Understanding Feline Leukemia Virus (FeLV): A Comprehensive Guide for Cat Owners

Reviewed by Dr. Fiona Lee

Feline leukemia virus (FeLV) is a serious disease affecting cats worldwide. In fact, it's one of the most common infectious illnesses affecting cats. While some cats may only have mild issues from the virus at first, others can develop life-threatening complications. The good news is that a vaccine and better testing have helped decrease FeLV's prevalence in recent years.¹

This comprehensive guide will help you better understand FeLV, including how it's transmitted, the symptoms, diagnosis and management.

What Is Feline Leukemia Virus (FeLV)?

Feline leukemia virus is a retrovirus.¹ Retroviruses use RNA in their genetic material, which they convert into DNA when they infect a host. This helps them better avoid the attention of the host's immune system. After infection, the viral genes remain in the DNA of infected cells, often causing long-term issues. In humans, the best-known retrovirus is HIV, which can cause AIDS (this has no relation to FeLV in cats).²

When cats are infected with FeLV, it can change a cell's genetic code, potentially leading to cancer. The most common cancer that FeLV causes is leukemia—cancer that affects the white blood cells. But FeLV can cause other serious diseases in addition to leukemia, such as:³

• Lymphosarcoma (also called lymphoma): A tumor of the lymphoid cells that can occur in multiple parts of the body
• Anemia⁴
• Reproductive issues leading to natural abortion during pregnancies⁵
• Intestinal inflammation
• Neurological issues
• Eye diseases
• Immune suppression (making the cat susceptible to many other illnesses, like urinary tract and respiratory infections)⁴

How Is FeLV Transmitted?

Feline leukemia virus is shed through saliva, feces, urine and milk when a cat is contagious. It can be transmitted through close contact such as:¹

• Bite wounds
• Mutual grooming
• Sharing litter boxes
• Sharing food or water bowls
• An infected mother transmitting to her kitten (either prenatally or via nursing)

Unlike some other viruses, FeLV only survives for a few hours once exposed to the environment outside a cat's body.¹

What increases the risk of transmission?

Cats are at greater risk of catching FeLV and developing complications if they're in the following types of scenarios:¹

• Prolonged close contact with other cats
• Bitten by another cat
• Living with infected cats or cats who haven't been tested for FeLV
• Allowed to roam unsupervised outside
• Kittens or elderly cats with compromised immune systems

Outdoor cats are particularly at risk because of how easy it is to encounter infected cats. In fact, one of the most common ways the virus is transmitted is through cat fights, when large amounts of the virus are transmitted via saliva, puncture wounds and bites.⁴

Who can catch FeLV?

While FeLV is contagious to other cats, it can't be transmitted to humans. It also can't be spread to any other species of animals, like dogs. So even if your kitty has FeLV, your other pets are safe.³

A 2022 study noted that the most recent statistics in the U.S. point to a 3% infection rate in cats overall,⁶ but the percentage can be higher since typically only sick cats are tested.⁷ In addition, the percentages are likely higher in cats living outside or in cat colonies.³

Cats Respond One of Three Ways to the Virus

Cats typically respond in one of the following three ways when exposed to the feline leukemia virus:

Some cats develop immunity and fight off the virus

Some cats successfully fight off the virus completely, developing immunity. This situation is called an abortive infection. They never test positive and the infection is fully eliminated. These cats may remain asymptomatic, never showing symptoms.¹ Others may have very mild symptoms for a few days, like fever or lethargy.⁴

Some cats have a partial immune response called a "regressive infection"

Other cats may have a partial immune response that leads to a regressive infection. In this case, the retrovirus does become part of their genome, but their immune system keeps it in check.¹

These cats are usually sick at first and can spread the virus. But after that, they can be asymptomatic for weeks or even years.⁴ Until the virus reactivates, they won't infect other cats or show symptoms.¹

Some cats don't fight off the virus, developing a progressive infection

Other cats don't fight off the virus, and they develop a progressive infection. In this case, the virus infects their bone marrow and continues to replicate. These cats are contagious and will develop symptoms from mild to severe.¹

Quite a few factors can influence whether a cat fights off the virus or not. These include:⁴

• The viral load they're exposed to
• The particular FeLV strain
• Their immune system (other infections at the same time may lower their resistance)
• Age (kittens and older cats are less able to fight off the virus)

Symptoms of FeLV Infection

The symptoms of FeLV can range from mild to severe, and some cats won't show any symptoms at all.¹

