Platelet function, antithrombin-III activity, and fibrinogen concentration in heartworm-infected and heartworm-negative dogs treated with thiacetarsamide.

Platelet aggregation and release, platelet number, mean platelet volume, antithrombin-III activity, and fibrinogen concentration were evaluated in heartworm-negative and heartworm-infected dogs at baseline and on days 3, 10, and 21 after treatment with thiacetarsamide. Platelet reactivity was enhanced in a group of dogs naturally infected with Dirofilaria immitis, compared with 2 groups of heartworm-negative dogs, but platelet reactivity was not further enhanced after treatment with thiacetarsamide. A significant decrease in antithrombin-III activity was detected 21 days after treatment. The platelets from a group of laboratory Beagles implanted with 50 adult D immitis displayed enhanced reactivity 6 months after implantation, but by 18 months, platelet reactivity had returned to near, or less than, baseline. Platelet reactivity was enhanced after thiacetarsamide treatment in this group. Thiacetarsamide-associated changes were not observed in platelet number or size; antithrombin-III activity decreased, but the change was not significant. Fibrinogen concentration was increased significantly (P less than 0.05) on day 10. Enhanced adenosine diphosphate (ADP)-induced platelet aggregation was observed on days 3, 10, and 21 after treatment in heartworm-negative dogs. This change was not observed in 6 control Beagles not treated with thiacetarsamide. Although antithrombin-III activity was decreased on day 3 and fibrinogen concentration was increased on day 10, paralleling changes observed in the heartworm-infected dogs, the changes were not statistically significant. In this study, thiacetarsamide was procagulatory in heartworm-negative dogs and may be an important contributing factor to the thromboembolism observed with adulticidal therapy.