What is FeLV in Cats- Test, Symptoms, Test & Vaccine – BarnCoop

What is FeLV in cats? Feline Leukaemia virus (FeLV) is a super important infection of cats happening all around the globe.

FeLV is part of a collection of viruses referred to as “oncornaviruses” and these viruses are able to cause the development of cancer (tumour) in infected felines. Cats struck with FeLV can grow lymphoma (a solid tumour of lymphocytes – a form of white blood cell), other tumours, or cancer of the bone marrow (leukaemia). But, other primary effects of FeLV infection are development of anaemia and extreme immunosuppression, and more cats will lose their life as a result of these complications than from tumour(s) development.

A cat consistently infected with FeLV has a high chance of developing clinical disease related to the virus, which can lead to painful suffering and cat fatality. But, the development of effective vaccines and readily accessible tests (to identify infected cats) have together reduced the frequency (prevalence) of infection with this virus to some degree, although it remains an important disease.


What is FeLV and how is it spread?

Feline Leukaemia virus (FeLV) is part of the retrovirus family of viruses, in a group called oncornaviruses. Oncornaviruses are a group of viruses (some infecting animals, some infecting humans) that lead to the development of cancers, among other effects. FeLV was first discovered in 1964 causing disease in cats.

FeLV is an important disease and death causing agents in cats. There is significant risk of taking on a lot of dangerous illnesses such as cancer, anaemia, and immunosuppression in a cat infected with the virus. Reports have shown that 80-90% of infected cats die within 3-4 years of FeLV diagnosis.

In persistently infected cat, large quantities of virus are shed in the saliva, and at times in the faeces, milk and urine. The virus is delicate and cannot survive in the environment for any period of time. It is thought that infection is maybe spread most commonly via prolonged social contact (sharing of litter trays, food bowls, and mutual grooming), where virus may be ingested). But, the virus can also be transmitted through biting and if an entire queen is infected with FeLV, any kittens she produces will also be infected (although many die or are aborted/resorbed before birth).

Generally speaking, not up to 3% of health cats are infected with the FeLV virus, but the infection is found to be more persistent in outdoor/sick cats, and mal cats are more prone.

Now, you know what is FeLV in cats, let’s move on to what happens after infection starts.

Symptoms of Feline Leukemia Virus

Cats infected with FeLV may exhibit one or more of the following symptoms:

  • Pale gums
  • Poor coat condition
  • Progressive weakness and lethargy
  •  Fever
  •  Diarrhea
  • Breathing difficulty
  • Reproductive problems like sterility in unspayed female cats
  • Yellow color in the mouth and whites of eyes
  • Enlarged lymph nodes
  • Bladder, skin, or upper respiratory infections
  •  Weight loss and/or loss of appetite
  • Stomatitis – Oral disease that includes ulceration of gingiva

Outcome Following Infection with FeLV

Cats are generally infected via the mouth, by taking in the virus. The virus replicates locally in tissues and quickly spreads to local lymphoid tissue (part of the immune system). The virus will spread through the body in monocytes (white blood cells involved in immune response) and lymphocytes and within a couple of weeks will spread to the bone marrow. Once the virus fully matures and creates a base of productive infection in cells of the bone marrow, many cats will remain persistently infected with the virus.

An immune response will fight against the virus, however, in many cases, it is not always effective. Many outcomes are possible after infection:

  • Cats may develop a super effective immune shield that can totally kill the virus soon after exposure. These cats that have recovered completely from infection are immune, but this may be relatively uncommon.
  • Cats may create a good immune response and contain effectively the virus, causing a “regressive infection” – the virus may still be alive in certain cells, but an effective immune response prevents widespread replication of the virus. These cats rarely shed the virus and rarely develop any FeLV-related disease and rarely shed the virus.
  • Cats may not be able to control replication of the virus within the bone marrow. Cells in the bone marrow used to form new blood cells may be already infected and the virus be inside these cells, in circulating blood cells and other sites such as intestinal tract, bladder, salivary glands etc. These cats are said to have persistent infections, “persistent virameia_  (meaning the virus is constantly present in the blood), or ‘progressive infections’. These cats are highly likely to develop FeLV-related disease.
  • On some occasions, cats may grow localised or atypical infections, where a partially effective immune response hugely prevents productive infection with the virus, but active replication may occur within certain tissues (like mammary glands or bladder).

