A.D. Donald

Responses to infection with Haemonchus contortus and Trichostrongylus colubriformis in ewes of different reproductive status

Abstract Groups of ewes which were either lactating, pregnant, non-lactating (lambs removed at birth) or unmated were subjected to twice weekly infection with constant numbers of H. contortus and T. colubriformis infective larvae for 8 weeks. From the size, structure and fecundity of their worm burdens, lactating ewes, and those in late pregnancy did not differ from each other and showed evidence of diminished immunological responses to infection compared with unmated ewes. The worm burdens of non-lactating ewes, slaughtered less than 30 days after parturition, resembled those of lactating and pregnant animals, but substantially fewer T. colubriformis were found in two ewes which had been deprived of their lambs for 33 and 40 days respectively. Increased numbers of mast cells, eosinophils and globule leucocytes were found in the gut mucosa of animals showing parasitological evidence of an immune response, and in particular, very few globule leucocytes were present in lactating and pregnant animals. Within animals over all groups there was a pronounced negative correlation between numbers of T. colubriformis and numbers of globule leucocytes in the small intestine.

Population dynamics of Trichostrongylus colubriformis in sheep: The effect of infection rate on the establishment of infective larvae and parasite fecundity

The establishment of Trichostrongylus colubriformis was estimated in helminthologically naive 20-week-old Merino sheep given third stage infective larvae (L3) at rates of 2000, 632 or 200 L3 per day, 5 days per week. After varying periods of continuous L3 intake, a levamisole-susceptible strain of T. colubriformis was replaced with a highly resistant strain for 1 week. The animals were then treated with levamisole to remove the susceptible population, and establishment of the cohort of resistant worms was estimated. In previously uninfected sheep, approximately 65% of the L3 given in the first week became established as adults. This fell to low levels (less than 5%) after 7, 10 and 14 weeks of continuous L3 intake for the high, medium and low infection rates, respectively. At the low infection rate, establishment remained at maximum levels for the first 4 weeks, but then fell at a rate similar to that observed for the higher infection rates. This implied that a threshold of worm exposure was required before resistance to establishment developed. Parasite egg production, expressed as eggs per gram of faeces, was proportional to infection rate and is explained by higher worm burdens occurring at high infection rates. However, estimates of fecundity in eggs per female per day showed the opposite relationship with rate of infection. Fecundity stayed high (approximately 600) for 5 weeks at the low infection rate but only maintained this level for 3 weeks and 1 week at the medium and high rates, respectively. This suggests that fecundity, like establishment, was similarly affected at threshold levels of immunological recognition.

Farm management and systems of helminth control

Abstract This paper examines relationships between farm management and gastro-intestinal helminthoses of grazing ruminants. Systems of grazing management and systems of treatment with anthelmintics are both considered. Management decisions, perhaps made for reasons unrelated to helminth control, may modify the extent and severity of helminthoses. Helminthoses may render necessary, or desirable, modifications of management. Management decisions aimed at helminth control cannot be considered in isolation from their effects on other parts of the enterprise, since they compete for labour, finance, land, skills, and perhaps other resources. The immediate effectiveness of integrated systems of parasite control is based on reducing infection rates by combining few anthelmintic treatments with the natural occurrence, or deliberate provision, of safe pastures. In the long-term, such systems may reduce the rate of selection for drug-resistant parasites by minimising drug use. Even if they do not, there is less dependence on the single factor of anthelmintic efficiency than if drugs are used alone. The kinds of management decisions which may influence the extent and severity of helminthoses in grazing animals are discussed. They include: (1) choice of stocking rate; (2) timing of reproductive events - especially parturition and weaning; (3) pasture resting, including rotational grazing; (4) alternate grazing by hosts of the same or of different species; (6) production and use of fodder crops, new sowings, and aftermath of hay and silage; (7) variations in the proportions of young and adult livestock; (8) choice of pasture species. The number of free-living parasites per unit area of pasture, referred to here as the pasture contamination status (PCS), is distinguished from pasture infectivity (PIF), the number of parasites of infective stages ingested by a grazing animal. Major changes in well-established systems of production may incur substantial departures from the objectives of the farmer. Hence systems of parasite control which depend on frequent dosing with anthelmintics, and are relatively independent of production systems, have been advocated widely, and have certainly given good immediate results. In the long term such systems may prove to be unsatisfactory because they invite drug resistance, are costly in drugs and labour, and are unlikely to be adhered to. The epidemiology of the parasites considered is such that effective control systems can be based on three to five dosings per year or one to two if combined with the provision of safe pastures, either through managerial manipulation or by timing of dosing to coincide with periods of rapidly diminishing or low PIF, though not necessarily low PCS. Multiple dosing should be necessary only when, because of unusual environmental conditions, susceptible animals cannot be protected against massive reinfections from heavily infective pastures. The effects of management on helminth control, and vice versa seem insufficiently understood not only by farmers, but by their professional advisers. Recommendations based on helminthological information frequently fail to take into account the ecology of farm management. Although there are signs of improvement, the current failure to use information which is readily available in considering strategies such as rotational grazing, emphasizes the gaps between helminthology, ecology and farm management decisions.

