Trends and international comparisons in infertility in circumpolar areas

A study was carried out at the International Livestock Research Institute (ILRI) Debre Berhan Research Station in Ethiopia from 1992 to 1995 to compare the peri-parturient rise (PPR) in faecal nematode egg counts (FEC) in ewes of two indigenous sheep breeds. A total of 1439 Menz and 1347 Horro ewes were single sire mated following oestrus synchronization to lamb in the wet and dry season. Three ewe treatment groups were constituted as mated/lactating/undrenched; mated/lactating/drenched; unmated/undrenched for three wet and three dry lambing seasons. All ewes grazed naturally contaminated pasture. Levels of faecal egg output were monitored at mating, 3 months after mating, 2 weeks before lambing, 4, 8 and 12 weeks post-lambing. A significant PPR in FEC occurred 2 weeks before lambing and peaked at 4 weeks post-parturition in ewes lambing just before the beginning of the dry season (October/November). There was no significant increase in FEC when lambing occurred before the onset of the long rainy season (May/June). The PPR in this study was associated with both lactation and seasonal availability of third-stage infective larvae on pasture. There was no consistent breed difference in FEC during the six sampling periods from mating to weaning. Faecal cultures and worm counts from both breeds confirmed the presence of Longistrongylus (Pseudomarshallagia) elongata, Trichostrongylus spp.and Haemonchus contortus. The role of the peri-parturient rise of FEC in ewes in gastrointestinal nematode transmission is discussed.     

False-positive Ascaris suum egg counts in pigs

False-positive Ascaris suum egg counts in pig faeces are frequently observed under both experimental and natural conditions. Data from 12 experiments with A. suum infections in pigs were summarized and showed that the percentage of false-positive faecal samples ranged from 4 to 36%. False-positive egg count values varied greatly between pigs and experiments (range 20-1060 eggs per gram faeces). Indoor experiments with pigs housed groupwise in pens generally produced more and higher false-positive egg counts, which may reflect differences in surface area and hence exposure to infective eggs, compared with pasture experiments. The positive predictive value (the number of pigs diagnosed positive by faecal sample that actually harboured worms) was low for indoor experiments (45%) compared with pasture experiments (89%). Differences in design for indoor experiments, such as floor type and use of bedding material, did not influence the positive predictive value (44-47%). A positive correlation was found (r = 0.56, P < 0.05) between faecal egg counts of true-positive and false-positive pigs that were penned together. The results of this survey strongly support previous suggestions that false-positive A. suum egg counts in pigs are the result of coprophagia in indoor experiments and coprophagia/geophagia in pasture experiments. False-positive A. suum egg counts in pig faeces may vary greatly in prevalence and magnitude, and depend in part on management/housing factors.     

The preventive effect of the fungus Duddingtonia flagrans on trichostrongyle infections of lambs on pasture

Four groups of 8 parasite-naive Dorset-crossbred lambs, 3-4 months old, were turned out on infected pasture on 2 May and allocated to 4 separate paddocks. From May to September, 2 groups received Duddingtonia flagrans (10(6) chlamydospores per kg body weight per lamb per day) mixed in 100 g of barley, while the other 2 groups received barley only. All groups remained set-stocked until slaughter for worm counts on 10 October. In late June, all lambs were treated with fenbendazole due to severe parasitic gastroenteritis in all groups. The faecal egg counts were comparable for the 2 treatments throughout the grazing period. Larval development of Ostertagia/Trichostrongylus spp. in faecal cultures was 1-28% in the fungi-fed groups compared with 60-80% in the untreated groups (P < 0.05). In September, pasture larval counts of Ostertagia/Trichostrongylus were 930 and 4400 L3 kg-1 on paddocks of fungi-fed and untreated groups, respectively. Corresponding figures for Nematodirus spp. were 7200 and 11600 L3 kg-1, respectively. At slaughter, the number of immature Ostertagia spp. was 62% lower in the fungi-fed groups compared with the untreated groups (P < 0.05). Four parasite-free lambs were introduced to each paddock during the period 3-23 October and slaughtered for worm counts after 3 weeks of housing. The total worm burden of tracers on paddocks previously grazed by fungi-fed lambs was reduced 86% (P < 0.05; geometric means) compared with control groups, while significant reductions were also seen in abomasal worm counts (68%; P < 0.05), N. spathiger (98%; P < 0.05) and for N. battus (97%; P < 0.01). It is concluded that dosing sheep with D. flagrans while grazing may limit the build up of pasture contamination in the late grazing season and subsequently limit the intake of larvae in sheep.     

