and tended to obscure changes that may have occurred until the beginning of April. As will be seen from Fig. 1 numbers of late 4th stage and 5th stage worms remained at a low level until the end of February. In March the number of late 4th stage larvae rose sharply and this I
I
1
I
Early 4th stage fl
L
I
I dun Fig.
1. Worm
I Fob
counts
at necropsy
I MW
in cattle
I APr
I May
used in 1972-73.
was accompanied by an increase in the number of adult worms. At the beginning of April the number of adults had reached a peak and the number of late 4th stage larvae, though still high, had apparently begun to decline; thereafter the number of worms of all stagesdecreased rapidly. If it is assumed that the time taken for development through the late 4th stage does not vary
RESUMED
DEVELOPMENT
OF
0.
ARRESTED
75
OSt&Ugi
greatly, then the number of worms at this stage during March could indicate that up to 450 000 larvae developed in that month. This could account for the disappearance of the entire burden of arrested larvae; it is not necessary to postulate a loss by any other route. TABLE DETAILS
Date killed
Year 28.12.72 18. 8. 1. 29. 5. 24. 26. 21. 2.
Yearling NO.
D D
CATTLE
Age on 2nd Jan Cd.)
USED
wt on 2nd Jan (kg. )
AND
1
OF THEIR
Wt change in March (kg/week)
WORM
COUNTS
AT NECROPSY
,vo. of 0.
ostertagi
recouere~I
5th stage
late 4th stage
Early 4th stage
15 500
4100 1800 2100 5200 11 900 61 300 36 500 3400 10 000 700 400
449 100 357 000 154 500 397 000 254 400 117400 334 000 27 000 72 300 24 200 1200
Total
1 456 459 462 459 455 455 455 455 455 459 455
1.73 2.73 3.73 3.73 4.73 4.73 4.73 5.73 6.73
Year 2 13.12.73 2. 1.74 15. 1.74 29. 1.74 12. 2.74 26. 2.74 6. 3.74 12. 3.74 18. 3.74 21. 3.74 25. 3.74 28. 3.74 2. 4.75 9. 4.74 16. 4.74 23. 4.74 7. 5.74 21. 5.74
D D
OF THE
Year 2. 14. 14. 16. 6. 9. 13. 13. 27. 10. 20. 1. 24. 1. 13.
3 1.75 1.75 1.75 1.75 2.75 2.75 2.75 2.75 2.75 3.75 3.75 4.75 4.75 5.75 5.75
D =
acute
313 326 337 375 313 354 292 338 320 318 295 315 347 314 339 413 609
110 241 130 232 152 160 111 200 132 150 112 172 168 205 177 225
360 350 364 424 356 412 392 426 372 432 474 428 358 393 436
101 104 233 140 152 142 230 154 266 130 236 119 144 169
329
ostertagiasis,
8300 8300 9800 26 100 58 200 87 000 17 500 13 100 600 1100
killed
on humanitarian
+ 10.5 +11.7 + 8.5 + 7.7 + 7.8 + 4.8 + 6.8 + 6.0 + 5.8 + 6.2 + 4-2
+ + +
5.5 2.3 4.0 l-4 3-o
grounds.
7500 15 400 20 250 4900 5300 6700 27 300 13 100 12 800 5000 4500 5300 12 400 16 400 2300 8300 4300 0
3400 47 100 39 000 30 200 11000 95 200 89 300 1800 81 500 44 600 199 100 119500 4800 2000 100
300 1200 4250 800 1100 2900 5300 3700 3400 1000 1500 2z 8700 300 2700 800 0
3300 62 000 126 000 7600 3000 27 100 71 900 1200 25 000 22 800 61 300 37 000
468 367 164 412 292 236 457 47 95 25
700 100 900 000 400 900 500 900 400 500 2700
100 98 75 10 16 37 70 24 55
700 100 750 000 000 900 400 600 200 2400 6000 3400 12 600 48 600 1100 10 000 1600 300
IO8 500 114 700 100 250 15 700 22 400 47 500 103 000 41 400 71400 8400 12 000 9500 27 300 73 700 3700 21000 6700 300
122 800 237 500 231 500 202 000 140 700 131 600 140 700 99 750 305 000 106 900 221 100 44 300 10 200 189M) 1200
129 500 346 600 396 500 239 800 154 700 253 900 301900 102 750 411 500 174 300 481 500 200 800 17 200 21900 1400
76
J. F. MICHEL
et a/.