Effects of FeLV infection in Cats

What are the effects of feLV in cats? The most evident effects of progressive FeLV infections/Persistent Viraemia are:

  • Immunosuppression – Immune responses getting suppressed of normal immune responses. This accounts for around 50% of all FeLV-related disease and allows for development of secondary infections and diseases.
  • Anaemia – FeLV-related anaemia can come alive in certain ways, including viral suppression of the red blood cell precursors in the bone marrow. Anaemia accounts for about 25% of all FeLV-related disease.
  • Neoplasia – FeLV infection can destroy the DNA (genetic material) of cells infected and can cause development of tumours (most commonly lymphoma or various leukaemias). This accounts for around 15% of FeLV-related disease. Although neoplasia is only a part of the disease spectrum caused by FeLV, an FeLV-infected cat is approximately 50 times more likely to develop lymphoma than a non-infected cat.
  • Other diseases – a group of other diseases including reproductive failure and skin disease develop in some infected cats.

We hope so far – we’ve been able to answer your question – What is FeLV in cats?

This disease type can develop in an individual cat infected with FeLV will depend in part on the strain of virus that infected the cat. A minimum of 4 different strains (or sub-types) of FeLV are known and these are termed A, B, C and T. Some of these sub-types are much more likely to lead to immunosuppression, for instance, while others are more likely to lead to anaemia.

Signs of FeLV infection

The single biggest cause of clinical signs in FeLV infected cats is Immunosuppression. Generally, a variety of persistent (chronic) and/or recurrent diseases develop in these cats, with continuous deterioration in their condition over time.

These symptoms show that a progressive deterioration in the cat’s immune response and ability to deal with other infections or diseases are strong. Clinical signs are really diverse but include weight loss, lethargy, fever, poor appetite, and recurrent or persistent respiratory, intestinal and skin problems.

Neoplasia and anaemia are also seen generally with FeLV, with a variety of clinical signs arising from these conditions too.

Diagnosis of FeLV infection in Felines

Good news! There are good diagnostic tests that are readily available for FeLV. FeLV test for cats are conducted by many vets (generally based on ELISA tests or immunochromatography) by blood test. These tests detect a protein produced during replication of the FeLV virus which is often present in the blood of cats persistently infected with the virus.

These tests are fast, relatively inexpensive, and commonly very reliable. Often the kits simultaneously test for FIV, as a lot of the clinical signs of FIV infection are similar to FeLV infection.

On some occasions, False FeLV in cats positive and negative results happen, so if an unexpected result is gotten, a confirmatory test is performed next. For a confirmatory test, a blood sample is often submitted to a specialist veterinary laboratory:

  • Virus isolation – this test detects the virus itself within the blood sample via laboratory culture of the virus
  • Immunofluorescence – this tests for viral antigens (proteins) present in blood cells
  • PCR (polymerase chain reaction) – this test detects the genetic material of the virus

Re-testing is a process required after 12-16 weeks to confirm the status of a cat – if the cat has only recently been exposed, it may test negative for the virus. Conversely, if a cat has only recently been infected, it can occasionally test positive, and may then be able to rid you of the infection.

Any feline/cat that tests positive to FeLV s should be separated from other cats to avoid transmission.


