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Laboratory strains of Haemonchus contortus and Ostertagia circumcincta selected for thiabendazole resistance were found to have a strong side resistance to a single dose of oxfendazole. The LD50 and LD95 in mg of drug per host liveweight were respectively 4.28 and 18.46 mg/kg for H contortus and 3.61 and 11.20 mg/kg for O circumcincta. A field strain of Trichostrongylus colubriformis that had not been selected with thiabendazole for seven years also had a strong side resistance to oxfendazole with approximately 50 per cent of its population resistant to the recommended dose rate of 5 mg/kg. Prolonged administration of oxfendazole by intraruminal controlled release capsules was found to be effective against both susceptible and resistant strains of the above parasites. The first observed effect of oxfendazole, from controlled release capsules, on resistant worms was a decrease in the percentage of eggs developing to third stage larvae. This was followed by a decrease in egg count and in worm numbers.

Four groups, each of six male Friesian calves, were set-stocked on separate 0.66 ha paddocks from May 7 until October 23 1986. Each of the animals in groups 1 and 4 was dosed with an oxfendazole pulse release bolus at turn out whereas the animals in groups 2 and 3 were left untreated. Parasite-free naive tracer calves were introduced into each paddock for a limited period 12 days after turn out and again at the end of the trial. No adverse reactions or clinical signs were observed in either of the groups of calves which received boluses. The development of clinical parasitic gastroenteritis in both the untreated groups necessitated the humane slaughter of two animals and emergency anthelmintic treatment of the remainder. The lower plasma pepsinogen concentrations, and lower faecal egg and larval counts and worm burdens post mortem, together with the absence of clinical signs of parasitic gastroenteritis and bronchitis in the treated calves, confirmed the high efficacy of the bolus treatment.

Composite strains of Trichostrongylus colubriformis and Ostertagia spp consisting of 0, 1, 10, 25, 50, 75, 90, and 100% of known resistant strains were prepared and tested for benzimidazole resistance using faecal egg count reduction tests, in vitro egg hatch assays and tubulin binding assays. All tests detected resistance where the proportion of the resistant strain in the composite was 50% or more, whereas none of the tests unequivocally detected resistance below 25%. Egg count reduction tests were no less sensitive than the in vitro tests in detecting low levels of resistance but the egg hatch and tubulin binding assays provided a better quantitative estimate of moderate to high levels of resistance. Faecal egg count reduction therefore, provides a suitable means of detecting resistance in the field but tests, more sensitive to low levels of resistance are required. Results indicate that the use of post-treatment counts alone provides an adequate indication of anthelmintic efficiency.

Sheep which had been either previously infected with 3. circumcincta or maintained worm-free, were surgically prepared with separated fundic pouches and abomasal cannulae and subsequently infected with 20,000 O. circumcincta larvae three times weekly. A reduction in food intake and increases in total acid output from the pouches and plasma pepsinogen levels were evident in both groups of sheep 4 days after repeated infections commenced; effects which increased in severity after 12 or more days. Except for a transient period of slight failure, previously infected sheep retained the capacity to acidify their abomasal contents whereas previously worm-free sheep lost this capacity. These changes were reversed between 2 and 7 days after treatment with thiabendazole (88 mg.kg-1). Secretory capacity of the fundic pouches was tested with histamine (40 mug.kg-1), the histamine antagonist (burimamide 8 mg.kg-1) and atropine (100 mug.kg-1). Ostertagiasis reduced or abolished the stimulatory effects of histamine. An increase in secretion volume and acid output was obtained after food was freshly provided, even though as little as 25 gm was consumed. Atropine and burimamide both caused a profound decrease in pouch secretion and acid output. These data are consistent with the hypothesis previously stated that in ostertagiasis the hypersecretion from fundic pouches is due to increased levels of circulating gastrin.

An experiment was carried out to assess the effect of morantel tartrate in the faeces of calves treated with a bolus on the survival and development of Ostertagia ostertagi eggs. Since the drug delivered from the bolus greatly reduces the nematode population in an infected animal and thus the number of eggs excreted it was necessary to mix O ostertagi eggs into the faeces of calves to which boluses had been administered. In three preliminary experiments it was shown that the methods used to extract the nematode eggs from faeces of infected cattle and remix them into faeces from uninfected cattle did not appear to affect their development into larvae or their even distribution in the faeces into which they had been remixed. The concentration of morantel tartrate lethal to O ostertagi eggs was in the range 0.0015 to 0.0025 M in vitro. It was demonstrated that the presence of the drug in the faeces of dosed calves prevented the maturation of approximately 99 per cent of O ostertagi eggs to infective larvae between days 7 and 84 after the administration of a bolus and of 75 per cent on day 91. These results help to explain the well recognised effect of the bolus in cleaning pastures of O ostertagi.