Period 19 7% 74 In the second year, the herbage larvae were very much smaller and Fig. 2, numbers of adult constant throughout the period stage larvae declined steadily. indicative of the development
infestation and the resulting burdens of arrested than in the first. As may be seen from Table 1 and late 4th stage worms remained roughly of observation while the number of early 4th The number of worms in the late 4th stage is of approximately 50 000 larvae during the
I
I D0c
Fig. 2. Worm
I JOI!
counts
I Feb
at necropsy
I M0r
in cattle
Fig. 3. 1974-75. Changes in mean live weight of the 4 yearlings ( - - - - ) and of the rest of the group (---).
I APT
MOY
used in 1973-74.
which
developed
acute
ostertagiasis
RESUMED
DEVELOPMENT
OF ARRESTED
0.
77
o.&?rta@
period of observation and is therefore sufficient to account for the loss of the entire burden of arrested early 4th stage larvae. There is no clear indication of an increase in the number developing in March, but the population of arrested larvae was already very small at this time.
2 I 0 I 2
2 I 0 I
2 t 3
I
1 ’ Fig. 4. Worm above,
I
Jan
I
I
fib
counts at necropsy in cattle the remainder below.
I
Mar
used in 1974-75.
I
I
APT
The 4 casts ofwinter
I
0
ostertagiasis
are shown
’ 4
250
c8 E -0 Y
0 0
I50
8
0
. 0
0 2loo
0
1
I ’
I
350
400
I 450
I
Aw
Fig. 5. Relationship of weight (kg.) to age (days), animals which developed acute ostertagiasis rest of the group by ( 0).
on 2nd January, of cattle in January and February
used in 1974-75. The 4 are shown by ( l ), the
78
J. F. MICHEL
et
al.
Period 1974-75 In the third year heavy herbage infestations were again achieved and not only did the cattle acquire large burdens of arrested Ostertagia, but their growth was adversely affected. When they were housed, most of the animals were holding their own, but 4 clearly differed from the remainder of the group in that they were in markedly poorer condition. Their liveweight, shown in Fig. 3, continued to decline and it was necessary to kill 2 in mid-January and 2 in February. In all 4 cases the number of developing and adult worms present was large (see Fig. 4) and it was clear that a large part of the burden of arrested worms had resumed its development. In the other cattle the number of late 4th stage and 5th stage worms remained low until the end of February. Both developing and adult worms greatly increased in number in March and the burden of early 4th stage larvae diminished. By May, few worms of any stage remained. It must be concluded that as in the first year of these observations, most of the arrested larvae resumed their development in March and that the resulting adult worms did not persist for very long. Although there was no change in feeding or in the conditions in which the cattle were maintained, their rate of live weight increase, shown in Fig. 3, was reduced during March and early April. DISCUSSION
While it is evident that arrested larvae can and do resume their development at other times, the present work reveals a marked tendency for nearly all those present in March to develop by the beginning of April. The adult worms resulting from this development are lost fairly quickly and by the end of May few worms of any stage remain. Resumption of development at a particular time of year is a necessary sequel to a seasonally induced arrest if this is to enable worms to postpone their adult life until conditions again favour the survival of their progeny. The mechanisms by which this seasonally synchronized development is brought about remain to be elucidated. Armour and Bruce (1974) suggested that arrested larvae develop spontaneously 16 to 18 weeks after their entry into the host. Equally, and perhaps more probably, their results might indicate spontaneous development 2 1 to 23 weeks after the start of the conditioning treatment to which the infective larvae were subjected. It may be relevant that the deconditioning of larvae, whereby they lose the ability to become arrested if ingested by the host, also occurs after about this period of time (Michel, Lancaster and Hong 1970, 1974). Spontaneous development after a fixed lapse of time need not be the only factor involved. Duplication of mechanism, discussed in another context by Barcroft (1934), is likely, and the possibility cannot be discounted that arrested larvae are sensitive to endocrine signals transmitted by the host. In all three years of these observations, developing, i.e. late 4th stage, worms were constantly present in the cattle from January until May. Since new infection was prevented, these must have been derived from arrested larvae. Their numbers indicate a loss from the reservoir of arrested worms of the order of 65 000 in 1973,50 000 in 1974 and 75 000 in 1975. This steady loss was most
RESUMED
DEVELOPMENT
OF
ARRESTED
0. ostertagi
79
