Emergence from inhibited development of cyathostome larvae in ponies following failure to remove them by repeated treatments with benzimidazole compounds.

The effect of three albendazole treatments at 5-week intervals, beginning at turnout in April, on cyathostome infections in Shetland ponies was compared with the effect of sequential treatments with albendazole, oxfendazole and oxibendazole. The results showed a substantial reduction in faecal egg output after the first albendazole treatment. Since faecal egg counts remained very low, no estimation of the effect of the second treatment was possible. The third treatment with albendazole and oxibendazole was followed by an increase in faecal egg counts to values of greater than 100 eggs g-1 within 4 weeks. A final albendazole treatment in December, 1 week before necropsy, failed to reduce faecal egg counts. These results suggest resistance to albendazole and oxibendazole in the cyathostome populations of the ponies. The increase in faecal egg counts after the third anthelmintic treatment in July occurred, although overwintered pasture infectivity was very low. The most likely explanation for this increase is resumption of the development of worms which overwintered as inhibited larvae in the host.