Why It's Important to Test Cats for Feline Leukemia Virus, Feline Immunodeficiency Virus, and Feline Heartworm
When thinking about basic preventive blood screenings in new cats, it's usually a no-brainer to recommend a "triple test," including the feline leukemia virus (FeLV), feline immunodeficiency virus (FIV), and feline heartworm (FeHW) tests. You might think that once those tests are done, pets are good to go moving forward, and if the cat subsequently gets in a fight or shows clinical signs, you'll check again.
However, there are other preventive reasons to test for these three health risks, and it may be time to think differently about more regular FeLV, FIV, and FeHW testing for feline patients. The two retroviruses, FeLV and FIV, are frequently lumped together. They do have some broad similarities, in that they both affect the cat's ability to have effective immune responses and can contribute to chronic diseases. In reality, they're very different diseases and should be handled as such. FeHW may not be at the forefront of your mind when it comes to regular wellness testing, but preventive testing can help keep your patients healthy, as well as the other pets they come into contact with.
Knowing how these diseases spread and what to do with cats who test positive helps with testing decision making.
Feline Leukemia Virus
FeLV is transmitted through close contact between cats over time. It can be transmitted horizontally—by grooming or fighting—or vertically from the queen to kitten—by nursing and grooming. Oronasal infection is most common, but transmission through bite wounds is also possible. Kittens are the most susceptible, but any cat can become infected. Once infected, 30-40% of cats will continue to test positive long term (progressive infection). That means the rest of the infected cats will either clear the infection (abortive infection) or will remain infected but not actively shed the virus (regressive infection). A FeLV-negative ELISA test does not differentiate between regressive or abortive infection.
Cats with regressive and progressive infections may be asymptomatic or have symptoms of nonspecific illness, such as fever, lethargy, and anorexia. More severe disease can include bone marrow dyscrasias or FeLV-associated lymphoma. Because of the absence of clinical signs in so many cats, as well as the ability of asymptomatic cats to spread the virus, regular testing of outwardly healthy cats allows for detection and action prior to developing illness or putting other cats at risk.
Feline Immunodeficiency Virus
FIV is transmitted through fight wounds. Recently infected cats may have subtle symptoms such as fever and lymphadenopathy. Cats who are chronically infected may have long and happy lives, but the possibility of spreading the virus is constant. Vertical transmission from queens is not as common as horizontal transmission from fight wounds; kittens who test postive for FIV antibodies before six months of age should be tested again after the six-month mark.
Symptoms of FIV infection, like those of FeLV, can be completely absent. Signs of infection can include periodontal disease, recurrent infections, or fever. However, the majority of cats who test positive for FIV show no illness at all, but they will still be able to spread the virus to other cats. Regular testing along with wellness blood screens can help keep FIV-infected cats safe and healthy.
Feline Heartworm Disease
Heartworm disease in cats is different from heartworm disease in dogs. Transmission occurs in an identical way, with infective larvae being deposited into the cat's bloodstream by an infected mosquito. Because cats are not a typical host for heartworms, it's more difficult for heartworms to thrive in a cat. Cats with heartworms typically have a fraction of the number of worms that an infected dog would have, but they still can have significant effects from them.
Many cats would be considered asymptomatic if using typical clinical signs for dogs, such as persistent cough and exercise intolerance. However, the signs of heartworm disease in cats can be subtle and easily attributed to something else. Affected cats may present with vomiting, asthma-like coughing, decreased appetite, or weight loss—all non-specific and not necessarily putting up red flags for heartworm disease. Knowing the heartworm status in cats with vague symptoms can also help guide treatment decisions and expectations.
Protecting Negative Cats
Cats who are FeLV-positive should be considered contagious, and either should live alone, with other FeLV-positive cats, or with cats who are properly vaccinated against FeLV. Cats who are FIV-positive, however, can easily live with other cats in a home where everyone gets along and there is no fighting. It may be prudent to keep a stable population in a household that includes an FIV-positive cat, as adding a new cat can create some drama and potential fighting.
With both viruses, cats who are positive should be limited to indoor living or outside on a leash or enclosed catio-type situation. Preventing free roaming not only prevents the possible spread of the diseases to other cats, but it also protects the positive cat from being exposed to diseases and dangers that could be more challenging for a cat with a potentially compromised immune system.
Prevention is the best option when it comes to FeHW. Unfortunately, mosquitoes definitely don't respect closed doors, and will happily come wherever they aren't wanted. For this reason, all cats are at risk, even those who live exclusively inside. In my own patient population, 40% of the cats who had a positive heartworm test were described by their owners as indoor-only.
Treating Positive Cats
There are no specific treatments for cats infected with FeLV, FIV, or FeHW. Treatments for sick cats are focused on addressing the specific associated disease processes, such as infection, anemia, or oral disease.
Cats infected with retroviruses and heartworm should have the same basic preventive care as cats who test negative. Maintaining a healthy weight, having regular blood and urine screens, and appropriate dental care are important to any cat's overall health. If a cat lives an at-risk lifestyle—like being exposed to cats with an unknown retroviral status—annual testing is recommended. Any cat who is presented for an unwell exam could also have retroviral testing as part of the unwell screening. But we should also especially consider testing cats who develop stomatitis and faucitis, cats with unknown fever or anemia, as well as cats who have been diagnosed with lymphoma or other neoplasia. Knowing the retroviral status when these diagnoses are made will help with creating expectations for the response to treatment and may help guide treatment decisions.
When Should Cats Be Tested?
Even for cats with no clinical signs of FeLV, FIV, or FeHW, regular screening is valuable. Have you ever had an owner tell you that their cat stays completely inside, but is covered with fleas? When you follow up, they might admit that their cat goes in the backyard, but it's fenced in, so they consider it to be an indoor cat. We may internally shake our heads, but this tells us that we're not always on the same page as pet owners. It's our responsibility to ensure we are making the best preventive decisions possible.
We owe it to the other cats in the household and in the neighborhood to prevent further spread of infectious disease. Having regular retroviral and heartworm testing allows us as veterinarians to make informed decisions. It also allows us to be able to consider all the possible reasons a patient may be having ill-defined symptoms or may not be responding to treatment as expected. Early detection of any disease is key to successful treatment and management, and regular testing of FeLV, FIV, and FeHW is a mainstay of any feline wellness program.