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PubMed1990

Haemonchus contortus with low inhibited development in sheep from the highveld of Zimbabwe.

The abomasa of sheep grazing on natural pastures in the highveld of Zimbabwe were examined for Haemonchus contortus. Of 304 abomasa, 213 (70%) harboured H. contortus. The worm burden increased during the rains to reach the peak in February-March. This was followed by a decline with low worm loads throughout the dry season. The fourth stage larvae (L4) accounted for 1-7% of the total H. contortus population, except during July and August when they comprised 22 and 54% of the worm burden, respectively. It appears that the inhibition of H. contortus is not common in the commercial farming sector where sheep are treated very frequently.

Abomasum↗
PubMed1990

The establishment and development of Haemonchus contortus in goats.

Twelve goats were inoculated with 40,000 third-stage Haemonchus contortus larvae and two were killed on each of Days 4, 7, 11, 14, 18 and 21 after inoculation (DAI). The number of worms that established, and the site of development were recorded. More worms established in the fundic, than in the middle or pyloric thirds of the abomasum. Early development occurred within the mucosa; emergence into the lumen started between 7 and 11 days after infection. By 4 DAI, all worms had completed the third moult to the L4 stage. At 11 DAI the majority of the worms were adults. A mean of 13.2% of the female worms had eggs in their uteri at 18 DAI; by 21 DAI more than half of the female worms had eggs in their uteri. The development of H. contortus was essentially similar to that described in sheep.

Abomasum↗
PubMed1990

Critical controlled test to evaluate resistance of field strains of Haemonchus contortus to thiophanate.

Following reports of suspected gastrointestinal strongylid nematode resistance to thiophanate in sheep and goats at Sokoine University of Agriculture (SUA), Tanzania, field strains of Haemonchus contortus, one from SUA and another from Kimamba, Tanzania, were selected for a critical controlled test to evaluate the efficacy of thiophanate. The SUA H. contortus strain in goats showed a marked resistance to thiophanate at an oral dose of 50 mg kg-1 liveweight (efficacy = 24.1%), while the Kimamba H. contortus strain showed very marked susceptibility (efficacy = 99.7%) to thiophanate at the same oral dose. This is the first report from East Africa of a field strain of H. contortus resistant to thiophanate. The incidence of resistant strains of strongyloid parasites to benzimidazole anthelmintics in sheep and goats in East Africa is on the increase. The magnitude of this problem should be evaluated and remedial solutions sought.

Abomasum↗
PubMed1990

A novel approach to the control of anthelmintic-resistant Haemonchus contortus in sheep.

This paper reports attempts to control a resistant strain of Haemonchus contortus on pasture by replacing it with a susceptible strain. By making use of artificially infected donor sheep, six camps (paddocks) were seeded with a resistant field strain of H. contortus until it was confirmed by means of worm-free tracer lambs that the grazing had become infective. Thereafter, using donor sheep infected with a susceptible laboratory strain of H. contortus for seeding the pasture, attempts were made at various times of the year to replace the resistant strain on the pasture with the susceptible strain in five of the camps. The sixth remained as a control camp, in which no attempt was made to replace the resistant strain. In two of the five test camps, the susceptible strain was introduced in the autumn after 8-10 weeks of nil grazing; in the remaining three camps the introduction was made in spring (two camps) or summer without having a period of nil grazing. The susceptibility of the worm strains introduced initially, as well as of those that developed in the various camps, was gauged both by controlled non-parametric anthelmintic slaughter trials at the beginning and at the conclusion of the trial, and by an in vitro egg hatch test. A reversion to susceptibility occurred in three of the five camps. These included both of the camps infested with the susceptible strain in the spring and one of the two infested in the autumn. Possible reasons for the failure to replace the resistant strain in the remaining two test camps are discussed.

Animals↗
PubMed1990

Effect of environmental stimuli on pre-infective and infective stages of Haemonchus contortus in the northern United States for the induction of hypobiosis.

The effects of both natural and artificial conditioning of pre-infective and infective stages of Haemonchus contortus were examined in two studies with 108 crossbred lambs, using an Ohio isolate of H. contortus known to exhibit up to 100% winter hypobiosis. The intensity of hypobiosis varied from 0 to 36% in 17 lamb groups given larvae subjected to various temperature and photoperiod conditions. The results in lambs given larvae conditioned for 4-8 weeks were not significantly different from those in lambs given freshly cultured larvae. It is suggested that winter hypobiosis of H. contortus in the northern U.S.A. is an obligatory survival mechanism that occurs without the need for external stimuli to trigger the onset of hypobiosis.

