Feline calicivirus (FCV) PCR in cats

What it shows

RT-PCR detects the genetic material (RNA) of FCV. Possible samples include conjunctival and oral swabs, blood, skin scrapings and lung tissue; swabs from the oropharynx and tongue were positive significantly more often than conjunctival swabs, so ABCD recommends the oropharynx. Quantitative tests are preferred because they also show the amount of virus.

Healthy cats can test positive: unlike herpesvirus, calicivirus is shed continually.

Higher than reference — what it may be a sign of

For PCR, “higher than reference” means the RNA was detected. Possible explanations:

• Current FCV disease — in a cat with typical signs a positive result fits an active infection.
• Healthy carrier — carriage was reported in 2.5% of healthy household cats kept in groups of fewer than four, and in 32% of healthy cats in groups of four or more. After infection most cats shed the virus for at least 30 days, and a few for several years, up to life-long.
• Vaccine virus — cats vaccinated with modified live vaccines can occasionally shed FCV. The sources do not say that a routine test separates vaccine virus from field virus, so the vet takes the vaccination date into account.
• Chronic mouth inflammation — in one study, RT-PCR was positive in 54% of 52 cats with chronic gingivostomatitis and in 24% of 50 healthy cats.

No test specifically detects the virulent systemic form of FCV; it is recognised by its clinical signs and confirmed by finding the same strain in blood, swabs or skin scrapings.

Lower than reference

“Not detected” does not rule out FCV. The virus genome is highly variable, and the sensitivity of RT-PCR depends on the primers and the strain: in a Swiss field study neither of two well-established tests could detect all isolates. The virus may also be present in amounts too small for the test to pick up. The vet decides whether to repeat the test or use another method.

Look at together with

• Herpesvirus PCR (FHV-1) — another respiratory virus; the two infections are often hard to tell apart by signs.
• Antibody tests — antibodies are not useful for diagnosing FCV infection.
• Vaccination record — the type and date of the last vaccine.
• Living conditions — the number of cats in the home or group changes how likely healthy carriage is.

Units and conversion

RT-PCR has no units: the form shows “detected” or “not detected” for calicivirus RNA. Because quantitative tests are preferred for FCV, some forms also give the amount of virus. The app stores the result as text, as printed on the form.

Reference range

The reference range is on your lab’s form; for this PCR it is usually “not detected”. Because healthy carriers occur, especially in groups of four or more cats, a positive result is read together with the signs and the vaccination date, and a negative one does not rule out infection. The result helps the vet choose the next steps.

Cats and dogs

Sources

1. ABCD (European Advisory Board on Cat Diseases) — Guideline for feline calicivirus infection
2. Hofmann-Lehmann R, Hosie MJ, Hartmann K, et al. Calicivirus Infection in Cats. Viruses 2022; 14: 937
3. Tonozzi CC. Feline Respiratory Disease Complex. Merck Veterinary Manual, professional version
4. Cornell Feline Health Center — Respiratory Infections
5. Cornell University Animal Health Diagnostic Center — Interpretation of PCR results
6. UC Davis Real-time PCR Research and Diagnostics Core Facility — Diagnostic FAQs