The seasonal prevalence of inhibited development of Haemonchus contortus in sheep in Senegal

Throughout a period of 13 months, the abomasa of 1024 sheep, living in a sahelian region, were examined for adult Haemonchus contortus and their inhibited L4 larvae. Of these 796 (78%) were found harbouring H. contortus, but with low worm burdens (geometric mean 37). Significant numbers of inhibited larvae appeared early in the dry season (November) and they represented 49% of the total in December. A similar proportion of inhibited larvae were present until April and after that their number declined abruptly. It was concluded that Haemonchus sp. survives the dry season as hypobiotic larvae as well as adults.

The effect of transfer factor treatment on two challenge infections of Haemonchus contortus in immuno-competent 7-month-old lambs

A number of immuno-competent Blackface lambs were infected with 10,000 Haemonchus contortus third stage larvae and were later challenged with a second dose of 10,000 H. contortus third stage larvae. One group of lambs was treated with dialysed transfer factor prior to the second challenge dose. Histological changes in the abomasal wall, reductions in abomasal worm burdens at slaughter and faecal egg counts monitored throughout the experiment suggested the presence in some animals of an innate resistance, a "self curing" effect, an enhancement of this "self curing" effect in the transfer factor treated group and a response in one transfer factor treated animal comparable to "classical self cure".

Some factors influencing the immunisation of sheep with irradiated Haemonchus contortus larvae

Over the last 6 years a number of experiments in which sheep were successfully immunised with 2 doses of 10 000 Haemonchus contortus larvae, irradiated at 60 krad in a gamma-source, have been reported by the authors. In this paper, the failure of such a regimen of immunisation, using the same strain of larvae, is reported together with an investigation of the possible causes. Technical errors were eliminated as a cause, on the grounds that the failure occurred in 2 separate laboratories using different 60Co sources, as was increased radiosensitivity of the continuously passaged strain of H. contortus as measured by the percentage development of irradiated larvae in 6 naive lambs. From experiments utilising 18 lambs and using both irradiated and normal larvae, it was postulated that the strain had lost its original degree of immunogenicity. Subsequently, in 2 experiments involving 27 lambs, it was shown that irradiation at 40 krad was completely successful in restoring immunogenicity and producing a degree of protection against challenge similar to that previously reported with 60 krad. It was concluded that, with the current strain of H. contortus, 60 krad is now too high a level of irradiation of larvae to produce a consistently high degree of protection against challenge.

The importance of endoparasite counts at necropsy

In a 2-year study of 49 Oklahoma cattle with a typical history of clinical endoparasitism, 12 animals had Ostertagia only, and Ostertagia was the predominant endoparasite in 21. Only 1 animal had a pure Haemonchus population. Though primary endoparasitism was diagnosed antemortem in 32 animals, postmortem worm counts confirmed the diagnosis in only 19. Worm counts documented inapparent disease in 8 animals. In 21 cases of suspected of toxicosis, 12 animals had endoparasitism none was poisoned.

The abomasal histology of worm-free sheep given primary and challenge infections of Haemonchus contortus

The histological changes in the abomasal mucosa of pairs of worm-free ewes and lambs were compared at various stages after infection and reinfection with 350 Haemonchus contortus L3 per kg bodyweight. In terms of worm burdens the ewes were relatively resistant to reinfection compared with the lambs and the histological changes in the abomasum at most stages after infection were more marked in the adult animals. These included increases in the numbers of mast cells, globule leucocytes, eosinophils and IgA plasma cells in the mucosa.

Experimental infection of sheep by the camel stomach worm, Haemonchus longistipes

An attempt has been made to infect sheep by Haemonchus longistipes with the objective of developing a relatively cheap and manageable model of camel haemonchosis. A large proportion of inoculated animals (28/34) developed patent infections; the prepatent period was short, lasting for 3 to 4 weeks in the majority of infected animals. Worm burdens were low and so was the total egg output. Most of the recovered worms were reduced in size.

Wool production of sheep chronically infected with Haemonchus contortus

A chronic infection was established in a group of twenty 18-month-old non-reproductive Merino ewes by oral administration of 3000 infective H. contortus larvae twice weekly for 12 weeks. Their live-weights and wool production were compared with those of 20 uninfected ewes grazing the same pasture. In the infected sheep, faecal egg counts increased over a period of 9 weeks to reach a mean of 5000 eggs per gram, accompanied by small, but significant effects on packed-cell volume and live-weight. The effects of infection on wool growth were also small, and not statistically significant, although there was evidence of a faster seasonal decline in wool growth in infected sheep. It was concluded that mortality induced by acute infection is the most important economic effect of this parasite.