Animals↗
PubMed1991

Efficacy of some anthelmintics on an ivermectin-resistant strain of Haemonchus contortus in sheep.

Following evidence of reduced efficacy of ivermectin in a field population of Haemonchus contortus in Brazil, this strain of the parasite was submitted to a controlled anthelmintic test. Eighty worm-free lambs were randomly split into two groups of 40. Each lamb in the first group was infected with 5000 third stage larvae (L3) of the ivermectin-resistant strain; the remaining 40 lambs were each infected with 5000 L3 of a H. contortus strain of known susceptibility to the major groups of anthelmintic compounds used in sheep. On Day 28 post-infection, each group was subdivided according to egg counts and at random into four sub-groups of ten lambs, each of which was treated with albendazole (ABZ) at 3.8 mg kg-1, levamisole (LEV) at 7.5 mg kg-1 or ivermectin (IVM) at 0.2 mg kg-1, or was left as untreated control. At slaughter, 7 days later, all the anthelmintics reduced worm burdens in animals infected with the susceptible strain (ABZ 98.9%, LEV and IVM 100%). By contrast, in the lambs infected with the ivermectin-resistant strain, LEV was 99.8% effective, but ABZ reduced worm counts by only 14.7% and IVM by only 10.4%. Interestingly at necropsy on Day 7 post-treatment, the egg counts in the resistant strain treated with ABZ had been reduced by 92.5%, although worm counts were reduced by only 14.7%.(ABSTRACT TRUNCATED AT 250 WORDS)

Albendazole↗
PubMed1992

Evaluation of ivermectin against experimental infections of Haemonchus contortus and Trichostrongylus colubriformis in goats.

Fourteen nematode parasite-naïve kids were given a mixed larval inoculum (approximately 17,250 infective larvae) via stomach tube. Twenty-two days after infection, the kids were randomly assigned to control and treatment groups of seven animals each. Mean eggs per gram for control and treatment groups were 4304 and 5315, respectively. Kids in the treatment group were given ivermectin (0.2 mg kg-1) administered by subcutaneous injection. At necropsy, the mean numbers of Haemonchus contortus and Trichostrongylus colubriformis for control and treatment groups were 2259 and 0, and 3033 and 773, respectively. This reduction was significant (P less than 0.05) for both species, resulting in an efficacy of 100.0% and 74.5%, respectively. The lower efficacy against T. colubriformis may be a sign of resistance, a reduced effectiveness due to route of administration, or a higher dose may be required with subcutaneous administration, as has been observed previously.

Animals↗
PubMed1992

Resistance of St. Croix lambs to Haemonchus contortus in experimentally and naturally acquired infections.

Parasitological and immunological parameters of experimental or naturally acquired infections with Haemonchus contortus were compared in St. Croix and Dorset lambs. In experimental infections, St. Croix lambs developed significantly greater levels of resistance to H. contortus, following primary exposure, as compared with Dorset lambs. This resistance was influenced both by age and by prior exposure to parasites. In grazing experiments on H. contortus-infected pasture, St. Croix lambs shed significantly fewer eggs as early as 5 weeks following initial exposure. Further, St. Croix lambs had more than 99% fewer worms in the abomasum at necropsy compared with age-matched Dorset lambs. Lymphoproliferative assays using peripheral blood mononuclear cells and antigen-specific serological tests demonstrated only minor differences in immune responsiveness between the two breeds despite the dramatic parasitological differences. Similarly, abomasal mucus from both breeds had elevated levels of parasite-specific antibodies and contained substances mediating larval paralysis. In contrast, St. Croix lambs which had become resistant to nematode infection had dramatically higher numbers of globule leukocytes in the abomasal mucosa compared with Dorset lambs.

Abomasum↗
PubMed1992

Humoral responses in rabbits immunized with two fractions of Haemonchus contortus: the antigen specificity of such antibodies.

Humoral responses were examined in rabbits immunized with either 28-40 kDa (Fraction 1) or a 19-24 kDa (Fraction 2) antigenic fraction from soluble antigens (Sol L3 Ag) from infective larvae (L3) of Haemonchus contortus. These fractions were eluted from electrophoretically separated Sol L3 Ag. Immunoblots revealed antibodies to Fraction 1 (fr. 1) or Fraction 2 (fr. 2) polypeptides as well as to several other molecular weight polypeptides of the Sol L3 Ag. The latter antibodies were shown by absorption studies not to be Sol L3 Ag cross-reactive anti-bacterial rabbit antibodies. When Sol L3 Ag was affinity-purified using monoclonal antibody to phosphorylcholine (PC) and the resulting fractions were further analysed by immunoblotting using rabbit anti fr. 1 or anti fr. 2 antiserum, the PC antigen was found to be shared between fr. 1 and other polypeptides of Sol L3 Ag. Using the rabbit antibody fractions eluted from nitrocellulose membranes containing fr. 1 or 2 polypeptides, it was found that these fractions contained antibody that bound mainly to fr. 1 and only to fr. 2 polypeptides of Sol L3 Ag. It is concluded that, from the present immune rabbit sera, antibodies specific for either fr. 1 or fr. 2 may be isolated and then used to purify small amounts of the corresponding antigens.

