Study data suggest true prevalence of adult heartworm infection in cats is likely higher, perhaps significantly so, than currently recognized.
NEW data were published Jan. 13 in Parasites & Vectors from a study conducted at Oklahoma State University that identified a method for improving the sensitivity of antigen tests for heartworm (Dirofilaria immitis) infection in cats.
The study — funded by the Bayer HealthCare LLC Animal Health division, which makes parasiticides — found that heat treatment of feline serum samples prior to testing can considerably improve the sensitivity of antigen assays in feline patients and, in turn, result in a more accurate diagnosis of heartworm infection in cats.
"Heartworm is a very serious disease in cats that can result in significant pulmonary damage and can be fatal," said lead study investigator Susan E. Little, Regents professor and Krull-Ewing chair in veterinary parasitology at the Oklahoma State Center for Veterinary Health Sciences. "We believe this study's findings will enhance detection of heartworm infection in cats, improving both patient care and veterinarians' understanding of the true extent of feline heartworm."
According to the researchers, the diagnosis of heartworm in cats has been complicated in the past by the difficulty associated with reliable detection of the heartworm antigen in feline samples. In fact, heartworm antigen tests have been considered much less reliable in cats than in dogs, they noted.
The lack of antigen detection has been attributed to low circulating antigenemia (the presence of antigens in the blood) resulting from the low number of worms often seen in feline infections. Other feline heartworm assays based on antibody detection can be difficult to interpret since they do not distinguish between past or current infections, Little said.
Aiming to determine if antigen/antibody complex formation — antigen blocking — interfered with detection of the heartworm antigen in cats using commercial assays, the study evaluated serum samples from six domesticated cats confirmed to be infected with a low number of heartworms. Four different commercially available assays were used before and after heat treatment of sera.
The study found that the heartworm antigen was detected in zero to one of six cats (0-16.7%) without heat treating the sera and using the assays according to manufacturers' directions. However, after heat treatment of the samples prior to testing, as many as five of six cats (83.3%) had detectable antigen — a "dramatic increase," the study's authors noted.
Thus, the researchers said the new data suggest that adequate antigen is present in samples from cats infected with heartworm to allow diagnosis, but formation of antigen/antibody complexes may prevent its detection on commercial assays.
The researchers concluded, "The data in the present study are exciting in that they suggest that once antigen/antibody complexes are disrupted, antigen tests may be of great value in confirming feline infection with D. immitis, particularly the sensitive microtiter well-based assays. Surveys conducted by antigen testing alone may be greatly underestimating the true prevalence of infection in cats."