What Is a Cat Upper Respiratory Infection?
A cat upper respiratory infection (URI), often called cat flu, is not a single disease but a complex of infections affecting the nose, throat, and sinuses. Despite the name, it is not the same as human influenza. Cat flu is caused by several different pathogens working alone or in combination, making it one of the most common illnesses seen in cats worldwide.
The condition is highly contagious between cats and spreads easily through direct contact, shared food bowls, grooming, and even airborne droplets from sneezing. Multi-cat households, rescue shelters, and catteries are the environments where cat flu spreads most rapidly.
Main Causes of Cat Flu
Several pathogens are responsible for feline upper respiratory infections, and identifying them helps guide treatment decisions.
Feline Herpesvirus-1 (FHV-1)
Feline Herpesvirus-1 is the most common cause of cat flu and is responsible for the majority of severe cases. A crucial characteristic of FHV-1 is that it causes a lifelong latent infection. Once a cat is infected, the virus remains dormant within the nervous system for the rest of the cat's life. It can reactivate during periods of stress, illness, or immunosuppression, causing recurrent bouts of symptoms. This reactivation is called recrudescence, and it is one reason why so many adult cats experience periodic respiratory flare-ups throughout their lives.
Feline Calicivirus (FCV)
Feline Calicivirus is the second most common cause. Unlike herpesvirus, FCV is particularly associated with painful oral ulcers on the tongue, lips, and palate. It can also cause joint pain and lameness in some strains. Certain virulent systemic strains of FCV can cause severe multi-organ disease, though these are less common.
Bacterial Pathogens
Bordetella bronchiseptica and Chlamydophila felis are bacterial agents that can cause or complicate respiratory infections. Chlamydophila felis is particularly associated with persistent conjunctivitis and responds well to antibiotic treatment. Secondary bacterial infections frequently develop on top of viral infections, which is why nasal discharge often changes from clear to yellow or green as the illness progresses.
Recognising the Symptoms
The signs of cat flu can range from mild to severe depending on the pathogen involved, the cat's age, and their vaccination status.
- Sneezing, often in repeated bouts
- Nasal discharge that starts clear and becomes thick and yellow or green with secondary bacterial infection
- Eye discharge and conjunctivitis, with red, swollen, or weeping eyes
- Oral ulcers on the tongue or lips, particularly with calicivirus
- Lethargy and general weakness
- Fever
- Loss of appetite, often because the cat cannot smell food through a blocked nose
- Difficulty breathing through the nostrils when severely congested
Which Cats Are Most at Risk?
Kittens are particularly vulnerable because their immune systems are not yet fully developed. Unvaccinated cats of any age face a much higher risk of severe illness. Cats living in shelters or multi-cat households are constantly exposed to other cats and have limited ability to avoid contact with infected animals. Elderly cats or those with underlying health conditions are also at heightened risk of developing serious complications.
The Herpesvirus Carrier Problem
Most cats that recover from a herpesvirus infection become lifelong carriers. During periods of stress — such as moving home, a new pet joining the household, boarding, or even routine veterinary visits — the virus can reactivate and cause symptoms again. Managing these cats long-term involves minimising stress where possible and considering L-lysine supplementation. L-lysine is an amino acid that may help reduce the frequency and severity of herpes flare-ups by interfering with viral replication, though its effectiveness varies between individuals. Reducing stressful events and maintaining consistent routines helps carriers stay symptom-free for longer periods.
Vaccination and Prevention
Vaccination is the single most important preventive measure. The core cat vaccine is a trivalent preparation covering Feline Herpesvirus-1, Feline Calicivirus, and Feline Panleukopenia Virus (FHV-1, FCV, FPV). This is often referred to as the FVRCP vaccine. Vaccination does not prevent infection entirely but significantly reduces the severity of illness and the likelihood of serious complications. Kittens require a primary course of vaccinations followed by booster doses throughout their lives.
Reducing overcrowding in multi-cat households and shelters, disinfecting shared equipment regularly, and isolating new or sick cats are all important preventive measures alongside vaccination.
Treatment Options
Treatment for cat flu is largely supportive, aimed at managing symptoms and preventing complications while the cat's immune system fights the infection.
Supportive Home Care
Keeping the nostrils clear is important for comfort and appetite. Saline nasal drops can loosen discharge, and placing the cat in a warm, steamy bathroom for short periods can help relieve congestion. A warm, draught-free environment aids recovery. Appetite stimulation is critical — cats that cannot smell often refuse to eat, and anorexia can lead to serious complications including fatty liver disease. Warming wet food, offering strong-smelling options such as rotisserie chicken, or trying fish-based foods can help. Hydration is equally important. Eye drops or ointment prescribed by a vet can manage conjunctivitis and help prevent corneal ulcers developing with herpesvirus eye involvement.
Medications
When secondary bacterial infections develop, antibiotics are required. Doxycycline is the most commonly prescribed antibiotic for cat flu because it is effective against both Bordetella bronchiseptica and Chlamydophila felis. For severe or recurrent herpesvirus infections, the antiviral drug famciclovir has shown real benefit in cats and can significantly reduce the severity and duration of FHV-1 symptoms.
When to See a Vet Urgently
Some situations require prompt veterinary attention rather than home management alone.
- A cat that has not eaten for 24 hours or more needs veterinary assessment
- Laboured or open-mouth breathing indicates significant respiratory compromise
- Any kitten showing respiratory symptoms should be seen quickly as they can deteriorate rapidly
- Eye ulcers, particularly corneal ulceration which is a known complication of herpesvirus eye infections, require urgent treatment to prevent permanent damage or blindness
Caring for a Cat with Cat Flu at Home
Rest, warmth, and gentle nursing make a significant difference to recovery speed. Keep the cat away from draughts and other cats to prevent spread. Wipe nasal and eye discharge gently with damp cotton wool several times a day. Encourage eating little and often rather than expecting the cat to finish a full bowl. Monitor for any worsening of symptoms and maintain close contact with your vet throughout the illness. Most cats with uncomplicated cat flu recover fully within one to two weeks with good supportive care.