Drought and flock isolation may enhance the development of anthelmintic resistance in nematodes.

A survey of anthelmintic resistant nematodes was conducted in sheep and goat flocks in Greece using in vivo and in vitro tests. Faecal egg count reduction tests in Macedonia were all greater than 99% indicating very high sensitivity of the nematodes to anthelmintics. In vitro tests showed benzimidazole resistant Teladorsagia circumcincta in 17 out of 106 flocks on small islands. On the mainland there were only three cases of benzimidazole resistance out of 310 flocks and animals had recently been introduced to the flocks. Flocks on the islands are isolated and there are higher temperatures than on the more mountainous mainland, where flocks tend to intermingle. It is concluded that drought and isolation are likely to be the major factors accounting for the development of anthelmintic resistance in nematodes in the island flocks.