Feline Calicivirus (FCV)

Overview

Feline Calicivirus (FCV) is one of the most common causes of upper respiratory infection and oral disease in cats. Most cases are mild-to-moderate, but in rare cases, the virus can mutate into a severe, life-threatening form.

• Very common and highly contagious, especially in shelters and multi-cat households
• Typically causes respiratory and mouth symptoms — usually manageable, not usually fatal
• A rare severe mutation called FCV-VSD (Virulent Systemic Disease) can be fatal in more than half of affected cats — this is different from typical calicivirus and important to know about
• Some cats become long-term carriers, shedding the virus even without visible symptoms

Common Symptoms

Typical / Mild-Moderate FCV

• Sneezing, nasal discharge, watery eyes
• Mouth ulcers (tongue, gums, hard palate) — often causing drooling and reduced appetite due to pain
• Fever
• Occasionally mild limping (a lesser-known but real symptom)

Severe / FCV-VSD (Rare)

• Swelling of the face, head, and legs
• Skin ulcers and crusting sores, especially around the nose, ears, and footpads
• Yellowing of the mouth/skin (jaundice) from liver involvement
• Bleeding under the skin or in the GI tract
• High fever, signs of multi-organ involvement

FCV-VSD most often appears in outbreak settings — shelters, catteries, or multi-cat households — rather than in single-cat homes.

Possible Causes

• Direct contact with infected cats, or contaminated surfaces, bowls, bedding
• Highly transmissible through respiratory droplets and saliva
• Outbreak risk is highest in high-density, high-stress cat environments
• Even vaccinated cats can catch calicivirus, since vaccines only offer partial cross-protection against different strains

Diagnosis

• Primarily diagnosed based on clinical signs (oral ulcers + respiratory symptoms are a strong indicator)
• PCR testing can confirm the virus in oral/nasal swabs or blood
• Bloodwork is used if FCV-VSD is suspected, checking for liver involvement and abnormal blood cell counts
• A veterinarian should evaluate promptly, especially if swelling or skin lesions appear alongside typical symptoms

Treatment

• No direct antiviral cure for standard FCV — treatment is supportive
• Pain management is important, since oral ulcers make eating painful
• Antibiotics if a secondary bacterial infection develops
• Fluids and nutritional support if the cat isn't eating
• FCV-VSD requires aggressive hospitalization — IV fluids, intensive supportive care, and immediate isolation from other cats

Nutrition & Food Intake

• Painful mouth ulcers often make cats reluctant to eat — warming food slightly or offering strong-smelling, soft food can help entice appetite
• Syringe feeding or appetite stimulants may be needed if a cat refuses food for more than a day
• Ensure easy access to water; dehydration can worsen recovery
• Cats that stop eating for an extended period are at risk of hepatic lipidosis (a serious liver condition) — this needs veterinary attention

Home Physical Checks (What to Look For Yourself)

• Mouth – gently check for ulcers on the tongue, gums, or roof of the mouth if drooling or reduced appetite is noticed
• Gums – jaundice (yellow tinge) is a red flag for the severe form and needs immediate vet attention
• Face and legs – any swelling, especially if paired with respiratory symptoms, should be evaluated quickly
• Footpads and skin – check for crusting sores or ulceration
• Appetite and energy – track whether the cat is still eating and drinking despite mouth discomfort

Supportive Supplements (General Info — Not a Prescription)

• L-lysine – historically used for feline herpesvirus/URI symptoms, though current evidence for effectiveness is mixed; ask your vet before use
• Probiotics – sometimes used to support overall immune health during recovery
• Omega-3s – general immune and inflammation support, not a treatment for the virus itself
• None of these replace veterinary care, especially if symptoms progress toward the severe form

When to Seek Emergency Care

Seek immediate veterinary care if you notice:

• Swelling of the face, head, or legs
• Skin ulcers or crusting sores
• Yellowing of the gums or skin (jaundice)
• High fever with lethargy
• Refusal to eat or drink for more than 24 hours
• Any of these symptoms in a multi-cat household — early isolation matters

Prevention

• Vaccination reduces severity of disease, though it doesn't guarantee full protection against all strains
• Good hygiene in multi-cat environments: separate bowls, regular disinfection, avoiding overcrowding
• Isolate new or sick cats before introducing them to a group
• In shelter/rescue settings, strict sanitation protocols are critical, since FCV-VSD outbreaks have caused hospital and shelter closures in documented cases

Prognosis

• Typical FCV: Good prognosis. Most cats recover within 1-3 weeks with supportive care, though some become lifelong carriers who can still spread the virus.
• FCV-VSD (severe form): Serious. Documented outbreak mortality rates have ranged from roughly 50% to 86%, according to multiple veterinary case studies and the Cat Fanciers' Association. This form requires aggressive, immediate treatment.
• The key distinguishing factor for outcome is how quickly the severe form is recognized and treated — early isolation and intensive care meaningfully affect survival in outbreak settings.

Sources: Cat Fanciers' Association; PMC veterinary case studies (Korea, Ireland, Australia outbreak reports); Vetster.