Cat flu in multi-cat homes: how it spreads and why kittens get it worst

A cat sneezing in the corner of a shelter can infect every kitten in the room before the day is out. Cat flu is not one disease but a cluster of upper respiratory infections caused mainly by two pathogens: feline herpesvirus type 1 (FHV-1) and feline calicivirus (FCV). Both spread easily, both persist in the environment longer than most owners expect, and both hit kittens disproportionately hard. Understanding the mechanics of that spread is the difference between containing an outbreak and watching it move through an entire household.

FHV-1 and FCV together account for the large majority of cat flu cases. FHV-1 causes the classic picture: watery eyes, nasal discharge, sneezing, and sometimes corneal ulcers. Like all herpesviruses, it establishes lifelong latency after the first infection. A cat that recovered months ago can shed the virus again during a stressful event, a new pet arriving, a move, a vet visit, without showing obvious symptoms itself. FCV tends to cause more oral ulcers and sometimes joint pain; certain virulent strains can cause severe systemic disease, though this is uncommon. Secondary bacterial infections from organisms such as Bordetella bronchiseptica and Chlamydophila felis complicate both, turning a manageable respiratory illness into something that needs antibiotic support. In a multi-cat home, the cat that looks merely a little off may be seeding the environment continuously.

Both viruses spread through direct contact with discharge from the eyes, nose, and mouth. Shared food bowls, water bowls, and grooming between cats are the most efficient routes. Droplets from a sneeze travel roughly a metre, which in a flat where cats sleep on the same sofa means almost every surface is a potential fomite. FHV-1 survives on surfaces for up to eighteen hours in a cool, humid environment; FCV is considerably tougher and can survive for weeks on hard surfaces. An owner who handles a sick cat and then touches a healthy one without washing hands is a vector. Bedding, litter scoops, and shared toys carry both viruses. In a shelter, the airflow between adjacent cages does the rest. The cats most at risk in a multi-cat home are those who are already stressed, immunocompromised, or unvaccinated, and kittens, for reasons that go deeper than simply being small.

A kitten under twelve weeks old has a partially functional immune system. It relies heavily on maternal antibodies received through the queen's colostrum in the first day or two of life. Those antibodies decline sharply after six to eight weeks, leaving a window where the kitten has lost its mother's protection but has not yet built its own. This is precisely the age at which most kittens enter shelters or new homes. FHV-1 in a kitten can progress from sneezing to severe conjunctivitis, corneal scarring, and pneumonia faster than in an adult. Dehydration sets in quickly because a kitten with a blocked nose will stop eating, it cannot smell its food. FCV in young kittens occasionally causes a limping syndrome alongside the oral ulcers, and the systemic load on a small body is proportionally far greater. A kitten that recovers from FHV-1 carries the virus for life, with periodic reactivation. This is why a kitten adopted from a shelter may seem fine for weeks, then begin sneezing after the stress of settling in.

Isolation is the first and most effective tool. A sick cat needs a separate room with its own food bowl, water bowl, litter tray, and bedding. The owner changes clothes or washes hands before moving between rooms. Surfaces in the sick cat's space should be cleaned with a disinfectant that is effective against non-enveloped viruses, FCV is a non-enveloped virus and survives products that kill FHV-1 easily, so a broad-spectrum disinfectant confirmed safe for use around cats matters here. Vaccination does not prevent infection entirely but significantly reduces severity and the duration of shedding. The WSAVA vaccination guidelines list FHV-1 and FCV vaccines as core vaccines for cats globally, meaning every cat should receive them regardless of lifestyle. In a shelter context, vaccination on intake and strict isolation protocols for new arrivals and symptomatic cats are the standard of care. For a flat with two or three cats, the practical reality is that full isolation is hard; keeping the sick cat in one room and limiting shared contact is still worth doing even if it is imperfect.

Most adult cats with mild cat flu, sneezing, clear discharge, eating and drinking normally, can be monitored at home for forty-eight hours. The situation changes when a cat stops eating for more than a day, when discharge turns thick and coloured, when breathing becomes laboured or audible, when the eyes show cloudiness or ulceration, or when a kitten of any age is involved. Kittens deteriorate faster than adults and can become dangerously dehydrated within hours of stopping feeds. Any kitten under twelve weeks with respiratory symptoms warrants a same-day vet call, not a wait-and-see approach. A cat that has recovered from previous bouts but is now showing corneal changes needs an ophthalmology-aware vet, because FHV-1 can cause progressive eye damage that is manageable if caught early. Cat flu is common, but common does not mean it always resolves without help.