FeLV and FIV Infections in Cats

Of all the infectious diseases in cats, none are as feared as FeLV and FIV—justifiably so.

Between 2-4% of feline population in the U.S. is a carrier or is infected with one or both of these potentially fatal viruses. Many clinics use an in-house test that checks for both viruses at the same time, and most feline infectious wellness conversations about infectious disease covers both topics, so it’s easy to see why owners might confuse the two. While FeLV and FIV are similar, there are some important differences in both transmission and how the virus functions in the cat’s body.


Are FeLV and FIV Preventable with Vaccines?

Vaccination against FeLV is recommended for all cats due to the prevalence of the virus and the efficacy of the vaccine. This is particularly important for young cats, which are at the highest risk of infection. It is a safe virus and saves feline lives and prevents a common but deadly illness.  As a cat ages, the decision on how often to administer a booster vaccine should be discussed with your veterinarian as the recommendations vary depending on a cat’s circumstances. FeLV vaccination does not interfere with the results FeLV testing.

An FIV vaccination exists but is considered more controversial, as its efficacy is less predictable. In addition, cats that have received the FIV vaccination may test positive for FIV during routine blood tests, even when they have not been infected. Testing positive for FIV can simply mean a cat has been vaccinated and shows antibodies for FIV. Certain at-risk populations may benefit from the FIV vaccine, but it is not routinely recommended for indoor cats.


What Are FeLV and FIV?

Both feline leukemia virus (FeLV) and feline immunodeficiency virus (FIV) are retroviruses. Unlike some forms of virus that infect cells and then kill them, retroviruses actually alter the genetic material of the infected cell and turn cells into makers of the virus. Basically, imagine the virus taking over a cell and using it to make copies of itself. This process takes time, so in both cases cats may be infected for many years before becoming clinically ill.


How Do Cats Get FeLV and FIV?

Both FeLV and FIV can be transmitted through bite wounds. In the case of FIV, saliva from an infected cat is the primary mode of transmission. Mutual grooming between an FIV+ and FIV- cat will likely not transmit the FIV to the non-FIV+ cat as once thought. It is not easily spread via saliva as cats do not groom with saliva but rather use the hairs on their tongues to smooth and tidy each other’s coats to show affection and bonding.

The FeLV virus is shed through saliva, nasal secretions, urine, feces, and milk; it may be transmitted through mutual grooming, from queen (mother) to kitten, bite wounds, or rarely, through shared litter boxes and feeding dishes.

The differences in transmission mean different populations of cats are at higher risk of infection. In the case of FIV, although both males and females get infected, unsprayed or neutered outdoor males are at the highest risk of infection because they are usually the ones getting in fights and biting and having saliva contact with other cats. An FIV-positive cat that lives with other cats and interacts with them in a casual, non-aggressive manner is unlikely to infect them. Unlike FeLV, grooming is still thought to play a significant role in transmission of FIV.

With FeLV, the fact that casual cat-to-cat contact can result in infection means it is easier for cats to become infected, especially cats in the same household that spend a lot of time together and are close. While cats of any age can become infected, kittens are much more susceptible to FeLV infection. The greater the virus exposure, the greater the risk of infection. In both cases, the virus is does not persist for a significant length of time outside of the body. Neither virus is infectious to humans.


What Happens When a Cat is Infected with FeLV or FIV?

In the early stages of both diseases, cats are often asymptomatic and perfectly healthy for a long time. It is normal for the cat to become mildly ill several weeks after infection only to return to an asymptomatic state for weeks, months, or even years. While it is believed the occasional fortunate cat can fight off an FeLV infection due to a genetic mutation, there is absolutely no evidence this happens with the FIV virus. Progression of both diseases is unpredictable; cats may become progressively ill over time or experience bouts of illness interspersed with healthy periods. Some cats become very ill and decline quickly while others thrive to old age with episodic periods of illness.

FIV causes a progressive destruction of the cat’s immune system through suppression of the white blood cells, so over time cats begin to show a variety of symptoms related to that immunosuppression. Feline Immunodeficiency virus symptoms often include:

  • Inflammation of gums/Dental infection/Tooth loss/Dental abscess
  • Diarrhea
  • Skin infections (opportunistic)
  • Upper respiratory infections and pneumonia
  • Weight loss/wasting
  • Poor coat condition/lack of grooming
  • Seizures or behavior changes such as lethargy and aggression or hiding


With FeLV, during the apparently healthy period, the virus may be completely dormant or may still be present in bodily secretions and a potential source of infection for other cats. In the later stages, FeLV causes a variety of symptoms based on the body systems targeted by the virus. Diseases associated with FeLV can include:

  • Anemia
  • Intestinal disease
  • Cancers such as lymphoma and leukemia
  • Reproductive problems
  • Secondary infections due to immunosuppression
  • Poor healing
  • Chronic respiratory infections
  • Inflammation of gums/abscess/tooth loss


How Are FeLV and FIV Treated?

Both FeLV and FIV cause a wide variety of symptoms in the cat and no two cases ever follow the same course. Veterinarians routinely recommend FeLV/FIV testing in cats because it is often an underlying contributing factor to a variety of diseases that appear unrelated. As there is no cure for the virus once contracted, treatment is focused on relieving the symptoms of diseases in the individual cat and is palliative care focusing on quality of life and comfort for the cat.


Despite the dire list of potential outcomes, it’s important to remember that many of these cats experience long and happy periods of health after the initial infection and often after diagnosis. A diagnosis of either FeLV or FIV should not be considered a death sentence.

Cats that have a confirmed diagnosis of either disease should have regular veterinary appointments twice a year, since they at risk of other diseases and can get opportunistic illnesses as they have weakened immune systems.

The following guidelines are recommended to owners to reduce the risk to their cats, as well as protecting other cats:

  • Schedule yearly blood work for your cat
  • Spay or neuter your cat
  • Keep your cats indoors
  • Do not feed a raw food diet to your infected cat as bacteria in the food could cause illness


While FeLV and FIV are serious feline conditions, we know more about them than we ever have. We now know more about prevention, but also the management of infected cats. We have vaccines and we have more treatments to ensure a cat with FIV or FeLV has a good quality of life and a long as life as possible. With proper attention and care, we can even minimize risk to other cats while giving FeLV or FIV positive felines the best chance at good health and a happy life.



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