Cat Flu (Feline Upper Respiratory Infection): Calicivirus and Herpesvirus

Cat flu is the common name for feline upper respiratory infection (URI), a highly contagious condition that primarily affects the nose, throat, and eyes. It is one of the most frequently seen illnesses in cats, particularly in shelters, catteries, and multi-cat environments. While the term "flu" suggests a single illness, cat flu is caused by a combination of pathogens, with two viruses responsible for the vast majority of cases.

Dr. Sarah Chen, DVM: "Cat flu can range from a mild sniffle that clears in a week to a life-threatening illness in kittens and immunocompromised cats. The key to good outcomes is early recognition, supportive care, and ensuring kittens receive their full vaccination course."

Feline Herpesvirus Type 1 (FHV-1)

FHV-1 accounts for approximately 50–60% of all cat flu cases. It is an alphaherpesvirus that targets the upper respiratory tract and eyes. Like all herpesviruses, FHV-1 establishes lifelong latent infection — once a cat is infected, the virus retreats into the trigeminal nerve ganglia and can reactivate during periods of stress, illness, or immunosuppression. Approximately 80% of recovered cats become latent carriers, and about 45% of carriers will intermittently shed the virus, potentially infecting other cats even when they appear healthy.

Feline Calicivirus (FCV)

FCV is responsible for approximately 30–40% of cat flu cases. Unlike herpesvirus, calicivirus exists in many different strains, which is why vaccinated cats can still become infected with strains not covered by the vaccine. FCV tends to cause more oral disease (mouth ulcers) and can occasionally cause a limping syndrome in kittens due to transient joint inflammation. A particularly virulent form — virulent systemic FCV (VS-FCV) — can cause severe systemic disease with a mortality rate exceeding 50%, though this form is rare.

Other organisms that contribute to the cat flu complex include Bordetella bronchiseptica, Chlamydophila felis (causing primarily conjunctivitis), and Mycoplasma species. These often act as secondary invaders or co-pathogens alongside the primary viral agents.

Understanding how cat flu spreads is essential for prevention, particularly in households with multiple cats or in environments like shelters where new cats are constantly being introduced.

Routes of Transmission

• Direct contact: The most common route. Infected cats shed virus in nasal and ocular secretions, saliva, and (for FCV) sometimes faeces. Close contact — nose-to-nose greeting, mutual grooming, sneezing — transmits the virus efficiently.
• Fomites (contaminated objects): Both FHV-1 and FCV can survive on surfaces, though FCV is significantly more environmentally resistant. FCV can survive on dry surfaces for up to 28 days at room temperature, while FHV-1 survives for only 18–24 hours. Shared food bowls, bedding, litter trays, and human hands can all serve as fomites.
• Aerosol spread: Sneezing can project infected droplets up to 1.5 metres, though this route is less efficient than direct contact.
• Carrier cats: Asymptomatic carriers are a major source of environmental contamination, particularly for FCV, where carriers can shed virus continuously for months or years.

High-Risk Populations

While any cat can contract cat flu, certain groups are disproportionately affected:

• Kittens (under 12 weeks): Most vulnerable due to immature immune systems. Maternal antibodies provide some protection in the first weeks of life, but a gap often exists between waning maternal antibodies and vaccine-induced immunity.
• Elderly and immunocompromised cats: Including those with FeLV or FIV.
• Unvaccinated cats: Vaccination does not prevent infection entirely but significantly reduces the severity and duration of illness.
• Shelter and cattery cats: Stress, overcrowding, and constant exposure to new cats create ideal conditions for cat flu outbreaks.
• Flat-faced (brachycephalic) breeds: Persians and similar breeds may experience more severe upper respiratory signs due to their conformation.

Cat flu symptoms typically appear 2–10 days after exposure and can vary in severity from mild sniffles to severe illness. The clinical presentation differs somewhat depending on whether herpesvirus or calicivirus is the primary agent.

Common Symptoms of Both FHV-1 and FCV

• Sneezing: Often the first sign noticed — frequent bouts of sneezing, sometimes producing nasal discharge.
• Nasal discharge: Initially clear and watery, becoming thick and purulent (yellow-green) if secondary bacterial infection develops.
• Reduced appetite: Cats rely heavily on their sense of smell to stimulate appetite. Nasal congestion can dramatically reduce food intake.
• Lethargy and fever: Particularly in the early acute phase.
• Drooling: Especially if mouth ulcers are present.

Signs More Typical of Feline Herpesvirus (FHV-1)

• Severe conjunctivitis: Red, swollen, watery eyes, often with the third eyelid (nictitating membrane) protruding. Discharge may become purulent.
• Corneal ulcers: FHV-1 can cause painful erosions on the cornea, visible as cloudiness or a film over the eye. These are a medical emergency and can lead to permanent eye damage if untreated.
• Severe nasal congestion: FHV-1 tends to cause more intense nasal signs than FCV.

Signs More Typical of Feline Calicivirus (FCV)

• Oral ulcers: Painful ulcers on the tongue, gums, lips, or hard palate are a hallmark of FCV infection. These cause significant discomfort and often result in drooling and refusal to eat.
• Limping syndrome: Some strains of FCV cause transient polyarthritis (joint inflammation), particularly in kittens. Affected kittens develop a shifting lameness that resolves within days to weeks.
• Milder nasal and eye signs: Compared to FHV-1, calicivirus often produces less dramatic upper respiratory signs.

If your cat is showing any of these symptoms, don't hesitate to consult a veterinarian. Our vet visit scheduling guide can help you determine when routine and urgent visits are needed.

