Migration of warble fly larvae in the yak and optimum timing of ivermectin treatment.

Sixty yaks were autopsied to determine the migration pattern of warble fly larvae. In August, first instars were observed in the body of yak for the first time. These larvae peaked in number in October. From November to February, second instars were detected and their number peaked in January. Third instars appeared in January and peaked in March. Forty-five yaks were administered with ivermectin: 15 animals in September, 15 in October and 15 in November. Between December and June, the number of warbles was checked by palpation. Although some warbles were observed in the September- and November-treated groups, no warbles were detected in the October-treated group. Treatment of yaks with ivermectin was most effective for warble fly in October.