Animals↗
PubMed1992

Isoenzyme analysis of Haemonchus contortus resistant or susceptible to ivermectin.

Three different strains of Haemonchus contortus (susceptible to ivermectin, S-IVM; selected for resistance to ivermectin, R-IVM; a multiple resistant strain, i.e. resistant to benzimidazole and ivermectin, R-IVM/SA) were examined for isoenzyme variation by starch gel electrophoresis. Using stains for seven enzymes separated in five different buffer systems, no differences in the electrophoretic mobility could be detected between any of the strains. Results demonstrate a low level of enzyme variation in H. contortus and no differences in enzyme electrophoretic profile between IVM-sensitive and IVM-resistant parasites. Differences between the ivermectin-sensitive and both ivermectin-resistant strains were observed with the propionyl esterases and although some of the differences are probably associated with benzimidazole resistance, others are associated with resistance to ivermectin. The three strains of H. contortus are generally identical; however, differences between all strains of H. contortus and a strain of Dictyocaulus viviparus were detected.

Animals↗
PubMed1992

The interaction of Trypanosoma congolense and Haemonchus contortus infections in trypanotolerant N'Dama cattle.

The interactions between Trypanosoma congolense and Haemonchus contortus infections were studied in N'Dama calves. A total of 38 N'Dama bulls was divided into four groups and each group infected either with H. contortus 1 week after infection with T. congolense or with T. congolense 4 weeks after infection with H. contortus, or with either infection singly. Parasitological (faecal egg counts, parasitaemia), haematological (packed cell volume, white blood cell counts, albumin) and clinical parameters (body weight change, mortality rate) were compared among the various groups. The results showed a reduced prepatent period and a markedly increased pathogenicity of H. contortus infections in animals with a concurrent T. congolense infection. The most harmful combination was a H. contortus infection 1 week after the T. congolense infection which resulted in a progressive and severe anaemia, accompanied by hypoalbuminaemia, increased weight loss and high mortality. The anaemia induced by dual infections showed a low responsiveness to chemotherapy and in several cases supportive treatment did not help recovery. The results also showed that animals with a concurrent T. congolense and H. contortus infection ran a higher risk of succumbing during the infection, and also during 10 weeks following treatment. Although infections with T. congolense alone produced no clinical signs, they were found to significantly reduce the ability of infected animals to mount a normal response to a subsequent H. contortus infection. It was concluded that the increased H. contortus egg excretion observed in animals infected with both parasites might significantly increase the risk of nematode infections and that the reduced prepatent period might necessitate more frequent anthelmintic treatments. These interactions should, therefore, be considered wherever attempts are made to control these two diseases.

Animals↗
PubMed1994

Reduction of Haemonchus contortus infective larvae by three nematophagous fungi in sheep faecal cultures.

The reduction of Haemonchus contortus (L3) infective larvae in sheep faecal cultures caused by the action of three nematophagous fungi conidia was evaluated in vitro. Addition of 20,000 conidia of Monacrosporium eudermatum, Arthrobotrys oligospora and Arthrobotrys robusta per gram of faeces caused a reduction of 95.7%, 98.3% and 10.1%, respectively, compared with the control group. A 97.4% reduction was observed when combined conidia of the three fungi were used. Total reduction of the larval population was observed by addition of the three fungi at 100,000 conidia per gram of faeces.

Animals↗
PubMed1993

Acute or chronic administration of prolactin alters ovine infections of Haemonchus contortus.