Immune responsiveness of nematode-resistant or susceptible Romney line-bred sheep to continuous infection with Trichostrongylus axei

Two divergent lines of Romney sheep have been selected on the basis of differences in faecal egg counts (FEC) to natural poly-generic parasite challenge in New Zealand. However, it is not known if the expression of resistance or susceptibility extends to parasitic nematodes that were not a major part of the selection challenge. To examine this, the immune response to infection with Trichostrongylus axei was examined in these sheep lines. Changes in the proportions of CD5+, CD4+, CD8+, T19+ and CD45R+ lymphocytes and parasite specific antibody titres in peripheral blood were monitored each week in six resistant and six susceptible line lambs that were maintained indoors in pens during the course of 14 weekly infections with 10,000 T. axei larvae. No difference in FEC was observed. Similarly, no significant difference in T. axei specific antibody titre between sheep lines was seen although antibody titre increased steadily from Week 4. Significant increases in the proportions of CD5+, CD4+, CD8+ and T19+ cells occurred in both resistant and susceptible line lambs during Weeks 1-4 of infection. Following peak levels, proportions of CD5+, CD4+ and CD8+ cells fell with the rate of decline of CD5+ and CD4+ cells significantly greater in the resistant line lambs. Proportions of CD45R+ cells showed significant changes with time that were the inverse of those of CD5+, CD4+ and CD8+ cells. Susceptible line lambs showed higher proportions of CD5+ and lower proportions of CD45R+ cells than resistant lambs before infection with T. axei. Overall, during infection, these differences were maintained while CD4+ and T19+ levels were higher in the susceptible line lambs.     

Depression in Parkinson''s disease. Pharmacological characteristics and treatment

An eight-year study was conducted to define the epidemiology of gastrointestinal nematode infection in Suffolk and Gulf Coast Native (Native) breeds of sheep, and to determine if the Native sheep is more resistant to infection. For the initial three years, each breed grazed separate pastures where anthelmintic treatments were administered to individual animals on a salvage basis. For the last five years, both breeds grazed concurrently; anthelmintic treatments were administered to individual animals on a salvage basis for the first three years, and to all animals, when treatment criteria were met, for the last two years. The fecal egg count (FEC) and blood packed cell volume (PCV) were monitored, and tracer lamb nematode burdens were determined. Overall, FEC for both breeds increased in the spring (periparturient rise) for most years and in the summer for all years. Under separate grazing conditions, Native ewes and lambs had consistently lower infection levels than Suffolk ewes and lambs. During the haemonchosis season (June-September) each year, Suffolk ewe and lamb PCV decreased, and Native ewe and lamb PCV remained relatively stable. The salvage treatment protocol resulted in 27 treatments for Suffolk and one for Native ewes; similarly for lambs, 13 for Suffolk and zero for Native. Tracer lambs grazed with their respective breed, and the FEC and mean total nematode burden corresponded with the pattern of infection for their respective breed. The predominant nematodes found in Suffolk and Native tracer lambs were Haemonchus contortus and Trichostrongylus spp., respectively. Under concurrent grazing conditions, the same seasonal repeatable pattern of infection was present and was exhibited by both breeds, with the Native ewes and lambs being consistently and significantly (p < or = 0.05) lower for FEC and higher for PCV. The salvage treatment protocol resulted in 57 and zero treatments for Suffolk and Native ewes, respectively; for lambs, 46 and 11. Tracer lamb nematode burdens again corresponded to their respective breed pattern of infection, with H. contortus and Trichostrongylus spp. being predominant in Suffolk and Native lambs, respectively. Data from all tracer lambs showed a relatively low level of hypobiosis (H. contortus only), and, although there was no consistent hypobiosis season, the tendency was for a higher level to occur in the fall. These results showed that the classic repeatable seasonal pattern of gastrointestinal nematode infection occurred in both breeds of sheep, and that Native sheep were more resistant to infection (specifically H. contortus) than Suffolk sheep.     

