Dogs, like cats, have generally been resistant to influenza infections. However, equine influenza virus was implicated in an outbreak of respiratory disease in racing greyhounds from Jacksonville, Fla., in early 2004.1 The outbreak was investigated by researchers at the University of Florida, in collaboration with Cornell University’s Animal Health Diagnostic Laboratory and the Centers for Disease Control and Prevention. Influenza virus was isolated from samples taken from affected dogs, and rising titers of anti-influenza antibodies were found in many of the dogs from the affected kennel.
As further proof that influenza virus was the cause of this disease outbreak, immunohistochemistry assays on affected lung tissue detected the presence of influenza virus H3 antigen.2,3 Genetic sequencing demonstrated that the virus resembled a strain of H3N8 equine influenza virus that had emerged in horses in the United States the previous year. The canine isolates shared more than 96% sequence identity with the H3N8 equine influenza virus.3 The initial outbreak of respiratory disease involved 22 dogs, and eight of these animals died of a peracute syndrome characterized by extensive hemorrhage in the lungs, tracheitis, bronchitis, and suppurative bronchopneumonia. Subsequent testing demonstrated seroconversion in 95% of the healthy contact animals, suggesting that subclinical infection had occurred in this population of dogs.3
Sporadic outbreaks of influenza-like illness during the late 1990s and early 2000s attributed to kennel cough might instead have been caused by influenza virus infections. Retrospective testing of samples from respiratory disease outbreaks from 2000 to 2003 detected antibodies to influenza A virus in nine dogs. No antibody-positive samples were detected before 2000.3 In the 16 months since the initial outbreak in 2004 was reported, additional influenza virus infections have been confirmed in greyhounds throughout the United States and in dogs in a shelter facility in north Florida and several veterinary clinics in Florida and New York.3
1. Carey S. UF researchers link equine influenza virus to canine susceptibility. DVM Newsmagazine 2004;35:1,52. Available at: www.dvmnewsmagazine.com/dvm/article/articleDetail.jsp?id=99227 . Accessed June 28, 2005.
2. Wheatley C. The detectives. Cornell Vet Med 2004;Fall/Winter:10-13. Available at: www.vet.cornell.edu/news/cvmagazine/Fall04/ . Accessed June 28, 2005.
3. Crawford PC, Dubovi EJ, Castleman WL, et al. Science, 26 September 2005 (10.1126/science.1117950).
Margaret C. Barr, DVM, PhD, College of Veterinary Medicine, Western University of Health Sciences, Pomona, CA 91766.