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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