A Guide for Veterinarians and their Clients
By Glenn Olah, DVM, PhD, DABVP (feline)
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 infection5. Here are some additional details:
An abortive infection occurs when a cat clears the infection. (Cats with this stage were formerly referred to as “regressor” cats.)
While a regressive infection causes a cat to become temporarily viraemic, the cat eventually clears the viraemia 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.)
Progressively infected cats shed virus in their saliva, ocular and nasal secretions, urine, faeces 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 diseases9. 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, anaemia 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 exposure11. 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 viraemia 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-positive12. Both regressively and progressively infected cats can test PCR FeLV provirus-positive as soon as two weeks after infection and they will remain positive thereafter13,14.
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 faecal 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 vaccination15; 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 lifestyle4. 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 vaccines16, and others are concerned that modified-live vaccine viruses may regain their pathogenicity in immunocompromised cats17,18.
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 beneficial19,20. 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 aetiology, 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 illness21:
- 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
- Malodorous breath
- Changes in sleeping habits
- Changes in vocalisation
- 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 behavioural changes and aren’t sure if they’re important, they should contact the veterinary clinic for advice. The best situation for an FeLV-infectedcat 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 diets22.
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 perspectives10. The AAFP Feline Retrovirus Guidelines are presently in the process of being updated23.
KEY points for FeLV infection
Terminology has changed: Persistent viraemia = progressive infection; Transient viraemia = regressive infection; Cleared virus = Abortive infection
Infections occur worldwide, but the prevalence and importance of FeLV in developed nations has decreased due to reliable tests, programmes to segregate viraemic carriers, understanding of FeLV pathogenesis and the introduction of effective vaccines.
The most widely used in-practice tests for FeLV are antigen ELISA and immunochromatography tests. As the prevalence of FeLV infection decreases the percentage of false positive test results tends to increase. Therefore, a positive result in a healthy cat should always be confirmed, preferably using provirus PCR offered by a reliable laboratory or by repeating up to `2 weeks after the first positive result.
A positive ELISA or immunochromatography result in a cat with clinical signs consistent with FeLV infection is more reliable, as in sick cats the prevalence of FeLV is considerably higher, and the higher the prevalence the higher the probability of a test to be correctly positive. Cats in which an antigen-positive test result has been confirmed can overcome viraemia (regressive infection) after weeks, usually by twelve weeks – in rare cases even later; in one documented case more than a year after infection.
Every antigen-test-positive healthy cat should be separated and retested after 6 or more weeks; if the cat still tests antigen-positive retesting can be repeated. Cats that clear infectious virus from the plasma will be negative by virus isolation, ELISA, immunochromatography and IFA, but will remain positive by provirus PCR. These cats should be considered regressively infected.
Often the development of viraemia as well as the establishment of a progressive infection may be overcome by a functioning immune system, resulting in transient viraemia. Such regressively infected cats are generally not at risk of developing disease, even with corticosteroid treatment in the future. In a multi-cat household without control of FeLV infection, 30-40 % of the cats develop persistent viraemia (progressive infection), 30-40 % exhibit transient viraemia (regressive infection) and 20-30 % develop antibodies without ever being detectably viraemic (abortive infection). A smaller proportion (~5 %) exhibits an atypical course of infection, displaying antigenaemia but no or only low-level viraemia.
FeLV does not survive long outside the host as it is easily destroyed by disinfectants, soap, heating and drying. Transmission via fomites is unlikely. The virus will however survive if it is kept moist at room temperature, so there is a potential for iatrogenic transmission via contaminated needles, surgical instruments or blood transfusions.
Viraemic cats are the source of infection, with shedding in saliva, nasal secretions, faeces, and milk. Risk factors are young age, high population density and poor hygiene. Transmission occurs mainly through friendly contacts like grooming. In viraemic queens, pregnancy usually results in embryonic death, stillbirth or in FeLV-viraemic kittens, which commonly fade away rapidly. In regressively infected queens, usually transmission does not take place during pregnancy. However, rarely some (but not all) kittens of these queens may become viraemic after birth, with transmission occurring rom individual mammary glands.
Young kittens are especially susceptible to FeLV infection. AS cats age they become increasingly resistant, especially to progressive infection, and this is in full force by 3 years of age. The cat’s age at the time of the infection is the most important factor determining the clinical progress. Viral and host factors, like the virus subgroup and the cell-mediated immune response, influence the pathogenesis in individual cats.
Cats with regressive infection appear to have the same life expectancy as cats that have never been exposed to FeLV.
The prognosis for cats with progressive FeLV infection is poor and most will develop typical clinical signs. Whether clinical signs are present or not, every cat with progressive FeLV infection is immune-suppressed with delayed and decreased primary and secondary antibody responses.
FeLV-infected cats may develop many different types of anaemia; most of them are non-regenerative and only few are regenerative. Regenerative anaemias, associated with haemolysis may be related to secondary opportunistic infections, for example by Mycoplasma haemofelis, or to immune-mediated destruction of red blood cells. Non-regenerative anaemias may be caused by the direct bone marrow suppressive effects of the virus, chronic inflammatory mechanisms, or myeloproliferative disease. Other cytopenias may be present, e.g., thrombocytopenia and neutropenia, caused by the same mechanisms. Many complications of FeLV infection respond well to treatment, such as secondary bacterial infections, especially with Mycoplasma haemofelis, which often improves with antibiotic treatment, such as doxycycline
In 2 recent studies FeLV antigen-negative cats with lymphoma had significantly longer remission times (472 ; 170 days) ) than FeLV antigen-positive cats (25 days, 27 days) following treatment. The prognosis of lymphoma in cats with progressive FeLV infection is poor because of bone marrow suppression, which is usually exacerbated by chemotherapy and can frequently delay treatment.
Although progressively infected cats can be housed in the same ward as other hospitalised patients, they should be kept in individual cages. Simple precautions and routine cleaning procedures will prevent transmission in the hospital. Since they may be immune-suppressed, they should be kept separated from cats with other infectious diseases. They should not be placed in a “contagious ward” with cats suffering from e.g. viral respiratory disease.
Source: Advisory Bureau of Cat Diseases (http://www.abcdcatsvets.org)