There is no specific antiviral cure for cat flu — treatment is primarily supportive, aimed at managing symptoms, preventing secondary complications, and keeping the cat comfortable while the immune system fights the infection. Most healthy adult cats recover within 7–14 days, though some may have lingering signs for several weeks.

Veterinary Treatments

• Antibiotics: While antibiotics do not treat the underlying viral infection, they are commonly prescribed to prevent or treat secondary bacterial infections that complicate cat flu. Doxycycline is often the first choice, as it is also effective against Chlamydophila and Mycoplasma.
• Antiviral medication: For FHV-1, the antiviral famciclovir has shown efficacy and is increasingly used in moderate-to-severe cases. It is given orally and is generally well tolerated. Topical antiviral eye drops (idoxuridine or cidofovir) may be prescribed for herpesvirus-related corneal ulcers.
• Anti-inflammatory pain relief: Non-steroidal anti-inflammatories (NSAIDs) formulated for cats (such as meloxicam) can reduce fever and oral pain, improving appetite.
• Eye medications: Topical antibacterial drops for conjunctivitis, lubricating drops for comfort, and antiviral drops if corneal ulceration is present.
• Fluid therapy: Cats that have stopped eating and drinking may become dehydrated. Subcutaneous or intravenous fluid therapy may be needed in severe cases.
• Nutritional support: Cats that refuse food for more than 48–72 hours may need appetite stimulants (such as mirtazapine) or, in extreme cases, feeding tube placement.

Home Care Essentials

• Keep the nose and eyes clean: Gently wipe away discharge with a soft, warm, damp cloth several times daily. This helps the cat breathe and smell food.
• Steam therapy: Place the cat in a bathroom with a hot shower running (not in the shower) for 10–15 minutes to help loosen nasal congestion.
• Warming food: Gently warming wet food enhances its aroma, encouraging cats with diminished smell to eat. Strong-smelling foods like sardines or roast chicken can help.
• Quiet, warm environment: Provide a warm, draft-free recovery space away from other cats to reduce stress and prevent transmission.

One of the most challenging aspects of cat flu is that many cats become chronic carriers after their initial infection, experiencing recurrent flare-ups throughout their lives. Understanding this aspect of the disease is crucial for long-term management.

FHV-1 Carriers

Approximately 80% of cats that recover from FHV-1 become lifelong carriers. The virus lies dormant in nerve cells and can reactivate when the cat is stressed — such as during boarding, moving house, the introduction of a new pet, or concurrent illness. Reactivation episodes typically manifest as mild-to-moderate upper respiratory signs and conjunctivitis, lasting 7–14 days. Some cats experience chronic nasal discharge and sneezing that never fully resolves, a condition sometimes called "chronic rhinosinusitis."

Long-term management strategies for FHV-1 carriers include:

• L-lysine supplementation: Historically recommended to reduce herpesvirus replication, though recent evidence suggests its efficacy is questionable. Discuss with your vet whether this is worth continuing.
• Feline interferon omega: Where available, this can help modulate the immune response during flare-ups.
• Stress minimisation: Pheromone diffusers, consistent routines, and avoiding known stressors are the most effective strategies for reducing flare-up frequency.
• Antiviral therapy during flare-ups: Famciclovir can shorten and reduce the severity of recurrent episodes.

FCV Carriers

Unlike herpesvirus, FCV does not establish latent infection in nerve cells. Instead, it persists in the oral tissues. Many cats shed FCV continuously for months to years after recovery, acting as a constant source of infection for other cats. Some cats eventually clear the virus, while others remain carriers indefinitely. FCV carriage is generally asymptomatic, though some carrier cats develop chronic stomatitis (painful inflammation of the mouth) that can be difficult to manage. In multi-cat households, understanding carrier status is important — see our cat vaccination schedule guide for recommendations on protecting all cats in the home.

Vaccination is the cornerstone of cat flu prevention and is considered a core vaccination for all cats, regardless of lifestyle. However, it is important to set realistic expectations about what vaccination can and cannot achieve.

What the Vaccine Does

Cat flu vaccines (typically combined FHV-1/FCV/panleukopenia in a single injection, often called FVRCP or F3) do not prevent infection entirely. What they do is:

• Significantly reduce the severity of clinical signs
• Shorten the duration of illness
• Reduce the amount of virus shed, lowering transmission risk
• Prevent life-threatening illness in the vast majority of cases

Vaccination Schedule

Kittens should receive their first cat flu vaccination at 6–8 weeks of age, with boosters every 3–4 weeks until 16 weeks of age (typically 3 doses). A booster is given at 12 months, then every 1–3 years depending on risk factors and the vaccine used. Indoor-only cats in single-cat households may be vaccinated less frequently than cats in multi-cat households or those that go outdoors. For a complete guide, see our cat vaccination schedule.

Environmental Prevention

For multi-cat environments, shelters, and catteries, environmental control is critical:

• Quarantine new arrivals: Isolate new cats for 7–14 days and monitor for signs of respiratory illness before introduction to the group.
• Disinfection: FCV is resistant to many common disinfectants. Accelerated hydrogen peroxide (such as Rescue/Accel) or dilute bleach (1:32) are effective against both FHV-1 and FCV. Clean food bowls, litter trays, and bedding daily.
• Ventilation: Good airflow reduces aerosol concentration of viral particles.
• Reduce overcrowding: Stress from overcrowding is the single greatest risk factor for cat flu outbreaks in group settings.
• Hygiene between handling cats: Wash hands and change clothing between handling sick and healthy cats.