The two viruses and who is at risk
FeLV and FIV are retroviruses, viruses that insert a DNA copy of their genome into host cells, and both occur worldwide [1][3]. In a referral laboratory dataset covering about 3 million point-of-care results from 68 countries between 2008 and 2016, North American prevalence was 4% for FeLV antigen and 5% for FIV antibody, with higher figures in several other regions [1].
According to the AAFP panel, FeLV passes mainly through close contact such as mutual grooming and shared food bowls and litter boxes, as well as from queens to kittens, and kittens are the most likely to become progressively infected [1]. Merck describes saliva as the main route of shedding and notes the infectious dose for oronasal spread is relatively high [2]. FIV, by contrast, spreads mostly through bite wounds; the guidelines describe transmission between non-fighting housemates as uncommon, though not without risk [1]. The Frontiers review identifies outdoor access, male sex, aggressive behavior, and older age as associated with FIV infection [3].
How infection presents
FeLV-associated disease includes nonregenerative anemia, lymphoma and leukemia, immunosuppression, immune-mediated disease, reproductive failure, and neurologic signs, according to Merck [2]. Merck reports that lymphoma and lymphoid leukemia occur in as many as 30% of progressively infected cats [2].
The Frontiers review describes FIV as moving through a primary phase, a long asymptomatic phase that may last the cat's lifetime, and a terminal phase marked by CD4+ lymphocyte loss [3]. Stomatitis, weight loss, lymphadenopathy, and chronic rhinitis are listed among common signs [3]. The AAFP panel notes that neoplasia is about five times more common in FIV-infected cats than in uninfected cats, and that some cats with very low CD4+ counts stay healthy [1].
Testing and confirmation
FeLV
The AAFP guidelines describe point-of-care ELISA or rapid immunomigration tests for soluble p27 antigen on whole blood, serum, or plasma as the usual first step [1]. The panel advises against using tears or saliva for antigen tests, citing one study in which saliva detected only 54% of infected cats [1]. Vaccination and maternal immunity do not interfere with FeLV antigen testing [1].
For a positive or questionable result, the guidelines list immediate retesting through a referral laboratory microwell ELISA, a PCR for FeLV provirus, or a point-of-care antigen test of a different brand [1]. In one comparison using 146 FeLV-positive and 154 FeLV-negative samples, sensitivity ranged from 85.6% to 100% across four brands; the panel cautions that other studies using different populations and reference methods found different results [1].
Immunofluorescent antibody (IFA) testing detects infection only after bone marrow involvement, and the guidelines note that subjective reading and differences between laboratories can produce errors in both directions [1]. Merck's summary recommends testing newly acquired cats and repeating the test 30 days or more later, and testing cats 30 days or more after the most recent risk of exposure [2]. Cats with discordant results should be treated as possible sources of infection until clarified, per both sources [1][2].
FIV
FIV is usually diagnosed by detecting antibodies, which most cats develop within 60 days of infection [1]. The AAFP panel recommends repeating a negative test no earlier than 60 days after the last possible exposure when recent infection is a concern [1]. Cats in the terminal phase may test antibody negative because antibodies become bound in immune complexes, and the guidelines suggest PCR or Western blot follow-up in high-risk cats with typical signs and a negative screen [1].
PCR is a common confirmatory test, but some infected cats are missed because of viral sequence variation or low viral load [1]. In an Australian study of 239 FIV-unvaccinated cats, the IDEXX FIV RealPCR test had 92% sensitivity and 99% specificity [1].
Vaccination complicates antibody testing. In a US study of 104 vaccinated, uninfected research cats, specificity was 98.1% for both Witness and Anigen tests, compared with 21.2% for VetScan and 1.9% for the IDEXX SNAP Combo [1]. A longitudinal study cited by the panel concluded that Witness and Anigen can be used in vaccinated cats provided primary vaccination occurred more than 6 months earlier [1].