Biological controls in mixed trichostrongyle infections

The parasite reaction phenotype of 80 lambs was determined with natural infection on pasture. These lambs were then assigned to outcome groups and randomized into treatment groups. Developing some simple but unusual analytical techniques it was possible to demonstrate that mixed natural infections of trichostrongyles with the chief biomass component Haemonchus contortus cayugensis protected against superinfections with 35000 infective larvae but protected against superinfections with 1000 infective larvae per day only within the limits characteristic of the enclosing barn ecosystem. The limits of the barn ecosystem were less constraining on more diathetic sheep than the natural pasture infection. Internal evidence from the experiment suggests that the interaction of sheep X pasture X infective trichostrongylid larvae constrains the parasite biomass in the host in more diathetic animals at significantly lower levels. Anaphylactoid 'self-cure' did not occur in this experiment but something like premunition certainly did.     

In vivo depletion of T-cells and cytokines during primary exposure of sheep to parasites

This study examined the role of CD8+ and WC1+ T-cells and of interferon (IFN)-gamma in the development of protective immunity against infection with the enteric nematode parasite Trichostrongylus colubriformis in sheep. Monoclonal antibodies (mAb) were administered during induction of the immune response to deplete or neutralise these components. Protection against the primary and challenge infections was assessed by faecal egg count and total worm count. Prolonged administration of mAb recognising IFN-gamma and CD8 resulted in significantly increased protection during the 6 week primary infection and following challenge. CD8+ cells were depleted from blood but not from intestinal mucosa. After injection of mAb (CC15) recognising the surface antigen WC1, WC1+ and Tcr gamma delta + cells were depleted from blood but not markedly from enteric mucosa, and protection against challenge, although variable, was increased by up to 88%. It appears that CD8+ and WC1+/gamma delta+ cells and IFN-gamma all retard the potential development of naturally acquired immunity against the parasite.     

A putative role for larval nematode infection in diarrhoeas of lambs which did not respond to anthelmintic drenches

Attempts to control a summer diarrhoea in grazing Finnish landrace lambs which had been unresponsive to anthelmintics and coccidiostats were made by supplementing them with cupric oxide particles and withdrawing a magnesium-rich mineral, while maintaining parasite control measures. The diarrhoea persisted from July to September and plasma pepsinogen activities were raised, suggesting that the anthelmintic did not prevent abomasal damage; the jejunum of an affected lamb showed lesions of parasitic gastroenteritis. Small responses to cupric oxide particles and larger responses to the withdrawal of magnesium were deceptive, possibly being confounded by differences in parasite challenge. In another experiment Finnish landrace lambs were more susceptible to diarrhoea than Suffolk cross lambs in autumn. The susceptibility was then linked to a strong inhibition of worm egg output and may have been caused by a hypersensitive mucosal response to the larval challenge. Plasma pepsinogen concentrations were again raised in the Finnish landrace lambs and did not decline after treatment with anthelmintic, whereas the concentrations increased later in the Suffolk cross lambs, and were apparently responsive to anthelmintic. The cases of diarrhoea were similar to 'July disease' and may have been caused by continuous nematode infections which were only briefly controlled by drenches. Anthelmintic-unresponsive diarrhoea is the term proposed for the disorder, which may be controllable by devices releasing anthelmintic continuously or by a move to less infected pasture. Faecal egg counts remained low in the condition and were diagnostically misleading.     

Acquired immunity against Cooperia spp. and Ostertagia spp. in calves: effect of level of exposure and timing of the midsummer increase

In two experiments, groups of calves were exposed to different levels and patterns of infection with Ostertagia spp. and Cooperia spp. The experimental design simulated the stereotypic pattern of herbage infestation, including both as normal and a delayed midsummer increase, under conditions of set-stocking. After this simulated 'first grazing season', calves were followed during the subsequent winter housing. At the end of that housing period some calves were challenged with 100,000 L3 Cooperia spp. and 40,000 L3 Ostertagia spp. and slaughtered 23 days later. All previously infected calves were protected against the establishment of the challenge infection with Cooperia spp., but not against Ostertagia spp. For the latter a significant negative correlation was found between worm count and previous level of exposure to infection. During the simulated first grazing season, changes in the ratio of Cooperia to Ostertagia eggs in the faecal egg output and the genus-specific egg count were influenced by both the level of exposure and the timing of the midsummer increase. It is concluded that acquired immunity against both parasite genera develops depending on the level of exposure to infection during a first grazing season, and that delaying the midsummer increase results in a delay of the acquisition of an effective immunity as measured by faecal egg counts and the ratio of Cooperia to Ostertagia egg output.     

