Submitted to: Veterinary Microbiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: November 1, 1999
Publication Date: May 22, 2000
Interpretive Summary: Influenza virus can infect and cause disease in a wide variety of animals including humans, pigs, horses, chickens and turkeys. However, the group of animals that influenza is thought to normally infect are ducks, gulls and other waterfowl. These wild birds are thought to be the natural reservoir for the virus. The virus readily infects these birds, but it doesn't cause disease. The virus also appears well adapted and only changes very slowly in this group of birds. In contrast, influenza changes rapidly in mammals, chicken and turkeys, which is evidence that the virus does not normally infect these animals. Previously the rate of change of influenza in wild birds and chickens and turkeys were compared together, but this work demonstrates that they must be compared separately to get meaningful results. To compare influenza viruses several conditions must be met. First, all the isolates examined must be from the same original source of virus. Second, multiple virus isolates from several different years must be available to measure rates of change. Many virus isolates are available for study, but only a few groups of isolates meet these criteria. Analysis of three different poultry outbreaks confirms that influenza changes more rapidly in chickens and turkeys than in wild waterfowl, and should be considered separately from them.
Technical Abstract: When influenza viruses infect chickens, turkeys, swine, horses, and humans, they are considered aberrant hosts for the virus. The distinction between the normal and aberrant host is important when describing virus evolution in the different host groups. The evolutionary rate of influenza virus in the natural host reservoirs of wild waterfowl, gulls and shorebirds is believed to be slow, but in mammals the rate is much higher. Chickens and turkey influenza isolates have previously and incorrectly been lumped together with wild birdinfluenza viruses when determining rates of evolutionary change. To determine mutational and evolutionary rates of a virus in any host species, two primary assumptions must be met: first, all isolates included in the analysis must be descended from a single introduction of the virus, and second, the outbreak must continue long enough to determine a trend. For poultry, three recent outbreaks of avian ninfluenza meet these criteria, and the sequence of the hemagglutinin and nonstructural genes were compared. Sequences from all three outbreaks were compared to an avian influenza consensus sequence, which at the amino acid level is highly conserved for all the internal viral proteins. The consensus sequence also provides a common point of origin to compare all influenza viruses. The evolutionary rates determined for poultry influenza viruses are similar to what is observed in mammals.