Feline Leukemia Virus (FeLV) in Cats: Vaccine, Signs, & More

Feline leukemia virus, or FeLV in cats, is an infectious disease that many pet parents may have heard of, whether from a vet, a cat shelter, or a cat boarding facility’s vaccine requirements.

While there is plenty to discuss regarding FeLV in cats, here are a few important points to start with:

• Feline leukemia virus is contagious, but it’s also preventable with the FeLV vaccine and other practices to prevent exposure.
• There is NO definitive cure for FeLV in cats, and the virus has long-term health implications, which is why prevention is crucial.

In this Sploot Vets guide, we take a closer look at FeLV in cats, how it’s diagnosed, when to get the FeLV vaccine for cats, and how this disease can impact the health of our feline companions.

I. What is Feline Leukemia Virus?

Feline leukemia virus is one of the most common infectious diseases among cats. It is a retrovirus that can affect the DNA, disrupting the cat’s genes and cellular functions. FeLV primarily affects the immune system, white blood cells, and bone marrow in severe infections.

FeLV has NO definitive cure and can have life-threatening complications in some cases. However, not all cats experience a progressive FeLV infection after exposure. Here are the main types of FeLV infections:

1. Progressive FeLV Infections

Progressive FeLV infections in cats are commonly observed and typically have the most impact on a cat’s health. Kittens and young cats are especially at risk of developing progressive FeLV infections.

In this type of infection, the retrovirus reaches the cat’s DNA after exposure, and the immune system is not strong enough to prevent its replication. Therefore, FeLV viral particles become present in the cat’s blood. 

A progressive FeLV infection in cats means…

• The cat will show symptoms of a FeLV infection.
• The cat will shed the virus and can infect other cats.

2. Regressive or Dormant FeLV Infections

Regressive or dormant FeLV infections in cats are also fairly common, but not easily detected. These occur when the retrovirus is incorporated into the cat’s DNA after exposure, but the immune system is strong enough to prevent viral replication. Therefore, FeLV viral particles are not present in the cat’s blood.

A regressive FeLV infection in cats means…

• The cat carries the virus but won’t show symptoms.
• They cannot infect other cats as long as the infection stays in a regressive state.

However, a regressive FeLV infection can reactivate or worsen (potentially becoming a progressive FeLV infection), especially if the cat’s immune system is strained with illness or certain medications.

3. Abortive FeLV Infections

Abortive FeLV infections are rarely detected because, in these cases, the cat’s immunity is strong enough to fight off and eliminate the virus before it can affect the cat’s DNA. While this is the best possible outcome after FeLV exposure in cats, unfortunately, it is not very common, which highlights the importance of preventive measures.

An abortive FeLV infection in cats means…

• The cat will not carry the FeLV infection nor develop clinical symptoms. 
• The cat can develop an immunity to FeLV.

II. FeLV in Cats: Symptoms & Ways of Spreading

What Are The Signs of FeLV in Cats?

Progressive FeLV infection in cats may NOT produce symptoms during its early stages, but various symptoms may develop over time, such as the following:

• Pale gums or anemia in cats – The virus interferes with bone marrow function, reducing red blood cell production and causing anemia, which is commonly seen as pale gums and overall weakness.
• Low energy & weakness – progressive FeLV in cats suppresses the immune system and can cause anemia, which reduces oxygen delivery to tissues; this makes cats feel tired and weak.
• Weight loss & poor appetite – Chronic infection, inflammation, and nausea reduce appetite and increase the body’s energy demands, leading to gradual weight loss in cats.
• Recurring infections – FeLV damages immune cells, so the body can’t fight off bacteria, viruses, and fungi as effectively; this leads to recurrent infections.
• Fever & enlarged lymph nodes - When a cat has feline leukemia virus, the immune system stays in a constant state of activation while trying to control the virus, causing fever and swollen lymph nodes.
• Gastrointestinal issues – FeLV can cause persistent vomiting and diarrhea in cats, which can be due to opportunistic GI infections or diseases, including cancer, as the immune system weakens.

Note: Cats that had an abortive FeLV infection (i.e., their immune system was able to fight it off) and a regressive FeLV infection (i.e., their immune system is still able to prevent the virus from replicating) will NOT show signs or symptoms of FeLV in cats.

How FeLV Spreads Among Cats

Feline leukemia virus (FeLV) spreads through close contact between cats, primarily via saliva and nasal secretions. Therefore, cats can be exposed to the feline leukemia virus through the following: 

• Mutual grooming with an infected cat
• Rubbing against or nose-to-nose with an infected cat 
• Shared food, water bowls, and litterboxes
• From mother to kittens (during pregnancy, birth, or nursing)

Can FeLV Live On Clothes & Surfaces?

