Feline Calicivirus

When Your Cat Gets “the Crud”

Nancy Kerr

Feline calicivirus, also known as “FCV,” is a highly contagious virus that causes mild to severe upper respiratory infections and oral diseases in cats and is one of the major causes of upper respiratory infections (URIs), or “cat flu.” It occurs worldwide, affecting both domestic and exotic cat species. “Together, FCV and feline herpesvirus (FHV, FHV-1) cause most upper respiratory infections in cats.”¹ “The virus often closely resembles feline viral rhinotracheitis (FVR).”² Though there has been reported to be “nearly 50 strains of feline calicivirus, causing disease of variable virulence and severity,”² this number has likely increased due to how the virus mutates.

What is the Calicivirus?

“Feline calicivirus belongs to a large family of viruses named ‘Caliciviridae,’ whose members infect a wide range of vertebrate animals, including rabbits, livestock, reptiles, birds, and amphibians. The human virus ‘norovirus,’ which causes a brief but unpleasant gastrointestinal disease, is also a member of the Caliciviridae family. Several strains of FCV circulate in domestic and wild cats. The virus mutates readily, leading to new strains that may not be fully covered by existing vaccines. Strains vary in the severity of the disease that they cause, with the majority causing only mild disease.”³

Who is Affected?

The virus is more common in multi-cat environments, shelters, or among stray cats; often infecting young cats. Those with weakened immune systems are at higher risk of contracting the virus. “Common traits in kittens and young cats that are particularly at risk include: living under stress, not enough feeding, suppressed immune system, and crowded and unhygienic life, as well other upper respiratory pathogens or having the herpes virus.”⁴

Symptoms

Symptoms vary in severity and may vary depending on the strain of the calicivirus. They may persist for “5 to 10 days in low-severity cases and up to 6 weeks in more severe cases.”⁴ Severe upper respiratory infection symptoms are normally seen, including: “sneezing, discharge from the nose, discharge from the eyes (clear or may become yellow/green in color), conjunctivitis, tongue ulceration, lethargy/tiredness, loss of appetite, high temperature (fever). In young kittens, the virus may also cause pneumonia (lung infection).”¹ In addition, infected cats often develop “ulcers on (the) hard palate, gums, lips, or nose.”⁵ In these situations, cats will usually exhibit salivation or excessive drool due to painful ulcers. “Other non-specific signs of an upper respiratory infection include anorexia, lethargy, fever, enlarged lymph nodes, and squinting.”⁵

Cats may also experience “inflammation of the tissue in the upper jaw of the mouth (gingivostomatitis), gingivitis and stomatitis (inflammation of the mouth and gums).”¹ Young cats/kittens may develop sudden painful lameness in one or more joints. Cats may alternate areas affected by the lameness. Some cats may temporarily experience painful arthritis.

How is it Spread?

The virus is spread through “direct contact with saliva or discharge from the eyes or nose, breathing in sneeze droplets (droplets can carry the virus for up to 1.5m), sharing food bowls, litter trays, bedding and grooming equipment. The virus can potentially survive up to a month in the environment, although it usually survives for around 7-14 days.”¹ “Humans that handle infected cats can inadvertently transfer the virus to new animals.”³

“Once a cat is exposed to calicivirus, it will go through an incubation period of 2-6 days before developing clinical signs which typically last for 14-21 days. During this entire time, the cat will potentially be infectious to other cats. At a minimum, infected cats will shed virus in their bodily secretions for 2-3 weeks.”⁵

Complications

In rare cases, the virus can mutate into a deadly, different form called “feline calicivirus virulent systemic disease” (FCV-VSD). This form can cause a very serious disease that involves multiple organ damage or even death. Cats that develop FCV-VSD will have much more “severe symptoms, including a high fever, swelling of the head and legs, as well as crusting sores and hair loss on the nose, eyes, ears and footpads. The mouth and ears may turn yellowish from liver damage, and there may be bleeding under the skin and in the gastrointestinal tract. FCV-VSD is fatal in up to 60 percent of cats that develop the disease.”³ Cats that develop FCV-VSD should receive veterinary care as soon as possible. Treatment may include intravenous fluids, antibiotics, and other treatments as needed.

Diagnosing

While a veterinarian may consider a cat’s symptoms, there is generally no need to make a definite diagnosis, as these infections are common and will resolve with supportive treatment. However, if multiple cats are infected or the cats are housed with others, swabs may be taken from the eyes, nose or mouth and sent to a lab to test for the presence of the virus.

