Emerging viral pathogens (Proceedings)

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Emerging viral pathogens (Proceedings)

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Apr 01, 2009

Canine Influenza

An outbreak of respiratory disease occurred in a kennel of racing greyhounds in 2004. While some dogs exhibited mild disease with fever and cough, some experienced peracute death with pulmonary hemorrhage (case fatality 36%). Virologic analysis revealed an influenza virus that was later found to be closely related to equine influenza virus subtype H3N8, sharing >96% genetic sequence identity. Retrospective studies on archival sera indicated infection had occurred in dogs as early as 2000. Additional outbreaks of respiratory disease have been associated with canine influenza since its identification.

Fortunately, most cases are mild, resembling kennel cough with a course of 14-21 days. Some manifest more severe disease with high fever and purulent nasal discharge; secondary bacterial infections are a major concern. Mortality rates of 1-10% have been reported, and are highest in puppies, elderly animals, and animals with concurrent disease.

Sustained transmission is occurring in dogs, and infection has been documented throughout the US. Racetracks, kennels, and other environments where dogs are housed together appear to be at greatest risk. As all dogs are susceptible and the virus appears to be quite contagious, continued spread is likely; aerosol as well as fomite transmission may occur.

Diagnosis is by serological analysis and virus identification by antigen detection, virus isolation, or PCR. Virus detection must be done early in the disease process. Supportive care including antibiotics appears to be very beneficial. Antiviral treatment using human medications such as Tamiflu have been done, but clinical studies have not yet been published. Currently, no vaccine is available for dogs; the equine vaccine should not be used in dogs.

Canine Respiratory Coronavirus

In 2003, an investigation into endemic respiratory disease in a large kenneled dog population in the UK identified a canine coronavirus distinct from the enteric canine coronavirus. The virus, a group 2 coronavirus closely related to bovine coronavirus, was found in tracheal washes and lung tissues, indicating infection of both upper and lower respiratory tracts. Subsequent studies showed significant prevalence in continental Europe and North America in addition to the UK. This pathogen must be considered in cases of respiratory disease in well-vaccinated dogs. Current diagnostics rely on serology (be sure to request canine RESPIRATORY coronavirus!) and virus identification by antigen detection, virus isolation, or PCR.

Canine Distemper

Canine distemper continues to be an important pathogen of dogs. The advent of vaccination for CDV in the 1950s led to a decrease in the incidence of distemper in dogs. However, infection with and outbreaks involving canine distemper virus (CDV) still occur. As with other RNA viruses, strains of CDV have genetic variability. Genetic characterization of CDV isolates has identified six major genetic lineages: America-1 and -2, Asia-1 and -2, European, and Arctic. Many commercial vaccines utilize strains from the America-1 lineage (Snyder Hill, Onderstepoort, Lederle) though these genotypes do not appear to be circulating in the field currently.

Novel CDV strains have been identified in recent years throughout the world. Oubreaks involving strains that appear to be derived from distant geographic locales have been documented. For example, the Arctic lineage has been found in Italy and isolates from dogs in Hungary have been found to resemble those from North America, likely due to extensive and often uncontrolled movement and trade of dogs. Distinct isolates have also been detected in North America. In 2004, phylogenetic analysis of virus from four clinical cases in the US identified three strains genetically distant from strains previously identified in North America. The dogs in three of these four cases had recently been vaccinated. Genetic characterization of the viruses from these cases found they were novel for the continental US. Outbreaks among raccoons in the Chicago area in 1998 and 2001 were also due to genetically distant lineages; in addition, the 2001 variant appeared to be more lethal. An investigation of North American isolates in 2007 identified genotypes distinct from vaccinal strains. Circulation in wildlife populations may lead to virus variants of antigenicity as well as virulence.

Contact with wildlife may result in CDV transmission to domestic dogs. An outbreak of CD in Alaska led to the death of several hundred dogs. The virus was isolated and characterized, and was found to be most closely related to a Phocine Distemper virus from an outbreak among Baikul Seals in Siberia. In the USA, significant seroprevalence has been found in raccoons in periurban regions. These raccoons are believed to have transmitted the virus to captive felids in an urban zoo, and may have been the origin of virus in a Chicago area outbreak among domestic dogs.

Genetic diversity has been associated with vaccine failures. As new strains of CDV continue to emerge, surveillance and characterization of isolates from field cases will be necessary. Antigenic, pathogenic, and genotypic descriptions of new isolates will provide important information about this important pathogen required to maintain safe and efficacious vaccines.