While the pig has been considered the leading candidate for the role as intermediate host for virus adaptation, recent information
suggests that terrestrial poultry, such as quail and chickens, also may play a central role in the emergence of viruses with
pandemic potential. Yet, as human and swine lineage viruses appear to replicate inefficiently in land-based poultry, the pig
remains the most likely domestic animal species in which genetic reassortment between human and avian viruses may occur.
Direct virus transmission between mammals
Influenza viruses of different genotypes and subtypes occasionally can be transmitted between two mammalian species. For instance,
direct swine-to-human transmission of influenza viruses has been documented on several occasions. Other examples include the
occasional human-to-equine transmission of H1N1, H2N2 and H3N2 viruses. The potential for interspecies transmission among
mammals is further highlighted by the recent transmission of an equine-like H3N8 influenza virus to dogs. The equine-to-canine
transmission event is particularly intriguing in light of the fact that dogs were not commonly regarded as hosts for influenza
Although circumstantial evidence from surveillance studies indicates that dogs are susceptible to infection with human-lineage
viruses, infection did not result in clinical disease and the viruses did not spread efficiently among dogs.
In contrast, the equine H3N8 strain that caused the canine influenza outbreak in the United States spread quickly among dogs.
Given that influenza A viruses do not establish persistent infections, the apparent maintenance of the H3N8 virus in the canine
population implies that this particular strain infects and replicates efficiently in dogs.
To date, the viral and host factors that determine influenza virus species specificity and the mechanism by which host-range
barriers are overcome are not completely understood. Nevertheless, evidence has accumulated over the years indicating potential
contributions by the products of all eight gene segments. Despite this, several virus genes appear to play dominant roles
in controlling influenza host range. Due to its role as the viral receptor binding protein, many investigators have focused
their attention on the HA as the primary determinant of host range. Over the years, a large body of evidence has accumulated
indicating that the HA is a key player in influenza virus species specificity.
Infections caused by influenza A viruses have burdened humans and animals since ancient times. Yet, due to the existence of
a global avian reservoir of all subtypes of influenza viruses, influenza's inherent ability to infect a broad range of animal
hosts, as well as the viruses' continued genetic evolution, influenza continues to represent a serious infectious disease
threat. Recent examples of interspecies transmission of viruses include the spread of equine H3N8 virus to dogs in the United
States as well as the resurgence of H5N1 avian influenza viruses in poultry, cats and humans throughout large parts of Asia,
North Africa and Europe. Although there has been an explosion of information on the molecular determinants of influenza virus
species specificity, much has remained unclear. The realization of the importance of interspecies transmission in the ecology
of influenza has importance for influenza control, since commercially available vaccines may not provide protection against
infection with viruses stemming from other species. Thus, virus surveillance in animal species must remain a priority. Surveillance
and genomic sequencing of large numbers of influenza viruses will help us comprehend the genetic basis of host adaptation
and will enhance our understanding regarding the extent and impact of the animal reservoir of influenza A viruses.
Dr. Hoskins is owner of DocuTech Services. He is a diplomate of the American College of Veterinary Internal Medicine with
specialities in small animal pediatrics. He can be reached at (225) 955-3252, fax: (214) 242-2200 or e-mail:
Serological Evidence for Canine Influenza Virus Circulation in Racing Greyhounds from 1999 to 2003:
T.C. Anderson; L. Grimes; J. Pompey; C. Osborne; W.J. Dodds; J.M. Katz; C.H. Courtney; P.C. Crawford University of Florida
College of Veterinary Medicine, Gainesville, FL, USA; Immunology and Viral Pathogenesis Section, Influenza Branch, Centers
for Disease Control and Prevention, Atlanta, GA, USA; Hemopet/Pet Life-Line, Garden Grove, CA, USA.