Feline leukemia virus (FeLV) and feline immunodeficiency virus (FIV) are retroviruses that represent two of the most important
infectious diseases of cats worldwide. The American Association of Feline Practitioners recommends the retrovirus status of
all cats should be known and has published guidelines for retrovirus testing and management (Levy, Richards et al. 2001).
Diagnosis of FIV
The most common method of diagnosis of FIV infection is screening for FIV antibodies using an enzyme-linked immunosorbent
assay (ELISA). Patient-side kits are commonly used in veterinary clinics and in shelters. Screening for viral antigen is not
possible since the amount of circulating virus is low after the acute stage of infection. FIV produces a persistent, life-long
infection so that the detection of antibodies is judged sufficient for diagnosis. Most cats will produce antibodies to FIV
within 60 days of exposure, but the length of time to seroconversion can be much longer in some cats.
False-positive results can occur with ELISA testing, so it is recommended that the Western blot be used as a confirmatory
test, especially in cats considered at low risk of infection. A recent study showed that the sensitivity and specificity for
FIV was very high in unvaccinated cats using a commercially available ELISA test kit (SNAP® FIV/FeLV Combo, IDEXX Laboratories,
Westbrook, Maine) (Levy, Crawford et al. 2004). A small number of cats will fail to produce detectable levels of antibodies
after infection and these cats will have false negative results with both ELISA and Western blot testing.
Positive FIV antibody tests in kittens under 6 months of age must be interpreted carefully. Kittens born to infected queens
may acquire FIV antibodies in colostrum(MacDonald, Levy et al. 2004). Since it is uncommon for kittens to acquire infection
from the queen, most kittens that test positive are not truly infected and will test negative when re-evaluated at 6 months
of age or older. Kittens that test positive for FIV antibody when over 6 months of age are considered to be infected. A negative
FIV antibody test is generally reliable at any age, especially in a low-risk patient.
Due to potential test interference, it is tempting to delay testing kittens for FIV until over 6 months of age. However, most
kittens test negative and can be reliably considered clear of infection. While FIV infection of kittens is uncommon, it has
been known to occur. Infected kittens could be a source of infection for other cats if they are not identified and isolated.
Compliance of both owners and veterinarians with retroviral testing recommendations is low, so that delaying testing of newly
acquired kittens would potentially result in many cats that never undergo FIV testing at all (Goldkamp, Levy et al. 2008).
The release of the first vaccine against FIV (Fel-O-Vax FIV®, Fort Dodge Animal Health) has complicated the ability of veterinary
practitioners to diagnose FIV infections. Vaccinated cats produce antibodies that cannot be distinguished from antibodies
due to natural infection using currently available tests (Levy, Crawford et al. 2004). Antibodies due to vaccination persist
for more than one year, and are also acquired by kittens nursing on vaccinated queens (MacDonald, Levy et al. 2004). Maternally
derived vaccine-associated antibodies may persist in kittens for longer than 8 weeks, giving another reason to interpret positive
FIV antibody tests in young kittens with caution and to recommend re-testing. Cats at risk of FIV infection, such as outdoor
cats, should be tested periodically throughout their lives if they are not FIV vaccinated. It is also recommended that cats
should be tested before vaccination against FIV.
In some patients, it may be difficult to determine if a positive FIV antibody test means the cat is truly infected with FIV,
is vaccinated against FIV but not infected, or is vaccinated against FIV and also infected. Polymerase chain reaction (PCR)
has been promoted by some commercial laboratories as a method to determine a cat's true status. In order to be useful to practitioners,
PCR tests should be able to detect a wide array of genetically divergent FIV strains and laboratories must maintain stringent
quality control. Published research indicates that PCR tests offered by commercial laboratories may be unreliable, with misidentification
of both FIV-infected and uninfected cats (Bienzle, Reggeti et al. 2004; Crawford, Slater et al. 2005).