Early-stage symptoms

Early-stage symptoms may include any (or none) of these:⁴

• Fever
• Loss of appetite
• Lethargy
• Swollen lymph nodes

Advanced-stage symptoms

As the disease advances and the illness becomes more severe, your cat may display the following symptoms. If you notice these, you should see a veterinarian right away:⁸

• High fever
• Severe diarrhea or vomiting
• Extreme weakness and lethargy, with little interest in getting up
• Little to no interest in drinking
• Dehydration
• Difficulty breathing

Cats can also develop serious complications from being immunocompromised from FeLV, such as catching a secondary infection from bacteria, viruses, fungi or protozoa that healthy cats might normally fight off.¹

Diagnosing FeLV

Diagnosing FeLV is very important to help ensure the virus doesn't spread. A cat with mild symptoms might still be contagious and should be isolated from other cats.

There are three types of FeLV tests. Often, a cat needs more than one test to determine if they have progressive FeLV due to how differently their bodies react to the virus. They may also need a follow-up test three to six weeks later,¹ since any of these three tests can give false negatives if given soon after exposure.⁹ A follow-up test can also be used to determine if the cat is able to clear the virus.

ELISA test

The ELISA (enzyme-linked immunosorbent assay) test is typically the first test your veterinarian will use. It can detect FeLV particles in the blood during early or late stages of infection.¹ It's considered a quick point-of-care test that can give results while you're in the office. In some cases, you may want another test 30 days later, since it sometimes takes 30 days from infection for antigens to test positive.¹⁰

IFA test

The IFA (immunofluorescent antibody assay) test is usually given second. After an ELISA test is positive, this one will be sent to a laboratory to confirm there's a FeLV infection. The IFA test detects the virus in white blood cells and platelets.¹

A positive result means your cat has a progressive infection or an early-stage infection. If it's progressive, it will be positive again when the test is run weeks or months later. This test (and ELISA) will later be negative if your cat develops a regressive infection.¹

PCR test

A PCR (polymerase chain reaction) test can tell if the virus has become part of your cat's genome in both regressive and progressive cases, even if the virus isn't currently active.¹

Treating FeLV-Positive Cats

There's no cure for FeLV, but there are treatments that can help your kitty overcome secondary infections that might arise.¹ The disease can be managed with supportive care and regular veterinary appointments that help catch any issues early.⁴

Sadly, cats with the progressive form of the disease may only live three years after diagnosis, especially if they have a severe case.¹¹ However, many cats can enjoy a longer, happy life if they get proper care, although they may still develop issues with FeLV later on.³

Since there's no cure, care usually involves treating the secondary infections or diseases caused by FeLV. That might include:¹

• Antibiotics for infections
• Blood transfusions for anemia
• Chemotherapy for cancer⁴

You can also ask your veterinarian about advances in treatment for the virus. For example, newer immune modulators are showing some promise in treating FeLV.³

Preventing FeLV Infection

The best way to "treat" FeLV is by preventing infection in the first place.

Vaccination against FeLV

A FeLV vaccine is one of the best ways to protect your cat, even if it isn't 100% effective. It's even now recommended as a core vaccine for kittens if you're planning to let them outside.¹

Typically, the vaccine is given initially as two doses, separated by three to four weeks, as early as eight weeks of age. A single dose is given a year later. High-risk cats may need boosters annually, while lower-risk cats may only need boosters every two or three years.¹¹

Your veterinarian may recommend the FeLV vaccine for your adult cat if they're at higher risk, such as if they go outdoors or frequently come into contact with cats who do. If your cat already has FeLV, the vaccine won't help, but it also won't hurt.⁴

Other preventive measures

If your cat stays indoors all the time and is never exposed to other cats, there's very little chance your kitty will ever catch FeLV. But if staying inside isn't possible, there are other steps you can take to help keep your cat safe:¹

• Provide outdoor access only in a secured enclosure.
• Test cats for FeLV before bringing them into your home.
• Don't let your cat play with kitties whose viral status isn't known.
• Keep infected cats isolated from noninfected cats (including not sharing food, water bowls or litter boxes).
• Consider spaying and neutering to reduce aggressive biting behavior that transmits the virus more easily.¹⁰

Living With a FeLV-Positive Cat

If your kitty has FeLV, it's natural that you will feel concerned. But with loving care and close attention, many cats can still have healthy and happy lives, even if their life expectancy may be shorter. In fact, many FeLV-positive cats live very similar lives to cats without the virus. They simply need you to keep a closer eye on them and help keep their environment stress-free, healthy and comfy.¹²

