Treatment of canine dirofilariasis: pulmonary thromboembolism caused by thiacetarsamide--microscopic changes.

Light and scanning electron microscopies were used to study the pulmonary embolism occurring in Dirofilaria immitis-infected dogs after treatment with thiacetarsamide. Lesions in control dogs (nontreated) which had been infected for 4 weeks with 9 Dirofilaria immitis adults were compared with lesions occurring in infected dogs at 2 weeks and at 4 weeks after they were treated with the adulticide. Extensive thromboembolism occurred in the caudal lobar pulmonary arteries of dogs at posttreatment weeks 2 and 4. Complicated villous proliferations were present at posttreatment week 2. The characteristic myointimal proliferation of dirofilariasis showed resolution in the large pulmonary arteries of the dogs at week 4. However, the caudal lobar pulmonary arterial and lung lesions were more severe in the later group. The pathophysiology of adulticide-induced thromboembolism and associated lung parenchymal changes were discussed.