Feline Leukemia Positive Cats
With proper management by the owner and healthcare from the veterinary team, cats with this retrovirus can live longer, more comfortable lives.
Feline leukemia virus (FeLV) is an RNA gamma-retrovirus of cats found worldwide, infecting anywhere from 3% to 14% of domestic cats depending on geographic location, sex, lifestyle and general health.1 Experts speculate that the virus evolved from rats during the late Pleistocene era up to 10 million years ago in the North African desert. Ancestral rats and cats roamed freely, and the virus was likely transmitted to cats through rat ingestion or bite.2,3
FeLV is highly contagious, particularly in kittens, and is readily spread among cats in casual close contact, which can include sharing food and water as well as mutual grooming. However, aggression (i.e. cat-fight bites) can also readily transmit the virus.
There are three primary outcome stages of FeLV infection: abortive, regressive and progressive.4 Approximately two-thirds of cats exposed to FeLV will experience either the abortive or regressive stage of infection, and about one-third of cats develop progressive infection.5 Here are some additional details:
Abortive stage. An abortive infection occurs when a cat clears the infection. (Cats with this stage were formerly referred to as “regressor” cats.)
Regressive stage. While a regressive infection causes a cat to become temporarily viremic, the cat eventually clears the viremia and does not become ill from FeLV-associated diseases. However, it does have viral DNA integrated into its genome.6-8 (Cats with regressive infection were formerly referred to as “latently infected” cats.)
Progressive stage. Progressively infected cats shed virus in their saliva, ocular and nasal secretions, urine, feces and milk and are thus infectious to other cats. (These cats were formerly called “persistently viremic” cats.)
Progressively infected cats can survive months to years, with a mean survival of 3.1 years, and may die of FeLV-associated diseases.9 However, with proper management and veterinary care, an FeLV-infected indoor-only cat may live much longer with a good quality of life. Focal infection may occasionally occur; it is characterized by persistent atypical local viral replication (such as in mammary glands, the bladder or the eyes).4,10
FeLV-associated diseases include lymphoma, leukemia, anemia and infectious diseases that are potentiated by the virus’ immunosuppressive effects. Outcomes of FeLV infection depend on an individual cat’s immune status, genetic makeup and age, the presence of any other infectious diseases, and the pathogenicity and infectious dose of the FeLV virus.
Determine FeLV status of all cats in a household
If a cat tests positive on a screening test for FeLV, it should be confirmed as “true positive” with a confirmatory test; both tests are typically performed on peripheral blood. Screening tests are usually ELISA-based tests designed to detect p27 FeLV antigen, and most cats will test positive within 30 days of exposure.11 Recommended confirmatory tests are either indirect fluorescent antibody (IFA) tests that detect p27 FeLV antigen in infected leukocytes or platelets, or polymerase chain reaction (PCR)-based tests that detect FeLV provirus. IFA tests don’t usually yield positive results until secondary viremia has occurred after infection of bone marrow (about 45 to 60 days after initial infection).
The stage (abortive, regressive or progressive infection) should be determined for all FeLV-infected cats. Abortive infected cats will test FeLV-negative and IFA- or PCR-negative, but they will seroconvert and test FeLV antibody-positive; however, antibody testing isn’t usually performed in a clinical setting. Regressively infected cats usually test FeLV antigen-negative no later than 16 weeks after infection, while progressively infected cats remain FeLV antigen-positive.12 Both regressively and progressively infected cats can test PCR FeLV provirus-positive as soon as two weeks after infection,13,14 and they will remain positive thereafter.14
Here are some additional principles for FeLV testing:
Any new cats or kittens should be screened for FeLV infection before being introduced into a household.
Household cats that go outdoors or share a house with cats that go outdoors should be FeLV-tested at least yearly. Also, any cat that becomes clinically ill should be tested for FeLV immediately if it shares a household with an FeLV-infected cat.
Household cats that may have been exposed to other cats with unknown FeLV infection status should be immediately tested for FeLV and retested six weeks after exposure. In some cats it can take up to four months to figure out the stage of FeLV infection. In a multicat household, it can be difficult for the owner to confine FeLV-exposed cats, assess risk to other cats, and decide how to manage the situation. Close partnership with the veterinary team is essential in these situations.
