The development of Haemonchus contortus, a nematode parasite of the ovine abomasum, in the laboratory rabbit

The development and growth of Haemonchus contortus was studied in the stomach of the laboratory rabbit. Less than 1% of the ensheathed infective larvae given to rabbits became established, whereas after exsheathment in vitro under physiological conditions, 20-30% of the larvae administered developed to the 4th stage. Following doses of up to 10000 larvae, nearly all worms were lost from the 16th to the 21st day after infection, but after doses of 50000 larvae considerable numbers of 4th-stage larvae were recovered 25 days after infection. No adult worms were recovered and no eggs were found in the faeces. Infection was accompanied by inflammation, haemorrhage, disruption and flattening of the mucus secreting cells and by mononuclear infiltration. No cellular reactions in the immediate vicinity of the nematodes in the mucosa or changes in the pH of the stomach contents were observed.

Developmental arrest of Haemonchus contortus larvae in lambs given larval inoculum exposed to different temperatures and storage conditions

The temperature (15 or 30 C) at which Haemonchus contortus larvae were allowed to develop to the infective (3rd) stage and the temperature at which they were subsequently stored (4, 15, or 30 C) had no effect on the occurrence of developmental arrest (larval impasse) of the larvae in infected lambs. If a large inoculating dose (10,000 larvae) was given to the lambs, a smaller percentage of nematodes was retained (14.2%) then if a small dose (1,000 larvae, 25.8% retention) was given, but the percentage of larvae in developmenttal arrest was greater (7.7% of the retained nematodes vs 0.02%). The temperatures of larval development and storage had no effect on the sex ratio of the adults that developed (46.3% male nematodes).

"Spring rise" in fecal nematode egg counts in sheep in Maine

A "spring rise" in fecal nematode egg counts occurred in Maine sheep housed under conventional conditions during 2 years. Nematodes causing this increase were Haemonchus contortus in one year and Ostertagia sp in the other. The increase in egg counts was greater in sheep that lambed than in unbred sheep. Most of the eggs in the spring rise were probably produced by worms that passed the winter in developmental arrest in the sheep. Some of the female worms probably came from contaminated bedding and developed in the sheep without arrest in growth. Acquisition from contaminated bedding, therefore, should be considered as an additional factor in the epizootiologic character of the condition in Maine.

Response of Haemonchus contortus cayugensis to a change in the ratio of smooth to linguiform

The equilibrium of vulvar phenotypes in Haemonchus contortus cayugensis was changed by infecting lambs with either a selected strain of smooth or of linguiform worms. Subsequently the lambs grazed the same pasture and observations were made on changes in worm populations in the lambs, and on pasture it appeared that the seasonal change in the relative frequencies of A and B linguiform worms was related to density-dependent factors in the host. In the spring, when lambs first become infected, the worm population is small and the majority of linguiform worms are A-type. As the season progresses the density of the parasite population increases. Once the population becomes crowded selection begins to favour the B-type females. The effects of crowding on linguiform A females can be seen as a positive correlation between the percentage of linguiform A and their weight. Population density also affected the survival of A-type worms since there was a negative correlation of percentage A-type on total worm numbers. There was a similar negative correlation between percentage A-type on total worm numbers. There was a similar negative correlation between percentage A-type and total worm numbers in both the continuously infected and tracer lambs. Population density did not affect the proportion of smooth worms. However, smooth worms increased in the tracers from approximately 10% at the beginning of the experiment to 55% at its conclusion 18 weeks later. The latter percentage is almost within range of the local subspecies, which has 60% or more smooth worms. It appears therefore that the proportion of smooth to linguiform worms is a stable equilibrium maintained by natural selection.

Experimentally induced Haemonchus contortus infections in the rabbit

Haemonchus contortus originally derived from an Ontario field strain was used to infect laboratory rabbits. Both enshealthed and artificially exsheathed larvae were given at doses of up to 10 million larvae per rabbit. Only artifically exsheathed larvae were infective and a dose of at least 50,000 was required to produce a significant level of infection. H. contortus completed its development in the rabbit, but at a slower rate than in the sheep. Although a few fecund females were found 35 days after infection no eggs were observed on faecal examination. A high proportion of early fourth stage larvae were recovered from numerous rabbits including some which were examined well beyond a postinfection period when all worms should have been in the adult stage. This phenomenon occurred when freshly harvested and exsheathed larvae were used. Post-exsheathed cold conditioning of larvae at 5degreesC further increased the proportion of early fourth stage larvae (maximum proportions were found after 21 and again after 79 days of cold conditioning and thereafter declined). Whether such larvae were inhibited cannot be stated definitely. However, several of these larvae contained crystalline rods within their soma, and the structure of the crystals was similar to those previously described from inhibited H. contortus larvae recovered from sheep. It was concluded that the laboratory rabbit could be a useful host for further studies on some aspects of the biology and pathogenicity of H. contortus.