The pathogenesis of experimental Haemonchus longistipes infection in camels

The pathogenesis and clinical signs of Haemonchus longistipes infection were studied in four experimentally infected camels two of which were adults and the other two were young. In the former animals, an acute infection developed, characterized by mucoid diarrhoea, anorexia, anaemia, loss of body weight, oedema of the lower parts of the limbs, general malaise and death at 8-10 weeks post-infection. In the two younger camels, a less dramatic disease was encountered with less severe symptoms and no oedema, but also terminating fatally at 19-20 weeks post-infection. Parasitological, haematological and biochemical parameters were determined during the course of the infection and were mostly comparable with those usually encountered in haemonchosis of other animals.

The pathogenesis of experimental Haemonchus longistipes infection in goats

Goats are highly susceptible to Haemonchus longistipes and could therefore serve as an inexpensive model to study camel haemonchosis. The course of the disease in goats is similar to that in camels and to H. contortus infection in sheep and goats. Unlike the age-dependency of camel haemonchosis, however, the severity of H. longistipes infection in goats is dose-dependent and varies from mild to hyperacute.

Radiolabeling of infective larvae of Haemonchus contortus (Nematoda: Trichostrongyloidea) with 75Se-methionine and their performance as tracers in sheep

Haemonchus contortus infective larvae incorporated between 5 and 12 pCi/larva for each muCi of 75Se-methionine added per gram of fecal sediment. Thorough admixture of 75Se-methionine and fecal sediment was necessary to obtain approximately normal distribution and low variance of individual larval radioactivities. Ecdysis induced by treatment with 0.025% HClO in vitro resulted in loss of approximately 40% of the 75Se label of infective larvae. Loss of 75Se by parasitic larvae and adult H. contortus in vivo conformed to a two-component negative exponential function with half lives of 3.1 and 56 days acting on compartments representing 90% and 10%, respectively, of the 75Se label remaining after ecdysis. Labeled and unlabeled worms were readily distinguished by autoradiography 37 days after infection. No effect of gamma radiation arising from decay of 75Se in the range 130 to 1,300 pCi/larva could be measured in terms of survival or sex ratio of worms recovered at 17 days PI.

[Observations on the effect of some measures for preventing gastrointestinal helminth infections in sheep]

Prophylactic anthelmintic treatment of ewes about parturition has only a negligible effects on the most important species of helminths when the ewes and their lambs are not grazed on clean pasture. Haemonchus contortus infections were completely prevented in some cases but usually severe infections occurred. As the build-up of these heavy infections in lambs was slightly delayed in comparison with that in the lambs of untreated ewes, treatment about parturition should still be regarded as useful. Treatment of lambs at the time of weaning combined with moving the animals to a clean pasture is a useful though not entirely effective preventive measure. Particularly when the lambs continue to stay on the farm, they have to be moved to clean pasture and/or treated with anthelmintics more frequently.

Observations on the fumarate reductase system in Haemonchus contortus and their relevance to anthelmintic resistance and to strain variations of energy metabolism

Fumarate reductase activity in a thiabendazole-resistant strain of Haemonchus contortus was found to be significantly lower than that from a susceptible strain. However, the fumarate reductase activity in a mebendazole-resistant strain did not differ from that in the susceptible strain, even though it was cross-resistant to thiabendazole. Published reports of fumarate reductase activity in strains of H. contortus susceptible or resistant to benzimidazoles were reassessed. A second, unrelated Australian thiabendazole-resistant strain also proved to have significantly diminished fumarate reductase activity, whereas two American strains, one resistant to thiabendazole and one to cambendazole, possess fumarate reductase activities indistinguishable from corresponding susceptible strains. It therefore appears that the phenomenon of benzimidazole resistance cannot be generally correlated with diminished fumarate reductase activity, although in the specific case of the Australian thiabendazole-resistant strains it may be a contributory factor.

The use of faecal egg counts for estimating worm burdens in sheep infected with Haemonchus contortus

Clun forest sheep, worm-free from birth, were given a single dose of 20 000 infective larvae of Haemonchus contortus. The total number of eggs/day in the faeces was recorded for 21 infections and data on population size, sex ratio and individual worm development were collected from 76 sheep. The female to male ratio was 1.28 +/- 0.07 (S.E.). The relation between increase in worm size and uterine egg content was linear. The number of eggs present in the uteri was found to be an accurate measure of eggs passed. It was shown that the daily faecal egg output is related to total parasite weight and is not a measure of the number of individuals present.

Effect of anthelmintic treatment of ewes at parturition and lambs at weaning on the build up of Haemonchus contortus populations in lambs in the Netherlands

In two groups of ewes and lambs, grazing pastures which were used respectively by calves and sheep in the previous year, infection with Haemonchus contortus was virtually prevented by a single treatment of the ewes with fenbendazole before turning out. In another group, grazing a pasture which was not grazed in the previous year, a delay of approximately eight weeks in the build up of high H contortus burdens in lambs was obtained by this treatment, compared with a group of untreated ewes grazing similar pasture. In two groups of lambs treated with fenbendazole at weaning and subsequently grazed on pastures which could be considered free or almost free of H contortus respectively, the build up of high pasture infectivity levels for H contortus also took at least six to eight weeks. It is suggested that treatment of ewes at parturition and lambs at weaning might delay the build up of high H contortus burdens in the lambs by less than the six to eight weeks observed in these experiments when such treatment is less than 100 per cent effective.

