Objective—To test the ability of a single injection of a
sustained-release formulation of moxidectin (moxidectin
SR) to protect dogs against heartworm infection
for 180 days after inoculation with infective thirdstage
larvae (L3) of Dirofilaria immitis.
Animals—32 adult mixed-breed dogs.
Procedure—Dogs were allocated to 4 groups on the
basis of weight and sex. Dogs were injected SC with
saline (0.9% NaCl) solution or moxidectin SR at the
rate of 0.06, 0.17, or 0.5 mg/kg of body weight (day 0).
Each dog was inoculated SC with 50 D immitis L3 180
days later. On days 330 and 331, dogs were euthanatized.
The heart, lungs, and thoracic cavity were examined,
and number and sex of heartworms were determined.
Results—A mean of 35.9 heartworms was recovered
from untreated control dogs. Fourteen worms were
recovered from 1 of 8 dogs given moxidectin SR at the
lowest dosage, and none of the dogs in the 2 highest
moxidectin treatment groups were infected. Small
barely palpable granulomas were detected at injection
sites of moxidectin-treated dogs. Frequency and size of
granulomas were positively correlated with dose of
Conclusions and Clinical Relevance—A single dose
of moxidectin SR at a dosage as low as 0.17 mg/kg
can safely and reliably confer complete protection
against infection after challenge-exposure with
D immitis L3, and protection lasts for at least 180
days. This mode of prophylactic treatment against
infection with heartworms effectively eliminates failure
of prophylaxis that results from erratic administration
of medications designed for monthly administration.
(Am J Vet Res 2001;62:1721–1726)