Are you a cat owner with questions or concerns about the Feline Leukemia Virus? The Meowing Vet explains the serious health complications of this untreatable and highly contagious feline disease. As August is National Immunization Awareness Month, The Meowing Vet informs cat moms and dads about the pros and cons of the current FeLV vaccination protocol. Read more below…
Transmission
The Feline Leukemia Virus, abbreviated FeLV and often pronounced “fee-Luke” in short-speak, is a retrovirus (meaning it converts RNA into DNA when it infects the cells of your cat’s immune system). It can lead to untreatable disease complications and a shortened lifespan in cats and is of serious concern in catteries and animal shelters. Euthanasia of FeLV-positive kittens and cats is common in shelter settings to reduce spread.
Most cases are exogenous, meaning that the virus passes from cat to cat via oronasal transmission (i.e. when the virus in saliva or nasal secretions from one cat is swallowed or inhaled by another cat). Kitties must be in very close contact with one another to allow for this type of transmission. For instance, grooming one another, eating or drinking from the same communal bowls, sharing toys, using the same litter box, and bite wounds from fighting can all lead to oronasal transmission. The virus can only be shed from one cat to another when the initially infected cat is in a viremic state. Viremia occurs when the virus is present in an infected cat’s bloodstream. A cat is also considered infected with FeLV if the virus is lurking in some other part of its body, often bone marrow or lymphoid tissue. However, if the virus is not currently in the cat’s bloodstream, it cannot be transmitted directly to another cat.
Much less commonly, some cases may be acquired when a kitten inherits the infection from one of its parents. This occurs via endogenous transmission in which the virus becomes incorporated into a parent cat’s genome and then passes onto offspring. Also uncommon is acquisition by a cat that receives a blood transfusion from an infected blood donor cat.
What cats are most susceptible to FeLV infection?
Younger cats and outdoor cats are most at-risk for becoming infected with the feline leukemia virus. Just like with human children, the immune systems of kittens and younger cats are often not as strong as adults. Therefore, younger “kitt-ehs” are more susceptible to infection and for entering a viremic state, increasing the chance that these juvenile cats in turn transmit the virus to other felines. Older cats tend to have a tougher immune system, better enabling them to ward off potential FeLV infection. Keep in mind, though, that some older adult kitties can still become infected in certain scenarios. However, mature adult cats that do unfortunately become infected with FeLV rarely become viremic, thus being much less likely to infect other cats.
Outdoor cats, especially feral cats but also outdoor companion kitties, are also at increased of FeLV infection. Outdoor cats are more likely to come into contact with other felines, some of whom may have FeLV with active viremia. This increases the odds of FeLV transmission. Stray outdoor cats are also likely to have other infections and diseases which may suppress their immune system, preventing them from fighting off infection if they are exposed to other cats with FeLV. Un-neutered (intact) male tomcats with access to the outdoors are also at increased risk of FeLV infection as these testosterone-fueled kitties are more likely to fight other cats and potentially acquiring the feline leukemia virus from these meowing adversaries.
What does the virus do in my cat’s body?
The feline leukemia virus is a complicated disease agent. Once the virus is inhaled or swallowed, it travels to nearby lymphoid tissue in the back of your cat’s throat (the tissue of your cat’s body responsible for the immune system). There, it multiplies and causes 1 of 3 possible scenarios:
1) Progressive infection; 2) Abortive infection; or 3) Regressive infection
Progressive Infection
Progressive infection with FeLV is the worst-case scenario and is most likely to occur when young kitties become infected. The replicated army of feline leukemia virus agents will move from the lymphoid tissue to white blood cells in the bloodstream (mononuclear cells to be exact). You guessed it: this is the viremia stage and can occur within 2-13 days after initial infection. Remember, during the viremia stage, the infected cat can transmit FeLV to other nearby cats.
As the viral particles travel through the body’s blood vessels, they can be carried to other lymphatic organs such as lymph nodes and the spleen. If a low level of virus agents entered the bloodstream, they can linger in white blood cells in the blood for up to 3-6 weeks. If, however, a large number of virus agents entered the bloodstream they can move into the bone marrow and cause a persistent viremia lasting much longer than 3-6 weeks. Because the bone marrow makes blood cells (white blood cells, red blood cells, and platelets), the virus organism can invade other cell types, including granulocyte-type white blood cells and platelets. This leads to immune system suppression and blood clotting disorders, respectively.
