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Understanding Feline Viruses – FIV and FELV

Understanding Feline Viruses – FIV and FELV
by Dr Caity Venniker

Read time: 5 min

Feline Immunodeficiency Virus (FIV) and Feline Leukaemia Virus (FELV) are two of the major viruses affecting cats around the world, and all too often they tend to be surrounded with a haze of fear and misunderstanding. While they are two distinct processes, they are sometimes grouped together because of their many similarities. Both result from infection with a virus which leads to compromised immunity and increased susceptibility to secondary conditions. Both diseases can under many circumstances be managed, with the aim of achieving both quality of life for the individual, as well as protecting other cats from becoming infected. Unfortunately, however, there is no cure for either FIV or FELV.

How Do Infections Happen?

Both FELV and FIV viruses do not survive for long in the environment outside of their host. FIV is spread mainly through fighting between cats, and for this reason is more common in cats that have not been sterilised, especially adult males(1). FELV is also commonly spread through fighting, but spreads readily in more subtle ways as well, through close contact between cats (such as grooming, as well as sharing bowls and litter trays). Infection of FELV occurs most commonly in young cats(1). Both diseases are more prevalent in cats that have access to roaming outdoors.

When Should an FIV and FELV test be done?

FIV and FELV are usually both tested for at the same time, as their effects on feline health can be similar. Cats with either a fever or anaemia of unknown origin should be tested; as well as cats that have chronic or recurring illness; or battle to recover from illness as expected.

Cats infected with either one of these viruses may become suddenly sick; or suspicion may be raised more gradually, if the cat appears to be in suboptimal health. Ongoing upper respiratory issues that do not respond to treatment; severe and non-responsive inflammation of the mouth and gums; non-healing wounds; or increased susceptibility to parasites such as mange, can all be signs of lowered immunity; and in these cases a test would be warranted. Cats with the freedom to roam outdoors and who are prone to fighting are of higher risk of infection, so periodic screening tests are advisable in these cases.

A snap test can be done to test for both viruses, which uses only a small amount of blood and is a quick procedure. For both viruses, a negative result almost always means that the cat is not infected (unless the infection is in the very early stages). For a positive result, confirmatory testing is sometimes advised.

This is especially true for a positive FELV result, as this disease is quite unusual in that infection can be:

  • Progressive (the cat remains persistently actively infected with FELV and sheds the virus)

  • Regressive/abortive (the cat clears the active infection); or in very rare cases

  • Focal infection occurs, where it is limited to certain organs.

For this reason, FELV tests are often repeated 60 days or more after the initial test; or other, more complex testing measures are used.

In the case of FIV testing, false positive results are rare but not impossible. The main categories where a positive result frequently does not indicate disease are:

  • In kittens below the age of six months (if their mothers were infected, they may carry antibodies for the disease without actually being infected); or

  • Cats that have been vaccinated against FIV (this is not standard practice in the UK).

With kittens, the test can be repeated after the age of six months, or alternative tests can be done. In cats vaccinated against FIV, more complex testing measures will need to be done in order to confirm their FIV status.

How Can I Protect My Cat Against these Viruses?

There is a vaccine available for preventing infection of FELV, which if boosted and maintained correctly offers good protection against the disease. It is not however an absolute guarantee of protection, especially in high risk environments(2) such as a home with an infected cat. There is also an FIV vaccine which is only available in some countries (it is not used routinely in the UK). Its efficacy is variable and its use is controversial(1).

Outdoor cats are at increased risk of infection, particularly if they are not sterilised, as this predisposes them to fighting. Any new feline additions to the home should be screened for both viruses before being introduced to the household, and cats that go outdoors should ideally be tested every year (2).

How Do I Manage an Infected Cat within the Home?

Infected cats should be sterilised and kept indoors to avoid transmission to other cats. Regarding the home environment, a single cat household; or separation of infected and healthy cats; are the safest options in terms of controlling spread. FIV is less likely to spread within a stable multi-cat household than FELV, but it is still a risk.

If it is not possible to keep cats separately then no additional cats should join the home, as this disruption of social balance can result in fighting which increases the risk of spreading disease. Within an FELV-infected home, non-infected cats should be vaccinated against the disease. Cats should have separate feeding stations and additional litter trays, and attention must be paid to increased frequency of cleaning and disinfection.

What Can I Do to Keep an Infected Cat Healthy?

Cats infected with the progressive and persistent form of FELV can survive for months to years (the mean survival time is approximately three years)(2). Cats infected with FIV can often live relatively normal life spans(3), depending on the occurrence of secondary conditions. For both viruses, good management and veterinary care can make a huge difference to both survival time and quality of life.

Infected cats that are asymptomatic should have biannual checks by a veterinarian and their weight must be monitored closely. A highly nutritious diet is helpful for overall health and immunity. Wet food is preferable due to the higher water content(2) and raw diets should be avoided because of their increased risk of bacterial or parasitic diseases(2) to these susceptible individuals.

Infected cats are immunocompromised; and so careful observation from their owners for early signs of concurrent illness can help to catch and treat secondary conditions before they become life threatening. Signs to look out for include changes in appetite or drinking behaviour; changes in weight, activity level, sleeping habits, elimination behaviour or vocalisation(2). Hiding, withdrawal and decreased grooming are also indicators of stress and may warrant a visit to the vet.

Antiviral therapies may be of help in some cases, but strong evidence of a survival advantage is limited(1). While many secondary infections due to compromised immunity can be successfully treated; conditions relating directly to FIV of FELV such as lymphoma or bone marrow suppression syndromes indicate a worse prognosis, and at this stage euthanasia on humane grounds should be considered.

Having a cat infected with FIV or FELV is a big responsibility, and it is important to be aware that they are at greater risk of succumbing to life threatening disease. However, with extra care and special attention, armed with knowledge of how the disease process works; these cats can often be managed in a way that both extends their survival and maintains quality of life.

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  1. Barrs, V. & Beatty, J. (2013). Infectious Diseases. In A. Harvey & S. Tasker (Eds.). BSAVA manual of feline practice: A foundation manual (pp. 446-448). Gloucester, UK: British Small Animal Veterinary Association.

  2. Olah, G. (2018, May 11). Living with FELV-infected cats: A guide for veterinarians and their clients. DVM360. Retrieved from:

  3. Feline Immunodeficiency Virus. (June 2019). Cornell University College of Veterinary Medicine. Retrieved from:

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