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Submitted to: Diseases at the Interface between Domestic Livestock and Wildlife Species
Publication Type: Abstract Only Publication Acceptance Date: 6/1/2003 Publication Date: 7/15/2003 Citation: Suarez, D.L. 2003. Influenza. Diseases at the Interface between Domestic Livestock and Wildlife Species. Interpretive Summary: Technical Abstract: Type A influenza viruses are common pathogens of humans, pigs, horses and poultry. Although the virus is endemic in a number of different species, all type A influenza viruses originate in wild birds, primarily ducks, gulls and shorebirds. Influenza viruses have eight gene segments, with six segments coding for conserved internal proteins and two coding for extremely diverse surface proteins. The hemagglutinin (HA) surface protein is divided into 15 antigenic subtypes and the neuraminidase (NA) protein has 9 antigenic subtypes. All 15 HA and 9 NA subtypes are found in wild birds, but only select subtypes have become endemic in mammalian species. Although many subtypes have been isolated from poultry, chickens and turkeys, only the H5 and H7 subtypes have been associated with the highly pathogenic phenotype. The reason for this subtype selectivity is not known. Influenza viruses are generally promiscuous and many replicate in birds and mammals. However, when these viruses transmit from the natural wild bird reservoir to aberrant species, most don't replicate or transmit well enough to cause either disease or establish a lasting infection in the new host species. On rare occasions the virus strain that crossed the species barrier will have a constellation of genes that allows sufficient replication and transmission in the new species. These viruses may cause disease, and are characterized by a high evolutionary rate as the virus becomes adapted to the new host species. Eventually, the virus will become so adapted to the new species that it will no longer replicate well in the original host species. Influenza because of its segmented nature can also reassort gene segments. This switching of viral genes allows an additional way for viruses to adapt, and if it involves the hemagglutinin gene can result in a "genetic shift" which can result in a new pandemic in humans. One common denominator for the species that influenza infects, is that these animals congregate or are reared in large numbers, which likely aids in viral transmission. |