The role of immunologically specific and non-specific components of resistance in cross-protection to intestinal nematodes.

Abstract Vaccination of 6–8-month-old Merino sheep with irradiated Trichostrongylus colubriformis larvae gave a high level of protection (81 %) against single-species challenge with normal infective larvae of the same species. The level of protection (34%) was substantially reduced against challenge with a closely related species ( T. vitrinus ) and no significant protection occurred against single-species challenge with a generically unrelated nematode (Nematodirus spathiger). These results suggest that antigen(s) which stimulate protective immunity are shared by the related Trichostrongylus species but not by N. spathiger . By contrast with the results obtained for single-species challenge, vaccination with irradiated T. colubriformis produced 98–100% protection against all 3 species in animals challenged simultaneously with infective larvae of the 3 species. Comparison of the levels of protection recorded following the 2 types of challenge indicate that although a specific antigenic trigger is required to provoke an appropriate response, the results obtained, particularly in the case of N. spathiger , suggest that the terminal effector mechanism is not immunologically specific. The implications of these conclusions are discussed in relation to theories of the mechanism of resistance to gastrointestinal nematodes and the potential efficacy of vaccination in the field.

An egg-hatch assay for resistance to levamisole in trichostrongyloid nematode parasites

An in vitro technique is described for detecting resistance of nematodes to the anthelmintic levamisole hydrochloride. Samples of eggs are developed under controlled temperature conditions until just prior to the commencement of hatching. They are then exposed to different concentrations of the drug and, when hatching is almost complete, the test samples are killed and preserved. The proportion of unhatched eggs at each drug concentration can then be counted at leisure. Provided a suitable range of drug concentrations is chosen for each test isolate, this assay provides results which may be satisfactorily fitted to a log-concentration-probit regression model. Comparisons with in vivo anthelmintic assays have shown that the technique provides an accurate reflection of the resistance status of parasite populations.

Selection studies on anthelmintic resistant and susceptible populations of Trichostrongylus colubriformis of sheep

Abstract Waller P. J. , Dobson R. J. , Donald A. D. , Griffiths D. A. and Smith E.F. 1985. Selection studies on anthelmintic resistant and susceptible populations of Trichostrongylus colubriformis of sheep. International Journal for Parasitology15: 669–676. A T. colubriformis population (BCK), formerly resistant to benzimidazole anthelmintics, but now highly resistant to levamisole after 6 years exposure to this drug alone in the field, was passed through 12 generations in the laboratory in three separate lines exposed either to selection with thiabendazole or levamisole, or to no selection. Another population (McM) not previously exposed to these anthelmintics was treated similarly in two lines, selected with thiabendazole or not selected. Selection with thiabendazole resulted in a return of benzimidazole resistance in the BCK line which occurred faster than in the McM line, but a similar level of resistance was reached in each by the twelfth generation. Resistance ratios in both selected lines compared with the unselected McM line were less than 20: 1, and only 1.5 times the recommended dose rate of thiabendazole was required to remove more than half of the resistant population. This suggests that a polygenic vigour tolerance rather than a specific resistance had been selected. In the case of levamisole resistance, the BCK population was found to contain two distinct subpopulations, one susceptible and the other highly resistant. Resistance ratios for the highly resistant subpopulation were greater than 4000: 1, implying a specific resistance controlled by a major gene. During the 12 generations of levamisole selection, the proportion of resistant phenotypes fluctuated about an average level of 70%, suggesting that susceptibility alleles were being maintained in the population through superior heterozygote fitness. This conclusion is supported by a significant decline in levamisole resistance in the absence of levamisole selection. Moreover, thiabendazole selection hastened the reversion to levamisole suceptibility. The results provide support for the reintroduction of a benzimidazole anthelmintic to control this helminth population, and for a slow rotation in the use of drugs with different modes of action.

