Tang Nguyen Mai Trinh

What is Calicivirus in Cat?

The Calicivirus of the family Caliciviridae can cause respiratory tract infection in feline family. Calicivirus can be isolated from 50% of infected cats [1]. When infected with calicivirus, the mortality rate in cats is 67% because the virus can be transmitted rapidly [2].

Feline Calicivirus (FCV) can persist in the environment for a long time, exposure is about 10% in cats in a household setting, whereas in shelters or pet stores it is about 25% to 40% [3].

Although most cats recover completely from  calicivirus infection, some rare strains can be fatal [4]. Funthermore, cats being infected for at least 30 days, after recovered may shed for several years or even till the end of their life [5].

The structure of Calicivirus

Figure 1. Electron microscopy of negatively stained calicivirus particles: virions (left) and Virus-like particles (VLPs) (right). Scale bar 1⁄4 100 nm [3].


Calicivirus has a single-stranded, positive, unsegmented RNA genome, about 35-39 nm in diameter in virion-free Calicivirus [6], genus Vesivirus. When observed under an electron microscope, it has a star or hexagonal shape (Figure 1) in Greek means cup [2].

Calicivirus is easily transmitted from cat to cat through contact with droplets, ocular or nasal secretions, or by sharing items used by an infected cat [7]. After 3 to 4 days the virus can enter the bloodstream, appear in different tissues and cause necrosis [8].

A previous study conducted by Mencke et al. (2009), showed that calicivirus can be found in flea feces and persists for up to 8 days, and kittens that come into direct contact with infected fleas or their faeces can become infected [9].

Clinical signs

The clinical presentation of FCV infection will be influenced by factors such as cat’s age, immune system, and the virulence of the infecting virus strain. Cats begin to exhibit the symptoms after being infected with FCV for at least one week.

Ocular, ulcers, and acute respiratory tract infection

Figure 2. Conjunctivitis in 2-month-old kittens with calicivirus infection. Appearance of multiple conjunctival erosions stained positive for Flureszein (a) and Lissamine-green (b) [13].


After 2 to 10 days of FCV infection, cats start to exhibit the typical signs of infections, including fever, body temperature around 38.60C [10,7], exhaustion, appetite loss coupled with diarrhea oozing [11], damaged skin, discharge from the eyes and nose, and in more severe cases, conjunctivitis (figure 2) [8, 12, 13]. Vesicles appearing on the mouth, the tongue’s edge, and the emergence of oral ulcers are the usual clinical manifestations (figure 3) [7, 8]. Usually, when infected with FCV, these symptoms will last for days to weeks.

Figure 3. Symptomatic cats develop mouth ulcers (a) and tongue ulcers (b) when infected with Calicivirus [8].


The syndrome of lameness when infected with FCV

Limping happens in kittens, although not frequently, but in some cases can lead to pneumonia [13]. This limp also can arise in kittens following immunization with FCV vaccine and several modified live virus vaccinations [14].

Viral systemic FCV disease

Figure 4. Systemic infection due to VS-FCV infection. (a) lesions of the nose and eyes; (b) hemorrhage in the soles of the cat’s paws [15]

Viral systemic FCV or highly virulent feline calicivirus disease is a variant of the FCV virus that was first described during outbreaks in Europe and the United States [15,16]. Studies have shown that VS-FCV-infected cases have an incubation period of 12 to 28 days in the home environment and 1-5 days in the clinic setting [10].

Athough the incidence of systemic virulent FCV is quite rare, infected cats will have more severe clinical manifestations. The infection will spread throughout the cat’s body, especially the legs and nose (figure 4) [17]. Pneumonia, hepatitis, pancreatitis, bloated, ulcerated skin, nosebleeds, and even intestinal bleeding, disseminated intra- vascular coagulation and multiorgan failure are some of the symptoms of infected with Vs- FCV. Up to 67% of infected cats may die [18].


Detection of nucleic acid

Although the RT-PCR method for nucleic acid detection is a conventional technique, it is steadily gaining popularity due to its speed and incredibly high accuracy. Cat blood, conjunctiva and oral swabs, scrapes of skin and lung tissue, and samples of ocular and nasal secretions can all be used to detect FCV RNA.

For identifying FHV in vaccinated cats or following a return of clinical symptoms, RT-PCR may be especially helpful. If samples are contaminated during sample collection or analysis, PCR may produce false-positive findings [19].

 Virus isolation

 Viral isolation is also employed in conjunction to other techniques like PCR or ELISA. Taking oropharyngeal samples for viral isolation, along with nasal and conjunctival swabs. This approach simply demonstrates the virus’s existence, and it is less sensitive than the PCR method. Additionally, virus isolation requires a lot of time and might fail if there are just a few infected virions in the sample [20]. One of the factors impacting the virus’s survival is the inactivation of the virus in vitro because of the presence of antibodies or faulty sample transport that inactivates the virus [21].

 Antibody detection

Antibody detection has gradually become popular because of its high specificity and sensitivity. Using ELISA or rapid test kit can determine the presence of antibodies against FCV in serum [22]. It should be emphasized that this inspection should not be done on cats that have had vaccinations. Due to antibodies produced following vaccination, false positive results are more likely to occur.


 When a cat showes the clinical symptoms, it is necessary to immediately take it to the nearest veterinary station for timely care. However, FCV disease is often quite complicated because the symptoms are quite similar to FHV, so tests are needed to confirm it.

Cats need immediate care, and may need fluids and nutritional supplements in some severe cases. When infected with FCV, the cat’s mouth, tongue, and nose are damaged, leading to the cat not eating or drinking. Antibiotics, non-steroidal anti-inflammatory drugs may be recommended to treat fever and sore mouth [8].

Moreover, it is necessary to immediately isolate cats suspected of being infected, and ensure strict hygiene conditions to avoid infection. Need to use separate eating utensils, sand trays and other items, etc.


 According to a research by Andréia Henzel et al. [24], the prevalence of FCV infection was 3.6% greater in unprotected cats than it was in vaccinated cats. It is advised to take the cat to the veterinarian for vaccinations in order to prevent it against the calicivirus. It is recommended to vaccinate kittens twice or three times by the time they are 8 weeks old since by this time the MDA passed from the mother has exhausted, increasing the risk of infection in the kittens [8]. After one year, booster shots and revision vaccinations every one to three years are advised.



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