“Commercial labs detect the presence of FCV in two ways: by growing the virus (found) in cells in a petri dish, or through reverse transcriptase PCR (RT-PCR), a procedure that detects a segment of genetic material that is specific to calicivirus. Both tests are equally effective, though the RT-PCR test may be more common in some areas, as part of a panel that tests for several organisms that cause respiratory disease.”³

“Test results should be interpreted carefully. Many cats that appear healthy, especially ones recently adopted from a shelter, pet store or breeder, will test positive for the virus due to previous exposure, so a positive result does not necessarily indicate that FCV is the cause of the problem. Recent vaccination with a modified live strain of the virus can also cause a false positive result. Incorrect negative results are more likely if the cat is swabbed more than a week after the start of the infection. Commercial testing cannot distinguish between mild strains of FCV and the more virulent strains that cause FCV-VSD.”³

Treatment

While there is currently no treatment to stop the virus, owners can offer supportive care for their cat while its immune system fights the infection. “Most (but not all) cats gradually recover from the acute, active phase of illness over a period of 7 – 10 days.”² While most cats can recover at home, severely affected cats may need to receive care at the veterinarian. Cats that are dehydrated, depressed, or have a severe case of illness will require hospitalization for more intensive treatment, including intravenous fluids and other supportive treatments. Eye medication maybe prescribed “to be applied topically if your cat has a purulent (green/yellow) eye discharge. Although viral infections do not respond to antibacterial drugs, broad spectrum antibacterial drugs (e.g., amoxicillin-clavulanic acid, brand name Clavamox, doxycycline or azithromycin) may be prescribed in an effort to prevent secondary bacterial infections from complicating the disease, particularly in kittens.”⁵

Steam inhalation or nebulization may also help cats with severe nasal congestion. Your cat will probably not be able to smell food well. Gently warming the food will help. Non-steroidal anti-inflammatory drugs can help to lower fever and reduce mouth pain. When required, broad-spectrum antibiotics can be used to help treat opportunistic bacterial infections. Anti-inflammatory medication may be prescribed or administered by a to relieve symptoms of lameness.

Thorough cleaning and disinfection are important when treating/handling infected cats to prevent accidental transmission of the virus. A diluted bleach solution made up of one-half cup of bleach per gallon of water is effective at killing the virus. Cleaning solutions, like Lysol, that contain phenol are also effective, but should not be used around cats because they are an irritate and are toxic. Owners may wish to relocate healthy cats from the home during this period to prevent exposure. The virus should die off naturally after one month.

Prognosis

Most cats will recover completely after a calicivirus infection; however, the rare strains can be especially deadly. The virus does not pose any threat to humans.

“Following apparent recovery from the disease, as many as half of all infected cats may develop a carrier state in which they will continue to shed the virus. In some of these cats the carrier state may only last for a few months, but in a small percentage of cats the carrier state may persist for life. Carrier cats may or may not show any signs of infection when they are actively shedding calicivirus and serve as an important source of infection to susceptible cats. Female cats that are carriers of this virus can pass the infection on to their newborn kittens.”⁵

“It can also cause persistent inflammation (chronic stomatitis) in some cats’ mouths that requires lifelong treatment.”⁴

Prevention

Vaccination for all cats for the virus is important. It is one of the most important ways to reduce the risk of calicivirus infection and to prevent it. “Two or three injections are recommended in kittens, starting at around 8 weeks of age. Cats should then receive a booster as a one-year-old, and after that, should receive further booster vaccines every 1–3 years depending on their risk of infection.”¹

It should be noted that since there are many different strains of the virus, vaccination is not able to provide 100% protection; however, it will be effective in reducing symptoms and risk of the disease progression when exposed. Try to prevent direct contact between your cat and other cats outside of your household, as this will greatly minimize the chance that your cat will pick up an infection. Additionally, good sanitation and hygiene practices, such as washing your hands thoroughly before and after petting another cat, will help to reduce the likelihood that you will spread the disease to your cat. If you bring a new cat into your household, consider isolating it from the existing cats in the household for one to two weeks to verify that it has no disease, etc.

 

Sources

1. Feline calicivirus (FCV), 03/19/2025, Accessed 07/12/2025, https://icatcare.org/articles/feline-calicivirus-fcv

2. Calicivirus in Cats: Causes, Symptoms, & Treatment, Dr. Pete Wedderburn, DVM, 04/17/2023, Accessed 07/12/2025, https://cats.com/calicivirus-in-cats
3. Feline Calicivirus, Accessed 07/12/2025, https://www.vet.cornell.edu/departments-centers-and-institutes/baker-institute/research-baker-institute/feline-calicivirus
4. VetAmerican Medical Veterinary Team, Feline Calicivirus (FCV) in Cats, 08/04/2024, Accessed 07/12/2025, https://www.vetamerikan.org/health-information/cat-diseases/feline-calicivirus-fcv-in-cats
5. Krista Williams, BSc, DVM, CCRP; Tammy Hunter, DVM; Cheryl Yuill, DVM, MSc, CVH; Feline Calicivirus Infection, 2022, Accessed 07/12/2025, https://vcahospitals.com/know-your-pet/feline-calicivirus-infection