Clinical and laboratory changes after administration of milbemycin oxime in heartworm-free and heartworm-infected dogs

Adverse reactions to oral administration of milbemycin oxime were investigated in heartworm (HW)-free and HW-infected dogs given either the minimal HW prophylactic dose (0.25 mg/kg of body weight) or the hookworm anthelmintic dose (0.5 mg/kg). In 12 HW-free control dogs treated with lactose excipient (100 mg/kg), abnormal signs were not observed. There were no differences between the 2 doses in prevalence of clinical signs of disease and laboratory test results. In 60 HW-free dogs (50 dogs administered the low dose, and 10 dogs given the high dose) and 46 nonmicrofilaremic HW-infected dogs (35 dogs administered the low dose, and 11 dogs given the high dose), only a transient and slight paleness of the visible mucous membranes, intestinal hyperperistalsis, or both were observed in some dogs. In 77 microfilaremic (MF) dogs (41 dogs administered the low dose, and 36 dogs given the high dose), weakness or loss of appetite was observed in 13 dogs (16.9%). Paleness of the visible mucous membranes was observed in 16 dogs (20.8%), intestinal hyperperistalsis was observed in 27 dogs (35.1%), and respiratory signs, such as mild labored respiration, were observed in 13 dogs (16.9%). Dullness of heart sounds was noticed in 4 dogs (5.2%). In 12 (9 dogs administered the low dose, and 3 dogs given the high dose) of 89 MF dogs (13.5%), adult heartworms migrated from the pulmonary arteries to the right atrium, causing signs of caval syndrome, including heart murmurs, jugular pulsations, and weakness. In HW-free dogs, rectal temperature tended to decrease gradually, and heart and respiratory rates were transiently decreased.(ABSTRACT TRUNCATED AT 250 WORDS)