Efficacy of ivermectin against ectoparasites of cattle in South Africa.

The efficacy of various formulations of ivermectin administered at the recommended dose rate of 200 micrograms/kg was evaluated in cattle infested with mange mites (Sarcoptes scabiei var. bovis.), lice (Linognathus vituli and Damalinia bovis) and ticks (Boophilus decoloratus, Amblyomma hebraeum, Rhipicephalus appendiculatus and Hyalomma spp.) in 8 trials conducted in South Africa. Mange mites (S. scabiei) were eliminated from animals treated subcutaneously, resulting in marked clinical recovery. Oral administration reduced the numbers of, but did not eliminate these mites. Sucking lice (L. vituli) were eliminated from animals treated orally or subcutaneously. Numbers of biting lice (D. bovis) were reduced but not eliminated, subcutaneous injection being more effective than oral drenching. Subcutaneous injection with ivermectin every 14 d over a period of 84 d significantly reduced numbers of engorged female R. appendiculatus and A. hebraeum. Numbers of Boophilus spp. and Hyalomma spp. were also reduced. A single subcutaneous injection resulted in significantly fewer engorged female B. decoloratus on treated animals for up to 28 d after treatment. In one trial significantly fewer A. hebraeum males occurred on treated animals for up to 28 d after treatment. Too few R. evertsi were present in these trials to evaluate the effect of ivermectin against this parasite.

The avermectins: A new family of antiparasitic agents.

The avermectins are macrocyclic lactones produced by fermentation of the soil micro-organism Streptomyces avermitilis. They show activity against a broad range of nematodes and arthropod parasites of domestic animals at dose rates of 300 microgram/kg or less. Unlike the macrolide or polyene antibiotics, they lack significant antibacterial or antifungal activity. By oral or parenteral administration, avermectins are active against gastrointestinal nematodes and lungworms, and important ectoparasites such as lice, mange mites, ticks and larval stages of flies. They show excellent activity against parasites resistant to existing anthelmintics or ectoparasiticides. The avermectins appear to cause paralysis of nematodes and arthropods by opening gamma-aminobutyric acid-mediated chloride channels at the neuromuscular junction.

Registration of veterinary drugs in Australia and New Zealand.

In both Australia and New Zealand, the objective of clearance for registration and licensing is to ensure safe and efficient use of chemicals and to recognise the special needs of several different interests, namely 1. end-users concerned with efficacy and possible hazards; 2. the consumers of food products from treated animals; 3. stock owners and vendors who need protection from false claims from competitive products; 4. wildlife and other ecological considerations; 5. protection of overseas markets, with special reference to drug and chemical residues. In Australia the Technical Committee on Veterinary drugs is responsible for the preliminary clearance of products for mass-medication with respect to efficacy and safety and, in association with the National Health and Medical Research Council Committees, sets residue limits and establishes poison schedules. This committee, however, can only make recommendations to the individual States, who are responsible for administering regulations through relevant State departments. In New Zealand, the Animal Remedies Act (1967) is responsible for the control of manufacture and importation of animal remedies and for the licensing of these for specific purposes. The Animal Remedies Act is administered by an Animal Remedies Board which operates within the New Zealand Ministry of Agriculture and and Fisheries.

Studies on nematode infections of beef cattle in New South Wales.

The occurrence and seasonal trends of nematode parasite infections in beef cattle on the Tablelands and North Coast regions of New South Wales are described, based on worm counts from 627 spring-born steers slaughtered at 2-monthly intervals from 6 to 24 months of age. The predominant parasites were Ostertagia ostertagi, Trichostrongylus axei and Cooperia spp. Large burdens of adult Ostertagia were seen in 12-month-old animals in late winter and early spring, and later in 18- to 20-month-old cattle in the following late summer and autumn. These infections often resulted in outbreaks of clinical parasitism. Massive numbers of inhibited early fourth stage larvae (EL4) also accumulated in the yearlings during their first spring, reached peaks in mid-summer and then declined. The possibility of their resumption of development to mature worms in the late summer and autumn period is discussed. Adult T. axei showed a similar seasonal trend to Ostertagia and may have increased the severity of outbreaks of clinical disease. Cooperia populations of EL4 and adult worms were highest in weaners during winter, but lower numbers thereafter indicated a strong resistance to re-infection. In addition, Haemonchus placei occurred frequently on the North Coast both as EL4 and adults in cattle of all ages up to 20 months. Of the other cattle nematodes, Oesophagostomum radiatum and Trichuris spp occurred in low numbers, mainly in weaners. Bunstomum phlebotomum and H. contortus occurred sporadically on the North Coast and Tablelands respectively. Intestinal Trichostronglyus spp, Oesophagostomum venulosum and Dictyocaulus viviparus were seen occasionally and Nematodirus spp were not seen. There was a poor relationship between worm counts and faecal egg counts. Cooperia spp dominated the egg counts, while those for other genera were generally low and did not reflect the relative abundance or seasonal changes in worm numbers.