Feline Leukemia, Feline Immunodeficiency Virus, and Heartworm Testing
- Feline leukemia virus (FeLV), feline immunodeficiency virus (FIV), and heartworm disease are untreatable diseases that can cause fatal complications in cats.
- Cats that go outside are at increased risk for exposure to FeLV, FIV, and heartworm disease, but indoor cats may also be exposed.
- Blood testing can help identify cats infected with FeLV, FIV, or heartworm disease. Repeat testing is sometimes recommended.
What Are Feline Leukemia, Feline Immunodeficiency Virus, and Heartworm Disease?
Feline leukemia virus (FeLV) is contagious among cats. Unlike many other viruses that enter specific cells in the body and destroy them, FeLV enters certain cells in a cat’s body and changes the cells’ genetic characteristics. This permits FeLV to continue reproducing within the cat each time infected cells divide. In some cats, FeLV becomes dormant (inactive), making disease transmission and outcome difficult to predict.
Like FeLV, feline immunodeficiency virus (FIV) is also contagious among cats, and a cat can be infected with FIV for many years without showing any clinical signs of illness. Although FIV is not contagious to humans, FIV has some similarities to the human immunodeficiency virus (HIV) and has been used to help researchers better understand HIV.
Heartworm disease is a serious and potentially fatal infection in cats. It is caused by parasitic worms (heartworms) living in the major blood vessels of the lungs and, occasionally, in the heart. These worms are transmitted (as microscopic larvae) through the bite of an infected mosquito. The scientific name for the heartworm parasite is Dirofilaria immitis.
How Do Cats Become Infected With FeLV, FIV, and Heartworm Disease?
FeLV is generally transmitted through contact with saliva from an infected cat. Certain social behaviors such as mutual grooming and sharing food or water bowls can spread the disease. Kittens can become infected during fetal development or during the first days of life as their mothers nurse and care for them.
Like FeLV, FIV is also transmitted through contact with saliva from an infected cat. However, most cats contract FIV through bite wounds sustained during fights with FIV-infected cats rather than through social behaviors. Because of the territorial behavior and related aggression of cats (particularly male cats), roaming outside tends to increase the risk for exposure to FIV.
Heartworms are spread by mosquitoes, not directly from cat to cat. Even though outdoor cats are at greater risk for exposure to mosquitoes, keeping a cat indoors does not guarantee freedom from infection.
What Are the Signs of FeLV, FIV, and Feline Heartworm Disease?
Not every cat that becomes infected with FeLV develops clinical signs or long-term complications associated with the virus. The immune system of some cats can eliminate the infection before the cat becomes sick. In other cats, the virus can “hide” in the bone marrow, where it is difficult to detect until it begins to cause problems later in life. Other cats become carriers of the disease or experience various illnesses before eventually dying of FeLV-associated complications.
Like cats with FeLV infection, FIV-positive cats don’t always show clinical signs of illness. Some FIV-positive cats can live a relatively normal lifespan after becoming infected. Similar to HIV in humans, FIV causes illness by attacking the patient’s immune system. Therefore, clinical signs of disease in FIV -infected cats tend to be related to illnesses other than FIV.
Clinical signs associated with FeLV or FIV infection can include fever, lethargy (tiredness), chronic respiratory infections, and chronic dental, oral, and gum infections. Some FeLV-positive cats also go on to develop bone marrow problems and certain cancers. Additional clinical signs associated with FIV infection can include chronic diarrhea, weight loss, and chronic eye and skin infections.
When cats infected with FeLV or FIV continue to spend time outside, they are at increased risk for exposure to other viruses, parasites, and infections that their bodies may be unable to handle. Additionally, they are likely to sustain wounds (through cat fights or other trauma) that may become infected or fail to heal properly due to the compromised immune function associated with FeLV or FIV infection. Most veterinarians recommend keeping FeLV- or FIV-positive cats indoors, which not only helps protect cats from injuries and other infections but also reduces the likelihood that these cats will transmit FeLV or FIV to other cats.
Some cats with heartworm disease never show any clinical signs. When present, the signs of heartworm infection in cats can be confused with signs of many other diseases, including feline asthma. Affected cats may vomit, cough, and have difficulty breathing. This condition is called heartworm-associated respiratory disease (HARD). Sometimes, the only sign of heartworm infection in cats is sudden death.
How Are These Diseases Diagnosed?
FeLV infection can be complicated to diagnose because there are several stages of disease and not every cat handles FeLV infection the same way. Blood tests detect the disease in many cats, but for other cats, the bone marrow must be examined to confirm infection. In contrast, FIV infection is usually diagnosed through blood testing alone.
Feline heartworm disease can be difficult to diagnose using blood tests, because negative test results do not necessarily rule out heartworm infection, and positive results (depending on the test and stage of infection) do not always confirm infection. Confirming a diagnosis of feline heartworm disease may involve other types of diagnostic tests besides blood work. Sometimes, evidence of heartworms can be seen on ultrasound images or radiographs (x-rays) of the heart and lungs. Unfortunately, these tests can also be inconclusive.
