Avian influenza

From MicrobeWiki, the student-edited microbiology resource

Characteristics of the symbiont/pathogen

Avian influenza is part of the Orthomyxoviridae group, which are single-stranded RNA viruses. There are two different classifications of avian influenza, highly pathogenic avian influenza (HPIA) and mildly or non-pathogenic avian influenza (MPIA). The starting place of the virus varies in HPIA and MPAI. In HPIA the HA proteins start in most of the cells in the body of the host and in MPIA the HA proteins start in the respiratory and enteric tracts of the host. There is a viral envelope that encases the surface proteins: Hemagglutinin (HA), which has 1,775 base pairs (bp) and Neuraminidase (NA), 1,413bp and Matrix 2 (M2), when paired with Matrix 1 has 1,027bp. (1) There are 16 HA subtypes that are the virus receptor-binding site and also allow the viral RNA to be released into the host cell. (3) The HA protein has to grow into the HA1 and HA2 subunits to be infectious. There are nine NA subtypes that are enzymatically active proteins that allow the virus to be released from the cell surface. The M2 proteins are an ion channel that activate the HA proteins to release the virus. Inside these surface proteins, there are six internal proteins that make up the viral genome and transcribe the virus. These proteins include: polymerase proteins PB1 with 2,341bp, PB2 with 2,341bp and PA with 2,233bp, the nucleoprotein (NP) with 1,565bp, Matrix 1 (M1), and nonstructural protein 1 and 2 (NS1 and NS2 with 890bp). NS1 is the only protein not located inside the virion. (4,6)

Chicken

Characteristics of the host

Avian influenza affects a variety of species. There are three types of the virus, type A, type B, and type C. Type A affects a range of animals including, but not limited to wild birds, including ducks, gulls, and seabirds, domesticated birds, such as chicken and turkey, mammals, such as horses, pigs, and humans. (1) At least 88 species of wild birds have been found to have type A avian inflenza. (6) Types B and C only affect humans. (4)

Avian Influenza is spread through the fecal-oral transmission, this makes water birds and surface feeders more suseptible to contract it than birds that forage. The virus spreads from the wild birds to domesticated birds, where it becomes highly pathogenic and then returns to the wild birds again. (5)

Avian Influenza, CDC

Host-Symbiont Interaction

Migratory waterfowl are the primary source of infection in poultry and mammals. Mammals are infected regularly, but since they are not the starting place for the virus, some of the subtypes have become extinct. Examples of this are the H2N2 subtype becoming extinct in humans and H7N7 becoming extinct in horses. Because the subtypes become extinct, more fit subtypes are developed. (4)

Most birds suffering from avian influenza have mild respiratory problems and decreased egg production. Chicken and turkey that have HPAI typically show a decrease in their vocalization, food and water consumption. They also have signs of depression and apathy. Birds that have avian influenza can develop significant physical problems, such as head tremors, wing paralysis, and a lack of coordination. (2)

Molecular Insights into the Symbiosis

One way to determine if the strain of the virus is HPIA or MPIA is to place the strain into fluid from embryonated eggs that are specific pathogen free (SPF) chickens. If six out of eight of the chickens die within 10 days the strain is determined to be highly pathogenic. If fewer than six die in the 10 days, it is classified as mildly pathogenic or non-pathogenic. (4)

Mallard Duck, National Geographic

Ecological and Evolutionary Aspects

Avian influenza was first discovered in free living ducks in 1972. Mallard ducks were found to have the most HA and NA subtypes; however, not all of the HA subtypes were found in abundance. This means that these ducks are probably not the starting place for all of the HA subtypes. HPIA was one of the first viral diseases found in poultry. The virus was found to have a rapid rate of evolution in mammals and domesticated birds, whereas, there is a low rate of evolution in wild birds. This is evidence that mammals are not primary hosts, but wild birds are. It is hard to know the full evolutionary history of how avian influenza has developed and evolved due to the viral interaction with many types of wild birds. (4)

References

[1] Boyce, W.M, Sandrock, C., Kreuder-Johnson, C., Kelly, T., and Cardona, C. 2008. Avian influenza viruses in wild birds: a moving target. "Comparative Immunology, Microbiology and Infectious Diseases." 32:275-286.

[2] Koch, G. and Elbers, A.R.W. 2006. Outdoor ranging of poultry: a major risk factor for the introduction and development of high-pathogenicity avian influenza. "Wageningen Journal of Life Sciences." 54:179-194.

[3] Stallknecht, D.E. and Shane, S.M. 1988. Host range of avian influenza virus in free-living birds. "Veterinary Research Communications" 12:125-141.

[4] Suarez, D. L. and Schultz-Cherry, S. 2000. Immunology of avian influenza virus: a review. "Developmental and Comparative Immunology." 24:269-283.

[5] Takekawa, J. Y., Prosser, D. J.,Newman, S.H., Muzaffar, S.B., Hill, N.J., Yan, B., Xiao, X., Lei, F., Li, T., Schwarzbach, S.C., Howell, J.A. 2010. Victims and vectors: highly pathogenic avian influenza H5N1 and the ecology of wild birds. "Avian Biology Research" 51-73.

[6] Tsai, K-N. and Chen, G-W. 2011. Influenza genome diversity and evolution. "Microbes and Infection" 13:479-488.

Edited by Nikki Byers, students of Grace Lim-Fong