Intraarticular Dirofilaria immitis microfilariae in two dogs

Dirofilaria immitis microfilariae were found in the synovial fluid of two dogs. One dog had clinical and cytological evidence of polyarthritis at the time of presentation. The second dog presented with severe effusion in a single joint and was later diagnosed with synovial sarcoma of the affected joint. These patients were not protected with heartworm prophylaxis and lived in heartworm endemic areas. Though there is documentation of D. immitis microfilaria in the synovial fluid of several clinically normal research dogs with cytologically normal synovial fluid, to our knowledge these are the first documented cases of intraarticular microfilaria in a dog with cytologically confirmed polyarthritis. Based on these unique cases, D. immitis infection should be considered a differential diagnosis in patients with polyarthropathies. Interpretive caution must be used when intraarticular microfilaria are present, as concurrent etiologies may also be present.