The two main causes are feline herpesvirus (FHV) (formerly known as feline rhinotracheitis virus) and feline calicivirus (FCV). They are both widespread across the world. They not only affect domestic cats, but also other members of the cat family. Different strains of FCV vary widely in their ability to cause disease, the severity and the symptoms they exhibit. Recently some very virulent strains of FCV have been seen which affect the whole body and may lead to death.
The cat flu viruses are spread very readily in air droplets when a cat sneezes, and also in eye and nasal discharges. This may be either direct from the infected cat, via a person’s clothing, or wherever the cat has rubbed it’s face. Most cats that recover will become carriers and shed virus after they have stopped showing symptoms. These cats are then a potential source of infection for other cats. FCV virus is shed continuously for a variable time after recovery. On the other hand, cats with FHV remain carriers for life, but shedding is intermittent and generally associated with periods of stress.
Although the viruses are fairly readily killed by disinfectants, they may remain active in discharges for up to a week. They can spread through a colony of cats very rapidly, and are of concern in catteries and rescue centres, where they can be very difficult to eradicate.