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Tissues of dogs treated with ivermectin were examined microscopically to learn the fate of microfilariae of canine heartworm that disappear from the peripheral circulation within a few days of treatment. Medicated dogs were killed 18 hours, 3 days, and 6 days after treatment with 0.5 mg of ivermectin/kg of body weight subcutaneously. Ivermectin was dissolved in 60% propylene glycol and 40% glycerol formal. In dogs killed at posttreatment hour 18, the peripheral microfilaremia had decreased by an average of 89%. At this time, a dense mass of RBC, WBC, and macrophages plus many microfilariae was found in pulmonary alveolar septae. Similar reactions were seen in liver, kidney, and spleen. Phagocytosis of microfilarial fragments was evident. In dogs killed at posttreatment day 3, many microfilariae were fragmented and phagocytosis of the fragments was common. In dogs killed at posttreatment day 6, microgranulomas were common, particularly in such vascular organs as lungs, kidney, and liver. Microgranulomas containing microfilariae were also seen in spleen, skeletal and cardiac muscles, diaphragm, lymph nodes, gastrointestinal tract, and pancreas. Small glial nodules were seen in the CNS. Denudation of the atrial epicardium was associated with fragments of microfilariae and granulomatous inflammatory cells. Renal epithelial crescents were observed in treated and nontreated dogs. Plasma cells were conspicuous in treated and nontreated dogs, especially in some livers and kidneys. Before treatment, all dogs were severely microfilaremic. At the end of the experiment, the peripheral microfilaremia was reduced by 98%.(ABSTRACT TRUNCATED AT 250 WORDS)

Ivermectin had no adverse effects on spermatogenesis, fertility, or reproductive performance of Beagle dogs when administered orally at 600 micrograms/kg (0.6 mg/kg) of body weight monthly for 8 treatments. Semen was collected every 3 days from 28 days before treatment began until 83 days thereafter from 6 ivermectin-treated Beagles and 6 similar water-treated controls (38 collections/dog). All dogs were then bred to 2 nontreated bitches each; litter size, birth weights, and pup abnormalities and mortalities were evaluated. After all pups were whelped, each dog was euthanatized and necropsied, and the testis and epididymis were examined microscopically.