Many veterinarians use a rapid-result test called a SNAP test to help diagnose FeLV, FIV, and heartworm infection in cats. The SNAP test is very accurate, can be performed in your veterinarian’s office using a very small amount of blood, and takes only a few minutes to complete. If your veterinarian obtains a questionable result on the SNAP test, additional testing may be recommended. Some of these tests must be performed at an outside laboratory, from which results take longer to receive.
How Are These Diseases Treated?
No medication can eliminate FeLV, FIV, or heartworm disease in cats. Most treatments involve managing the clinical signs and associated complications. Your veterinarian will determine how to monitor your pet and manage the signs of disease.
Many cats can live reasonably normal lives with FeLV, FIV, or heartworm infection, so if your cat tests positive, do not despair! This result does not necessarily mean that your cat will soon become sick and die. However, infected cats may need frequent, long-term medications to control their illness. Infected cats should be monitored closely at home and should receive regular veterinary examinations to help detect signs of illness. Precautions should also be taken to protect FeLV- or FIV-positive cats from wounds, parasites, and other infections that can make them sick and shorten their lifespans.
When Should Cats Be Tested for FeLV, FIV, and Heartworm Disease?
Because FeLV or FIV infection can have many clinical presentations, your veterinarian may want to test your cat if he or she seems to be ill—especially if a fever is present. Similarly, your veterinarian may recommend testing your cat for heartworm disease if coughing, breathing problems, or other suspicious clinical signs are observed. Some veterinarians also recommend testing a cat for heartworm disease before starting heartworm preventive medication.
Kittens or cats being introduced into the home should be tested for FeLV and FIV, especially if they are ill. Kittens whose mothers were infected with FIV may test positive when they are very young but test negative later as the antibodies they received while nursing from their mother wear off. Some veterinarians, therefore, recommend retesting young kittens when they are older (for example, at 6 months of age) to verify whether they are still positive. With FeLV infection, some kittens may test positive at first but test negative later if their immune system has been able to eliminate the infection. Similarly, some cats may be FeLV-negative at one point and test positive later as the virus progresses through various stages in the body. Because infection with FeLV or FIV can be complex, your veterinarian may recommend retesting at some point.
How Can I Vaccinate Against and Prevent These Diseases?
Because FeLV, FIV, and heartworm disease are not treatable in cats, prevention is the best option for protecting cats from these dangerous diseases. Vaccines can prevent disease associated with FeLV and prevent infection with FIV. Kittens are generally vaccinated against FeLV around 8 to 9 weeks of age. A booster vaccination is given 3 to 4 weeks later, according to the vaccine label, followed by boosters each year as long as the risk for exposure remains. Similarly, vaccination against FIV can begin when kittens are around 8 weeks of age. Two additional boosters are given 2 to 3 weeks apart, followed by boosters each year as long as the risk for exposure remains.
Cats that go outside are at greater risk for exposure to FeLV and FIV compared with cats that stay indoors. If your cat’s exposure risk is low, your veterinarian may not recommend these vaccines, so be sure to discuss this important question with your veterinarian.
Current FeLV testing technology (including the SNAP test) can differentiate FeLV-infected cats from FeLV-vaccinated cats. However, current FIV tests cannot tell the difference between FIV antibodies obtained through vaccination and those obtained through natural exposure to the disease (such as from a bite wound). This means that once a cat is vaccinated against FIV, there is no reliable way to tell if the cat is truly FIV positive or merely FIV vaccinated. This can become a cause for concern if a roaming FIV-vaccinated cat is picked up by a shelter and tested for FIV, which is a common practice at shelters. Until this issue can be resolved, many veterinarians recommend implanting identification microchips in FIV-vaccinated cats. This can help shelters identify the cat and avoid euthanasia or another unfortunate consequence of mistaken FIV status.
Protecting your cat from exposure to FeLV and FIV involves minimizing exposure to other cats and knowing the FeLV and FIV statuses of all the cats in your home. Any new kitten or cat being introduced into the home should be examined by a veterinarian as soon as possible and separated from all other household pets for a quarantine period of at least a few weeks. During this time, the new cat should be tested for FeLV and FIV and monitored closely for signs of illness. Any problems should be reported to your veterinarian before introducing the new cat to your other pets.
There is no vaccine against feline heartworm disease, but heartworm preventive medications are highly effective at preventing infection and protecting cats from the disease. Heartworm preventive medications are administered monthly in oral or topical (“spot on”) formulations. These medications are safe, easy to give, and inexpensive compared with the cost of managing heartworm disease in a sick pet. Heartworm preventive medication should be initiated in kittens and continued for the life of the cat. Ask your veterinarian which method and schedule of heartworm prevention are best for your pet.
This article has been reviewed by a Veterinarian.