Feline Respiratory Disease Complex (Feline Viral Rhinotracheitis, Feline Calicivirus)

Feline respiratory disease complex is a set of respiratory illnesses caused by a group of organisms that can cause infection alone or together. It includes those illnesses with signs including inflammation of the nasal and sinus linings, inflammation of the eye lining (conjunctivitis), excessive tear production, salivation, and mouth sores. The principal diseases are feline viral rhinotracheitis (also called feline herpesvirus type 1) and feline calicivirus, although other diseases may also be involved ( see Table: Organisms Found as Part of Feline Respiratory Disease Complex).

The majority of feline upper respiratory infections are caused by feline viral rhinotracheitis, although the incidence of feline calicivirus may be higher in some populations of cats. Infection with both these viruses at once may occur. Natural transmission of these agents occurs through small droplets in the air (such as from a sneeze) and contaminated objects, which can be carried to a susceptible cat by a handler. Recovering cats may spread the virus for many months. Stress may trigger a relapse.

The onset of feline viral rhinotracheitis is marked by fever, frequent sneezing, inflamed eyes (conjunctivitis), inflammation of the lining of the nose (rhinitis), and often salivation. Excitement or movement may cause sneezing. The fever may reach 105°F (40.5°C) but subsides and then may come and go. Initially, the disease causes a clear discharge from the nose and eyes; it soon increases in amount and contains mucus and pus. At this point, depression and loss of appetite become evident. Severely affected cats may develop mouth inflammation with sores, and inflammation of the cornea occurs in some cats.

Signs may persist for 5 to 10 days in milder cases and up to 6 weeks in severe cases. The outlook is generally good except for young kittens and older cats. When the illness is prolonged, weight loss may be severe. Bacteria often infect cats that are already ill with feline viral rhinotracheitis. After the signs resolve, the virus usually remains in an inactive state in the cat's body for the rest of its life. The virus can reactivate and cause flares of infection, especially during times of stress.

Feline calicivirus most often affects the lining of the mouth and the lungs. There are many related strains of feline caliciviruses. Some caliciviruses cause few or no signs. Some strains cause sores in the mouth, while others produce fluid buildup in the lungs (pulmonary edema) and pneumonia. It is often impossible to distinguish feline viral rhinotracheitis from feline calicivirus infection. Two feline calicivirus strains may produce a transient “limping syndrome” without mouth sores or pneumonia. These strains produce a short fever, leg lameness, and pain on handling of affected joints. Signs occur most often in 8- to 12-week-old kittens and usually improve without treatment. The syndrome may occur even in kittens that have been vaccinated against feline calicivirus, because no vaccine protects against both of the particular strains that produce the “limping syndrome.”

Calicivirus has also been found in cats with inflammation of the gums and mouth. The lesions heal rapidly, and the infected cat regains appetite 2 to 3 days after onset, although the course of disease may last 7 to 10 days. Fever, poor appetite, and depression are common signs. Nasal and eye inflammation also can occur.

The veterinarian’s initial diagnosis is based on the typical signs as described above. These characteristics may be difficult to sort out when more than one infection is present. A definite diagnosis is based on isolation and identification of the organism through appropriate tests and microscopic examination of samples from the oral and nasal mucous membranes, nostrils, or conjunctival sacs. However, diagnosis of feline viral rhinotracheitis may be difficult because the virus is shed only periodically, and because even cats without symptoms can show presence of the virus.