The anthelmintic efficacy of fenbendazole against thiabendazole-resistant strains of Haemonchus contortus and Trichostrongylus colubriformis in sheep

The anthelmintic efficacy of fenbendazole (methyl 5-(phenylthio)-2-benzimidazole carbamate) was tested in sheep against standardised strains of Hcaemonchus contortus and Trichostrongylus colubriformis, known to be resistant to thiabendazole (LD90 for thiabendazole against H ontortus was 200 mg/kg bodyweight and against T colubriformis was 150 mg/kg). Fenbendazole at dose rates of 5, 10 and 20 mg/kg per os reduced total worm counts in H contortus infected sheep by 66, 90 and 100 per cent respectively, with similar reductions recorded for worm egg outputs. For the thiabendazole resistant strain of T colubriformis, fenbendazole reduced total worm counts in infected sheep by 4, 44 (40-48), 79 (75-83), 96 and 100 per cent at dose rates of 5, 10, 20, 40 and 80 mg/kg per os. Significant suppression of worm-egg production by thiabendazole resistant T colubriformis was obtained with fenbendazole at dose levels of 5 mg/kg and above. The implications of these results are discussed in the light of the increasing occurrence of strains of trichostrongylid nematodes resistant to currently available benzimidazole anthelmintics.

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.

Effect of nematode parasite infection on the performance of stocker cattle at high stocking rates on coastal bermudagrass pastures

Studies were conducted during the summer of 2 successive years (1971 and 1972) to determine the effects of gastrointestinal nematode parasitism on performance failure of stocker beef steers grazed at high stocking rates on Coastal bermudagrass pastures. During each of the 2 years, 3 groups of steers were rotated at 7- to 10-day intervals on sets of three 1.63-ha pastures from mid-May to late October. (Each group was maintained on a given set of pastures.) In the 1971 program, from mid-May to early August (phase 1) each group consisted of 48 steers for a stocking rate of 29.6 steers/ha. From early August to late October (phase 2) the number of steers per group was reduced to 36 (stocking rate of 22.2 steers/ha) to improve the plane of nutrition. In the 1972 program, 2 groups were stocked at 48 steers per group (29.6 steers/ha) and a 3rd group was temporarily stocked with 58 steers (10 additional steers) to provide 2 animals each month for necropsy evaluation of worm burdens. With exception of the monthly removal of 2 steers from the 3rd group, the stocking rate in 1972 remained intact for the entire grazing season. In 1971, the 3 groups (groups 1, 2, and 3) were treated with thiabendazole (2 doses, 16 days between doses) before grazing. Parasite burdens and levels of pasture contamination were low initially and body weight gains were good during the 1st 3 months of grazing (phase 1). In the 2nd half (phase 2) of the grazing program, 1 group of steers given free-choice supplemental feed (1% of body weight/steer each day) and treated with anthelmintic (thiabendazole; 2 doses, 18 days between doses, during August) and another group, given supplemental feed only, had significantly better weight gains than did the 3rd group, not given supplemental feed nor treated with anthelmintic. There was no weight gain advantage derived from the reduced stocking rate in phase 2. Tracer calves, which were allowed to graze on pastures, developed an increased level of infection after August. Fecal egg counts in the principal steers and recovery of infective larvae from pasture paralleled the infection level in tracer calves. Necropsy evaluation of steers from groups 1, 2, and 3 at the end of the 1971 program indicated a similarity in total worm counts, but in the group not given supplemental feed nor treated with anthelmintic there was more generalized inflammation and a greater number of lesions in the abomasum (Ostertagia) and the caudal portion of the intestine (Oesophagostomum). Steers in the 3rd group also were in poorer condition. Initial worm burdens in the 3 steer groups (groups 4, 5, and 6) were low at the start of the 1972 program. One group was given freechoice supplemental feed (0.5% of body weight/steer each day) and treated with anthelmintics (thiabendazole in the 1st dose, levamisole in the 2nd dose, 12 days between doses, in May and August) and another group was given supplemental feed throughout the summer; the 3rd group was treated with anthelmintics (in May and August)...

A possible relationship between haemonchosis and haemoglobin polymorphism in Merino sheep in Kenya

It is known that sheep show polymorphism for haemoglobin in that three phenotypes (HbA, HbB AND HbAB) are normally found. Since it has been suggested, on the basis of haematological values, that HbA type sheep are more able to survive in Haemonchus contortus endemic environments, a preliminary study was conducted on the performance of set-stocked Merino ewes and lambs in relation to their haemoglobin phenotypes. The results provided some evidence that, in Merino sheep at least, those with HbA type show 'self-cure' more frequently and effectively than those with HbB and HbAB types. Examination of the PCV values and total body weights of Merino eres showed a similar trend, ie, the HbA types had the highest haematological parameters and the heaviest body weights, while the HbB types were consistently lowest. It would appear that the practical significance of these results justifies investigation on a larger scale.

