Feline calicivirus (FCV) is a highly contagious virus that causes oral ulcers, upper respiratory infections, and sometimes limping in cats. It’s one of the most common infectious diseases in cats worldwide, and because the virus mutates at an exceptionally high rate, hundreds of strains circulate at any given time. Most cats recover within one to two weeks with supportive care, but certain strains can cause severe, life-threatening illness.
How FCV Spreads and Survives
Calicivirus spreads through direct contact with an infected cat’s saliva, nasal discharge, or eye secretions. Sneezing is a major route: viral particles can travel through the air over short distances and land on surfaces, food bowls, bedding, and clothing. The virus is hardy enough to persist on these surfaces, which is why shelters, boarding facilities, and multi-cat households see frequent outbreaks.
Once a cat is exposed, symptoms typically appear within 2 to 10 days. But here’s what makes FCV especially tricky to control: after recovery, most cats continue shedding the virus for at least 30 days. Some shed for years, and a small number become lifelong carriers. These carrier cats often look perfectly healthy while quietly spreading the virus to other cats around them. In large colonies or breeding households, this creates a cycle of ongoing infection, with the virus constantly evolving new strains under immune pressure from the group.
Common Symptoms
The hallmark of calicivirus is mouth ulcers, particularly on the tongue, gums, and roof of the mouth. These sores make eating painful, so many infected cats stop eating or drool excessively. Alongside the oral ulcers, you’ll usually see signs of an upper respiratory infection: sneezing, nasal congestion, runny eyes, and sometimes a fever. Lethargy and loss of appetite are common. In mild cases, these symptoms last 5 to 10 days. More severe infections can drag on for up to six weeks.
Some cats develop pneumonia, particularly kittens and those with weakened immune systems. This is the most dangerous complication of a standard FCV infection and requires prompt veterinary care.
The Limping Syndrome
One of the more surprising presentations of calicivirus is a transient lameness that can appear without any mouth sores or respiratory signs at all. Certain FCV strains cause a short fever along with leg lameness and pain in the joints. This “limping syndrome” shows up most often in kittens between 8 and 12 weeks old and typically resolves on its own without treatment. If your kitten suddenly starts limping after a vaccination or exposure to other cats, calicivirus is a likely explanation.
Chronic Mouth Inflammation
Calicivirus plays a significant role in a painful condition called feline chronic gingivostomatitis, a severe, ongoing inflammation of the mouth and gums. This isn’t a simple infection. It results from an overactive immune response to chronic stimulation in the mouth, and the condition is considered multifactorial. However, FCV is found in nearly 100% of cats with this condition, compared to a much lower rate in the general cat population. Cats with chronic gingivostomatitis often have red, swollen gums that bleed easily, and the pain can make eating so difficult that they lose significant weight.
Virulent Systemic Disease: The Severe Form
Rarely, a cat encounters a highly virulent strain that causes systemic disease, meaning the virus attacks multiple organ systems rather than staying in the mouth and respiratory tract. This form produces the usual oral ulcers but also causes swelling under the skin (particularly around the face and legs), ulcers on the ears and paw pads, and organ damage. The mortality rate for virulent systemic calicivirus reaches approximately 50% even with intensive veterinary treatment. Outbreaks of these strains tend to occur in clusters, often in veterinary hospitals or shelters where cats are housed close together.
The virus’s genetic plasticity is what allows these dangerous variants to emerge. FCV has one of the highest mutation rates ever recorded for any virus, with its outer shell protein evolving rapidly enough to escape immune defenses and occasionally produce far more aggressive strains.
Diagnosis
Veterinarians often diagnose calicivirus based on clinical signs alone, since the combination of mouth ulcers and upper respiratory symptoms is fairly distinctive. When lab confirmation is needed, a swab from the mouth or throat can be tested using a method that detects viral genetic material. The catch is that carrier cats and recently vaccinated cats can also test positive, so a positive result doesn’t automatically mean calicivirus is causing a cat’s current illness. Your vet will interpret the result alongside the cat’s symptoms and history.
Treatment and Recovery
There is no antiviral drug that eliminates calicivirus, so treatment focuses on keeping the cat comfortable and supported while the immune system does its work. For most cats, this means ensuring adequate hydration and nutrition. Cats with painful mouth ulcers often refuse food, so softened or warmed food can help, and some cats need appetite stimulants or even temporary feeding support. Pain management is important, especially for cats with severe oral sores. If a secondary bacterial infection develops on top of the viral illness, antibiotics may be prescribed for that specific complication.
Cats with pneumonia or the virulent systemic form need more intensive care, including intravenous fluids, oxygen support, and close monitoring. Recovery timelines vary widely. A straightforward case in an otherwise healthy adult cat resolves in about a week. Kittens, elderly cats, and those with compromised immune systems face longer recovery times and a higher risk of complications.
Vaccination: Essential but Imperfect
Calicivirus vaccination is considered a core vaccine for all cats worldwide, regardless of lifestyle. Kittens typically receive their first dose at 6 to 8 weeks of age, followed by boosters every 2 to 4 weeks until they’re at least 16 weeks old, with an additional dose at 26 weeks or older. After that initial series, low-risk indoor cats can be revaccinated every 3 years or longer. Cats that board, go outdoors, or have regular contact with other cats may benefit from annual boosters.
The vaccine provides cross-protective immunity against multiple strains, which reduces the severity of illness even when a cat encounters a strain that doesn’t perfectly match the vaccine. However, no FCV vaccine protects against all strains. Vaccinated cats can still get infected and develop symptoms, though the disease is generally milder. If a vaccinated cat develops calicivirus-related illness, switching to a different vaccine brand containing different strains for future boosters is sometimes recommended.
Preventing Spread in Multi-Cat Homes
Because calicivirus is so persistent in the environment and so many recovered cats become carriers, controlling it in multi-cat settings requires consistent effort. Separate food and water bowls for each cat, regular cleaning of surfaces with a disinfectant effective against non-enveloped viruses (bleach diluted at a 1:32 ratio works well), and isolating any cat showing respiratory symptoms are all practical steps. New cats entering the household should be kept separate for at least two weeks and ideally tested before mingling with resident cats.
In shelters and catteries, calicivirus is nearly impossible to eliminate entirely because of the carrier state. Even a cat that appears completely recovered can silently transmit the virus for months or years, seeding new infections in susceptible animals. This reality makes vaccination the single most effective tool for reducing the overall burden of disease, even though it can’t prevent every infection.