Prolactin has been implicated as a modulator in the phenomenon of periparturient egg rise during ovine nematodiasis. In the first experiment, lambs (4 months of age) with patent infections (10,000 infective larvae) of Haemonchus contortus were injected daily (Days 28-39 post-inoculation (PI)) with prolactin (20 or 200 IU) or saline. Fecal egg concentrations and total daily egg production were monitored for each lamb. After 12 days, lambs injected with the lower dose of prolactin had the highest daily egg production. Fewer adult worms were collected from prolactin-treated lambs, and the female worms were larger than those from the saline-injected controls. Worm size and number were not correlated. In the second experiment, lambs were injected daily with ovine prolactin (25 IU) or saline. Five days after initiation of treatments, each lamb was inoculated with 3000 infective larvae of H. contortus. After 42 days of injections (Day 37 PI), prolactin-treated lambs had larger and fewer worms, with higher fecundity, than saline-treated controls. Increases in endogenous circulating prolactin during late pregnancy and lactation in ewes, therefore, might contribute to periparturient egg rise irrespective of the developmental stage of the parasite when the hormone exposure occurs.

Animals↗
PubMed1993

Model predictions for anthelmintic resistance amongst Haemonchus contortus populations in southern Brazil.

A computer model developed to study Ostertagia circumcincta resistance to anthelmintics in UK sheep flocks has been adapted for use with Haemonchus contortus under southern Brazilian conditions. The model simulates the effect of different anthelmintic control regimens on the year-to-year pattern of resistance in breeding ewes. The nematode control regimen most used by Brazilian sheep farmers was found to increase the frequency of genes which confer resistance from approximately 3% to 14% in an H. contortus population over a 20 year period. The effect of early versus late season anthelmintic treatment was investigated. This indicated that early season treatment would select for resistance rapidly, whereas late season treatments would not, owing to large numbers of untreated parasites accumulating at the beginning of the season. A model which can predict the development of anthelmintic resistance in parasites of ewes is a valuable tool in the understanding of the effect of different strategies on nematode control programmes and merits further consideration.

Animals↗
PubMed1993

The ecology of Haemonchus contortus in a winter rainfall region in Australia: the development of eggs to infective larvae.

The seasonal pattern of development of Haemonchus contortus eggs to infective larvae was investigated on the south coast of Western Australia. Sheep faeces containing H. contortus eggs were deposited on pasture plots at intervals over a 3-year period, and pasture grass and faeces were sampled periodically for infective larvae (L3). The maximum recoveries occurred in late autumn and in late spring, when adequate moisture coincided with warm temperatures. Larval development was low and sporadic over the hot and dry summer period, and depressed during winter, although most egg depositions in winter yielded L3 at some time. The proportion of L3 recovered was related to temperature and moisture parameters, and the major constraint appeared to be the availability of moisture at pasture level. The best indicator of the suitability of conditions for the development of H. contortus was the proportion of green pasture material present, based on a visual assessment.

Animals↗
PubMed1993

The ecology of Haemonchus contortus in a winter rainfall climate in Australia: the survival of infective larvae on pasture.

At sites near Albany, on the south coast of Western Australia, sheep faecal pellets containing either eggs or infective larvae (L3) of Haemonchus contortus were deposited onto natural pasture plots at intervals over 3 years. Faecal and pasture samples were collected periodically and processed to detect L3. On annual pasture plots, L3 were recovered for from 10 to over 20 weeks from depositions made between autumn and early spring, at which time temperatures were mild or low, rainfall was frequent and the pasture was visibly green. During the hot and dry summer, when the pasture was completely dry, L3 often failed to develop or were recovered on only a single occasion, and the mean larval survival period on positive plots was less than 5 weeks. In contrast, L3 deposited on green perennial pasture plots in summer were recovered for up to 4 months in faecal pellets and on pasture. The longest periods of larval survival were associated with the lowest temperature and highest rainfall recordings, and with the greatest quantity of green plant material in the pasture. It is suggested that the poor survival of L3 during the dry summer period could be used in a strategic treatment programme to interrupt the transmission cycle of H. contortus in winter rainfall climates. However, areas of green pasture which persist in summer may provide a sufficiently moist microclimate to permit the survival of H. contortus L3 despite otherwise unfavourable environmental conditions.

Animals↗
PubMed1993

Lamb serum recognition of infective larvae and adult Haemonchus contortus antigens.

Sixteen- to eighteen-week-old lambs were infected with 2500 3rd stage larvae (L-3) of Haemonchus contortus or kept as uninfected controls. Two months later all animals were challenged with 5000 L-3 of this parasite. Soluble antigens of H. contortus L-3 and adult worms were analyzed by SDS-PAGE and Western blotting during experimental infection and challenge. Antigens from both sources, particularly of low molecular weight under reducing conditions, were recognised by the pooled sera of infected lambs. No single L-3 antigen was recognised by all infected lambs, whereas two peptides having around 25 and 26 kDa from adults were recognised by infected animals during the patency and could be of potential use in the diagnosis of lamb haemonchosis.

Animals↗