Changes in anthelmintic resistance status of Haemonchus contortus and Trichostrongylus colubriformis exposed to different anthelmintic selection pressures in grazing sheep

This experiment was designed to study, over a 5-year-period, the effect of different frequencies of treatment with three different anthelmintic groups, namely, benzimidazoles, levamisole and ivermectin, and different frequencies of alternation between them, on existing levels of anthelmintic resistance in the nematode parasites Haemonchus contortus and Trichostrongylus colubriformis of grazing sheep. No evidence of ivermectin resistance emerged, even in suppressively treated groups. Likewise, H. contortus failed to develop resistance to levamisole under a similar selection regimen. Thiabendazole was shown to select positively against levamisole resistance in T. colubriformis resulting in significantly greater susceptibility to this drug than for the natural reversion which occurred in the untreated control. There was no evidence that an anthelmintic treatment combined with a movement of sheep to pastures of low infectivity selected more rapidly for resistance than where the same number of treatments were given to set-stocked sheep. Rotation between anthelmintic groups at yearly intervals appeared to be more beneficial in delaying resistance than rotation of drugs with each treatment.

The effect of selection with levamisole on benzimidazole resistance in Ostertagia spp. of sheep.

A field population of Ostertagia spp. (predominantly O. circumcincta) in sheep, found to be resistant to oxfendazole, was exposed to selection with1 levamisole in the laboratory and the field. Progeny of the survivors of a single dose of levamisole in penned sheep showed a significantly lower level of resistance to oxfendazole in an anthelmintic assay, and a similarly lower level of resistance to the ovicidal activity of thiabendazole in vitro compared with the progeny of the survivors of a single dose of oxfendazole, although they remained more resistant than a known susceptible strain of 0. circumcincta. The progeny of worms present in grazing sheep after three doses of levamisole at 4-weekly intervals, and subjected to the same tests, behaved in a similar fashion. They also showed a substantial fall in benzimidazole resistance compared with the progeny of worms not exposed to any anthelmintic for 6 months. The results suggested that levamisole selected positively against benzimidazole resistance. In these circumstances some form of alternation in the use of different anthelmintics could delay the development of a high level of resistance and maintain the practical usefulness of existing anthelmintics.

Predicting populations of Trichostrongylus colubriformis infective larvae on pasture from meteorological data

A long-term field study was carried out at Badgerys Creek, New South Wales, Australia to investigate the ecology of the free-living stages of Trichostrongylus colubriformis. Results included weekly estimates of the number of infective larvae on pasture arising from single applications of contaminated faeces. These were used to construct a mathematical model to predict larval availability on pasture from standard meteorological measurements. The model has three components predicted from meteorological variables—p, the probability that an egg develops to infective larval stage and migrates to the herbage, E(X1), the average time that the egg takes to develop to infective larval stage and migrate to the herbage, and E(X2), the average lifetime of an infective larva on the herbage. The meteorological variables used to predict p were evaporation and rainfall in the first 2 days after the eggs were deposited on pasture, and the length of time until an effective fall of rain. E(X1) was described by a function of the average temperature in the first week after eggs were deposited on pasture and the length of time until an effective fall of rain. E(X2) was predicted by the rainfall and average temperature in weeks 7–10 after the eggs were deposited on pasture. A value of R2 = 0.39 was obtained over a set of 39 plots. The optimal value for this set of data is R2 = 0.76. The model was adjusted to simulate the pattern of larval availability on pasture arising from continual contamination by grazing sheep with naturally acquired infections. This gave a value of R2 = 0.60 when tested against published larval availability data obtained in grazing experiments.

Population dynamics of Trichostrongylus colubriformis in sheep: the effect of infection rate on loss of adult parasites.

Rejection of adult T. colubriformis was examined in lambs given 2000, 1124, 632 or 200 L3 day-1, 5 days per week for up to 20 weeks. Rejection of adults began at approximately the same time for the first three infection rates, and took about 9 weeks to complete. Approximately 20% of adults were rejected by week 10 of infection at the higher infection rates and it was estimated that rejection commenced at about week 7. This coincided with a decline in establishment of larvae to about 1%. For the low infection rate, there was a delay of about 5 weeks before adult worm rejection commenced and reached equivalent levels to those observed at the higher rates. This delay also coincided with an estimated 5-week delay in larval establishment declining to 1% at the low rate.