Observations on the development of Haemonchus contortus in young sheep given a single infection

Worm free lambs 2-4 months old were given a single infection of 50,000 Haemonchus contortus larvae. The changes in packed red cell volume were compared with those of uninfected sheep and the infected sheep were divided into two groups according to the rate of change of packed cell volume. In group 1, packed cell volume fell sharply from the 10th day of infection and the sheep eventually died. In group 2, the fall was less marked and the packed cell volume returned to normal after about 40 days. The rates of increase of parasite body length and dry weight were compared for the two groups. Parasites in group 1 sheep grew more slowly than their group 2 contemporaries. Accurate planning of metabolic studies on H. contortus was made possible by using this information to predict the course of an infection and the size of the worms. Parasite development was also measured in sheep for which the packed cell volume was not recorded. Computer analysis showed that adult worm length increase did not follow a simple growth pattern starting from the last ecdysis, but consisted of a rapid elongation followed by a simple negative exponential increase. Dry weight increase also followed a negative exponential during the second phase. Measurements of dry weight as a percentage of wet weight indicated that the rapid elongation was possibly linked with cell enlargement by the uptake of water.

Field studies on inhibition of Haemonchus contortus

The epidemiology of Haemonchus contortus was studied in north east England in lambs reared under field conditions with infected ewes. The parasite population was monitored by pasture larval counts, faecal egg counts and lamb postmortems. From 25 June to 11 November, eight lambs were slaughtered each month, together with eight worm-free tracer lambs. Postmortem worm counts showed that on 25 June, small numbers of adult H. contortus were present but no inhibited larvae. However, subsequently the number of worms recovered rose rapidly to 1000-5000 and the percentage inhibition increased to 57% in July, 75% in August and virtually 100% in September, October and November. On each sampling occasion no significant differences in worm count or percentage inhibition were recorded between experimental and tracer lambs. These results suggest that neither autumn climatic effects nor host immunity were responsible for inhibition in this strain of H. contortus.

Inhibited development in Haemonchus contortus

Inhibited development of Haemonchus contortus was studied in single experimental infections of worm-free lambs. Chilling of the infective larvae at +4 degrees C was without effect on the percentage of larvae subsequently becoming inhibited and a period of exposure to autumnal conditions was unnecessary to induce a high rate of inhibition. It was concluded that seasonal inhibition of H. contortus in East Anglia is brought about primarily by an environmental stimulus acting upon the preparasitic stages but that, unlike Obeliscoides cuniculi and Ostertagia ostertagi, this was not cold. It could be provided in a culture kept in the dark at 25 degrees C for 12 days. While the age of the host did influence the phenomenon, in that larvae were less inclined to inhibition in very young animals, it was concluded that this was not a primary factor in the aetiology.

The use of disophenol in studies of the pathogenicity of the arrested fourth-stage larvae of Haemonchus contortus in the sheep

Disphenol was administered to sheep infected with Haemonchus contortus in order to prevent the development of populations of adult worms, and studies were made of the pathogenicity of the arrested fourth-stage larvae which remained. The treated sheep showed elevated plasma pepsinogen and abomasal pH, predominantly negative dry-matter balance, and evidence of greater fluid loss, including plasma, into the gastro-intestinal tract. It was concluded that arrested larvae may cause damage to the abomasal mucosa.

Age group interaction of Haemonchus contortus in premunized lambs

Lambs premunized with a relatively less pathogenic (RLP) isolate of Haemonchus contortus were challenge exposed each day with 500 3rd-stage larvae (L3) of a normally pathogenic isolate of the same worm, starting on day 9 of the premunition-induced infection. In 6-month-old lambs, there was a fourfold decrease in egg production by the premunizing RLP isolate with challenge exposure compared with egg production in lambs given only the premunizing RLP isolate and not challenge exposed. Premunized lambs were able to carry larger worm burdens with less hemoglobin loss than were nonpremunized challenge-exposed control lambs. Although challenge exposure had affected parasitic egg production by the 9-day-old premunizing RLP worms, the same young infective isolates were able to survive and protect lambs against the additional parasitism.

The effect of fenbendazole on nematode parasites in experimentally infected lambs

Fenbendazole given orally to experimentally infected lambs at a dose rate of 5 mg per kg was found to be 100 per cent effective against three and 10-day larave and also against 20-day adults of H contortus, O circumcincta, N battus and T colubriformis. The same dose was also 100 per cent effective against 10-day, 17-day larvae and 27-day adult D filaria.