Abortive Infection
During cases of abortive infection, the feline leukemia virus stays in the initial lymphoid tissue, never incorporating into your cat’s genome nor spreading to the bloodstream. Therefore, viremia does not occur, so these cats do not transmit the virus to other kitties. The infected cat’s immune system is sufficient to clear the FeLV, resulting in a natural “cure” (one of the only times that FeLV is considered curable; sadly, most cases are not).
Regressive Infection
Regressive infections begin similarly to progressive FeLV infection cases. The virus agents move into mononuclear white blood cells in the bloodstream, causing an initial viremia that can last for 3-6 weeks (potentially infecting other cats during this time frame). However, after this time the cat’s immune system clears the virus, ending viremia and preventing infiltration into the bone marrow (unlike in progressive infections). Therefore, granulocyte white blood cells and platelets in the bone marrow are unaffected.
Instead, the DNA of the feline leukemia virus intertwines itself with the DNA of your cat’s chromosomes inside the feline cell that it invaded. Therefore, when your cat’s infected cells replicate, the incorporated FeLV DNA (called a provirus) replicates along with them. This process is how rare cases of inherited FeLV infection can occur.
1 of 2 possible outcomes may then occur:
a) Your cat’s immune system may detect these infected cells and destroy both the cells containing proviral DNA as well as the entire viral infection. This occurs in over 90% of such cases, resulting in a natural “cure” within 24-30 months.
b) A latent (or dormant) infection may occur instead. Instead of quietly hiding out in the genome of your cat’s infected cells, the proviral DNA may pop back out of these cells, leading to a reactivated FeLV infection with viremia in the bloodstream that can cause virus transmission to other cats once again. Viral reactivation may be triggered by stress or a suppressed immune system (such as a co-infection with another disease, cancer, or the use of immunosuppressive drugs). Though other cats are at risk of potential FeLV spread, the original infected cat is unlikely to suffer from clinical disease by the feline leukemia virus during this latent infection period.
Clinical Signs of FeLV Infection
The feline leukemia virus can cause a wide variety of different disease syndromes in your cat. Some cats may suffer from several syndromes while other individuals may only have one or another in various degrees of severity. Such disorders include immunosuppression (poor functioning of the immune system, increasing the risk of developing other infections), blood disorders (a.k.a. hematopoietic disorders), and cancer (or neoplasia). Be suspicious of FeLV infection in your cat and speak with your personal vet if your kitty is experiencing lethargy, weight loss, decreased appetite or not drinking, difficulty breathing, pale or yellow gums and skin, bleeding, or signs of secondary infection such as a green nasal or ocular discharge, a red mouth and gums, or a distended abdomen.
Less common disease manifestations include osteochondromatosis (in which joint cartilage is transformed into bone), various neurologic disorders (or neuropathies), and “fading kitten syndrome” in which kittens less than 2 weeks old may become severely ill and die due to FeLV infection.
Let’s go into a bit more detail regarding the most common FeLV consequences…
Immunosuppression
Because FeLV affects the body’s immune system, which is vital for warding off other infectious diseases, an FeLV-positive kitty will be at increased risk for acquiring other infections. Mycoplasmosis (spread by fleas), feline immunodeficiency virus (FIV), feline infectious peritonitis (FIP), stomatitis (inflammation of the inside of the mouth), abscesses, and upper respiratory infections are all more likely to occur in cats with FeLV than in cats with normally functioning immune systems.
Check back later for future articles on the Feline Immunodeficiency Virus (FIV), Feline Infectious Peritonitis (FIP), and Mycoplasmosis.
Blood Disorders
Because the feline leukemia virus can invade bone marrow during periods of progressive infection, all blood cell types (which are produced in the bone marrow) can potentially be destroyed or thwarted from developing properly. Cats with FeLV may have anemia (low red blood cells leading to poor oxygen circulation), thrombocytopenia (low platelet count, which can cause blood clotting disorders), and leukopenia (or low white blood cell count, thus adding to the risk of secondary infections since white blood cells function in the immune system).
Does FeLV really cause feline cancer?!
Yes… in some cases. It’s all in the name: feline LEUKEMIA virus. Roughly 10% of cats with FeLV will develop some form of cancer (or neoplasia). Cancer can develop in FeLV-positive cats during regressive infections in which viral DNA is incorporated near an oncogene contained in your cat’s cellular DNA. When activated (be it by FeLV infection or natural processes), oncogenes cause the expression of abnormal and unregulated cellular activity and division, which cause tumor development and cancerous processes. When the DNA of the feline leukemia virus combines with the DNA found in the oncogene of your cat’s infected cells, the feline leukemia virus alters to become the feline sarcoma virus. When the feline sarcoma virus then spreads to other cells in your cat’s body, it causes them to become cancerous (or neoplastic) as well.