Temporal fibrous dysplasia with labyrinthine involvement. Apropos of a case and review of the literature

A survey to detect anthelmintic resistance in nematode parasites of sheep was conducted on 10 randomly-distributed farms in the Chivhu District, Mashonaland East Province, Zimbabwe. Before the survey, a questionnaire was circulated to the farmers concerning nematode parasite control. Results showed that parasite control using anthelmintic treatment was the only method practised and that the benzimidazoles were the most frequently used anthelmintic drugs. The faecal egg count reduction test was used to detect resistance. The anthelmintic groups tested were benzimidazoles, levamisole and ivermectin. Resistance to benzimidazoles was detected on 6 of 10 farms and levamisole resistance on 2 of 3 farms. Ivermectin resistance was not observed on the farms surveyed. Post-treatment larval cultures indicated that Haemonchus contortus survived administration of fenbendazole, albendazole, oxfendazole and levamisole. A Cooperia sp. strain resistant to albendazole was detected and this is the first report in Zimbabwe of a resistant parasite in this genus.     

Epidemiology of gastro-intestinal nematodes of sheep in wet tropical conditions in Malaysia

A study on the seasonal variations in the population structure of Haemonchus contortus and Trichostronglyus colubriformis was conducted for a period of 12 months in a typical large scale sheep farm on improved pasture in Peninsular Malaysia which has a wet tropical climate. Successive groups of helminth-free tracer lambs were grazed for 4 weeks together with naturally infected sheep and were necropised for worm counts 2 weeks after their removal from the pasture. The monthly populations of H. contortus fluctuated slightly except in May and August during which more worms were found in the tracer animals. The numbers of T. colubriformis were comparatively high from October to December 1992 and again in March 1993, low during April and June 1992. Small numbers of hypobiotic larvae of H. contortus were detected in the tracer animals. Development and survival of infective larvae of H. contortus and T. colubriformis on pasture were investigated by spreading faeces containing eggs on grass plots in October 1993, February and May 1994. Development of the eggs to the infective larvae occurred within one week and their survival times were 7 weeks in the 3 experiments. The potential for control by rotational grazing is discussed.     

Some factors affecting the transverse strength of repaired denture acrylic resin

The reappearance of strongyle eggs in faeces after treatment with ivermectin or pyrantel embonate was investigated in 22 foals, 36 yearlings, and 45 adult horses on five Dutch horse farms. The results confirmed earlier studies which showed an egg reappearance period of 9 and 6 weeks after ivermectin and pyrantel treatment, respectively. There were no differences between the egg reappearance periods of foals, yearlings, and adult horses. The mean egg counts of the yearlings were, however, consistently higher than the mean egg counts of the adult horses and foals in both ivermectin- and pyrantel-treated animals. It is concluded that shorter treatment intervals in foals and yearlings are not obligatory. However, longer intervals must be prevented in yearlings because their contribution to pasture contamination is relatively high.     

Teratogenicity and toxicity of coniine in cows, ewes, and mares

Cows, ewes, and mares varied considerably in susceptibility to toxicoses from the oral administration of the piperidine alkaloid, coniine. Cows were most susceptible and ewes least. Only calves had teratogenic effects from maternal administration of coniine during gestation; lambs and foals were apparently resistant. Results suggest that the marked differences between cattle and sheep are probably not due to variation in gut absorption or rumen metabolism.     

Sweat conductometry in cellulose acetate foil--a simplified method for diagnosis of cystic fibrosis

A trial was designed to test the efficacy of vaccination of lambs against field challenge with T. ovis eggs using 3 different levels of pasture contamination. Three concentric paddocks (1.62 hectares each) were used and the inner paddock was heavily contaminated with T. ovis eggs by tethering 2 dogs, experimentally infected with T. ovis, at 5 different points in the paddock over a period of 1 month. Four groups of lambs were used. Sixty lambs, from a property known to be free of T. ovis for 2 years previously, were divided into 2 groups of 30 each; one group was vaccinated with T. ovis culture antigen (vaccinated group=V) while the other was vaccinated with culture medium without antigen (sham-vaccinated group=SV). Thirty mature sheep were obtained from a property with a recent record of T. ovis infection (previous natural exposure to infection group=PNE) and 15 lambs were artificially reared from birth under cestode-egg-free conditions (artificially reared groups= AR). Ten animals from groups V, SV, and PNE, and 5 AR lambs were grazed on each of the 3 concentric paddocks for 6 weeks, and then all animals were slaughtered and their total muscle mass (including heart, diaphragm, tongue and masseters) examined, by slicing, for T. ovis cysticerci. The mean total numbers of T. ovis cysticerci recovered were as follows: Inner paddock, PNE 45.5, AR 47.6, SV 107.8, V 6.5; Middle paddock, PNE 5.2, AR 9.0, SV 8.2, V 0.3; Outer paddock, PNE 2.9, AR 2.8, SV 2.3, V 0.2. There were 4 major conclusions from this study. Firstly, vaccination was more effective than previous natural exposure to infection in preventing establishment of new cysticerci. Secondly, although vaccination effectively reduced the numbers of cysticerci, those that became established were mostly viable at the time of slaughter. In contrast, PNE sheep had greater numbers of cysticerci but the majority were degenerating. Vaccination, therefore, did not seem to stimulate a complete immunological response. Thirdly, the presence of T. hydatigena in lambs did not prevent subsequent infection with T. ovis. Fourthly, T. ovis eggs were dispersed from the inner paddock to the outer paddock.     