FeLV tests detect infection, not clinical disease. A decision for euthanasia should never be based solely on whether a cat is “confirmed” FeLV-infected. While FeLV infection can be life-threatening, proper management and prompt veterinary care can help regressively and even progressively infected cats have long, healthy lives with good quality.
Which cats should be vaccinated for FeLV?
The decision to vaccinate an individual cat against FeLV is based on risk assessment for infection and lifestyle. Cats that should be vaccinated include:
Kittens, because they’re more susceptible to infection and their lifestyle is still in flux. Note that although FeLV infection susceptibility decreases as cats get older, the risk does not necessarily reach zero; it depends highly on a cat’s lifestyle and degree of viral exposure.
Cats with access to the outdoors and cats that have contact with cats with access to outdoors.
Cats that live with FeLV-infected cats.
Cats that may encounter other cats with unknown FeLV status.
Managing healthy FeLV-positive cats
If a cat is FeLV-positive but displaying no clinical signs, it should receive a physical examination at least twice a year and at each veterinary visit, with attention paid to unintentional weight loss, enlarged lymph nodes, clinical signs of upper respiratory infection (e.g. ocular or nasal discharge) and oral health. All cats should have the anterior and posterior segments of the eye thoroughly examined. Complete blood count, biochemical profile, urinalysis, urine culture and fecal examination are indicated at least once a year. Some fragile FeLV cats may need bloodwork, urinalysis and fecal examination more frequently.
Infected queens and toms should not be bred, and they should be spayed or neutered, respectively, to reduce behaviors that increase risk of disease exposure or transmission, such as escaping, fighting and roaming. Routine gastrointestinal and external parasite controls should be provided. Some FeLV-infected cats have been shown not to mount an adequate protective response to rabies vaccination;15 therefore, it’s prudent to advise owners that FeLV-infected cats should not have outdoor access, especially in rabies-endemic areas.
Regardless, FeLV-infected cats should still be vaccinated with core vaccines (rabies, feline herpesvirus, calicivirus and panleukopenia virus) and possibly vaccinated more frequently (for example, every six months) based on an individual cat’s risk assessment and lifestyle.4 There is controversy surrounding the use of inactivated, modified-live or recombinant vaccines. Some researchers and clinicians suspect an increased risk for the development of injection-site sarcomas with the use of adjuvant killed vaccines,16 and others are concerned that modified-live vaccine viruses may regain their pathogenicity in immunocompromised cats.17,18
Managing clinically ill FeLV-positive cats
Early therapeutic intervention is key to a successful treatment outcome in FeLV-infected cats that display clinical signs. First, the clinician should determine whether the illness is directly associated with FeLV infection (for example, lymphoma or anemia) or a secondary disease associated with immune dysfunction (opportunistic infection or oral inflammatory disease). Intensive diagnostic testing should occur earlier during a diagnostic workup as opposed to a “wait-and-see” approach.
Most FeLV-infected cats respond well to appropriate medications and treatment strategies, but they may require a longer or more aggressive course of treatment and need to be more closely monitored during recovery. While several antiviral drugs, immunomodulators and alternative therapies have been investigated for efficacy in FeLV treatment, most have been shown to be ineffective or only marginally beneficial.19,20 To date, no treatment has been shown to reverse or cure FeLV infection in cats.
Educating owners of FeLV-infected cats
Initial diagnosis may illicit quite a bit of anxiety in an owner. To alleviate this anxiety, it’s helpful to educate the owner about FeLV infection etiology, its clinical effects, and how proper home management and veterinary care can provide the best health and quality of life for the cat. Remember the old adage that people often fear what they don’t understand. Be sure to empower owners regarding management of the cat at home and have them view the veterinary clinic as a source for medical and management advice. In addition, alert owners to the 10 common feline signs of illness:21
Inappropriate elimination (urination, defecation or both)
Changes in social interaction
Changes in activity level
Changes in sleeping habits
Changes in food and water consumption, changes in chewing and eating habits
Unexpected weight loss or gain
Changes in sleeping habits
Changes in vocalization
Signs of stress (hiding, withdrawal, changes in appetite, decreased grooming, decreased social interaction, more awake time and so on).
If owners observe any of these signs, or if they notice other behavioral changes and aren’t sure if they’re important, they should contact the veterinary clinic for advice.
