The Difference Between FeLV and FIV

Jessica Vogelsang, DVM
By Jessica Vogelsang, DVM on Oct. 23, 2016

Of all infectious diseases in cats, few are as feared as FeLV and FIV—and with good reason. 

Between 2-4% of feline population in the U.S. harbors 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 wellness conversations about infectious disease covers both topics, so it’s easy to see why owners might confuse the two. But while they are similar, there are some important differences in both transmission and how the virus works in the body.

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 little virus factories. 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. 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 litterboxes and feeding dishes.

These 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, intact outdoor males are at the highest risk of infection because they are usually the ones getting in fights. 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 not 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. 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 very fragile in the environment and 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 often show no symptoms at all. It is common 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, there is 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.

In the case of FeLV, during this apparently healthy period the virus may be completely dormant or may still be present in excretions and a potential source of infection for other cats. In the later stages, FeLV causes a variety of symptoms based on the cells 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

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. In addition to the low white blood cell count, symptoms often include:

  • Inflammation of gums
  • Diarrhea
  • Skin infections
  • Upper respiratory infections and pneumonia
  • Weight loss
  • Poor coat condition
  • Seizures or behavior changes

How Are FeLV and FIV Treated?

As you can see from the list above, both FeLV and FIV cause a wide variety of symptoms in the cat; no two cases 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, but because there is no cure for the virus, treatment is focused on relieving the symptoms of disease in the individual.

Despite this dire list of outcomes, it’s important to remember that many of these cats experience long and happy periods of health after the initial infection. A diagnosis of either FeLV or FIV should not be considered an automatic death sentence. Cats that have a confirmed diagnosis of either disease should be evaluated by a veterinarian twice a year, since they are so susceptible to a variety of diseases. In addition, the following is also recommended to owners to reduce risk to their cats, as well as to other cats:

  • Schedule yearly bloodwork
  • Spay or neuter your cat
  • Keep your cats indoors, infected or not
  • Do not feed a raw food diet to your infected cat

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. As a cat ages, the decision on how often to boost the vaccine should be discussed with your veterinarian as the recommendations vary depending on the individual 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. Certain at-risk populations may benefit from the FIV vaccine, but it is not routinely recommended for household cats.

While FeLV and FIV are dangerous and scary diseases, we know more than we ever have, not only in regards to prevention, but also the management of infected cats. With proper attention and care, we can minimize risk to other cats while giving FeLV or FIV positive felines the best chance at good health and a happy life.

See Also:


Cornell Feline Health Center


Why FIV is Not a Death Sentence for Cats

Feline Vaccination Series: Part 1, Part 2, and Part 3

Blood Disorders Related to FeLV Infection in Cats

Jessica Vogelsang, DVM


Jessica Vogelsang, DVM


Jessica Vogelsang, DVM, is a person who loves too many topics to be able to stick to one descriptor: writing, dogs, communication, cats,...

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