boosters are recommended for cats at high risk of FeLV infection in order to maintain high immunity. Cats that are at low, but not zero, risk of FeLV may benefit from vaccines that are licensed to be given every two to three years, allowing the adverse effects of vaccines to be minimized while still providing a moderate level of protection against FeLV infection (Stone et al., 2020). A number of commercially available vaccines are available for the prevention of FeLV infection. Available vaccines do not induce sterilizing immunity. Vaccines do, however, inhibit viral replication, viral persistence, and FeLV-associated disease (Stone et al., 2020). Therefore, they are considered effective in the fight to prevent FeLV. None of the commercially available FeLV vaccines interfere with obtaining accurate results on a later FeLV test; vaccinated cats will not test positive for FeLV. Despite this fact, it is important to test cats before vaccination. Veterinarians must document the cat’s disease status in the unlikely event of a future vaccine failure, while also determining whether vaccination is necessary/beneficial for their patient (Litster, 2015a). Adverse events associated with FeLV vaccines include localized swelling or pain, transient lethargy or fever, and granuloma formation (Stone et al., 2020). Feline vaccine- associated sarcoma (VAS) has been associated with FeLV vaccination in the past, but it has also been associated with other injections in cats. There is no evidence that FeLV vaccine is associated with a higher risk of VAS than other vaccines. However, the administration of FeLV vaccination should be based on individual patient risk and lifestyle factors (Litster, 2015a).
is considered to have protective immunity against FeLV within ten days of their second vaccination dose (Stone et al., 2020); if there is a FeLV-positive cat in the home, they should be separated for 10 days after the kitten’s final FeLV vaccination. After the initial vaccination series during kittenhood, the decision to continue vaccination should be made based on lifestyle and risk assessment, after discussion with the owner. In adult cats, the decision of whether to initiate or continue FeLV vaccination should be based on lifestyle. Cats that may be exposed to FeLV-positive cats should be vaccinated for FeLV (Stone et al., 2020). While FeLV vaccination is obviously indicated for cats that go outdoors, there are other cats for which the decision to vaccinate may be less clear. The American Association of Feline Practitioners also recommends vaccinating cats in the following categories: cats that live with FeLV-positive cats and cats that may be exposed to cats of unknown FeLV status (Stone et al., 2020). This includes cats living in households with frequent new feline additions to the home (such as in the case of owners who foster cats, where FeLV could easily be introduced to the household) and cats that may have a tendency to escape outdoors. Cats that live indoors in a stable household group with other FeLV-negative cats do not require FELV vaccination once they reach adulthood (Stone et al., 2020). If an adult cat with no prior history of FeLV vaccination is determined to be at risk of infection, an initial series of two vaccines should be given 3 to 4 weeks apart. The FeLV vaccine should then be boosted one year later (Stone et al., 2020). For previously vaccinated adult cats, most FeLV vaccines are labeled for a one-year duration of immunity. Yearly vaccine
FELINE IMMUNODEFICIENCY VIRUS
Feline immunodeficiency virus (FIV) is another retroviral disease of cats with worldwide distribution. Like FeLV, FIV is an immunosuppressive virus. However, FIV tends to be far less immunosuppressive than FeLV, and therefore is associated with a better prognosis. While these two diseases Epidemiology of FIV A 2006 study of 18,038 cats in veterinary hospitals and animal shelters noted a FIV prevalence of 2.5% in the United States (Levy, 2006). However, marked regional variation occurs and locally higher numbers are observed. The worldwide prevalence of FIV varies from 1% to 31%, depending on the study population (Litster, 2015b). A 2024 study examined 90 different studies of FIV prevalence conducted between 1980 and 2017 and found a global prevalence of 4.7%, with lower prevalences reported in North America and Europe (Nehring et al., 2024). Unlike FeLV, FIV affects both adults and kittens with equal frequency (American Association of Feline Practitioners, 2013). Feline immunodeficiency virus is most common in intact male cats with outdoor access, likely due to their tendency to fight with other cats (American Association of Feline Practitioners, 2013b). Feline immunodeficiency virus is transmitted horizontally in the saliva, most commonly via bite wounds. It may also be transmitted via sustained close contact among household contacts (Addie, 2000), though this is not common. In one study examining cohabiting rescue cats of both FIV-positive and FIV-negative status, FIV transmission was not observed despite years of exposure (Litser, 2014). In the environment, the virus is very unstable and dies within minutes, so fomite transmission and casual contact are not significant causes for concern.
are often grouped together in conversation, it is important to be aware of how they differ. Feline immunodeficiency virus is a lentivirus. It causes progressive impairment of the immune system and eventual AIDS-like immunodeficiency in affected cats (Little, 2013). Feline immunodeficiency virus can also be transmitted vertically, from mother to kittens. Vertical transmission can occur both transplacentally and via the milk (American Association of Feline Practitioners, 2013b). It is important to note, however, that not all kittens of an infected mother will be infected with the virus (Little, 2013). Therefore, individual kitten testing is necessary. Blood transfusions have also been documented as a source of FIV transmission (Litster, 2015b). Mucosal transmission via the vaginal, rectal, or oral route can occur under laboratory conditions, but this requires such large numbers of virus particles as to be clinically insignificant in natural infections (Litster, 2015b). Feline immunodeficiency virus, like FeLV, is divided into subgroups. There are five unique subgroups, or clades, of FIV. These clades are characterized based on their envelope glycoprotein, gp120. Prevalence of the different clades varies geographically, and there are genotypic and phenotypic variations within each clade. Subtypes A and B are the predominant subtypes within the United States, and there are large differences within and between subtypes (Little, 2013). Feline immunodeficiency virus has been observed to have a high mutation rate because of characteristics of its reverse-transcriptase enzyme, and these mutations can affect antigenicity and virulence (American Association of Feline Practitioners, 2013b).
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