Feline Immunodeficiency Virus (FIV) in Cats

FIV gradually destroys white blood cells used for fighting infection, which makes cats more vulnerable to minor illnesses that can take a more serious turn.

By: humbert15
Cats get FIV by bites from infected cats. By: humbert15

Feline immunodeficiency virus (FIV) is closely related to the human immunodeficiency virus (HIV), but these 2 infections are species specific — in other words, your cat cannot infect you, and vice versa.

This illness gradually destroys the white cells used for fighting infection, which makes the cat more vulnerable to minor illnesses taking a more serious turn (e.g., a cold turning into pneumonia).

FIV requires close contact to spread, and the most common route of infection is a bite during a catfight.


The symptoms fall into 2 phases: short term and long term.

In the short term, immediately after receiving an infected bite, the cat may be generally off-color for a few days and show nonspecific signs such as a mild fever and loss of appetite. Often this clears up uneventfully.

In the longer term, the cat may be well for years. Some of these cats are prone to sore gums (gingivitis or stomatitis) but otherwise show no ill effects for a very long time.

Eventually, however, the cat may pick up a mild infection, such as a cold, that the body cannot fight and quickly turns to pneumonia. Thus the cat is disproportionately ill given the nature of the initial infection. In addition, his lymph nodes swell, and his coat becomes unkempt.


FIV is excreted in body fluids such as saliva, and to infect another cat it must enter that cat’s bloodstream. The most common source of infection is catfights, where an FIV-positive cat gets into a scrap and bites another cat. It is thought that in a group of 100 “healthy” cats, around 6 carry FIV. In sick cats, this goes up to around 1 in 6.

There is no evidence to show that FIV is sexually transmitted, which means an infected tom does not cause illness if he mates with a healthy queen. Infected females, however, can pass the virus onto their kittens via the placenta.

The good news is that some infected kittens are able to shake the virus off and not become sick later in life.


There are a number of different blood tests available to diagnose FIV. The majority of these look for the body’s immune response to the presence of virus, whereas more specialist tests register if the virus itself is present.

The problem with diagnosis is in interpreting test results. Most vet clinics use an in-house test that gives a yes/no answer as to whether the cat has come into contact with FIV.

If a healthy cat tests positive, however, it cannot be concluded that the cat will necessarily become sick. It might be that his immune system is doing its job and fighting off and getting rid of the virus. To be certain, a second test is necessary a few weeks later to see if the result is still positive or if the body has ejected the virus.

The gold standard test is looking for the virus in the bloodstream. These tests are accurate, but the downside is they are more expensive, and it takes several days to get the answer back from a specialist lab.


The options for killing the virus are limited. Azidothymidine (AZT) has been used to stop the virus from breeding, but the risk of side effects is high, and the drug is expensive.

More commonly it is a case of taking your FIV-positive cat promptly to the vet at the first sign of illness. Catching infections early and treating with antibiotics is likely to stop things from getting out of control.

In this video, veterinarian Dr. Scheller of the Charlottesville Albemarle SPCA explains that FIV cats can be adopted and live normal lives:

YouTube player


FIV cats are only a danger to other cats if they are likely to get into a fight.

Thus all toms should be neutered to decrease territorial aggression.

For many years the advice has been to keep FIV-positive cats indoors, but opinions are now starting to swing toward letting docile cats outside if they are not going to stray far or look for trouble.

Unfortunately, there is currently no vaccine available against FIV.


  • “Feline immunodeficiency virus.” Hopper, Sparkes & Harbour. Feline Medicine and Therapeutics. 2nd edition. 488–505.
  • “Advances in therapy for retroviral infection.” McCaw. Consultations in Feline Internal Medicine. 2nd edition. P21–25.


This pet health content was written by a veterinarian, Dr. Pippa Elliott, BVMS, MRCVS. It was last reviewed Dec. 17, 2018.