Little S, Levy J, Hartmann K, Hofmann-Lehmann R, Hosie M, Olah G, et al. 2020 AAFP Feline Retrovirus Testing and Management Guidelines. J Feline Med Surg [Internet]. 2020 Jan 9 [cited 2020 Jan 13];22(1):5–30.
The 2020 retrovirus guidelines published by the AAFP give a summary of the current state of knowledge on feline retroviruses, and discuss the pathogenesis, diagnosis, and management of disease in retroviral-positive cats. Though no new research is presented, this article gives an excellent, in depth review of current data. The highlights are summarized below, though much more information is present in the article itself.
Feline Leukemia Virus (FeLV)
The pathogenesis of FeLV is complex. Several strains of FeLV exist, both in the wild and through recombination with endogenous retroviruses, the details of which are not discussed here. The virus is initially inoculated, usually by the oronasal route but also by bite wounds or iatrogenically. There is initial replication in local lymphoid tissues (ie tonsils, lymph nodes), followed by spread through the blood by monocytes and lymphocytes in a “primary viremia”. At this point animals are PCR positive, IFA negative, and have variable antigen positivity. The virus reaches the bone marrow, leading to replication in leukocytes and platelets, followed by a “secondary viremia” with PCR, antigen, and IFA positivity.
Diagnosis of FeLV is usually based on detection of soluble p27 antigen in blood. This antigen is produced in excess during viral replication. While present in low levels in tears and saliva, use of these for diagnosis is not recommended. Most cats are antigen positive within 30 days of exposure. Antigenemia is generally equivalent to viremia, and only viremic cats shed virus.
Three basic outcomes of FeLV infection may occur: abortive, regressive, or progressive infection.
Abortive infection (similar to previous “regressors” category), occurs in 20-30% of cats experimentally but is likely much higher in the wild, and is probably the most common outcome of infection. These cats never become IFA + and have high level of FeLV neutralizing antibodies. They are consistently negative for antigen, viral RNA, viral DNA, and viral culture.
Regressive infection (similar to previous “latent” and “transient viremia”) occurs in 30-40% of cats. They have high viral loads initially but decrease to undetectable levels due to an immune response that contains but does not eliminate infection. No viral shedding occurs after the first antigenemic phase. Proviral DNA is present in blood and integrated into the genome, and so virus may be spread by blood transfusion. These cats may have a prolonged survival and a low risk of FeLV associated disease, but reactivation may occur, especially with immunosuppression. Risk of reactivation decreases with time but can happen many years later. Regressive infection may increase the risk of LSA and bone marrow suppression without reactivation. Regressive cats are initially antigen positive and PCR positive, but lose antigen positivity with time (usually 16 weeks). Quantitative PCR shows lower viral loads than progressive cats. They are generally IFA negative.
Progressive (similar to previous “persistent viremia”) infection occurs in 30-40% of cats. Replication spreads from the bone marrow to mucosal and glandular epithelium and virus is shed in saliva, milk, nasal secretions, urine, and feces. These cats are positive by PCR, IFA, and antigen tests. Neutralizing antibodies are not produced. Progressively infected animals succumb to retroviral associated disease in several years, with survival of greater than 5 years uncommon. Despite this, diagnosis of FeLV positive status should never be a sole indicator for euthanasia.
Discordant cats (ie test results which disagree) should be considered potentially infectious. Infection may be spread through bites, but generally through grooming and resource sharing, or iatrogenically through surgical instruments and supplies. Environmental survival is very poor, and so infected cats in hospital do not need to be isolated (though may require reverse isolation). Any common disinfectant will kill retroviruses. Needles, IV lines and bags, ET tubes and anesthesia circuits, etc may spread disease. Blood donors should be tested by ELISA and PCR to ensure they are negative.
FeLV positive cats should be preventative healthcare exams every 6 months, with special attention to the oral cavity. Infected animals should not be fed raw meat or dairy products. A CBC should be performed every 6 months, and a biochem and urinalysis by cystocentesis is recommended every 12 months. All infected animals should be neutered. Perioperative antibiotic use should be dictated by the procedure and is no different than for uninfected cats unless clinically immunosuppressed.
Vaccination is protective against progressive infection and related diseases but may not prevent viral integration (ie regressive infection). Studies on vaccination are difficult due to inability to reliably infect adult cats. Cats should be tested before vaccination, not because of increased risk of reaction but to avoid unneeded vaccination. Vaccination of all kittens with a two-dose series between 8-16 weeks and a 1-year booster is highly recommended. Annual vaccination recommended in high risk cats and biannual in medium- risk. Some vaccines with three-year duration are available and so should be used every 3 years in all cats at risk if chosen.
Ideally infected cats should be housed indoors, though this may depend on quality of life factors. Should be isolated from negative cats. Any negative cats in a household with positive cats should be vaccinated for FeLV, even if isolated. Infected queens should not be used for breeding. (MRK)
Studer N, Lutz H. Pan-European Study on the Prevalence of the Feline Leukaemia Virus Infection – Reported by the European Advisory Board on Cat Diseases (ABCD Europe). Viruses. 2019 Oct 29;11(11).