Pituitary and plasma levels of growth hormone in Booroola sheep that are either homozygous carriers or non-carriers of the FecBB fecundity gene

The aim of this study was to examine the effect of the FecBB fecundity gene on plasma concentrations and pituitary content of growth hormone (GH) in sheep. No differences were found between homozygous carriers (BB) and non carriers (++) of the FecBB gene with regard to pituitary GH contents in both ovariectomized and intact ewes. However, ovariectomized ewes had higher levels of pituitary GH than intact ewes (P < 0.01). There were no differences between FecBB genotypes with respect to plasma concentrations of GH in 6-year-old ovariectomized ewes bled every 10 min for 12 h or in ram lambs bled weekly during their first year of life. GH levels in the rams decreased until week 27, increased to a peak at week 31 then decreased before increasing again at week 43. Mean plasma GH concentrations in the ewe lambs bled weekly for a year decreased until week 19 then remained at approximately this level for the remainder of the year. Mean GH plasma concentrations in the ram lambs were higher than in the ewe lambs (P < 0.001). Ewe lambs that were homozygous for the FecBB gene had lower body weights (P < 0.05) and had higher levels of GH (P < 0.01) than non carrier ewe lambs during their first year. Before the average age of first behavioural oestrus (36 weeks) GH levels in the ewe lambs were negatively correlated with body weights (r = -0.69, P < 0.001, n = 22). When body weight was included as a covariate in analysis of variance the genotype difference in ewe lamb plasma GH concentrations was no longer significant.(ABSTRACT TRUNCATED AT 250 WORDS)     

Infection-induced protection against Haemonchus contortus in merino and manchego sheep. Relationship to serum antibody response

Resistance to primary and secondary infections with Haemonchus contortus was studied in 10-month-old Manchego and Merino sheep. No notable interbreed differences were observed after primary infections in the parameters determined (prepatency period, faecal egg output, abomasal worm burden). Previously infected sheep (200 L-3/kg live weight (lw)) from both breeds showed notable protection after challenge (400 L-3/kg lw), evidenced by lower eggs/g faeces (epg) values and worm burdens. A protective response in the Manchego breed was associated with arrested development of fourth stage larvae in the abomasal mucosa, whereas in the Merino breed a more rapid expulsion mechanism seems to be involved. Serum antibody levels (IgG, IgA) were infective dose-dependent and protection from re-infection was not clearly related to the parasite-specific IgG response estimated by enzyme-linked immunosorbent assay (ELISA) and Western blotting.     

Relationships among ewe milk production and ewe and lamb forage intake in Suffolk and Targhee ewes nursing single or twin lambs

