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Understanding a cat's cough
Internal medicine experts weigh in on feline asthma, influenza, bacterial infections, heartworm


DVM360 MAGAZINE


More causes to consider

There are more likely causes for an acute onset of respiratory signs in cats (e.g., herpesvirus, calicivirus, bacterial pneumonia), but veterinarians should be vigilant to the possibility of influenza infection. Viral isolation or RT-PCR from oropharyngeal or rectal swabs or necropsy specimens is the typical method of confirmation. Immunohistochemistry can be used on infected organs. Serologic diagnosis using hemagglutination inhibition is also possible. Treatment would be largely supportive.

Chronic rhinosinusitis in cats is a common and frustrating condition. Often, veterinarians reach for antibiotics to treat this condition without pursuing a specific diagnosis in a belief that all or nearly all rhinosinusitis is the result of persistent or recurrent viral infection with secondary bacterial infection. However, many nonviral disease conditions may result in similar clinical signs of chronic nasal discharge, including nasal neoplasia (carcinoma and lymphoma), inflammatory rhinitis, mycotic rhinitis (e.g., cryptococcosis, aspergillosis), dental disease, nasal polyps and foreign bodies.

In certain geographic regions, lung parasites account for a substantial proportion of respiratory disease in cats. Important parasitic respiratory pathogens include Paragonimus kellicotti, Aelurostrongylus abstrusus and Eucoleus aerophilus (previously known as Capillaria aerophilia). A common misconception holds that these parasites can be ruled out through fecal flotation. Pulmonary parasites are shed through the feces only on an intermittent basis. Immature pulmonary parasites (ova or larvae) must be expectorated and swallowed prior to fecal shedding. For this reason, shedding is only intermittent at best. Even when using the most sensitive, most appropriate fecal examination techniques, intermittent shedding means that infections may be missed from a single sample. Therefore, it is recommended that fresh feces be examined on at least three different occasions before ruling out lung parasites as a cause of respiratory disease. Alternatively, a course of anthelmintic sufficient for the treatment of lung parasites can be employed as a therapeutic trial.

Acute upper respiratory infections (URIs) are common in cats. Traditionally, amoxicillin has been used to treat these infections. It is inexpensive, safe and available in convenient dosage strengths, and it has activity against most gram-positive and anaerobic pathogens. Recently, many veterinarians have switched from these traditional treatments with amoxicillin or doxycycline to azithromycin. Azithromycin has a long tissue half-life in cats, allowing it to be dosed infrequently. The drug would be expected to be efficacious for treating mycoplasmosis as well as infection with many gram-positive and gram-negative organisms. Despite the fact that some infections would be predicted to not respond to amoxicillin based on in vitro culture results, there was no difference in outcome between cats treated with amoxicillin and those treated with azithromycin. Azithromycin remains a reasonable antimicrobial choice for treating URIs in cats. However, it is considerably more expensive than some more traditionally used medications and may not offer a substantial advantage in regard to treatment efficacy.

Veterinarians have been taught that pyothorax follows cat fight wounds. This seemed to fit with the predominance of infections caused by mixed bacterial populations of anaerobes and facultative anaerobes plus oral flora. Recently, it has been proposed that infections are more likely the sequelae of parapneumonic spread of infection after colonization and invasion of lung tissue by oropharyngeal flora related to upper respiratory infection.

Unfortunately, a variety of both bacterial and viral organisms can be found as normal flora in healthy cats, after recovery from infection of previously ill cats, or as pathogens involved in current infections. Simply finding an organism doesn't prove cause and effect. This is true even if the organism is a known pathogen, such as feline herpesvirus 1 (FHV-1). Cats infected with FHV-1 remain latently infected long after clinical recovery from any illness; cats previously vaccinated for FHV-1 may be infected but never develop any clinical illness yet remain latently infected. Therefore, FHV-1 may be identified in either healthy cats or in cats with upper respiratory signs due to some altogether different cause. To even make matters more complicated, commonly used assays such as PCR cannot distinguish naturally occurring virulent FHV-1 infection from modified-live vaccine strains.

A similar problem occurs in relation to calicivirus. Cats may be healthy carriers of the virus for prolonged periods. One of these carrier cats might easily develop upper respiratory signs for some reason completely independent of calicivirus infection and yet "test positive" for calicivirus.

Similar issues arise with bacterial infections. Most organisms that cause bacterial rhinitis as secondary, opportunistic pathogens are found as part of the normal nasal or oropharyngeal flora. Simply growing these organisms from nasal culture does not prove that they are the primary, or even a secondary, cause of nasal or upper respiratory signs. Veterinarians who wish to obtain nasal cultures should request anaerobic and Mycoplasma species culture since these types of pathogens may have relevance and are not detected by routine aerobic culture.


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Source: DVM360 MAGAZINE,
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