Cats with feline leukemia virus (FeLV) often show no signs at all during the early stages of infection. That’s what makes this virus so dangerous: a cat can carry it for weeks, months, or even years before any symptoms appear. When signs do develop, they tend to start subtle and worsen over time, cycling between periods of illness and relative health.
Early Signs Are Easy to Miss
The acute phase of FeLV infection occurs roughly two to six weeks after exposure. During this window, symptoms are rarely detected, but a cat may run a mild fever, seem slightly “off,” or have mildly swollen lymph nodes. Most owners won’t notice anything wrong at this stage. The virus is quietly replicating and spreading through the bloodstream, and the cat’s immune system is mounting its initial response.
Some cats successfully fight off the virus and never progress further. Others enter a prolonged period where the virus persists in the body but hasn’t yet caused visible damage. This latent phase is one reason routine testing matters, especially for cats that go outdoors or have contact with cats of unknown status.
Common Signs as the Disease Progresses
When FeLV does start causing problems, the signs are often general enough that they could point to many different illnesses. The pattern to watch for is a combination of these symptoms, or symptoms that keep returning after seeming to resolve:
- Loss of appetite and progressive weight loss. A cat that gradually stops finishing meals or loses body condition over weeks is showing one of the most common early visible signs.
- Poor coat condition. The fur may look dull, rough, or unkempt, even in a cat that normally grooms well.
- Persistent or recurring fever. You may not be able to measure this at home, but a cat that feels warm to the touch, sleeps more than usual, or seems lethargic could be running a low-grade fever.
- Enlarged lymph nodes. These can sometimes be felt as firm lumps under the jaw, in front of the shoulder blades, or behind the knees.
- Pale gums. Healthy cat gums are light pink and moist. Gums that look white, very pale, or have a yellowish tint suggest anemia or poor circulation, both of which FeLV can cause.
- Recurring infections. Frequent upper respiratory infections, skin infections, urinary tract infections, or eye infections that keep coming back are a hallmark of the immune suppression FeLV causes.
Many owners first notice something is wrong because their cat has an infection that clears up with treatment but returns unusually fast, or because two or three of these vague symptoms appear together.
Mouth and Digestive Problems
Inflammation of the gums and mouth is one of the more recognizable signs of FeLV. Cats may develop severe gingivitis or stomatitis, where the gums and tissues inside the mouth become red, swollen, and painful. This can make eating difficult, leading to drooling, dropping food, or refusing to eat altogether. In chronic cases, tooth loss can follow.
Persistent diarrhea is another common sign. Some FeLV-positive cats develop a condition that resembles feline panleukopenia, with progressive appetite loss, vomiting, bloody diarrhea, and rapid weight loss. This syndrome tends to be more severe and is associated with a dangerous drop in white blood cells, leaving the cat vulnerable to secondary infections.
Anemia and Breathing Changes
FeLV attacks the bone marrow, which is where red blood cells are produced. Over time, this can lead to anemia, a condition where the blood can’t carry enough oxygen. The most visible sign is pale or white gums. You can check this yourself by gently lifting your cat’s upper lip: if the gums look washed out rather than a healthy pink, that’s worth immediate attention.
Anemic cats may also breathe faster than normal or even pant with an open mouth, which is unusual for cats at rest. Weakness, disorientation, or collapsing are signs that anemia has become severe. If you notice pale gums combined with rapid breathing or lethargy, treat it as an emergency.
Neurological and Eye Problems
FeLV can affect the nervous system, though these signs are less common than the ones above. Some cats develop seizures or noticeable behavior changes, becoming unusually aggressive, disoriented, or withdrawn. These neurological effects happen when the virus damages nerve tissue directly.
Eye problems are another possibility. FeLV can cause inflammation inside the eye (uveitis), which may make the eyes look cloudy or cause the cat to squint. One distinctive sign is uneven pupil sizes, where one pupil is noticeably larger than the other. This happens because the virus can damage the nerves that control pupil constriction. Kittens infected before birth may develop abnormal retinal growth, which can affect vision permanently.
Cancer: Lymphoma and Leukemia
The “leukemia” in feline leukemia virus is somewhat misleading. The virus doesn’t always cause leukemia (cancer of the blood cells), but it significantly raises the risk of several cancers, particularly lymphoma. Lymphoma is a cancer of the immune system and is the most common FeLV-associated cancer in cats.
Historically, FeLV-linked lymphoma most often appeared as a mass in the chest cavity or as widespread disease across the lymphatic system. The signs depend on where the cancer develops. Chest lymphoma can cause difficulty breathing, while lymphoma in the digestive tract may cause vomiting, diarrhea, and weight loss. Skin lymphoma may appear as visible lumps or masses. Across all types, the early signs tend to be nonspecific: weight loss, poor appetite, and low energy.
Signs in Kittens
Kittens are especially vulnerable to FeLV. Newborns infected by their mother may develop what’s called “fading kitten syndrome,” where they fail to nurse, become cold to the touch, grow dehydrated, and decline rapidly in the first days or weeks of life. Pregnant cats with FeLV may also abort their litters or have stillbirths.
Older kittens infected through contact with other cats tend to progress to illness faster than adult cats, because their immune systems are less equipped to contain the virus.
How FeLV Is Diagnosed
Because the signs of FeLV overlap with so many other conditions, a blood test is the only way to confirm infection. The most common initial test is a rapid screening that detects a specific viral protein in the blood. This can be done at most veterinary clinics during a regular visit. A positive result on this screening test means viral protein is circulating, but it doesn’t always mean the infection is permanent.
A second, different type of blood test checks whether the virus has reached the bone marrow, which indicates a deeper, more established infection. This test is typically run at an outside lab. Results from the two tests together help determine whether a cat has a progressive infection that will likely cause ongoing health problems, or a regressive infection where the immune system has partially controlled the virus. Cats with regressive infections may test positive on the first test but negative on the second, and some eventually clear the detectable virus from their blood entirely.
What to Expect After Diagnosis
A progressive FeLV infection does shorten a cat’s life, but the timeline varies enormously. Some cats live comfortably for years after diagnosis, especially with attentive care. Others decline within months. The pattern tends to involve recurring health problems, with each episode potentially more serious than the last as the immune system weakens.
The cycling nature of the disease is one of its most confusing features for owners. A cat may seem perfectly fine for months, then develop a serious infection, recover with treatment, and return to normal again. Over time, these episodes typically become more frequent and harder to bounce back from. Keeping an FeLV-positive cat indoors, maintaining regular veterinary checkups, and watching closely for any of the signs listed above gives the best chance of catching problems early and managing them effectively.