The most common form of cancer caused by FeLV is lymphoma (uncontrolled growth and division of lymphocytes, a type of white blood cell). Lymphoma is termed leukemia when these cancerous lymphocytes enter the bloodstream. Immune system impairment and death can occur despite proper oncologic treatment. (Keep in mind that although FeLV can cause lymphoma in some cats, most cases of feline lymphoma are not caused by feline leukemia virus infection but by other processes.)
Fibrosarcomas (tumors of connective tissue, often found in multiple places under the skin) can also arise secondary to FeLV infection. These tumors can quickly metastasize, meaning they can spread to multiple organs of the body, leading to multi-organ shut-down. Younger cats infected with FeLV are at increased risk of fibrosarcoma development. (These fibrosarcomas are not to be confused with vaccine-associated fibrosarcomas as can occur in some cats after vaccination against FeLV. Keep reading below to learn more about vaccine risks and vaccine-related tumors.)
Diagnosis
All kittens, newly adopted or previously stray cats, as well as cats known to be exposed to other felines with FeLV should ideally be tested for the feline leukemia virus. FeLV testing should also be performed before any previous unvaccinated cat receives its first FeLV vaccine.
Testing can be quite complicated for diagnosing FeLV. Current tests may sometimes show false positive results (the test appears positive for FeLV yet the tested cat is not actually infected) while others may display false negative results (the test reads negative yet the tested cat indeed has FeLV). For these reasons, several tests may be run at various time frames to grant your vet a better picture to reach an accurate diagnosis. I recognize how frustrating this can be for both your veterinarian as well as you — the pet owner. However, please be patient and bear with your vet if he or she recommends running a series of tests to meet a true diagnosis. Because FeLV is such a worrisome disease, knowing your cat’s status for certain is vital.
The ELISA test (a.k.a. FeLV antigen test) and the IFA test are the most commonly used diagnostic tools. The ELISA test detects a molecule called an antigen found on the surface of the virus in your cat’s bloodstream during periods of viremia. (A combination ELISA test may additionally test for feline immunodeficiency virus/FIV as well as feline heartworm disease. Such test options include the IDEXX SNAP® Triple Test for FeLV, FIV, and heartworm infection as well as the WITNESS® FeLV-FIV Test Kit by Zoetis.)
To confirm the validity of a positive ELISA test, an immunofluorescent antibody test (IFA) is ideal for full confirmation. This test is performed by a specialty laboratory. It detects the presence of antibodies formed by your cat’s immune system against the feline leukemia virus, as occurs during FeLV infection.
A cat confirmed to be FeLV-positive (with progressive infection) should have an ELISA test re-performed in 10 weeks. If the test is negative now, the kitty has entered an abortive or regressive infection state.
Treatment???
As mentioned at the beginning of this article, FeLV is not treatable, meaning there’s no magic potion that can cause a cat to spontaneously be cured. Only if a kitty enters into an abortive infection state or in some cases of regression can it potentially become cured. Although there’s no known effective treatment, several medications and lifestyle changes can help to manage the disease consequences and severity of FeLV. The bottom line is to try to support your cat’s overall health by providing optimal care to mitigate potential problems induced by FeLV infection. This starts with routine veterinary check-ups at least annually (every 6 months is better) to alert your vet to any emerging health problems complicated by FeLV infection, allowing for prompt diagnosis and therapy. Be aware that your FeLV-positive kitty will require more frequent veterinary attention for the remainder of its life. Keeping your cat up-to-date on all other vaccines is also vital in preventing other secondary infectious diseases.
Additionally, your FeLV-positive cat should be kept strictly indoors. Doing so protects your own kitty from acquiring other infections from other cats or from the environment (since FeLV compromises the integrity of your cat’s immune system function, leaving him or her more susceptible to other infectious diseases). Moreover, an ethical and responsible pet owner will keep his or her FeLV-positive cat indoors to prevent it from exposing other cats it may contact outside with the feline leukemia virus. Cats with FeLV should also be prohibited from breeding to reduce the chance that they transmit the disease to their offspring.
