Infection in cats with Dirofilaria immitis (canine heartworm) is present in the United States; in fact, cases have been documented in a majority of the states.
Identification of heartworm infection in live cats can be challenging and usually requires a close clinical review of serologic assays, thoracic radiography and echocardiography. While microtiter well-based ELISAs for D immitis antigen are viewed as the most sensitive and specific means of identifying heartworm infection in dogs, these assays can be unreliable in detecting active infection in cats. The reasons for this potential for unreliability are due to a low number of worms, presence of only male worms or low concentrations of circulating antigen in many cats. In addition to these factors, the presence of only immature worms in some cats appears to result in high frequency of false negative antigen test results and underestimation of the true prevalence of D immitis in cats.
With commercial assays, pretreatment of canine and feline serum with heat improves detection of heartworm antigen. A prior study in cats has indicated that the pretreatment of sera of cats results in improved detection of antigen in the majority of samples.
In this study, feline serum (n=281) and plasma (n=104) was collected from adult cats at animal shelters or part of a trap-neuter-return (TNR) program. Each sample was tested by commercial microtiter plate assay and then all positive results were confirmed using a second antigen-detecting microtiter assay both before and after heat pretreatment of the samples. An antibody assay was used on unheated serum with a subset of samples which were selected on the basis of adequate sample volume.
The results of the study were that antigen of D immitis was found in 1/220 (0.5%) cats from animal shelters and 4/165 (2.4%) of free-roaming cats prior to heat treatment of samples. After heat treatment, antigen was detected in 13/220 (5.9%) shelter and 13/165 (7.9%) free-roaming cat samples. In testing for antibody to D immitis, antibody was detected in 32/176 (18.2%) of samples heat treated. Of these antibody positive samples, 1/32 (3.1%) was antibody positive before heat treatment.
Surveys for D immitis in cats by conventional antigen testing suggest a prevalence of 0.9% in the US (range of 0.5% and 1.4%). Antibody tests are more likely to be positive, with prevalence estimates of 5.1-32.7%.
The authors state that pretreatment of feline serum and plasma resulted in a more than five-fold increase in antigen detection in samples from shelter and free-roaming cats. The mechanism responsible for blocking antigen from detection is not known but cats develop a significant inflammatory response to D immitis infection which suggests immune complexes may play a role in blocking antigen detection in some infections. Antibody tests are a valuable tool for the identification of past or current heartworm infection in cats.
In conclusion, this study suggests many cats in endemic heartworm areas may test negative on routine antigen tests for heartworm yet will convert to positive when heat pretreated samples are used in the assay, suggesting the cat could have a current or recent D immitis infection. Additional study is needed to determine sensitivity, specificity and predictive values of the assays when the samples have been heat-treated.