Classifying FeLV infection
The AAFP guidelines classify FeLV outcomes into three groups [1]:
| Outcome | Description in the sources |
|---|
| Abortive | Negative for antigen, RNA, and proviral DNA; antibodies are "the only indication of FeLV infection" [1] |
| Regressive | Immune response "contains, but does not eliminate, virus replication"; no shedding after initial antigenemia, but reactivation possible [1] |
| Progressive | Infection not contained; virus shed mainly in saliva; shorter survival [1] |
Field studies in owned cats suggest abortive infection is the most common outcome under typical conditions, according to the panel [1]. In two Australian sanctuaries that mixed infected, uninfected, healthy, and ill cats (38 and 51 cats), 21% were classified as progressive, which the authors linked to intense infection pressure, comorbidities, and stress [1].
Merck describes quantitative real-time PCR as a way to separate the two infected groups: antigen-positive, PCR-positive cats with 1 million or more proviral DNA copies/mL are more likely progressively infected, and those below that level more likely regressively infected [2]. The guidelines note that regressively infected cats can transmit FeLV through blood transfusion, which is why donors are screened by both antigen test and PCR [1].
Prevention and vaccination
The 2013 AAFP vaccination guidelines, as summarized in the 2020 retrovirus document, recommend FeLV vaccination for all kittens up to and including 1 year of age and for at-risk adults [1]. The panel describes a two-dose primary series starting as early as 8 weeks, a second dose 3 to 4 weeks later, and a booster 1 year after the series [1]. Testing before vaccination is advised, and vaccinating an infected cat has no therapeutic value [1]. The panel also notes some vaccines did not consistently prevent proviral integration [1].
The only commercially available FIV vaccine, Fel-O-Vax FIV, showed a 56% protective rate in the sole published field study, from Australia [1][3]. The AAFP guidelines state it was discontinued in Canada and the USA in 2015, while the Frontiers review gives 2017; it remains available in Australia, New Zealand, and Japan [1][3].
In shelters, the AAFP panel calls testing optional for individually housed cats and recommended before group housing, and it does not recommend routine testing in trap-neuter-return programs [1].
Care of infected cats
The AAFP guidelines recommend preventive checkups at least every 6 months, with attention to the mouth, lymph nodes, eyes, skin, and body weight [1]. They call for an annual complete blood count in FIV-infected cats and at least every 6 months in FeLV-infected cats, plus annual serum biochemistry and urinalysis for both [1]. The panel advises against raw meat and raw dairy, supports parasite control, recommends neutering, and states that vaccination should not be avoided in infected cats, with vaccine selection based on individual risk assessment [1].
On housing, the guidelines favor indoor living or secure outdoor enclosures, with case-by-case judgment for cats that adapt poorly [1]. A 22-month study cited by the panel found FIV-infected cats in a crowded sanctuary more likely to show disease, with 51% dying, than those living alone or with one other cat [1].
The panel describes zidovudine (AZT) as one of the few antivirals used for both infections, stating it can be given at 5-10 mg/kg PO q12h when illness is attributed to the retrovirus, with caution at the higher dose in FeLV-infected cats because of nonregenerative anemia [1]. Evidence for interferons is limited; one study found better 9-month survival in FeLV-infected cats given parenteral feline interferon omega than in placebo-treated cats [1]. The Frontiers review notes reports of FIV-infected cats becoming resistant to long-term zidovudine therapy and of toxic neuropathy with didanosine [3]. Merck states that "no cure exists for FeLV infection" [2].
Prognosis and client communication
In a large study cited by the AAFP panel, more than 1,000 FIV-infected cats were compared with over 8,000 matched controls, and median survival after testing was 4.9 years versus 6.0 years; for over 800 progressively FeLV-infected cats, it was 2.4 years versus 6.3 years [1]. The panel notes that a high euthanasia rate in the first year likely reflected the illness that prompted testing or infection-control euthanasia [1].
The panel recommends that owners be told about care options and about the possibility of false-positive results, with additional testing offered where feasible [1]. For adoption, it suggests FeLV-infected cats go to single-cat homes or homes with other FeLV-infected cats, while noting little FIV transmission among compatible co-housed cats in one shelter report [1].
