What Is Cat Upper Respiratory Infection?
Cat upper respiratory infection (URI) — colloquially called cat flu — is an infectious illness affecting the nose, throat, sinuses, and eyes of cats. It is not caused by the same viruses that cause influenza in humans and cannot be passed to people, but the term has stuck because the clinical signs — sneezing, runny nose, watery eyes, and general malaise — mirror those of a human cold or flu.
The condition is extremely common, particularly in kittens, unvaccinated cats, and cats living in multi-cat households or rescue shelters. In otherwise healthy adult cats, most episodes are self-limiting, but the illness can be serious and even life-threatening in very young kittens, elderly cats, and immunocompromised individuals.
The Two Main Causes: FHV-1 and FCV
While several pathogens can contribute to feline upper respiratory infection — including Chlamydophila felis and Bordetella bronchiseptica — two viruses are responsible for the overwhelming majority of cases.
Feline Herpesvirus Type 1 (FHV-1)
Feline herpesvirus type 1, also known as feline rhinotracheitis virus, is a highly contagious virus that causes acute upper respiratory signs including sneezing, nasal discharge, conjunctivitis, and corneal ulceration. In severe acute infections, particularly in kittens, FHV-1 can cause significant corneal damage and even permanent eye injury.
One of the most clinically important features of FHV-1 is its ability to establish lifelong latency. After the initial infection resolves, the virus does not leave the body. Instead, it retreats into nervous tissue — particularly the trigeminal ganglion — where it remains dormant. When the cat experiences a period of stress, such as rehoming, the introduction of a new pet, illness, or anaesthesia, the virus can reactivate and the cat develops clinical signs again. This pattern of latency and stress-induced reactivation means that FHV-1 carriers can experience recurrent episodes of sneezing, eye discharge, and nasal congestion throughout their lives.
Feline Calicivirus (FCV)
Feline calicivirus is the other major cause of cat flu and tends to produce slightly different clinical signs from FHV-1. While respiratory signs do occur, FCV is particularly associated with painful oral ulceration — sores on the tongue, gums, hard palate, and lips that can make eating very painful. Affected cats may drool excessively, paw at their mouth, or refuse food due to the discomfort of swallowing.
Unlike FHV-1, FCV does not typically establish lifelong latency, but some cats become persistent carriers that shed the virus continuously without always showing obvious signs of illness themselves. These carrier cats are an important source of infection in multi-cat environments. A small number of FCV strains are associated with a severe systemic illness known as virulent systemic FCV, which can cause fever, oedema, and high mortality, though this form is uncommon.
The Role of L-Lysine: A Controversial Supplement
For many years, L-lysine supplementation was widely recommended as a supportive measure for cats with FHV-1, particularly during periods of stress or recurrence. The theory was that lysine competes with arginine — an amino acid that the herpesvirus requires to replicate — thereby slowing viral multiplication.
However, the evidence base for L-lysine in cats has come under serious scrutiny. A comprehensive review published under the auspices of the World Small Animal Veterinary Association (WSAVA) concluded that there is no convincing clinical evidence that L-lysine supplementation is effective in preventing or reducing the severity of FHV-1 infection in cats. The review noted that the mechanism does not translate reliably from in vitro studies to living animals, and that high supplementation could potentially be harmful. As a result, WSAVA no longer recommends L-lysine as a routine supplement for cats with herpesvirus, and many veterinary dermatologists and internal medicine specialists have followed suit. If your vet previously recommended it, it is worth discussing the current evidence at your next appointment.
Shelters as High-Risk Environments
Rescue shelters and catteries represent the highest-risk environment for feline upper respiratory infection. The combination of many cats from unknown health backgrounds living in close proximity, the stress of rehoming and unfamiliar surroundings (a potent trigger for FHV-1 reactivation), and the practical difficulty of maintaining strict biosecurity creates ideal conditions for rapid spread.
Reputable shelters vaccinate incoming cats as early as safely possible, isolate new arrivals, and separate sick cats into dedicated hospital areas. Despite these precautions, URI remains one of the most common reasons for veterinary attention in shelter populations. If you adopt a cat from a rescue, ask about its health history and be aware that signs of cat flu may emerge in the first few weeks at home as the stress of moving triggers FHV-1 reactivation.
Supportive Care: What Helps a Cat with URI
Most cases of feline upper respiratory infection are managed with supportive care rather than specific antiviral treatment, as there are no licensed antiviral drugs widely available for routine use in cats with FHV-1 or FCV in the UK.
Nebulisation
Nebulisation — the use of a device that creates a fine mist of saline or medication — can be very effective in loosening thick nasal secretions and helping congested cats breathe more easily. It is particularly useful in kittens and cats with chronic nasal discharge. Some cats can be treated at home using a nebuliser in an enclosed space such as a cat carrier, or simply by placing them in a steamy bathroom for short periods to benefit from the moisture in the air.
Appetite Stimulants and Nutritional Support
A congested or painful cat often loses interest in food, partly because it cannot smell what is being offered and partly because eating may be uncomfortable. Maintaining adequate nutrition is critical, especially in young or debilitated cats. Warming food slightly to enhance its aroma, offering strongly scented foods, and trying a variety of textures can help encourage eating. In cats that are significantly anorexic, a vet may prescribe mirtazapine, a licensed appetite stimulant available in tablet or transdermal gel form, to encourage eating while the cat recovers.
Eye drops or ointments are often prescribed for cats with significant conjunctivitis or corneal involvement. Keeping the face clean by gently wiping away discharge with damp cotton wool reduces discomfort and helps prevent secondary skin irritation. Secondary bacterial infection is common in cats with URI, and antibiotics are frequently prescribed to address this even though they do not target the primary viral cause.
Chronic FCV and Oral Ulceration
In some cats, particularly those that become persistent carriers of feline calicivirus, chronic oral ulceration becomes a long-term management challenge. The gums may be persistently red and inflamed (gingivostomatitis), and eating can remain painful for months or years. In severe cases, extraction of some or all teeth — while it sounds drastic — can provide significant relief, as removing the dental surfaces on which calicivirus antigen accumulates can dramatically reduce oral inflammation. Cats that have had full mouth extractions often eat far more comfortably than they did before the procedure.
If your cat has been diagnosed with FCV-associated stomatitis, a referral to a veterinary dental specialist is worth considering, as this is a complex condition that benefits from specialist input.