Feline leukaemia virus (FeLV) is a very important viral infection of cats occurring worldwide.
FeLV belongs to a group of viruses known as ‘oncornaviruses’ and these viruses have the ability to cause the development of tumours (cancer) in infected individuals. Cats infected with FeLV can develop lymphoma (a solid tumour of lymphocytes – a type of white blood cell), leukaemia (cancer of the bone marrow) and some other tumours. However, other major effects of FeLV infection are severe immunosuppression and development of anaemia, and more cats will die of these complications than from development of tumours.
A cat persistently infected with FeLV is highly likely to develop clinical disease related to the virus, which can cause suffering and death in cats. However, the development of effective vaccines and readily accessible tests (to identify infected cats) have together significantly reduced the prevalence (frequency) of infection with this virus, although it remains an important disease.
What is FeLV and how is it spread?
Feline leukaemia virus (FeLV) belongs to the retrovirus family of viruses, in a group known as oncornaviruses. Oncornaviruses are a group of viruses (some infecting humans, some infecting animals) that cause development of cancers, among other effects. FeLV only causes disease in cats, and was first discovered in 1964.
FeLV is an important cause of disease and death in cats. In a cat persistently (permanently) infected with the virus, there is significant risk of developing many severe illnesses such as anaemia, immunosuppression and cancer. It has been estimated that 80-90% of infected cats die within 3-4 years of FeLV diagnosis.
In a persistently infected cat, large quantities of virus are shed in the saliva, and potentially the faeces, urine and milk. The virus is fragile and does not survive in the environment for any length of time. It is thought that infection is perhaps spread most commonly through prolonged social contact (mutual grooming, sharing of food bowls, litter trays etc., where virus may be ingested). However, 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 are aborted/resorbed before birth).
In general, less than 1-2% of healthy pet cats are infected with FeLV, however the infection is found more commonly in sick/outdoor cats, and it is slightly more common in males.
Outcome following infection with FeLV
Cats are most commonly infected orally, by ingestion of the virus. The virus replicates locally in tissues and rapidly spreads to local lymphoid tissue (part of the immune system). The virus will spread through the body in lymphocytes and monocytes (white blood cells involved in immune responses) and within a few weeks will spread to the bone marrow. If the virus establishes a productive infection in cells of the bone marrow, most cats will remain persistently infected with the virus.
An immune response will develop against the virus, but this is not always effective. Several outcomes are possible after infection:
- Cats may mount an effective immune response that can completely eliminate the virus soon after exposure. These cats that have recovered fully from infection are immune, but this may be relatively uncommon.
- Cats may mount a good immune response and contain infection effectively, resulting in a ‘regressive infection’ – the virus may still be present in some cells, but an effective immune response stops widespread replication of the virus. These cats rarely develop any FeLV-related disease and rarely shed the virus.
- Cats may be unable to control replication of the virus within the bone marrow. Cells in the bone marrow used to form new blood cells may be infected and the virus will be present in these cells, in circulating blood cells and other sites such as the bladder, intestinal tract, salivary glands etc. These cats are described 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 rare occasions, cats may develop atypical or localised infections, where a partially effective immune response largely prevents productive infection with the virus, but active replication may take place within certain tissues (such as the bladder or mammary glands).
Effects of FeLV infection
The most common effects of progressive FeLV infections (persistent viraemia) are:
- Immunosuppression – suppression of normal immune responses. This accounts for around 50% of all FeLV-related disease and allows for development of secondary diseases and infections
- Anaemia – FeLV-related anaemia can develop in a number of ways, including viral suppression of the red blood cell precursors in the bone marrow. Anaemia accounts for around 25% of all FeLV-related disease
- Neoplasia – FeLV infection can damage the DNA (genetic material) of infected cells and can lead to 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 variety of other diseases including skin disease and reproductive failure develop in some infected cats.
The type of disease that develops in an individual cat infected with FeLV will depend in part on the strain of virus that infected the cat. At least four different strains (or sub-types) of FeLV are recognised and these are termed A, B, C and T. Some of these sub-types are much more likely to cause immunosuppression, for example, while others are more likely to cause anaemia.
Signs of FeLV infection
Immunosuppression is the single biggest cause of clinical signs in FeLV infected cats. Typically a variety of chronic (persistent) and/or recurrent diseases develop in these cats, with progressive deterioration in their condition over time. These features all suggest a progressive deterioration in the cats immune response and ability to deal with other diseases or infections. Clinical signs are extremely diverse but include fever, lethargy, poor appetite, weight loss, and persistent or recurrent respiratory, skin and intestinal problems.
Anaemia and neoplasia are also seen commonly with FeLV, with a variety of clinical signs arising from these conditions too.
Diagnosis of FeLV infection
Fortunately, good diagnostic tests are readily available for FeLV. Simple ‘in clinic’ blood tests are used by many vets (generally based on ELISA tests or immunochromatography). These tests detect a protein produced during replication of the FeLV virus which is usually present in the blood of cats persistently infected with the virus. These tests are quick, relatively inexpensive, and generally very reliable. Often the kits simultaneously test for FIV, as many of the clinical signs of FIV infection are similar to FeLV infection.
Occasional false positive and negative results occur, so if an unexpected result is obtained a confirmatory test is usually performed. 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 through laboratory culture of the virus
- Immunofluorescence – this tests for viral proteins (antigens) present in blood cells
- PCR (polymerase chain reaction) – this test detects the genetic material of the virus
Re-testing is sometimes 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 eliminate infection.
Any cat that tests positive for FeLV should be isolated from other cats to prevent transmission.
Treatment of FeLV infection
There is no cure for FeLV infection, and management is largely aimed at symptomatic and supportive therapy. This include:
- Prompt diagnosis and treatment of secondary infections – more prolonged therapy may be needed as the immunosuppression may mean response to therapy is slower
- Maintaining good quality nutritional support, and avoiding of raw foods that may carry a health risk
- Maintaining a good preventive healthcare programme with routine veterinary visits at least twice a year and regular worming, flea treatments and vaccination
- Keeping infected cats indoors to prevent spread of infection to other cats and to reduce the exposure to other infections agents
- 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
Although no treatments can cure FeLV infection, some drugs may help to reduce viral replication and improve the condition of infected cats:
- Interferon – recombinant feline interferon omega is available in some countries – studies have suggested 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 suggests this might be of some help to them. Although this drug appears well tolerated and to reduce FeLV replication, its clinical efficacy still needs to be determined.
Control of FeLV infection
Efforts should be made to prevent cats being exposed to FeLV. These include:
- Where possible, the FeLV and FIV status of any cat should be known.
- Any FeLV (or FIV) positive cat should be kept apart from other cats and kept indoors to prevent spread of infection to other cats
- FeLV vaccination has been proven to be successful. Several vaccines are available and these generally appear to provide 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 all kittens against FeLV (ideally with a booster at a year of age). Subsequent vaccinations should be based on 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 a persistently infected cat, 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 6.5 years for similarly aged uninfected cats.
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