Feline leukemia virus typically dies within minutes to hours outside of a cat’s body. This is why close contact is the most common way this virus spreads. However, if the cat comes in contact with a surface very soon after the virus is released onto a surface (i.e., via nasal secretions or saliva), FeLV infection is still possible, especially if the surface has not been cleaned.

III. FeLV Vaccines for Cats [Prevention]

Feline leukemia virus is mainly prevented with the FeLV vaccine for cats. In addition, the following practices help reduce the risk of exposure and infection:

1. Restricting outdoor access for pet cats
2. Testing new cats before adoption to determine their FeLV status. Many shelters do not routinely test apparently healthy cats before adoption, so it is always a good idea to schedule a check-up with your primary veterinarian for an FeLV/FIV test right away after adoption. 
3. Refraining from adopting FeLV-positive cats for multi-cat households, especially those with FeLV-negative cats
4. Choosing cat boarding establishments that are committed to preventing the spread of FeLV (i.e., requires the FeLV vaccine, separates cats, has proper hygiene practices).

Is The FeLV Vaccine Necessary For Cats?

Yes, for cats less than 1 years of age, the FeLV vaccine is considered a core cat vaccine, meaning it is generally recommended for all. We recommend a series for all kittens and then a booster at 1 year of age. After that, the determination of whether or not they need a booster is decided based on the following risk factors:

• Cats that have access to the outdoors
• Cats that have had a history of finding ways to escape to the outdoors
• Cats that socialize with any cat that has access to the outdoors
• Cats that frequently encounter new cats
• Cats that travel frequently or periodically
• Cats that stay in cat boarding establishments or pet sitters’ homes, especially if the cats are not kept separate from one another

Note: As for adult cats that haven’t been given the FeLV vaccine at all, your veterinarian can best determine whether or not they need it, based on the above risk factors.

Can Vaccinated Cats Still Get FeLV?

The FeLV vaccine for cats helps reduce the risk of contracting feline leukemia virus; however, there is still a chance that vaccinated cats can get FeLV. While this rarely happens, it can happen in the following cases: 

1. High viral exposure due to living with one or more FeLV-positive cats. Close contact and frequent fighting can compound this risk.
2. The FeLV vaccination series (2 shots) was NOT completed, and the cat got exposed to FeLV.
3. The FeLV vaccination was NOT updated according to the recommended FeLV vaccine schedule (which is typically every 1 to 3 years, depending on lifestyle and exposure risk), and the cat got exposed to FeLV.
4. The cat had a regressive or dormant FeLV infection before vaccination. Screening tests may sometimes miss this type of infection, so it could have been undetected the whole time and only worsened later on. Confirmatory lab testing may help prevent this scenario, depending on your vet’s recommendation.

IV. Diagnosing Feline Leukemia Virus (FeLV)

Diagnosing FeLV in cats is a combination of 1) observing clinical symptoms (e.g., weakness, lethargy, pale gums, weight loss), 2) a review of possible instances of exposure (e.g., outdoor access, contact with a new cat), and 3) veterinary diagnostics.

The Screening Test for FeLV in Cats: ELISA

The primary screening test for feline leukemia virus is a blood test known as ELISA (Enzyme-Linked Immunosorbent Assay). Vet clinic staff may also refer to this as a ‘SNAP FeLV Test’, a branded name for a commonly-used diagnostic kit that utilizes ELISA technology.

This test detects FeLV antigen circulating in the bloodstream, which indicates a progressive infection. It’s commonly used in the following situations: 

• Before cat adoption – FeLV status is crucial information for potential adopters, especially if they have healthy cats at home that may get infected.
• Before FeLV vaccination – The vaccine presents no benefits for cats that already have an active FeLV infection; therefore, screening is typically done first.

Note: The main limitation of the FeLV screening test is that it can miss regressive FeLV infections in cats. If you think your cat has a high likelihood of having been exposed to FeLV, you can ask your vet if a confirmatory test will be helpful.

Confirmatory Tests for Feline Leukemia Virus

Confirmatory testing for FeLV in cats is used when screening test results are positive or not aligned with a cat’s symptoms or history. These tests typically include the following:

• PCR (Polymerase Chain Reaction) testing – detects FeLV genetic material; especially helpful if a regressive FeLV infection is highly suspected based on a cat’s history or recent activities.
• IFA (Indirect Immunofluorescence Assay) testing – identifies FeLV infections that have reached the bone marrow. This test indicates that a FeLV infection is progressive and beyond its early stages. 

