FeLV and FIV infection are important viral diseases of cats. Their initial prevalence reported over 10 years ago was 13% and 7%, respectively. After widespread client education, testing, and vaccination development, prevalence has decreased. A recent study involving a small number of cats (1763) found the prevalence of FeLV to be 1.3% and that of FIV to be 0.9%. Another study involving 1876 feral cats reported the prevalence as 4% for each virus. In this prospective study, 18,038 cats from 345 veterinary clinics and 145 animal shelters were tested between August and November 2004 to determine the prevalence and to identify the risk factors associated with infection. Cats were tested by using point-of-care ELISA test kits (SNAP Combo FeLV antigen/FIV antibody, IDEXX Laboratories). This study found the prevalence of FeLV infection to be 2.5% and for FIV to be 0.3%. Age was a risk factor for infection, with adult cats at greater risk than kittens and juveniles. Sexually intact adult male cats were at greater risk than intact females. Finally, outdoor cats that were sick at the time of testing were more likely to be positive than healthy indoor cats. Even though age, sex, and lifestyle were found to be associated with risk factors, the authors cautioned that all cats are at risk for infection and any new or sick cat should be tested.

COMMENTARY: Because testing is voluntary, determining the true prevalence of FeLV and FIV infection is a challenging goal. Different groups of care providers have different reasons for testing, thus budgeting considerations differ. Veterinarians are most likely to test sick cats and will recommend testing kittens and newly adopted cats. Staff managing homeless cats at shelters may selectively test healthy cats for suitability for adoption. Many large spay-neuter programs for feral cats are likely to test a debilitated cat to determine suitability for releasing the cat back into the colony. False-positive (and less commonly, false-negative) results also affect determination of the true seroprevalence of these viral infections.

A point of clinical interest for the practitioner is to note that, despite age-induced immunity, adult cats are at greater risk for FeLV infection. Thus, vaccination of "at risk" cats must continue in adults.

The ultimate responsibility of whether to test lies with our clients. However, as our clients' advisors, it is our responsibility to recommend testing cats when they are first acquired as pets, when they are exposed to cats known to be infected, when they are exposed to cats with unknown infection status, and when they become ill, regardless of previous test results, as per the AAFP Retrovirus Testing Guidelines.

Seroprevalence of feline leukemia virus and feline immunodeficiency virus infection among cats in North America and risk factors for seropositivity. Levy JK, Scott HM, Lachtara JL, Crawford PC. JAVMA 228:371-376, 2006.