Suffolk and Targhee ewes (30 each) with single or twin lambs were used in four periods beginning in late gestation and continuing through weaning to evaluate breed differences in milk production, lamb BW, and DMI by ewes and lambs. In Periods 1 (late gestation) and 2 (early lactation), ewes (Period 1) and ewes with lambs (Period 2) were individually penned, fed .45 kg of barley x ewe(-1) x d(-1) and allowed ad libitum access to chopped alfalfa. Ewes and lambs grazed native range in Periods 3 and 4. Grazed forage DMI was estimated using chromic oxide. Estimates of milk production were obtained by handmilking. Average lamb age was 4, 45, and 73 d at the beginning of Periods 2, 3, and 4, respectively. Milk production tended (P = .20) to be greater for Suffolk than for Targhee ewes. Targhee ewes produced 85% more (P = .001) wool than Suffolk ewes. From 33 d prepartum to 89 d postpartum, Suffolk ewes consistently weighed more (P = .001) than Targhee ewes. Suffolk ewe BW loss (-.15 kg/d) was greater (P = .01) than Targhee ewe BW loss (-.02 kg/d) from 33 d prepartum to 6 d postpartum. From 6 to 89 d postpartum BW gain did not differ (P = .69; .05 kg/d) between breeds. From birth to 89 d postpartum, Suffolk lambs consistently weighed more than Targhee lambs (P = .003). From birth to 89 d postpartum, ADG was greater for Suffolk than for Targhee lambs (P = .006). Targhee ewes consumed 25% more (P = .01) feed over the course of the study than did Suffolk ewes. Grazed forage DMI by Targhee lambs was 26% greater (P = .01) than DMI by Suffolk lambs. When meat production is the primary income from sheep, one potential advantage of Suffolks compared with Targhees is more rapid gain with less feed intake.     

Efficacy of moxidectin as a 1% injectable solution and a 0.1% oral drench against nasal bots, pulmonary and gastrointestinal nematodes in sheep

Thirty ewes, 3-10 years old, known to be naturally infected with internal parasites, were allocated to three homogeneous groups of ten ewes each based on faecal nematode egg counts. The following experimental treatments were administered on Day 0: (A) moxidectin 1% injectable solution at 0.2 mg kg-1 body weight; (B) moxidectin 0.1% oral drench solution at 0.2 mg kg-1 liveweight; (C) untreated control. Faecal samples were taken on Days -7, 0, 1, 2, 3, 7, 14, 21, 28 and 35 to obtain counts of nematode eggs. One-half of the ewes in each treatment group were slaughtered 14 days after dosing, while the remainder were slaughtered 35 days after treatment to count the numbers of nasal bots, pulmonary nematodes and gastrointestinal nematodes. Moxidectin 1% injectable solution and moxidectin 0.1% oral drench solution were highly effective against gastrointestinal nematodes and against a variable infection of Dictyocaulus filiaria. Moxidectin 1% injectable solution was effective against first stage larvae of Oestrus ovis, whereas moxidectin 0.1% oral drench was ineffective.     

Variability of egg excretion in Schistosoma mansoni infection in Ethiopia: a case report

An Ethiopian boy, aged 18 years, and heavily infected with S. mansoni (1250 eggs per gram of faeces), gave stool sample for microscopy three times a day (at 9:00 am, 1:00 pm and 4:00 pm) for 5 consecutive days. Each time two slides were prepared by the modified Kato's thick-smear technique. The maximum egg count at each examination was converted to eggs per gram of faeces (EPG). There was no significant variability (at 10% level, F-value = 0.04) in in egg counts made at different times of the day. However, the coefficient of variations between the egg counts made on different days were fairly high ranging from nearly 61% to 73% and the differences being highly significant at 10% level (F value = 4.076). The implications of this day-to-day variability of S. mansoni faecal excretion in "selected" chemotherapy in Ethiopia is discussed.     

Relationships between individual differences in salt appetite of sheep and their plasma electrolyte status

A pasture was prepared so that an equilibrium of fresh and ageing eggs of Taenia hydatigena was present. This was done by placing dogs with patent infections on it for six months. Five months after their introduction, groups of lambs were grazed there for two, four, eight, 16 or 32 days. The dogs were then removed. Three and six months later, further groups of sheep were grazed there for a similar period. The infection pressures and the indices of clustering of the eggs were calculated by an examination of the lambs for cysts at autopsy. When the dogs were present, the daily rate of infection was about 60 per cent of the flock. This was reduced to 6.5 per cent then to 3.5 per cent within three and six months respectively after their removal. Very high cyst counts were obtained in some animals and the indices of dispersion diverged from a Poisson model during the period when the lambs were maintained on the pasture in the presence of the dogs. There was a trend for a reduction in the mean cyst counts and the index of dispersion tended to conform more closely to a Poisson model the longer the dogs were excluded. There was also a corresponding reduction in the survival rate of the cysts. It was suggested that an asymptote of less than 100 per cent in the prevalence rate may be accounted for by the acquisition of resistance without the persistence of infection and this was induced in some animals by the initial ingestion of ageing organisms which gradually formed the predominant type after the removal of the dogs from the pasture.