Heartworm disease manifested by encephalomyelitis and myositis in a dog

A 2-year-old Boston Terrier was referred because of depression and hindlimb and tail paresis. Clinical examination and serum biochemical analysis resulted in a diagnosis of encephalomyelitis and myositis. Results of testing for Dirofilaria immitis were positive. Gross and histologic examination revealed an aberrant adult heartworm infection resulting in thrombosis of the femoral artery and multiple muscular branches, with subsequent muscle necrosis and inflammation in one hindlimb. Additionally, an unusual larval-tissue interaction to microfilariae yielded a multifocal encephalomyelitis in the brain and spinal cord.