Thiophanate administered daily at low dosages reduced nematode faecal egg output, egg hatchability and parasitic worm burdens in treated lambs and ewes. Six daily doses of 1 or 3 mg per kg thiophanate (approximately 1/25th to 1/75th of the median therapeutic dose), given to lambs experimently infected with Trichostrongylus colubriformis, were partially effective in suppressing faecal egg output and egg hatchability. Six doses of 5 mg per kg per day were effective in lambs infected with Haemonchus contortus and Nematodirus spathiger. Daily doses of thiophanate (50 or 200 mg per head) given over 14 weeks to lambs grazing contaminated pasture resulted in improved productivity (the higher dosage) and suppression of output of viable eggs and reduced worm burdens (both dosages). Reduced output of viable eggs was also obtained in housed, lactating ewes receiving 5 or 7 mg per kg thiophanate dispersed daily in the feed for 11 or nine weeks respectively after lambing.

A new anthelmintic candidate, p-toluoyl chloride phenlylhydrazone, was administered to lambs in a paste formulation. The efficacy of this drug was determined at dose rates of 20, 30, 40, and 50 mg of active ingredient per kilogram of body weight. Helminths expelled by these dosages included Haemonchus contortus, Ostertagia circumcicta, Trichostrongylus colubriformis, T. axei, Nematodirus filicollis, Cooperia curticei, Strongyloides papillosus, Oesophagostomum columbianum, Bunostomum trigoncephalum, Chabertia ovina, Moniezia expansa, and M. benedeni.

Slow release from intraruminal capsules of 0.48 mg/kg oxfendazole per day for 5.5 days gave percentage efficiencies of 99 +/- 0.6, 86.6 +/- 6.3 and 93.1 +/- 4.5 for the removal of adult worms, developing 4th stages and inhibited larvae of Ostertagia ostertagi respectively. These efficiencies were not significantly different from those obtained from a single oral dose of 2.5 mg/kg. Peak plasma levels of oxfendazole were similar for the 2 types of administration but high levels were maintained longer in cattle given capsules.

Changes in anthelmintic resistance in nematode parasites were monitored in sheep grazing on 2 separate farms, but with the same anthelmintic treatment program, over 16 years. High levels of benzimidazole resistance emerged in Ostertagia and Trichostrongylus spp populations on both farms following 9 years of continuous use of this class of drug. Subsequently, variations in the levels of resistance occurred for the same species between farms and between species on the same farm. A change to levamisole for 2 years resulted in a significant reversion towards benzimidazole susceptibility, but a concomitant rise in levamisole resistance, in Ostertagia on one farm. However, benzimidazole resistance increased rapidly following the re-introduction of oxfendazole into the anthelmintic treatment program. Results from both farms illustrate the pitfalls of using one anthelmintic class for an extended period and provide indirect support for the alternation of anthelmintic classes at approximately yearly intervals.

The control of Ostertagia ostertagi infections in Australia is aimed specifically at young cattle in their first and second year of grazing after weaning. Mature breeding stock are not routinely treated. The recommended strategy is preventive, using an integrated approach of timed anthelmintic treatments in relation to the epidemiology of ostertagiasis in different environments. Best results are obtained when treatments, given at weaning and 6 months later, are combined with a move to 'safe' pastures which have not been grazed by cattle for the previous 6 months. Good results are also obtained in the winter rainfall regions when set-stocked cattle are treated before and during autumn to prevent contamination of pastures at this time. More complicated grazing management, involving the spelling of pastures during autumn and winter, combined with anthelmintic treatments, is needed in some summer rainfall regions where weather conditions are especially favourable for the development and survival of the free-living stages of O. ostertagi.

The tolerances of individuals (insects, parasites) in a population have a frequency or probability distribution called a tolerance distribution. Many tolerance distributions in bioassay studies can be the result of a rather heterogeneous population of individuals and can often be modelled as a mixture of a number of standard unimodal distributions. A probit analysis can be generalized to the case where the tolerance distribution is a mixture of location and scale parameter distributions. In this article, the existence and determination of the maximum likelihood estimates are investigated. An expectation-maximization (EM) algorithm for probits of mixtures is developed and it is shown that by application of the EM algorithm, the problem of probits of mixtures can be separated into a series of probits of individual component tolerance distributions.

An unidentified coccidia was found in chinchillas. Treatment with sulfamethazine apparently eradicated the infection from several herds. Ostertagia females and 1 Trichostrongylus colubriformis male were also found in chinchillas, causing emaciation. A dose of thiabendazole of 100 mg./kg. of body weight was effective against the roundworms, but it was toxic. A dose of trichlorphon, 100 mg./kg. of body weight, was effective and safe. Hymenolepis infection caused emaciation and death. Treatment with doses of niclosamide, 200 mg./kg. of body weight, was successful in removing tapeworms, ending deaths, and improving the condition of the host. It did not eradicate the infection.