plainly seen in 1974 when the initial level was low and the decrease in the number of early 4th stage larvae was apparently linear. Michel (1970)) working with a system in which constant recruitment to the population was provided by the daily administration of infective larvae, concluded that the mean life span of adult 0. ostertagi was about 25 days. A turnover at this rate implies a population structure with between four and five times as many adults as late 4th stage larvae and this ratio is seen in the present results except where, as in March 1973 and March 1975 or in the four acute cases in 1975, the numbers resuming their development were greatly increased. The size of a population of adult worms depends on their life span and on the rate of recruitment which, in the present case, is the rate at which arrested larvae resume their development. How this rate is determined is still obscure. Neither unpublished work by Michel (1967) nor the present data suggest that there is any simple relationship between the number of early 4th stage larvae and the number re-embarking on their development per unit time. Meanwhile, the results of Michel (1963, 1971) provide evidence that the removal of adult worms can lead to the development of arrested worms though probably only in numbers sufficient to replace the adults removed. The somewhat circular relationship implied by these findings, namely that the number of adult worms is determined by the rate at which arrested worms resume their development which in turn, depends on the number of adults, must present conceptual difficulties. The simultaneous development of large numbers of arrested larvae can also occur at other times than in March. Outbreaks of clinical ostertagiasis referable to the development of arrested larvae occur in the field from January onwards. In the third year of the present observations, four animals reacted differently from the remainder of the group. Their weight decreased sharply, they showed symptoms of acute ostertagiasis, two had to be killed in January and two in February, and it was plain from their worm burdens that large numbers of arrested worms had resumed their development. These four animals were among the youngest and the smallest of the group and they were light for their age (Fig. 5). These circumstances suggest that development was prompted by some failure of host resistance, using the term in its widest sense. It is significant in this connection that host resistance has been shown by Michel, Lancaster and Hong (1973) to be a possible cause of arrested development and unpublished work by the same authors (1975) shows that it can be as effective a cause as environmental conditioning of the free-living stages. It is clear that just as arrested development of 0. ostertagi may be due to more than one cause so development may be induced to recommence in at least three circumstances: ( a ) a small constant number resumes its development every day, (b) the rate is very greatly increased during March (c) this increase may also occur earlier, apparently through a breakdown of host resistance. The question arises whether distinct mechanisms are involved and whether worms arrested on account of one cause will only recommence their development in response to the corresponding stimulus. The present observations indicate that this is not so.
80
J. F. MICHEL
et
Uf.
SUMMARY
In three successive years, groups of yearling cattle were exposed in autumn to grazing infested with Ostertagia ostertagi. They were housed at the end of December and the fate of the burdens of arrested early fourth stage Ostertagia larvae which they had acquired was studied by means of post mortem worm counts on animals killed at intervals until early summer. Arrested worms resumed their development at a steady rate of approximately 500 per day. Nearly all the arrested worms remaining in March developed during that month. In a small number of animals whose history and condition suggested an impairment of resistance, the development of very large numbers of arrested worms in January and February led to fatal disease. REFERENCES
Anderson, N., Armour, J., Jennings, F. W., and Ritchie, D. S. (1966). Clinical bovine ostertagiasis. Proceedings, First International Congress of Parasitology, (1964) II, 549. Rome. Anderson, N., Armour, J., Jennings, F. W., Ritchie, J. D., and Urquhart, G. M. (1965). Inhibited development of Ostertagia ostertagi. Veterinary Record, 77, 146147. Armour, J., and Bruce, R. G. (1974). Inhibited development in Ostertagia ostertagi infections-a diapause phenomenon in a nematode. Parasitology, 69, 161-174. Armour, J., Jennings, F. W., and Urquhart, G. M. (1969). Inhibition of Ostertagia ostertagi at the early fourth larval stage. II. The influence of environment on host or parasite. Research in Veterinary Science, 10, 238-244. Barcroft, J. (1934). Features in the Architecture of Physiological Function. Cambridge University Press. Michel, J. F. (1963). The phenomena of host resistance and the course of infection of Ostertagia ostertagi in calves. Parasitology, 53, 63-84. Michel, J. F. ( 1970). The regulation of populations of Ostertagia ostertagi. Parasitology, 61,435-447. Michel, J. F. (1971). Adult worms as a factor in the inhibition of development of Ostertagia ostertagi. International Journal for Parasitology, 1, 3l-36. Michel, J. F., Lancaster, M. B., and Hong, C. (1970). Observations on the inhibition of Cooperia oncophora in calves. British Veterinary Journal, 126, xxxv-xxxvii. Michel, J. F., Lancaster, M. B., and Hong, C. (1973). Ostertagia ostertagi: Protective immunity in calves. The development in calves of a protective immunity to infection with Ostertagia ostertagi. Experimental Parasitology, 33, 179-l 86. Michel, J. F., Lancaster, M. B., and Hong, C. (1974). Studies on arrested development of Ostertagia ostertagi and Cooperia oncophora. Journal of Comparative Pathology, 84, 539-554. Ross, J. G. (1965). The seasonal incidence of ostertagiasis in cattle in Northern Ireland. Veterinary Record, 77, 16-19. J. F. (1953). The parasitological and pathological Taylor, E. L., and Michel, significance of arrested development in nematodes. Journal of IIelmintholopy, 27, 199-205. [Received for publication,
June 16th, 19751
