Feline upper and lower respiratory tract infection is the most prevalent and difficult to manage infectious problem in cat populations. The five most important pathogens in feline upper respiratory infection (URI) (in increasing order of importance) are Bordetella bronchiseptica, Chlamydophila felis, Mycoplasma spp., feline calicivirus (FCV) and feline herpesvirus (FHV).

Bordetella bronchiseptica is a non-enteric gram-negative bacterium that can cause mild infections, especially if the cat is co-infected with FCV or FHV. The bacterium can survive in moist environments for weeks. Cats may develop mild conjunctivitis and oculonasal discharge or occasionally pneumonia.

Feline calicivirus is an RNA virus with high rates of mutation and antigenic and genetic diversity. The infection is transmitted by aerosol via the oral and nasal routes. Respiratory shedding occurs for 10 days to 2 weeks; commonly in shelters and in kittens, shedding can occur for months. Calicivirus infection causes conjunctivitis and rhinitis with serous ocular discharge, vesicular stomatitis/faucitis (aphthous stomatitis), pneumonia, occasional fever and limping, and immune complex polyarthritis and gingivitis. Recently, a very pathogenic series of caliciviral infections were reported, where cats developed facial and limb oedema with vasculitis secondary to calicivirus.

Feline herpesvirus is an enveloped DNA virus of cats. Kittens commonly acquire the infection shortly after birth because parturition efficiently induces recrudescence of the virus in the queen. In older cats, transmission of FHV requires intimate contact among cats. Following infection, the virus persists in the trigeminal nerve and can periodically be reactivated to cause classical URI clinical signs, especially during stressful periods or if the cat becomes immunosuppressed. Disease signs may include conjunctivitis, anterior uveitis, serous ocular discharge with secondary bacterial infection and mucopurulent discharge, or keratitis with dendritic ulceration. Ulcerative and necrotising nasal dermatitis resembling shingles has been described in some cats with herpes.

Occasionally, URI signs in cats are due to infection with C. felis, an obligate intracellular bacterium. This feline pathogen resides in conjunctival and genital mucosa (and possibly gastrointestinal). Chlamydial disease often is acquired shortly after weaning. The typical sign is conjunctivitis, which is often unilateral.

Mycoplasma spp. are degenerate, obligate parasitic bacteria with no cell wall, reduced genomes and minimal ultra structure. The reservoir for mycoplasmas is respiratory and genital mucosa. Transmission is via aerosol. Mycoplasmas in cats can cause URI, conjunctivitis and arthritis. However, mycoplasmas also are commonly recovered from well cats.

Making a Clinical Rule-Out

A cat with some or all of: conjunctivitis, anterior uveitis, ocular or nasal discharge, gingivitis, faucitis, stomatitis, glossitis, fever and lymphadenomegaly has URI.

 Vesicular stomatitis and faucitis cat probably has FCV

 Lameness and joint pain cat probably has FCV or Mycoplasma

 Greenish coloured or tenacious ocular or nasal discharge cat has bacterial contamination in the site, but this does not help determine the underlying pathogen

 Cough cat may have bronchitis (allergic or infectious) or pneumonia; far less commonly a cat with a cough may have 'kennel cough', i.e., Bordetella bronchiseptica tracheitis

 Keratitis and corneal ulceration cat probably has FHV

 Chronic sinusitis often with turbinate destruction cat probably has FHV or FCV

 Fever cat probably has FHV or FCV

Diagnosis and Management

Usually diagnosis in URI is based on clinical presentation, using the rule-out list above. Tests one can run include cytology of conjunctival cells, bacterial, viral and mycoplasmal culture, and polymerase chain reaction (PCR). The best way to manage URI is to minimise its impact. Infected and exposed cats may have recurring disease and there is little specific treatment available. Individual cats with URI should receive nursing care and fluids if needed, and be encouraged to eat. Chronically symptomatic cats should be managed for the sequelae of virus infection: secondary bacterial infection, gingivitis, inappetence, etc. Reducing stress helps reduce the risk of recrudescence of chronic or latent disease. Unfortunately, some of the modified live vaccines will induce mild to moderate URI which is clinically indistinguishable from disease caused by 'field strain' pathogens. Commercial vaccines are available for feline bordetellosis, calicivirus, herpesvirus and C. felis. Pregnant queens should be vaccinated only with killed virus vaccines. Some veterinarians use partial doses of intranasal (high antigen mass) vaccines to protect kittens as young as 1-2 weeks old.

It is inappropriate to treat all affected cats with antibiotics, as most are infected with viruses, antibiotics can destroy normal flora, increase susceptibility to further infection and promote antibiotic resistance. Antibiotics should be reserved for cases where there is green purulent discharge, where there is a concern of sepsis, or where there is a suspicion of Clamydophila, Mycoplasma or B. bronchiseptica. Good broad-spectrum drugs with activity against pathogenic staphylococci are cefalexin or amoxicillin- clavulanate. For possible sepsis, drugs with excellent broad and enteric spectra should be included, such as amoxicillin-clavulanate or ticarcillin-clavulanate, enrofloxacin or a penicillin with an aminoglycoside. The drug of choice for B. bronchiseptica, Mycoplasma spp., and C. felis is doxycycline, although enrofloxacin and azithromycin are useful for mycoplasmas and C. felis, while trimethoprim-sulfamethoxazole is good for B. bronchiseptica. B. bronchiseptica isolates often have multiple drug resistance. Cats with suspected C. felis or Mycoplasma spp. infection may be treated with topical ophthalmic antibiotics, and usually improve clinically within a few days.

There are no specific drug treatments for calicivirus, but several drugs may mitigate infection with herpesvirus, including idoxuridine, trifluridine and vidarabine, all given topically five to seven times a day. Aciclovir is an excellent systemically active drug against human herpesvirus infection, but is relatively ineffective against feline herpesvirus.

Lysine sometimes is used to treat human and feline herpes, with the justification that it will antagonise utilisation of arginine in herpes viral protein synthesis (dietary arginine must be restricted). However, restriction of arginine is not safe for cats. Nevertheless some clinicians report that lysine treatment improves the clinical status of some cats with herpes. Alpha-interferon has been used with some success in feline herpes but cats quickly become resistant to the recombinant human protein.

URI agents are usually susceptible to many commonly used disinfectants, but the main reservoirs for URI are the other cats.