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Shoutout to Dad for suggesting calicivirus as a topic.
Upper respiratory infections are common in cats. Often called cat flu, these infections are a lot like the colds and flus that humans get in the winter. Cat flu is highly contagious, but not very serious for most healthy adult cats. It can be dangerous for young kittens, seniors, and cats with chronic illnesses or weak immune systems. Like human colds, cat flu can be caused by more than one virus, but one of the most common culprits is feline calicivirus.
What is Feline Calicivirus?
Feline calicivirus (FCV) is part of the Caliciviridae family of viruses [1, 2]. There are viruses from this family that afflict a wide range of vertebrate animals, including humans [1, 2]. Norovirus is a member of the Caliciviridae, as are some diseases of rabbits [1, 2]. The name calicivirus comes from the Greek work “calyx,” meaning “cup” or “goblet” [2]. This refers to the cup-shaped depressions on the surface of the hexagonal or star-shaped virus particles [2].
Feline calicivirus is specific to cats, both housecats and wild species [1, 2]. It is a single-stranded mRNA virus, a type which is particularly prone to mutation [2]. This has led to there being many different strains of FCV around the world.
How Are Cats Infected with Calicivirus?
FCV spreads most often in environments where a large number of cats live in close proximity to each other, such as boarding kennels, pet stores, shelters, breeding facilities, and dense feral colonies [1-5]. The risk begins to increase dramatically when as few as four cats are in close proximity [2]. In groups of fewer than four healthy cats, the infection rate is 2.5%, but groups of four or more healthy cats have an infection rate of 32% [2]! In colonies and shelters, the rate can range from 50-90% [2].
The virus is spread through direct contact with body fluids from an infected cat, primarily saliva and eye and nose secretions [1-5]. Blood, urine, and feces can also spread the virus, as can aerosolized droplets from sneezes [1-5]. This means sharing things like food bowls, water bowls, litterboxes, and bedding can spread FCV [1-3]. So can just sharing an environment, or human caretakers can accidentally spread the virus around by getting it on their hands or clothes [1-3]. Most infected cats shed virus for 2-3 weeks, but some become carriers and continue to spread FCV for months or years [1, 2].
Symptoms of Feline Calicivirus
The typical presentation of FCV is an upper respiratory infection [1-5]. The virus attacks the inside of the mouth and the lungs [1, 2]. The symptoms include the usual upper respiratory signs like sneezing, stuffy/runny nose, runny eyes, and sleepiness [1-3]. However, FCV may also cause ulcers inside the mouth, which may in turn lead to drooling and refusal to eat due to oral pain [1-5]. Occasionally vulnerable cats, usually young kittens, will develop pneumonia [2, 3]. It is rarely fatal, however, and most cats will recover in a week or so, although severe cases may take up to six weeks [1].
Some cats who become infected with feline calicivirus go on to develop chronic gingivitis [1-5]. Researches do not yet know why this happens, or why it only happens to certain cats [2]. If your cat has recently recovered from cat flu and is still having problems with their mouth, this is probably something to talk to your veterinarian about.
Limping Syndrome
FCV can sometimes attack the joints and result in a strange–and upsetting–manifestation known as limping syndrome [1-5]. It mostly occurs in kittens [3, 4], and it can look very scary. Affected kittens develop a fever as well as pain, stiffness, and lameness in their joints, but which limb(s) are affected may change over time [2-4]. Sometimes limping syndrome occurs as a side effect of an FCV vaccination, not from an infection [2, 3, 5]. Although it is painful, limping syndrome only lasts 2-3 days and usually clears up on its own [2-4]. However, during that time, your veterinarian may be able to prescribe anti-inflammatory and/or pain medication to make your kitten more comfortable [2, 3]. Kittens that are severely affected may be reluctant to move at all [3] and need your help getting to their food, water, and litterbox until they’re feeling better.
Virulent Systemic Feline Calicivirus (VS-FCV)
In rare instances, the FCV virus mutates into a much, much more dangerous form called virulent systemic feline calicivirus (VS-FCV) [2, 3]. VS-FCV is able to infect multiple organ systems and the lining of blood vessels [2, 3]. It causes high fever, inflammation throughout the body, swelling of the head and limbs, sores on the face and feet, jaundice, and multiorgan failure [1-3]. About 60% of cats infected with VS-FCV die [1-3]. Fortunately, outbreaks are very rare [1-3] and have only been observed in some countries [3]. Researches have not found any genetic traits that distinguish VS-FCV strains from regular FCV strains, however, making it difficult to specifically vaccinate against VS-FCV until there’s a breakthrough [2].
Treatment and Prevention
FCV usually goes away on its own in healthy cats [1-4]. There are currently no antiviral drugs that are proven to be effective against FCV, although a couple have shown promise in preliminary research [2]. As such, treatment focuses on relieving symptoms and supporting your cat’s body while it fights off the virus [1-4]. Your veterinarian might decide to prescribe medication like a non-steroidal anti-inflammatory drug or anticongestant [1, 2, 4]. Keeping the eyes and nose clean is important–nebulizers and saline eye or nose drops can help [1, 2, 4]. Some cats may need intravenous fluids [1, 2, 4]. Since many cats lose their appetite or simply find eating difficult, you may have to try different foods or ways of presenting their food to make sure they eat [1, 2, 4]. In severe cases, they may require a feeding tube [1, 2, 4].
The bad news is that FCV is extremely widespread. The good news is that there is a vaccine for it, which the American Association of Feline Practitioners [5] and European Advisory Board on Cat Diseases [2] both consider a core vaccine that all cats need to have. FCV is usually included in a combination vaccine with other common viruses [2].
Because there are so many different strains of FCV, the vaccine doesn’t always prevent a cat from becoming infected, but it is effective at preventing them from getting sick [2, 5]. Kittens lose the protection of the immune cells they get from their mothers somewhere between 6 and 14 weeks of age, so the American Association of Feline Practitioners recommends vaccinating kittens against FCV at 6 weeks, or as early as is allowed in your country [5]. Kittens should receive a primary vaccination series of 2-3 shots, and adult cats should receive a booster every 1-3 years, depending upon their risk level [2, 3, 5].
Works Cited
- Baker Institute for Animal Health. (2016). Feline calicivirus. Cornell University College of Veterinary Medicine. https://www.vet.cornell.edu/departments-centers-and-institutes/baker-institute/our-research/feline-calicivirus
- Hofmann-Lehmann, R., Hosie, M. J., Hartmann, K., Egberink, H., et al. (2022). Calicivirus infection in cats. Viruses, 14(5), 937. https://doi.org/10.3390%2Fv14050937
- International Cat Care. (2018, August 17). Feline calicivirus (FCV) infection. https://icatcare.org/advice/feline-calicivirus-fcv-infection/
- Kuehn, N. F. (2022, October). Feline respiratory disease complex (feline viral rhinotracheitis, feline calicivirus). Merck Veterinary Manual. https://www.merckvetmanual.com/cat-owners/lung-and-airway-disorders-of-cats/feline-respiratory-disease-complex-feline-viral-rhinotracheitis,-feline-calicivirus?query=calicivirus
- Scherk, M. A., Ford, R. B., Gaskell, R. M., Hartmann, K., et al. (2013). Disease information fact sheet: Feline calicivirus. Journal of Feline Medicine and Surgery, 15, supplementary file. https://catvets.com/public/PDFs/PracticeGuidelines/Guidelines/Vaccination/FelineCalicivirus_FactSheet.pdf
Published February 12th, 2023
