Many cat owners will have heard of Feline Immunodeficiency Virus (FIV) and Feline Leukaemia Virus (FeLV). They are two serious viral diseases of cats, and both can cause suppression of the immune system, leading to an increased risk of other severe infections and also the development of certain forms of cancer. The two viruses are often considered together, as they have many similarities; but there are also some important differences between them. So firstly let’s have a look at the two viruses and how they affect our feline friends.
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What is FIV?
Feline Immunodeficiency virus (FIV) is in many ways similar to the HIV virus that causes AIDS in people. But importantly the two viruses are species specific, so it’s impossible to contract FIV from your cat. FIV is most commonly spread through fighting (via saliva) and less commonly mating. The virus can also occasionally be passed from a female cat to her kittens via the placenta.
Initially, cats infected with FIV may have a very brief mild bout of illness which resolves quickly, they then show no further signs until months, or often years, later. Once a cat is infected, the disease is not curable. But it can be carefully managed with help from your vet. Some infected cats will develop other symptoms, such as severe gum inflammation, blood disorders, cancers and neurological problems caused by the virus. Despite this, affected cats can often, with good care, live for many years following their diagnosis.
What‘s FeLV?
Feline Leukaemia virus (FeLV), unlike FIV, is more likely to infect cats that are friends and live together in a household. It is spread through saliva, urine and faeces. Cats may be infected through prolonged close contact, such as mutual grooming, and occasionally via bite wounds. Kittens can also be infected by their mothers via her milk. Young cats and kittens are more susceptible to infection than older cats.
Like FIV, the signs of illness are not generally apparent for months or even years following infection. And the virus affects the immune system, leading to the development of severe or recurrent infection with other bacteria, fungi and viruses. It is, sadly, also a common cause of cancer in infected cats. Once FeLV takes hold there is no cure and sadly the disease is usually fatal.
What makes a test reliable?
Obviously being such potentially serious diseases, it’s really important that we have good tests available, that can reliably tell us if a cat is infected with FIV or FeLV. When thinking about how reliable a test is we need to look at a few different factors. Firstly, we need to make sure that our test correctly identifies as many animals as possible that really do have the virus we are testing for. This is called the sensitivity of a test. We also want to make sure that animals that don’t have the virus, test negative for it – called specificity. The perfect test would correctly show positive for all affected animals and negative for all unaffected animals; high sensitivity and specificity.
How is the infection detected?
There are a few ways we can look for viral infections in an animal. Firstly, we can look for the virus itself. One way is by identifying small bits of the virus (antigens) in the bloodstream or tissues of infected animals. This method is commonly used but can miss infections if there are low numbers of viruses present, reducing how sensitive the test is.
Another way of looking for the virus is with PCR testing which looks for DNA (or, in this case, RNA) from the virus. This method is extremely sensitive and specific but is expensive and time consuming to do.
Lastly, rather than looking for the virus itself, we can look for the animal’s immune response and we do this by detecting antibodies. Antibodies are small proteins that the body’s immune system makes and releases into the bloodstream after it identifies an invader. These antibodies are specific to each different invader. The main drawback of this type of testing comes when the animal has been previously vaccinated against the virus. Vaccination will also cause the release of antibodies and give a false positive result. In the UK, cats are frequently vaccinated against FeLV but not FIV. However, in other countries an FIV vaccine is also available. FIV tests often look for antibodies. So it’s important to know if a cat has received such a vaccine in the past as this may cause a false positive result.
What else can cause errors?
Another source of error in testing is sometimes seen with FeLV infections. When a cat is first infected with FeLV they will test positive for the disease for a few weeks. However, a proportion of these cats will have an effective immune response and completely clear the virus from their system, never developing the full-blown disease. This is a particularly important consideration when testing healthy cats rather than those already showing symptoms.
With FIV there is a different issue because we are generally testing for antibodies. At the very beginning of the infection, it can take up to 2 months for the body to produce sufficient antibodies to be detected on a test. Conversely, due to the virus’ effect on suppressing the immune system, at the end stages of infection the immune system can be too damaged to produce enough antibodies for the test to detect. It is also worth noting that kittens born to an FIV positive queen will receive antibodies from her milk early on; so may initially test positive without being infected. These kittens should be retested at around 5-6 months of age.
How are the tests done?
Most cats are initially tested for the viruses using something called a “snap” test; which looks for either antibodies or antigen. These tests require a blood sample and are fast, inexpensive tests which can be performed “in-house” at the veterinary clinic. These tests are generally reliable and have a very high sensitivity (they rarely miss a positive case) but a slightly lower specificity (they occasionally incorrectly identify a cat not infected as positive). If there is any doubt about the result, especially positive results in healthy cats, your vet will recommend that a further test will be sent off to a laboratory to confirm the result using a different test, sometimes a few weeks later.
When will a cat need to be tested?
There are two main circumstances when a cat might be tested for these viruses. And commonly they will be tested for both FIV and FELV; due to the similarities in signs and the fact that cats can be carrying either virus for a period of time before they show any symptoms. The first circumstance that might lead to testing is a rescue cat, or a cat with an unknown history that is being rehomed. And the second is when there is an unwell cat showing signs that could be associated with either disease. In both circumstances it is vital that the test is reliable, and all of the described factors are taken into account.
If you have any worries or questions about FIV or FeLV tests for your cat, speak to your vet who will be happy to advise you further.
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