The best situation for an FeLV-infected cat is to live in an indoor-only environment and be the only cat in the household.10 A nutritionally balanced diet is also essential. Cats are obligate carnivores and evolved from a desert environment; thus they thrive on high-quality (animal-based) protein (more than 45% by dry matter), low-carbohydrate, moderately low-fat and high-moisture diets.22
Canned cat foods are ideal because they have high water content. It’s possible to transition cats that prefer dry food to a canned food diet, but this should be done cautiously. Remember that many cats would rather starve to death than eat unfamiliar foods or foods they don’t like. It’s better to have a cat eat than not eat, so if dry foods must be fed, then research dry foods with a good nutrient profile. Raw diets should be avoided in FeLV-infected cats because of the increased risk of foodborne bacterial and parasitic diseases.
Although it’s preferable for FeLV-infected cats to live in single-cat households, thereby avoiding viral transmission to cat housemates and preventing high-risk behavior such as cat fights, this isn’t always possible. If they’re to be part of a multicat household, then separation of FeLV-infected cats is ideal. If an owner is unwilling to separate the FeLV-infected cat from non-infected cats, then the non-infected cats should be adequately FeLV-vaccinated. Warn owners that vaccination does not guarantee 100% protection, especially in high-exposure environments.
No new cats should be added to the household because this would disrupt the social structure and possibly increase the risk of cat fights and bites. Since FeLV is primarily transmitted by close contact (both friendly and aggressive) and the sharing of food bowls, water bowls and litterboxes, it’s unlikely that an owner will create an environment completely void of FeLV infectious virions. However, providing separate feeding stations for infected and non-infected cats may help decrease the degree of exposure. FeLV is also labile outside of the host, remains infectious for only minutes in the environment and is readily inactivated with soap and disinfectants, so frequent cleaning of litterboxes and other potential fomites with soap and disinfectant may decrease viral load. FeLV is not zoonotic.
Recommendations in this article are based on 2008 American Association of Feline Practitioners (AAFP) Feline Retrovirus Guidelines but also include some updated material and perspectives.10 The AAFP Feline Retrovirus Guidelines are presently in the process of being updated.23
The American Association of Feline Practitioners (AAFP) improves the health and welfare of cats by supporting high standards of practice, continuing education, and scientific investigation. The AAFP has a long-standing reputation and track record in the veterinary community for facilitating high standards of practice and publishes guidelines for practice excellence which are available to veterinarians at the AAFP website. Over the years, the AAFP has encouraged veterinarians to continuously re-evaluate preconceived notions of practice strategies in an effort to advance the quality of feline medicine practiced. For more information, visit catvets.com.
Winn Feline Foundation is a nonprofit organization established in 1968 that supports studies to improve cat health. Since 1968, Winn Feline Foundation has funded almost $6 million in health research for cats at more than 30 partner institutions worldwide. This funding is made possible through the support of dedicated donors and partners. Research supported by Winn Feline Foundation helps veterinarians by providing educational resources that improve treatment of common feline health problems and prevent many diseases. Grants are awarded at least twice yearly with the help of the foundation’s expert review panel. For further information, go to winnfelinefoundation.org.
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Franchini M. Die Tollwutimpfung von mit FeLV Infizierten Katzen. Thesis, University of Zurich, 1990.
Srivastav A, Kass PH, McGill LD, et al. Comparative vaccine-specific and other injectable-specific risks of injection-site sarcomas in cats. J Am Vet Med Assoc 2012;241(5):595-602.
Buonavoglia C, Marsilio F, Tempesta M, et al. Use of a feline panleukopenia modified live virus vaccine in cats in the primary stage of feline immunodeficiency virus infection. Zentralbl Veterinarmed B 1993;40(5):343-346.
Scherk MA, Ford RB, Gaskell RM, et al. 2013 AAFP Feline Vaccination Advisory Panel Report. J Feline Med Surg 2013;15(9):785-808.
Greggs WM, Clouser CL, Patterson SE, et al. Discovery of drugs that possess activity against feline leukemia virus. J Gen Virol 2012;93(pt 4):900-905.
Hartmann K. Efficacy of antiviral chemotherapy for retrovirus-infected cats: What does the current literature tell us? J Feline Med Surg 2015;17(11):925-939.
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Little SE. Personal communication. AAFP FeLV/FIV management guidelines are in process of being updated.