Prophylactic activity of ivermectin against Dirofilaria immitis infection in dogs: Establishment of effective dose and administration schedule.

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Abstract
To establish a recommended dose and prophylactic program of ivermectin for preventing D. immitis infection, antilarval activity of ivermectin at 3 or 6 .mu.g/kg against one day, 30 and 60 days old larvae was evaluated. The worms were recovered from all dog given ivermectin at 3 .mu.g/kg one day after inoculation with an average infection ratio of 21.9%. One male worm was recovered from one of 6 dogs given ivermectin at 3 .mu.g/kg 30 days after inoculation with an extremely low average infection ratio of 0.1%. Out of 6 dogs given ivermectin at 3 .mu.g/kg 60 days after inoculation, 3 animals yielded worms with a low infection ratio of 4.0%. The results suggest that ivermectin at 3 .mu.g/kg demonstrates incomplete antilarval activity against D. immitis larvae of one day, 30 and 60 days old and is not suitable to monthly or bimonthly interval medication for preventing D. immitis infection. Small numbers of the worms were recovered from 5 of 6 dogs given ivermectin at 6 .mu.g/kg one day after inoculation with an average infection ratio of 5.7%. No worms were recovered from all dogs given ivermectin at 6 .mu.g/kg 30 and 60 days after inoculation. The results show that a single oral dose of ivermectin at 6 .mu.g/kg, although the dose demonstrates incomplete antilarval activity against one day old larvae, would be a recommended dose which can completely kill the larvae which are naturally infected at any time when the dose is given once a month at one month interval during the period from one month after the first infection to one month after the last infection of D. immitis.

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