Note: Not all cats need confirmatory testing for FeLV. Vets recommend the above tests based on health risks and the necessity for shaping further care.

V. Treating or Managing FeLV in Cats

What To Do If A Cat Is FeLV-Positive?

Once a cat is confirmed to be FeLV-positive, veterinarians can recommend a personalized care plan, which can include the following: 

• An immediate veterinary check-up if any signs of illness are noted; even mild infections need to be treated proactively.
• High-quality cat food to support your cat’s overall health
• Lifestyle changes to minimize your cat’s stress (stress can further aggravate their condition)
• Regular pet wellness exams and bloodwork to help treat infections, anemia, and other possible complications early.
• Avoiding immunosuppressant medications as much as possible (these can reactivate or worsen FeLV infections)
• Minimizing the risk of infections with proper hygiene, careful food preparation, and a regular cat care routine
• Immune-supporting cat vitamins or supplements

Your veterinarian may also discuss screening and protecting other cats if you have a multi-cat household.

Can A Cat Recover From FeLV?

While this is possible, it is not very common. Most cats that have either a regressive or progressive FeLV infection will carry the virus indefinitely because the virus has already reached their DNA

However, in rare cases, cats can have enough immune defenses to fight the feline leukemia virus right after exposure; this is known as an abortive FeLV infection.

VI. Adopting a FeLV Positive Cat: Things to Know

What Does FeLV-Positive Really Mean in The Context of Cat Adoption?

Most cat shelters use the ELISA or SNAP test for FeLV to determine which cats are FeLV-positive. Therefore, most of the FeLV-positive cats for adoption have a progressive infection, as this is the type that’s usually detectable by SNAP tests. 

Cats that have a progressive FeLV infection are at risk of developing complications later on, which means they require special care. They are also contagious to FeLV-negative cats.

Can a FeLV-Positive Cat Live With Other Cats?

It is NOT advisable for a FeLV-positive cat to share a home with FeLV-negative cats, even if the FeLV-negative cats are vaccinated. This is because a constant high viral exposure can put FeLV-negative cats at risk of getting infected. 

Additionally, stress can aggravate the condition of FeLV-positive cats. Therefore, their companions, if any, must be chosen carefully for their well-being. Most FeLV-positive cats need a loving environment with minimal stressors and consistent, compassionate care.

How Long Can Cats With FeLV Live?

Most cats with a regressive or dormant FeLV infection can live long lives, especially with a personalized care plan and a stress-free environment. These can help maintain the regressive or dormant state of their FeLV disease.

As for cats with a progressive FeLV infection, while a personalized care plan and stress reduction can also help, most will still have a shorter lifespan than healthy cats. The age of the cat upon exposure to FeLV plays a part in their expected lifespan, as detailed below:

• Progressive FeLV during kittenhood – lifespan of around 2 years or possibly less, especially if the kitten gets infected at a very young age
• Progressive FeLV as an adult cat – remaining years can be around 2.5 years or possibly several years, partly depending on the cat’s overall health and supportive care.

Final Thoughts on the FeLV Vaccine & Prevention

To conclude this article, we’d like to remind pet parents about the importance of FeLV vaccines for cats. It is a core vaccine series for kittens, best started at around 8 to 12 weeks of age. Meanwhile, it can be given at any time for adult cats that are at risk of exposure. 

The FeLV vaccine for cats is most effective when it is updated regularly (or yearly), according to the recommendations of your veterinarian. An ounce of prevention can truly go a long way for your feline companion’s health because FeLV is a lifelong and potentially life-threatening disease.

Additionally, we recommend limiting your cat’s exposure to the outdoors, where they may get FeLV from ‘community’ or feral cats. We also recommend choosing a cat boarding or cat grooming establishment carefully, opting for ones that have practices and policies that help prevent the spread of FeLV.

Sploot Vets: Your All-in-One, Fear Free Cat Vet for Preventive Care & More

Whether you’d like more information about feline leukemia virus or the FeLV vaccine for cats, we’re always here to help!

Sploot Veterinary Care offers all-in-one veterinary care for cats. Along with cat vaccines and complete preventive care, we also provide urgent care and emergency vet services, conveniently under one roof.

Our experienced vets are here to help your cat stay at their healthiest and happiest. Plus, we offer a Fear Free veterinary experience that prioritizes your cat’s comfort and well-being in every visit.

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