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Emerging respiratory infections (Proceedings)

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Apr 01, 2010

Recent years have seen the emergence of previously undescribed respiratory infections in dogs and cats. Although these infections remain rare, the potential exists for substantial morbidity and mortality. While there are several respiratory infections that might be described as "emerging" (eg, Pneumocystis jirovecii, Streptococcus equi subspecies zooepidemicus, respiratory coronavirus), we will focus on only several specific infections including influenza and virulent systemic calicivirus (VS-FCV).

Influenza viral infection

Until less than a decade ago, dogs and cats were not thought to be susceptible to illness due influenza. Influenza viruses, single-stranded negative-sense RNA viruses in the family orthomyxovirdae, are named numerically according to the hemagglutinin (H) and neuraminidase (N) expressed on the virus. While many influenza viruses are adapted to a particular species, they may be able to infect multiple different species or add species affinity. All influenza viruses are highly susceptible to genomic change through either genomic drift (mutation) or genomic shift (interchange of entire segments of genome from one virus to another). Canine influenza is a high morbidity, low mortality infection that is reported from most of the USA. Feline infection with virulent avian influenza has not been reported in the USA, but is important not only as a very high mortality infection, but as a possible zoonotic risk. Both cats and dogs have been infected with the triple reassortment H1N1 virus often described as "swine flu".

Canine influenza

Canine influenza was first described in a kennel of racing greyhounds in 2004. Apparently, equine influenza was able to "jump species" from the horse to dog in a fashion that allowed subsequent spread within the canine species. The H3N8 virus seems to be adapting well to dogs, and it growing slightly more distant from the original equine virus. While the original description was of a severe hemorrhagic pneumonia with a relatively high mortality, it now seems that the typical course of infection is less severe. In fact, unless dogs are densely housed or concurrently infected with other pathogens, canine influenza may be largely another potential cause of Canine Infectious Respiratory Disease complex (i.e., CIRD, aka, infectious tracheobronchitis, aka kennel cough). The incidence and prevalence of CIV as a cause of CIRD is yet to be determined, although there are a few epidemiologic studies now beginning the early stages of investigation. It remains to be determined how often kennel cough is due to H3N8 infection in the USA.

The list of differential diagnoses for acute cough in dogs includes numerous cardiac and respiratory diseases. Infectious causes are also many, and include viruses (e.g. canine influenza virus, respiratory corona virus, distemper virus, parainfluenza virus, adenovirus 2), bacteria (e.g. Bordetella bronchiseptica, Mycoplasma spp., Streptococcus equi subsp. zooepidemicus), protozoa, mycoses, and parasites. A history of recent exposure to other dogs with cough or to group housing situations is supportive of a diagnosis of infectious tracheobronchitis. Influenza has a shorter incubation period (< 1 week) compared to Bordetella bronchiseptica (up to 2 weeks). Because most dogs are naïve to influenza antigens, spread of disease within an exposed population of dogs may be more extensive than the spread of other common causes of CIRD. Compared to CIRD associated with Bordetella, dogs with influenza are often described as more likely to act lethargic, have a soft, moist cough, have purulent nasal discharge, and are more likely to be febrile. Dogs with influenza may be more likely to develop pneumonia than dogs with Bordetella, but this tendency is not well documented.