Calicivirus in Cats: Symptoms and Vaccination

Calicivirus in Cats

Feline calicivirus (FCV) is a highly contagious upper respiratory infection that is often found in facilities with multiple cats such as catteries, pet stores, and shelters.

Vaccines against FCV exist with some caveats about efficacy and safety. First, FCV mutates efficiently into new strains that may not be fully or even partially by the current vaccines. Secondly, the vaccines do not produce so-called sterilizing immunity, which means a vaccinated cat could still become infected with a strain covered by the vaccine and possibly shed the virus to other cats.

These caveats raise a valid question: Why cats should be vaccinated against FCV?  These are the reasons for vaccination:

  1. Vaccination against FCV greatly reduces severity of symptoms and may tamp down viral spread.
  2. Severe cases due to infection with the natural strain or non-vaccination need expensive intensive care and supportive therapy. Not only does the infected cat suffer, but also treatment protocols could include nebulization, intravenous fluids for dehydration, antibiotics for a secondary bacterial infection, and non-steroidal anti-inflammatory drugs to induce fever reduction. Several studies have looked at the efficacy of various antivirals. Currently, no antivirals exist for the specific treatment of FCV.
  3. The virus is hardy and endemic.

We review the basics of FCV below, discuss a recent study that demonstrates the difference between vaccinated and unvaccinated cats when challenged with the virus, and lastly our position on the vaccine.

Basics of FCV

An infected cat can spread the virus to other cats through saliva, secretions from the nose or eyes, or sneezing. This hardy virus can live on uncleaned objects (bowls, furniture, etc.) for approximately one month. Indeed, caregivers can spread the virus to uninfected cats after contact with an infected cat. Secondary sources of infection may be urine or feces.

Once exposed and prior to the appearance of clinical signs, FCV incubates for approximately 2-6 days. Eventually, clinical signs last for 14-21 days. During this whole time, the cat is infectious to other cats. A subset of infected cats will continue to shed the virus for months or life.

Symptoms and severity often depend on the contracted strain, whether it is covered or partially covered by the vaccine, and the cat’s vaccination status.

  • Sneezing
  • Nasal discharge
  • Ulcers on the nose, lip, tongue, or mouth
  • Eye discharge
  • Conjunctivitis (eye inflammation)
  • Fever
  • Lethargy
  • Dehydration
  • Inappetence

Signs of more severe cases are:

  • Pneumonia
  • Cough
  • Difficulty breathing
  • Limping due to joint inflammation

Signs of the most severe forms of FCV are deemed virulent systemic disease (FCV-VSD) and include:

  • High fever
  • Swelling
  • Multiorgan failure including liver failure
  • Hair loss and crusty skin sores
  • Gastrointestinal tract bleeding
  • Death (60%)

FCV Challenge Study

The research challenge study found that a vaccine that has been used for decades against FCV in Europe still protects against the traditional (non-virulent, systemic) disease or reduces severity of symptoms.

This study lasted a few years and was multilayered.

What is a challenge study?
A challenge study – with institutionally approved animal welfare oversight – takes disease-free animals and splits them into a vaccinated group and a control group (unvaccinated). After vaccination, the researchers challenge both groups with a strain of a particular virus.

Where was this study conducted?
University of Zurich in Switzerland

What was the vaccine used?
The vaccine used was FCV vaccine F9 strain. FCV F9 has been used in Europe since the 1950’s. One of the scientific questions needing resolution was whether the prolonged use of FCV F9 might have driven the circulating FCV viral strains have evolved to become vaccine-resistant variants.

What viral variant was used?
Actually, two variant isolates were used in this study, FCV 273 and FCV 27. FCV 273 was isolated from a cat that did not have outdoor access and was the sole cat in the household. FCV 27 came from another cat that lived in a breeding colony and had outdoor access. Both strains were obtained in 2013 and 2012, respectively. The goal of the study was to find out if the vaccine developed in the 1950’s still affords protection to newer variants of the virus.