Your vet may also suggest supplementary medications, such as oral or injectable interferon, to aid your cat’s immune system. The effectiveness of interferon is questionable, but some vets and pet owners have reported improvement in FeLV-positive cats treated with it. AZT (a.k.a. zidovudine), an antiviral medication, may theoretically help decrease the number of virus particles in your cat’s body (and subsequently improve the immune system). However, because the efficacy of AZT has not been proven, it is not commonly used.
Prognosis
I must share these sad statistics with you, my fellow cat-lovers, in order to highlight the urgency of reducing your cat’s risk of acquiring FeLV, an untreatable and often incurable disease. Half (50%) of cats with the progressive infection form of FeLV will die within 2 years after becoming infected, and 80% will die within 3 years! (Infected indoor-only kitties in a single-cat household who receive good veterinary and at-home care may live slightly longer.) Any of the clinical syndromes described above under the “Clinical Signs” section may claim your cat’s life. Therefore, the key to maintaining your cat’s health and longevity is to prevent feline leukemia virus infection in the first place. How? Through proper vaccination and by keeping your cat indoors!
Prevention
The FeLV vaccine is not fool-proof, but it does offer protection against feline leukemia virus complications in 85-90% of cases. It doesn’t actually prevent infection, but rather it prevents viremia in exposed yet vaccinated cats and aids that kitty in clearing the virus without getting sick from it. The FeLV vaccine is not a core inoculation for all cats. (A core vaccine is one against very common and highly contagious disease agents that can cause severe disease in pets.) Rather, the feline leukemia virus vaccine is optional based on your individual kitty’s risk of being exposed to the virus. As discussed, cats with access to the outdoors are at increased risk of acquiring FeLV, so these cats should receive annual boosters of the FeLV vaccine. Indoor-only cats with no exposure to other unvaccinated kitties are unlikely to be at-risk for contacting FeLV, so these cats may not need annual vaccination against FeLV. Your personal veterinarian will be able to advise you on the risk vs. benefit of administering the FeLV vaccine to your individual kitty.
The current vaccine protocol recommends a series of two vaccines (given by your vet 3 weeks apart) to all kittens regardless of whether or not they will later be indoor-only or outdoor cats. Since kittens are at increased risk of infection in comparison to older adult cats, ensuring protection in these youngsters is a necessity. Cats that go outside, felines in a cattery or shelter setting, or cats that live in the same household as another kitty with FeLV should all receive annual boosters of the FeLV vaccine as adults. Cats that are already infected with the feline leukemia virus should not receive the FeLV vaccine, as it will be useless.
The American Association of Feline Practitioners (AAFP) has additional excellent resources on the most up-to-date information regarding FeLV and many other feline diseases. Check out their site.
Click here to learn more about the Feline Leukemia Virus vaccine and other important vaccinations and preventative medicine tips for your adult cats and kittens.
Vaccine Risks
Because the FeLV vaccine is an optional (rather than core) vaccine, the risk vs. benefit of inoculation should be carefully assessed in each individual cat prior to vaccinating. In 1-2 out of every 10,000 kitties receiving vaccinations (most commonly the feline leukemia virus vaccine or the Rabies vaccine), a malignant tumor known as a sarcoma (fibrosarcoma) may develop at the injection site. A genetic predisposition may exist in some cats to developing these tumors following inflammation caused by injections.
Though a rare complication, these tumors can be quite aggressive, requiring surgical excision and oftentimes chemotherapy or radiation. Amputation of the affected limb may also be necessary. For this reason, your vet should only inject the lower portion of the back legs (vs. the trunk of your cat) so that if a vaccine-associated sarcoma does develop, it can more easily be surgically removed, perhaps taking your cat’s limb yet saving his or her life.
However, don’t go skipping your at-risk cat’s FeLV (or Rabies) vaccines solely out of fear of these rare tumors developing. The benefit in cats likely to be exposed to the feline leukemia virus far outweigh the risk. While 1 or 2 kitties out of 10,000 vaccinated cats may unfortunately develop a sarcoma, 100-500 of those 10,000 would contract FeLV if not vaccinated, leading to a progressive infection that causes severe immune system impairment, blood disorders, perhaps cancer, and a shortened lifespan. For the sake of your cat’s health, ask your veterinarian if the FeLV vaccine is right for your feline companion and take additional steps to preventing the spread of FeLV in your kitty.
– Maranda Elswick, DVM
The Meowing Vet urges you to visit your local veterinary hospital today to discuss the Feline Leukemia Virus vaccine with your veterinarian.
