If you have a feline family member, chances are that you have heard of Feline Leukemia (FeLV) and Feline Immunodeficiency Virus (FIV). FeLV was first discovered in 1964 in a group of cats from the same household with a type of cancer called lymphoma. After development of a reliable diagnostic test that could detect the virus in the blood stream, scientists found that cats with FeLV had an 888 times greater incidence of lymphoma than in non-infected cats. This strongly suppor
ted that FeLV caused lymphoma. With more studies, it was found that the virus also caused blood abnormalities such as severe anemia. Because there was not yet a vaccine to prevent viral infection, “test and remove” (euthanasia of the positive) was employed to prevent spread to others.
Soon after, a study of 45 multi-cat households demonstrated that direct contact between cats, rather than exposure to virus in the environment, was how FeLV was transmitted. We would later find out that the virus does not last for more than a couple of hours in the environment and is mainly transmitted via saliva and nasal secretions. We now know, from direct experience and the scientific literature, that the age of infection is of the utmost importance in transmission. FeLV decreases significantly with age, with almost 100% of newborn kittens developing persistent infection compared to 15% of kittens over four months of age. What I have learned from my experience working with thousands of cats, is that kittens testing positive for FeLV will typically die from the virus by one and a half to two years of age. These kittens often have sudden breathing difficulties due to fluid around the lungs (secondary to lymphoma) or become extremely weak due to severe anemia. FeLV-infected kittens also have a higher risk of other infections and are especially at risk for deadly Feline Infectious Peritonitis (FIP).
There is good news, though. Most adult cats with strong immune systems can fight off FeLV and clear it from their bodies. Knowing this, if we have an adult cat test positive on our IDEXX snap test (the most accurate by our assessment), we always retest in 6 weeks to see if they become negative. In our experience, if a cat tests positive again 6 weeks after the initial test, they are persistently infected, meaning that they continue to test positive in the future, and most will eventually develop FeLV-related illnesses. Other confirmatory tests are available, but due to lower sensitivity (ability to correctly identify cats with FeLV) and specificity (ability to correctly identify cats without FeLV) these expensive tests have not been as useful to us as the tried-and-true IDEXX snap tests. More good news about FeLV is that repeated cat-to-cat contact is usually necessary for a cat to become infected, and the virus becomes quickly non-infectious, or “dies,” when outside of the host. Unfortunately for kittens, this is the main reason they have such poor outcomes - in addition to weak immune systems, they have extensive exposure to the virus if their mother is infected. Kittens can get infected in utero, through the milk, or from the mother’s saliva during grooming.
So, what about FIV? Like FeLV, it is a retrovirus, but it is very much different in its transmission and outcomes for infected cats. It is much harder to transmit, as it is almost entirely passed from cat to cat via deep bite wounds. Naturally, sexually intact male cats that have been outside fighting with others have the highest risk of infection and a large percentage of older tomcats that we test are positive for FIV antibodies (the FIV test is for antibodies and the FeLV test is for the viral antigens). FIV was discovered in 1986 in a facility housing 43 previously stray cats, ranging from six months to 13 years of age. The researchers demonstrated antibodies against FIV in 10 of 25 “sick” cats that “were either very thin and rough-coated or had one or more of a number of chronic infections including gingivitis, periodontitis, pustular dermatitis, ear infections, chronic rhinitis, chronic conjunctivitis, or diarrhea” and in one of the 18 healthy cats in the group. Even though fewer than half of the sick cats had FIV, the researchers concluded that FIV caused the illnesses. They also tested for antibodies, the body’s reaction to the virus, rather than the virus itself in these cats.
From this and other articles I reviewed, the only completely convincing and repeatable evidence is that FIV causes a decrease in certain white blood cells, and as such, could diminish the immune response of the cat. The discoverers of FIV write that cats ultimately die from “progressive secondary or opportunistic infections of the oral cavity, upper respiratory tract, gastrointestinal tract, skin, or urinary tract.” As I mentioned, this makes sense in theory, but was not clearly demonstrated by the researchers. Additionally, a study following FIV positive cats over a period of four years demonstrated that despite the lowered white blood cell counts, all the cats in the study remained “outwardly healthy.” Other studies even show no difference in prevalence of illnesses historically associated with FIV between positive and negative cats. This is consistent with what we see in practice.
Because we treat thousands of cats each year at the clinic, I am fortunate to be able to make observations that allow me to question what I was taught in veterinary school. Cats with FeLV can live happy and fulfilling lives. We do see a decreased lifespan in most of these cats, especially in kittens as previously mentioned. I have seen several FeLV positive cats that lived well into their teenage years. I have seen FeLV positive cats that came into the hospital with fractured jaws, severe upper respiratory infections, and other significant issues completely recover and have good quality of life for many years. FeLV cats should be kept separate from those without the virus but can safely live in the same household. Positive cats do not need to be automatically euthanized as was one advised in the “test and remove” era.