FeLV infection Treatment

There is really no cure for FeLV infection, and management is hugely aimed at symptomatic and supportive therapy. This include:

  • Quick and timely diagnosis and treatment of secondary infections – more prolonged therapy may be required as the immunosuppression may mean response to therapy is slower
  • Maintaining a good preventive healthcare programme with routine veterinary visits at least two times per year and frequent worming, vaccination and flea treatments.
  • Keeping infected cats indoors to prevent spread of infection to other cats and to reduce the exposure to other infections agents
  • Maintaining good quality nutritional support, and avoiding of raw foods that may come with health risk(s).
  • In some cases, supportive therapy may include use of blood transfusions and drugs to manage anaemia
  • Chemotherapy – may be used to manage FeLV-associated lymphomas. Although the prognosis for cases associated with FeLV infection is always much more guarded, some may still respond to therapy

Sadly, there are no treatments that can cure FeLV infection, however, some drugs may be able to reduce viral replication and improve the condition of infected cats:

  • Interferon – In some countries, recombinant feline interferon omega is available– studies have recommend it may have some clinical benefit in treating FeLV-infected cats, but the effect (if at all) is likely to be small
  • AZT (azidothymidine) – is one of the anti-viral drugs used to treat HIV-infection in humans and may be helpful in some cases of FIV infection. Although it also helps to reduce FeLV replication, there is little evidence that it has any useful clinical effect in cats
  • Raltegravir – is a drug used to treat HIV infections in humans and recent experience in FeLV-cats shows this might be of some help to them. Although this drug appears well tolerated and to reduce FeLV replication, its clinical efficacy are still yet to be determined.

Control of FeLV infection

Efforts can be made to stop cat from being exposed to FeLV. These steps include:

  • Whenever possible, the FIV and FeLV status of any cat should be known.
  • Any FeLV (or FIV) positive cat should be separated from other cats and kept indoor to avoid spread of infection to other cats.
  • FeLV vaccination has a high success rating. Many vaccines are available and these commonly appear to offer a valuable level of protection against infection. Kittens tend to be more susceptible to FeLV infection, and as the environment a kitten will end up in is usually uncertain, there is good rationale in routinely vaccinating every kitten against FeLV (ideally with a booster at a year of age).

Vaccination after that should be based on the risk of exposure (e.g., a single housed indoor cat is at no appreciable risk of exposure to FeLV so vaccination may not be warranted, whereas it may be important in an indoor-outdoor cat.


For cat that is persistently infected, the prognosis is very guarded. In one study FeLV infected cats survived on average around 2.5 years after their infection was diagnosed, compared with around 6.4 years for similarly aged uninfected cats.

Difference Between FIV and FeLV

FIV and FeLV have some major difference that sets them apart.

So, what is FeLV in cats and how is it different from FIV?

Feline Leukemia (FeLV) is much more life-threatening than FIV. This is because FeLV generally leads to cancer (e.g., lymphoma), leukemia (e.g. cancer of the bone marrow or circulating red and white blood cells), and extreme bone marrow suppression (e.g. anaemia) in young cats.

Note: Cats suffering from FeLV only enjoy a few years life expectancy as a result of the disease, as compared to cats with FIV. Cats can live with FIV for a much longer period of time (even up to 10 years or more), provided their other inflammation/infections of the body can be treated.

Plus, FeLV is much more contagious to your other cats. The already positive-tested ones should be isolated with quick effect from other cats and kept indoors-only!).

Other FAQ

  • How can I prevent my cat from getting FIV?

The best and fastest means to protect your cat from contracting FIV is by screening and testing every cats in your household at your veterinarian. Before you introduce a new cat to the household, this common FeLV and FIV test is a must do. If your cats are negative, great!

The next safest thing is to keep your cats indoors to avoid fighting with a stray neighborhood cat that may carry the disease.

  • What if my cat was just diagnosed with FIV?

If you learn that your cat is infected with FIV, do not fear. Again, cats can live with this disease type for years (as compared to FeLV). That being said, veterinary visit at least twice a year is a must – the sooner your veterinarian can detect inflammatory changes in your cat (in the eyes, skin, etc.), the sooner your doc can treat it and help make your cat more comfortable.

We hope you now know what is FeLV in cats. However, when in doubt, speak to your veterinarian about this. There’s also some great information about FIV and FeLV at our blog – Marley Fund so ensure to subscribe and drop off your comments below.


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