The efficacy of ivermectin (pour-on) against the eyeworms, Thelazia gulosa and Thelazia skrjabini in naturally infected cattle.

Sixteen crossbred beef cattle, 6-8 months of age, were used to determine the efficacy of ivermectin in a pour-on formulation against natural infections of Thelazia spp. Cattle were weighed on Day 0, ranked in descending order by bodyweight and sequentially paired. Within each pair, one animal was randomly assigned to an untreated control group, while the other animal was treated with a pour-on formulation of ivermectin applied topically at 1.0 ml per 10 kg bodyweight to achieve a dosage of 500 mg ivermectin kg-1 body weight. On Day 14 all animals in both treatment groups were slaughtered. The eyes and surrounding tissues were removed and examined for eyeworms. A total of 86 adult worms (74 Thelazia skrjabini and 12 Thelazia gulosa) were recovered from the untreated control animals. T. skrjabini was found in seven, and T. gulosa in three, of the eight untreated control animals. Two T. skrjabini, one from each of two animals, were found on the surface of the eyes of cattle that had received ivermectin treatment. The reduction in the geometric mean number for T. gulosa collected from treated animals was 100% (P > 0.1) relative to controls, and 97.02% (P = 0.0025) for T. skrjabini relative to controls.