The feline retroviruses, FeLV and FIV, today are well recognized for their ability to cause profound immune-suppressive disease
in cats throughout the world. Clearly among the most complex infections affecting the cat, a retroviral infection demands
an immune response that is robust and sustained if the infected cat is to survive long-term. Both innate immune responses
(neutrophils, macrophages, and antigen presenting cells [APCs]) as well as adaptive immunity (B-lymphocytes and T-lymphocytes)
In general terms, FeLV causes the most significant clinical infection, compared to FIV. In the past, FeLV was considered
the most common cause of death from infectious disease. Today, however, the incidence of disease has clearly declined from
what it was just 20 years ago. While vaccination accounts for some of that decline, the ability to perform rapid, accurate
retrovirus testing of cats is recognized to be the principle reason behind the decrease in FeLV incidence. It's important
to note that susceptibility to FeLV infection is greatest during the first 6 months of a cat's life. After that, "natural
resistance" develops…which is attributed to maturation of the immune system in an adult cat and production of one of the interleukins.
In the immunologically na´ve kitten, exposure to FeLV is likely to result in life-long infection. With FeLV, 'life-long'
may be months or it may be years…it's a disease that is very difficult to prognose due to the wide range of clinical outcomes
possible once the virus reaches the infected cat's bone marrow.
Susceptibility to FIV infection, on the other hand, does not change with a cat's age. Risk among adults is similar to that
in kittens. Furthermore, FIV infection is not associated with the same spectrum of clinical consequences seen in FeLV-infected
cats. Although FIV does cause an acquired immunodeficiency syndrome, one that shares many similarities with HIV infection
in humans, the prognosis of FIV infection is generally going to be better than that of FeLV, especially when level of medical
care the cat receives during the course of infection is high. Opportunistic infections are common, but FIV-infected cats
tend to live longer (given the opportunity to do so) than FeLV-infected cats. Many FIV-infected cats are known to die of
age-related causes not linked to their retrovirus infection. And…as such, FIV has not had the impact on the feline population that FeLV has.
FeLV and FIV: Diagnostic testing
One of the most significant technologic advances introduced into clinical practice within the last 20 years has been the ENZYME-LINKED
IMMUNOSORBENT ASSAY, or ELISA. The ability to perform in-hospital, same-day testing for feline leukemia virus (FeLV), feline
immunodeficiency virus (FIV), canine parvovirus (CPV), along with several others, are among the most important value-added
laboratory services offered by veterinarians today.
In accordance with current guidelines, it is recommended that the FeLV and FIV status of all cats seen by the practice be established and that all cats be tested (and determined to be NEGATIVE) prior to vaccination. However, the commitment to perform routine
screening of cats for retroviral infection raises important issues pertaining to interpretation of test results and follow-up
actions needed to further manage those households with confirmed FeLV and/or FIV positive cats. Current testing recommendations
outlined by the AAFP's Advisory Panel on Retrovirus Testing have recently been updated and be reviewed/downloaded at:
http://www.catvets.com/ (search: Practice Guidelines: 2008 Retrovirus Management Guidelines).
Fundamental to the proper use of the SNAP (ELISA-based) testing for the diagnosis of FeLV and FIV infected cats is an understanding
that FeLV tests are designed to detect the p27 antigen (viral core antigen) while FIV tests detect the presence of antibody.
In clinical practice, these facts have important implications. For example, the FeLV test can detect FeLV virus in the blood
(serum or plasma) of kittens, even as young as 1-day of age; a positive test is consistent with infection. It is important to note that FeLV tests designed to detect the presence of virus in tears and/or saliva are also ELISA-based
tests. However…these tests are significantly less sensitive/specific (ie, large numbers of false-positive and false-negative
results) than tests utilizing blood, serum, or plasma.