Ivermectin as a possible control agent for the tsetse fly, Glossina morsitans

Ivermectin produced 100% mortality in adult teneral males, mature males and fertile females of Glossina morsitans morsitans following a single meal of defibrinated pig blood containing concentrations of 0.1, 1.6 or >1.6 μg ml⁻¹ respectively. The lethal concentration was reduced to <0.04 μg ml⁻¹ for teneral males when fed repeatedly on treated blood. When pregnant females were fed a single blood meal containing ivermectin (0.08 μg ml⁻¹) on the day after their first larviposition, followed by normal blood meals, no offspring were produced in the subsequent reproductive cycle but full recovery occurred thereafter. A dose dependent decline in fecundity was measured and data were subjected to Probit analysis. Thus estimates were made of ivermectin concentrations in the peripheral blood of treated animals by measuring the reduction in fecundity induced in flies fed on such blood. Indications are that with subcutaneous injections at least, amounts greatly in excess of the recommended clinical dose would be required to achieve levels lethal to feeding flies following a single blood meal. Oral treatment of a horse with twice the anthelmintic dose of ivermectin (0.4 mg kg⁻¹) produced a maximum concentration in the blood of about 0.14 μg ml⁻¹ within 24 h and this was adequate to reduce tsetse fecundity to zero following a single meal. Such levels in a single blood meal were also sufficient to shorten the life expectancy of teneral male flies. The half-life of ivermectin in the horse was approximately 5-6 days with a maximum of 2.4% of ingested material entering the peripheral circulation. A cow treated with injectable ivermectin (0.2 mg kg⁻¹) produced maximum blood levels of about 0.005 μg ml⁻¹ after one week; this was only 0.17% of the administered dose and sufficient to reduce fecundity in female flies to 50% of normal following a single blood meal. Such levels in a single blood meal had no effect on the longevity of flies. However, at least half the maximum activity was present in the circulation between 3 and 14 days following injection. Repeated feeding on the blood of a treated animal reduced considerably the dose of ivermectin required to produce a given effect. The fecundity of female flies was reduced to zero by repeated feeding on blood taken from the horse 8 days after treatment, and even after 15 days the blood of the horse contained sufficient drug to reduce fly fecundity to 50% of normal. Thus where domestic animals constitute major hosts of tsetse, treatment with ivermectin can be expected to achieve some measure of fly population reduction.