Effect of repeated oxfendazole treatments on small strongyle infections in Shetland ponies

Overwintering of horse cyathostomes as inhibited third stage larvae (L3) and the effect of repeated oxfendazole (OFZ) treatment on strongyle infections were studied in an experiment with two groups of three Shetland ponies. Both groups were grazed together from May 28 to November 11, 1986 and subsequently housed. Treatments with 10 mg OFZ kg-1 were given on May 26, July 1 and July 28 and again one week before each group was necropsied in December and April, respectively. Worm populations of both groups were dominated by inhibited early L3. The proportion of fourth stage larvae (L4) was significantly higher in April than in December. The faecal egg counts showed that the OFZ treatment in May was more effective than those in July. This resulted in high pasture larval counts from the end of August onwards. The final treatments revealed a low efficacy against L3 and L4 but not against adult worms.