The pathogenesis of a single challenge dose of Haemonchus contortus in lambs under six months of age

The pathogenesis of H. contortus infection in lambs under 6 months of age challenged orally with 10,000 third stage infective larvae is described. The development of the parasite and its relationship to haematological and pathological changes are discussed, with particular reference to specific cellular mobilizations, and detailed descriptions are given of the haematology and parasitology, gross pathology and histopathology at 4, 7, 12, 18, 22 and 35 days after infection. Dramatic changes had developed by day 12.

Quantitative studies of ovine haemonchosis. 2. Relationship between total worm counts of Haemonchus contortus, haemoglobin values and bodyweight

The relationships between Haemonchus contortus burdens, haemoglobin levels and bodyweight, were examined in 50 commercial Merino sheep, 9-18 months of age, in south-east Queensland where haemonchosis was endemic. Worm counts and haemoglobin levels were predictable (P less than 0.001, r = 0.68) from the equations log worm count = B.wt. X 0.06942 + log worm count X (-2.18896) + 12.51557. Bodyweight was not predictable from either worm counts or haemoglobin levels. Although bodyweight was represented in both equations, it was only a small and insignificant contributor. When haemoglobin levels became moderately depressed (less than 10.5 g%) or severely depressed (less than 8.0 g%), then the number of worms associated with these haemoglobin levels varied with the bodyweight of the sheep. For sheep up to 20 kg, 10.5 g% Hb was associated with 112 worms and 8 g% with 355 worms. However, 355 worms caused only moderate depression of Hb levels in sheep over 50 kg, and 1259 worms were required to cause severe depression (less than 8.0 g%) in sheep over 50 kg. Three sheep were found to be severely anaemic in the presence of less than 100 worms. It was postulated that additional factors of time and nutrition may have affected these sheep. The sheep were of low bodyweight, in poor condition (low fat score) and were subject to high stocking rates on long grass pastures and high rainfall in autumn. Other internal parasite species present were Nematodirus spp. and Trichostrongylus spp. However, their numbers were low (57 and 218 worms, respectively) and did not contribute to anaemia. The correlation between H. contortus and Nematodirus spp. was found to be negative but non-significant (r = -0.26, P less than 0.01), while the correlation between H. contortus and Trichostrongylus spp. was positive and significant (r = 0.38, P less than 0.01).

Quantitative studies of ovine haemonchosis. 3. The interpretation and diagnostic significance of the changes in serial egg counts of Haemonchus contortus in a sheep flock

An examination of the level and distribution of egg counts of H. contortus within an extensively managed flock of Merino sheep in south-east Queensland was carried out on three occasions over a ten week period. During this time the geometric mean egg count increased from 527 to 1331 eggs per gram, and although 26.3% of the flock were responsible for this increase, clinical evidence of haemonchosis was not observed. Examination of the distribution of counts supported the hypothesis that counts followed a negative binomial distribution. The values obtained for "k" (the index of over-dispersion), ranged from 0.5 to 1.4. However, on the two latter examinations, counts showed a divergence from the expected pattern (P = 0.015; P = 0.014). Further examinations of flocks at higher mean levels of count are required to establish if the trend towards a more normal distribution were maintained. The evidence from the present study and from outbreaks previously reported, suggested that in extensively managed sheep, the level of variance remained high as the mean egg count increased. It therefore appeared likely that in outbreaks of acute haemonchosis under grazing conditions, only a small proportion of the flock may become seriously affected.

Comparison of changes in resistance of Haemonchus contortus eggs following withdrawal of thiabendazole selection

Two strains of Haemonchus contortus with different levels of thiabendazole resistance were developed from the same parent strains by being subjected to different selection intensities. These two strains were each subdivided into two; one selected by thiabendazole in each generation, the other unselected for four generations. Levels of thiabendazole resistance were monitored in each generation by an in vitro egg hatch technique. During this time the unselected strain with the higher level of resistance maintained an LC50 similar to its selected control. The LC50 of the unselected strain with the lower level of resistance actually increased until at the end of the fourth generation it had an LC50 similar to the thiabendazole selected strains.

Haemonchus contortus third stage larvae in sheep: kinetics of arrival into the abomasum and transformation during rumino-omasal transit

The kinetics of passage of Haemonchus contortus third stage larvae through the omasoabomasal orifice were analysed in sheep for six hours after their administration by the nasooesophageal route. The larvae moved into the abomasum more slowly than the liquid phase of the ruminal contents. They arrived in three different forms: sheathed, exsheathed or coiled. A very small number of exsheathed larvae seemed to be necessary for inducing early modifications in permeability of the abomasal mucosa.