Testing kittens for FeLV and FIV (Proceedings) - Veterinary Healthcare


Testing kittens for FeLV and FIV (Proceedings)


Recently, a discriminant ELISA detecting antibodies against formalin-treated FIV whole virus and untreated transmembrane peptide was shown to have high sensitivity (97.1%) and specificity (100%) for distinguishing uninfected from infected cats, regardless of vaccination status (Levy, Crawford et al. 2008). Should a discriminant ELISA become commercially available, a new testing strategy could be devised where existing commercial FIV antibody tests would be used as screening tests and positive results would be confirmed with the discriminant ELISA. If the discriminant ELISA is negative, the cat is probably vaccinated against FIV but not infected. Positive results with the discriminant ELISA are likely to represent infection.

Diagnosis of FeLV

Diagnosis of FeLV relies on detection of the core antigen p27 in peripheral blood. ELISA test kits detect soluble circulating antigen and are recommended for routine in-clinic use. They may be used with whole blood, serum or plasma, although the test kit should be checked for the manufacturer's recommendations on sample type. Tests performed on tears and saliva are less reliable and are not recommended. ELISA tests can detect infection early, during primary viremia. Most cats will test positive on ELISA within 1 month of exposure, although detection of antigenemia may take much longer in some cats. Immunofluorescent antibody (IFA) tests on smears from blood or bone marrow detect p27 antigen within infected neutrophils and platelets and are recommended as confirmatory tests. IFA tests do not detect infection until secondary viremia is established due to infection of bone marrow (6 to 8 weeks after initial infection).

Ideas on possible outcomes of infection with FeLV are currently undergoing re-evaluation. In the past, it was believed that about 1/3 of cats became persistently viremic and about 2/3 would clear infection. New research using PCR technologies suggests that most cats remain infected for life following exposure to FeLV. However, they may revert to a non-viremic state that is termed regressive infection. In regressive infections, there is no antigen present in the blood and virus cannot be cultured from blood. But FeLV proviral DNA can be detected in blood using PCR (Pepin, Tandon et al. 2007). The significance of PCR-positive but antigen-negative regressive infections is not yet clear. These cats are unlikely to shed infectious virus in saliva, but may transmit proviral DNA via blood transfusion if used as a blood donor. Prior to the advent of PCR technology, the term "latency" was used for antigen-negative cats where virus could not be cultured from blood, but could be cultured from bone marrow or other tissues. It now appears that "latency" is a phase through which cats pass during regressive infection.

Kittens can be tested at any age, as passively acquired maternal antibody does not interfere with testing for viral antigen. Newborn kittens infected via FeLV-positive queens may not test positive for weeks to months after birth. While it may be tempting to test only a queen and not her kittens in an attempt to conserve resources, it is inappropriate to test one cat as a representative for others. If a queen or any one of her litter of kittens tests FeLV-positive, all should be considered potentially infected and isolated, with follow up testing to resolve status. Susceptibility to FeLV infection is age-related, with the highest infection rates in very young kittens. Shelters sometimes test pooled blood samples from litters of kittens in order to save money, but this practice should be discouraged as the reliability of this method is unknown.

Kittens or cats that test negative but have a known or suspected exposure to FeLV should be retested no earlier than 1 month after exposure to rule out false negative test results obtained during incubation of the virus. Periodic testing of cats at ongoing risk of FeLV infection is justified and is not generally compromised by vaccination. However, blood collected immediately following vaccination may contain detectable FeLV antigens from the vaccine, so samples should be collected prior to FeLV vaccination (Levy J, unpublished data). It is not known how long this test interference persists.

Since the consequences of a positive test are significant and false positive test results can occur, confirmatory testing with IFA is recommended, especially in low-risk patients (Hartmann, Werner et al. 2000). Some cats may be only transiently viremic and may revert to ELISA-negative status (regressive infection). However, a positive IFA test at any time on blood or bone marrow generally indicates a cat is persistently infected.


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