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

My Kitten has FIV

Problems with current tests for FIV

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

We hear it all the time. “I found a kitten and was going to keep it, but my vet just said she has FIV!” No, she probably doesn’t. Really.

There’s no question that all cats should be tested for FIV and FeLV, but the age at which they are tested is a source of some debate. The labs that make the tests say kittens can be tested at any age and the test will be accurate, but more and more rescues and vets are learning that this isn’t really the case. To understand why, let’s look at how the immune system works.

Antibodies are a part of the immune system that your body makes in response to infection. Once they have been produced in response to a specific pathogen (germ), the body can later recognize that pathogen and quickly attack it, preventing illness. There are several ways in which you can develop antibodies to a pathogen:

  • You catch a disease and your body produces antibodies and fights the infection. Depending on the disease, you either fight it off completely or (as in the case of FIV) remain infected for life with your immune system keeping the infection suppressed. 
  • You are vaccinated against a disease and your body produces antibodies in response to the vaccine. You are then protected from future infection.
  • Babies (or kittens) get antibodies from their mothers for a period after birth.

The most common test for FIV is an ELISA test (commonly called a SNAP test, although there are others on the market now) that is performed in house at your veterinarian’s office. This test looks for the antibodies to FIV in the cat’s blood. The problem is that this test cannot distinguish between the 3 ways of developing antibodies. So when a cat tests FIV+ on an ELISA test, it actually means one of 3 things:

  1. The cat is infected with FIV.
  2. The cat was once vaccinated against FIV.
  3. A kitten has antibodies to FIV from its mother (either because the mom was infected or vaccinated).

Are you beginning to see the problem here? Cats that come into rescues and shelters often come in as strays with no medical history. We have no way of knowing if a cat has been vaccinated for FIV; the FIV vaccine is not a “core” vaccine and is not usually recommended by the AAFP, but cats (especially ones that are indoor/outdoor) are still vaccinated.

How can you tell who actually has FIV?

We can’t. Adults that test FIV+ have the test repeated immediately to rule out test error, and then are retested again in a month. If they are still positive, then we have to consider them FIV+.

So why doesn’t my kitten have FIV again?

You have to keep in mind that FIV is very hard to transmit – it’s usually only spread through deep, intramuscular bites like the kind seen between unneutered males. Kittens younger than 6 months don’t really get in those types of fights. Stray kittens are also young enough that they probably haven’t had a previous owner that vaccinated them for FIV. So by process of elimination, the most likely reason that a kitten is testing FIV+ is because it still has antibodies to FIV from its mother. As the kitten gets older, those maternal antibodies fade and leave their system as the kitten’s own immune system develops. Once those maternal antibodies fade, the kitten will test negative for FIV. This typically happens by 6 months of age, although it is possible for it to take longer. For this reason, any kitten that tests positive for FIV should be retested at 6 months of age. Calvin’s Paws typically retests “FIV” kittens one month after the original test, because that is often long enough for the antibodies to clear, but any kitten that still tests positive is retested again at 6 months, and again at 1 year if needed. A diagnosis of FIV really cannot be made until the cat is a year old.

Well if this is true, why didn’t my vet tell me this?

FIV was only discovered around 30 years ago. Since then, numerous studies have shown that what we initially believed about FIV (that it’s very contagious and cats with FIV will live short, painful lives) is wrong. New research on FIV is coming out all the time, but some vets simply haven’t been well educated on the topic.

We have had countless kittens come into our program testing FIV+ over the years, and almost every single one of them has ended up being negative by 6 months of age. Here are just a few that we’ve had recently:

Snowden, Sheridan, and Sheldon: Found in a cemetery. Refused by other rescues because they were testing FIV+. All eventually tested negative and are in loving forever homes.

Siamese fluff

Catsby: Found as a stray and tested positive at finder’s vet. Tested negative within days of us taking him into the program.


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.


My Kitten has FIV