FIV versus Feline Leukemia

The difference between the two most common retroviruses in cats.

by Brittany Roth, Calvin’s Paws Medical Care Coordinator and FIV/Felv Advisor

In response to our last post about FIV testing in kittens (link here), we got the question:

FIV is another name for Feline Leukemia, right?

No, FIV and FeLV are most definitely NOT the same thing. FIV stands for Feline Immunodeficiency Virus, while FeLV is the Feline Leukemia Virus.

The two viruses are commonly confused because they are both retroviruses that infect cats. A retrovirus is a type of virus that causes disease by inserting itself into the genome (DNA) of its host. The most famous retrovirus is HIV, Human Immunodeficiency Virus, but not all retroviruses are the same. There are several different types of retroviruses that have different effects on the organisms that they infect. These different types of retroviruses are related, but not the same.

To understand what this looks like, think of your own family. In your family, you might have brothers, sisters, parents, aunts, uncles, grandparents, etc. You’re all part of the same family, but none of you are exactly the same and you are more closely related to some family members than others (for example, you’re more closely related to your parents than your uncle).

FIV and FeLV are in the same family (Retroviridae), but are different types of retroviruses (FIV is a lentivirus and FeLV is a gamma-retrovirus). They can be thought of as distant cousins in the family tree of retroviruses. It is because of this that the symptoms and progression of the two diseases are quite different.

FIV is a type of retrovirus called a lentivirus.  Lentiviruses are typically slow-moving viruses with long incubation periods that affect their host over a long period of time. Most cats with FIV lead normal, healthy lives and show no symptoms for many years (if at all).  Eventually, FIV can weaken the immune system of infected cats, leaving them susceptible to other diseases. A cat usually does not die from FIV, but it may die from a secondary infection that they were unable to fight off because of the FIV. FIV is fairly difficult to transmit between cats; the most common mode of transmission is through very deep, intramuscular bite wounds like the kind seen between unneutered males.

FeLV is a type of retrovirus called a gamma-retrovirus. This type of retrovirus is a “distant cousin” to the lentivirus. Cats with FeLV can have a variety of illnesses, ranging from anemia to leukemia and other cancers (or they may show no symptoms at all for many years – it just depends on the strain of the virus and how well the cat’s immune system reacts to it). FeLV commonly shortens the lifespan of infected cats, although there is a lot of variation in how long infected cats live and many things that factor into this (like the age at which they are infected). FeLV is most commonly spread either through bite wounds or prolonged close contact with an infected cat (usually through repeated mutual grooming, but it is also possible – but less likely – to spread through shared food/water bowls and litter boxes).

Summary of Similarities and Differences between FIV and FeLV

• Is a lentivirus • Are retroviruses • Is a gamma-retrovirus
• Affects cats slowly over a long period of time • Can cause immune dysfunction in cats • Affects cats more quickly over a period of months or years depending on a variety of factors
• Typically does not cause severe illness (although it can leave a cat more susceptible to secondary infections) • Can cause a variety of illnesses that range in severity
• Hard to transmit; does NOT spread through casual contact like mutual grooming or shared food/water bowls and litter boxes • Spreads more easily between unvaccinated cats; can be spread through casual contact
• Has a vaccine that is not recommended and does not provide much protection from the virus • Has a vaccine available that is very effective (depending on the type of vaccine)
• Can live with other cats that do not have FIV • Should only live with other FeLV+ cats or cats that have been vaccinated (although as with any vaccine, there is always some amount of risk)

So which one is more serious?

FeLV (feline leukemia) is more serious than FIV. Cats with FIV typically live normal, healthy lives, while cats with FeLV are expected to have a somewhat shortened lifespan, and are at a higher risk of developing certain cancers. However, if there’s one thing that we’ve learned in our work with FIV+ and FeLV+ cats, it’s that each cat is an individual, and there is no telling how a cat will respond to either virus (although this is ESPECIALLY true for FeLV).  This is why we are committed to helping both FIV+ and FeLV+ cats; we believe that every cat deserves a chance at a wonderful life filled with love and joy, regardless of their retroviral status. Many people don’t want to adopt FIV+ or FeLV+ cats, thinking “oh, they’re sick and will die soon, it’s not worth it…” But they’re wrong. A cat is not “sick” just because it has FIV or FeLV; remember, they may not show symptoms for years! It’s not fair to judge anyone, human or animal, based on what might happen in the future, and being positive for either virus does not mean that a cat is not as equally deserving of a loving home as any other cat.

Calvin’s Paws is a 501(c)(3) rescue. We work through a network of foster homes throughout the Triangle area to save homeless cats and dogs. We are a dedicated group of volunteers with common goals: rescuing animals (both positives and non-infected felines), finding the best fitting homes for each animal, and educating the public on animal health and responsible pet ownership.



FIV versus Feline Leukemia

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