Veterinary anthelmintics : their efficacy and effects on abomasal physiology : a thesis presented in partial fulfilment of the requirements for the degree of Master of Veterinary Science in Veterinary Clinical Pharmacology at Massey University

Abstract

PART 1. A Review of the Veterinary Anthelmintic Literature A comprehensive review was undertaken of the pharmacology, efficacy, side effects and toxicity of veterinary anthelmintics used against nematode parasites. Anthelmintics reviewed for use in cattle, sheep, goats, horses, dogs and cats include copper, nicotine, arsenic, tetrachlorethylene, phenothiazine, diethylcarbamazine, piperazine, toluene, cyacethydrazide, bephenium, thenium, organophosphates, and methyridine. The review was limited to cattle for the benzimidazoles, pyrantel, morantel, tetramisole, levamisole, avermectin and milbemycins anthelmintics. Efficacy data is provided in a tabular format which classifies each anthelmintic according to method of administration and dose. PART 2 Efficacy of two formulations of moxidectin pour-on and the effects of treatment on serum pepsinogen and gastrin levels and tissue gastrin in cattle Three groups of eight yearling Friesian bulls were used to compare the efficacy of two 5 g/L pour-on formulations of moxidectin applied at 1 ml/10kg (500 mcg moxidectin per kg bodyweight) in removing naturally acquired gastrointestinal parasites. At slaughter, 14-16 days after treatment, the burdens of Ostertagia spp. and Trichostrongylus axei were significantly lower in both the treated groups versus the controls (P<0.01). Anthelmintic efficacies (based on geometric mean worm burdens of treated and control groups) were all greater than 99.8% for Ostertagia spp. and Trichostrongylus axei in both treated groups compared with the controls. A significantly lower burden of adult Cooperia spp. was recorded for Formulation 1 (P<0.05). The anthelmintic efficacy of the two treatments against Cooperia adults, early L4 and late L4 were 96.25% (P<0.05), 97.31% and 91.08% respectively in calves treated with Formulation 1, and 71.44%, 67.14% and 64.29% respectively for calves treated with Formulation 2. Low numbers of large intestinal worms, Trichuris ovis and Oesophagostomum spp. in the control cattle precluded any valid efficacy assessment of these species. Based on these results, Formulation 1 is distinguished from Formulation 2 by its significantly greater efficacy against adult Cooperia spp. Serum pepsinogen and gastrin levels were monitored in the three groups of calves after treatment. All groups showed a steady decline in levels of both pepsinogen and gastrin until termination on day 14. On only one occasion was there a significant difference in serum pepsinogen between treated and untreated calves. Following treatment there was a more rapid and significant decrease in gastrin levels in calves treated with Formulation 1 than Formulation 2. At no stage was there a significant difference between the controls and calves treated with Formulation 2. There was no correlation between pepsinogen and gastrin levels for any of the groups. Neither was there a correlation between numbers of Ostertagia spp. and serum pepsinogen or gastrin. It is suggested that the decreases seen in the control group were due to lack of larval challenge and normal loss of adult worms resulting insufficient numbers to sustain a hypergastrinaemia. Gastrin concentrations were also measured in tissue samples from the pyloric antrum or the proximal duodenum. There was no significant difference in antral and proximal duodenal gastrin concentrations between the treatment and the control groups. Antral levels were between 1148- 1323 pM/g which were 25-35 times those found in duodenal tissue (32.3-50.9 pM/g).