Three studies were conducted in Latin America--one in Venezuela, one in Argentina and one in Brazil--using a common protocol to investigate the efficacy of a single
subcutaneous injection of
doramectin in the prevention and control of
Cochliomyia hominivorax infestations in castrated cattle. In each study, two groups of 20-28 animals each were allocated to a treated (T1) or to a control (T2) group on the basis of
body weights. Animals of T1 received
doramectin at 200 micrograms kg-1 (1 ml per 50 kg) and animals of T2 received
saline solution at 1 ml per 50 kg of live weight.
After treatment all cattle were castrated surgically. Animals were examined on treatment day and at 2, 4, 6 and 12 days post-treatment. At each observation day, the presence of C. hominivorax infestations was recorded.
Doramectin was 100% effective in the prevention and control of screwworm strikes in castrated cattle exposed to continuous field infestations of C. hominivorax in tropical and subtropical areas of Latin America. Over the 12 day duration of the studies, 85%, 60% and 65% of animals in the control groups had infested
wounds in Venezuela, Argentina and Brazil, respectively. Affected animals required repeated therapeutic treatment, whereas none of the
doramectin-treated cattle were infested (P < 0.0001). A high proportion of the
castration wounds in
doramectin-treated cattle had the presence of characteristic C. hominivorax eggs but none developed into larvae. There were no clinical signs of adverse reactions to treatment in any of the three studies.