Results of First Challenge
The first challenge was with FCV 273.

  • All cats showed some clinical signs after the first experimental infection (FCV 273).
  • Oral ulcerations: 2/5 vaccinated cats; 5/5 control cats.
  • Fever: 2/5 vaccinated cats; 5/5 control cats.
  • Enlarged lymph nodes: 5/5 vaccinated cats and 5/5 control cats.
  • The total clinical scores and maximum scores were significantly lower in the vaccinated group than in the control group.
  • No sneezing, open-mouthed breathing or rhinitis was observed in any of the cats, and none of the cats died during the observation period.
  • All 10 cats shed the virus after challenge. The duration and frequency analyzed by virus isolation was not significantly different between the vaccinated and the control group.

Results of the Second Challenge
The second challenge was performed one year after the first challenge with the FC 27 isolate. All of the vaccinated cats were revaccinated.

With the exceptions of mild increases in body temperature and lymph nodes, no clinical signs were observed after the second challenge in any of the cats.

2/5 vaccinated and 2/5 control cats were shedding FCV on day 3. Three cats stopped shedding by day 15.

Why were there no clinical signs during the second challenge?
The researchers hypothesized that the first experimental infection elicited a strong, cross-protective immune response towards the second FCV challenge virus.

Why were the vaccinated cats shedding the virus in the first challenge?
According to the researchers, the duration and frequency of replication-competent – virus particles capable of infecting cells and replicating to produce additional infectious particles – were not significantly different between the groups.

However, the viral RNA loads (amounts of virus replication) shed from the oropharynx were significantly lower in the vaccinated cats compared to the unvaccinated controls, a noted shorter duration of RNAemia (detection of RNA in blood).

Some studies have documented a reduction in the extent of shedding in vaccinated animals, but these studies did not use FCV F9 as a vaccine strain.

Other FCV Vaccine Strains

Our goal here is to show you an example of a recent challenge study using the oldest strain, FCV F9, to assess the efficacy against the newer variants. Other vaccines use strains FCV 255, G1 or 431.

The European Advisory Board on Cat Diseases suggests that if an FCV-associated disease is found to be circulating in fully vaccinated cats then switching to a different FCV vaccine strain should be considered.

Virulent Systemic Disease of FCV and Vaccines

This area of research does not have complete answers yet, and needs further exploration. Vaccines against the traditional strains of FCV did not protect cats against field infections in many recent FCV-VSD outbreaks. However, some protection has been shown and prior vaccination resulted in a milder form of the disease.

But, were some of the cats unvaccinated or else sufficiently vaccinated against the traditional FCV to help reduce severity of VSD?

Side note: one company produces a vaccine against traditional FCV and virulent-systemic FCV.

Position on Vaccination Against FCV

Dr. Jean Dodds recommends vaccinating against FCV.

Her preferred protocol is to give the combination vaccination of panleukopenia (feline parvovirus), calicivirus, feline viral rhinotracheitis (feline herpesvirus-1) between 8-9 weeks of age and then boostered at 12-13 weeks of age. As well, she recommends giving an additional booster of this combination at 1 year of age, or one year later. This vaccine is written as FVRCP.

After that, it is wise to revaccinate every one to three years, or, measure serum antibody titers against this virus instead.

Dodds, W Jean. “Early Life Vaccination of Companion Animal Pets.” Vaccines vol. 9,2 92. 27 Jan. 2021, doi:10.3390/vaccines9020092,

Hofmann-Lehmann, Regina et al. “Calicivirus Infection in Cats.” Viruses vol. 14,5 937. 29 Apr. 2022, doi:10.3390/v14050937,

Spiri, Andrea M et al. “Modified-Live Feline Calicivirus Vaccination Reduces Viral RNA Loads, Duration of RNAemia, and the Severity of Clinical Signs after Heterologous Feline Calicivirus Challenge.” Viruses vol. 13,8 1505. 30 Jul. 2021, doi:10.3390/v13081505,

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