Efficacy of doramectin in the protection of neonatal calves and post-parturient cows against field strikes of Cochliomyia hominivorax.

Two studies were conducted in Latin America, one in Venezuela and one in Argentina, using a common protocol to investigate the efficacy of a single s.c. injection of doramectin in the prevention and control of field infestations of Cochliomyia hominivorax in newborn calves and post-parturient cows. In both experiments, pregnant cows were paired on the basis of their calving time. The first cow that calved was assigned to a medicated group, and both cow and calf were treated with doramectin. The cow received a dose of 200 micrograms kg-1 and the calf 1 ml of a 1% doramectin solution. The second cow and calf were assigned to a saline-treated group, and the cow received saline at 1 ml per 50 kg and the calf 1 ml. The procedure was repeated until all animals were allocated to the two treatments. Animals were examined on the treatment day and at 2, 4, 6 and 12 days post-treatment. At each observation day, the navel of newborn calves and external genitalia of the cows were inspected and the presence of C. hominivorax infestations was recorded. Doramectin was 100% effective in the prevention and control of screwworm strikes in newborn calves and in post-parturient cows exposed to continuous field challenge of C. hominivorax. Over the 12 day duration of the studies, 48% and 57% of the saline-treated calves had screwworm navel strikes in the studies in Venezuela and Argentina, respectively. Infested animals required repeated therapeutic treatment, whereas none of the doramectin-treated calves were affected (P < 0.005).(ABSTRACT TRUNCATED AT 250 WORDS)