Here are some strategies that can help you manage the disease and give your sweet kitty a better quality of life:⁴

• Maintain regular veterinary appointments to check for secondary infections.
• Closely monitor your cat's weight and behavior (like food intake and litter box habits) to catch any progression or illness early.¹²
• Avoid raw diets since your cat can't fight off infections as well.
• Feed wet food regularly to help with hydration.¹²
• Keep your cat indoors to avoid spreading the disease or catching something else.
• Minimize stress, such as using calming pheromone products.¹⁰
• Make sure your cat has high-quality, nutritious food.¹⁰
• Join a network of other cat owners whose kitties have FeLV, so you have emotional support and resources.¹⁰
• Ask your veterinarian about any supplements or vitamins that might help your cat's health. Some may recommend an immune-boosting supplement.¹²

Pets Best Can Help You Keep Your Cat Healthy

Understanding FeLV, including how to prevent and treat it, is vital for any loving cat owner. The best first step is to ensure your cat is vaccinated for all the core vaccines, along with noncore vaccines your veterinarian may recommend.

If your cat is diagnosed with the virus, don't despair. While some cats may have a lower life expectancy from FeLV, they can still live happy and fulfilling lives. You'll simply need to provide extra love to your precious cat and keep a closer eye on their health, visiting your veterinarian regularly. Ask your veterinarian for advice tailored to your cat's specific situation, and about any advances in treatment that may be on the horizon.

At Pets Best, our cat insurance plans can be customized for your pet's unique needs. In addition, Pets Best policyholders can speak to a veterinary expert anytime through a 24/7 Pet Helpline. This can be a big help when you have questions about your pet's health.

Sources/Citations

1. "Feline leukemia virus," (2024), Cornell University College of Veterinary Medicine, https://www.vet.cornell.edu/departments/cornell-feline-health-center/health-information/feline-health-topics/feline-leukemia-virus

2. "Retroviruses," Linda Geddes (n.d.), New Scientist, https://www.newscientist.com/definition/retrovirus/

3. "Feline leukemia virus disease complex," Krista Williams et al. (n.d.), VCA Animal Hospitals, https://vcahospitals.com/know-your-pet/feline-leukemia-virus-disease-complex

4. "Feline leukemia virus (FeLV)," (n.d.), Drake Center for Veterinary Care, https://www.thedrakecenter.com/resources/materials/cats/feline-leukemia-virus-felv

5. "Feline leukemia virus disease," Kelly A. St Denis (2/2022), Merck Veterinary Manual, https://www.merckvetmanual.com/generalized-conditions/feline-leukemia-virus/feline-leukemia-virus-disease#Clinical-Findings_v3276569

6. "Prevalence and risk factors of feline immunodeficiency virus and feline leukemia virus infection in healthy cats in Thailand," Fabienne Sprißler et al. (1/2022), Frontiers in Veterinary Science, https://www.frontiersin.org/articles/10.3389/fvets.2021.764217/full

7. "Seroprevalence of feline leukemia virus and feline immunodeficiency virus infection among cats in North America and risk factors for seropositivity," Julie K. Levy et al. (2/2006), Journal of the American Veterinary Medical Association, https://avmajournals.avma.org/view/journals/javma/228/3/javma.228.3.371.xml

8. "Signs & symptoms of feline leukemia: What cat owners need to know," (n.d.), Veterinary Medical Center of St. Lucie County, https://vetmedcenterslc.com/blog/signs-symptoms-of-feline-leukemia/

9. "2020 AAFP feline retrovirus testing and management guidelines," Susan Little et al. (1/2020), Journal of Feline Medicine and Surgery, https://journals.sagepub.com/doi/10.1177/1098612X19895940

10. "Feline leukemia virus (FeLV)," (n.d.), Alley Cat Allies, https://www.alleycat.org/resources/feline-leukemia-virus-felv/

11. "Feline leukemia virus vaccination," Lori Teller (10/2021), Today's Veterinary Practice, https://todaysveterinarypractice.com/preventive-medicine/feline-leukemia-virus-vaccination/

12. "Living with feline leukemia virus (FeLV)," Lauren Giplin (n.d.), Ferndale Cat Shelter, https://ferndalecatshelter.org/blog/living-feline-leukemia-virus-felv/