Feline heartworm (Dirofilaria immitis) infection: detection of specific IgG for the diagnosis of occult infections.

Sera from 54 cats, 53 asymptomatic and one symptomatic (chronic dyspnoea and coughing), living in a hyper-endemic area for canine heartworm (Dirofilaria immitis) infection were studied to evaluate the reliability of two ELISA-based antibody tests coupled with both somatic (ELISASA) and excretory/secretory (ELISAE/S) antigens. All cats were examined by echocardiography and radiography. In addition, an ELISA-based test to detect adult female heartworm circulating antigens and a modified Knott test for microfilariae in the blood were carried out on all cats. No cat was positive for microfilariae in the blood. Heartworms were visualized in 12 of 54 cats by echocardiography. Of these, three asymptomatic cats and the symptomatic one had radiographic signs of infection and were the only ones positive for heartworm circulating antigens. All sera except two were positive when analyzed in ELISA(SA). In ELISA(E/S), nine sera were positive but three were negative. No sera from the 42 echocardiography-negative cats was positive in ELISA(E/S), but 11 were positive in ELISA(SA). Western blot analyses with somatic antigens of sera from echocardiography-positive cats showed at least four bands of recognition between 19 and 30 kDa and one of about 40 kDa. With E/S antigen, a large band of about 22 kDa and one of about 25 kDa were recognized; these appear to be most specific.