Influenza A virus in wild birds

Abstract: Influenza virus is a RNA virus that exists as different types and subtypes. Influenza A virus strains are known to cause disease in several bird and mammalian species. Wild birds are believed to constitute the natural reservoir for influenza A virus.In humans, influenza A virus causes yearly seasonal influenza epidemics of respiratory disease resulting in high morbidity and severe economic consequences. Due to the virus’ ability to change its antigenic properties by mutation, yearly vaccination is required for protection from the disease.There are many different subtypes of influenza virus which are characterized according to two surface structures - the hemagglutinin and neuraminidase proteins - , for example; H5N1. These subtypes have the ability to recombine, and thereby creating new variant combinations. If a subtype that the living population of humans has not encountered before starts to spread among humans, it can result in a pandemic. Pandemic outbreaks have occurred at irregular intervals throughout history and have had a devastating impact on mankind. For example the Spanish influenza pandemic of 1918 is thought to have killed more than 50 million people.Influenza A virus is also an important cause of disease in poultry where virus strains of some subtypes may change into forms that are highly pathogenic. These virus strains may transmit directly to man and multiple other species. This has been the case in the ongoing outbreak that started in South East Asia in 2003. All known subtypes of influenza A virus have been isolated from wild birds living in aquatic environments, mainly dabbling ducks. These species are considered to be the reservoir for influenza A virus. The virus causes sub clinical gastrointestinal infection in ducks. High amounts of virus are excreted in the feces and spread via the fecal-oral route through water where it can persist for a prolonged time.There are still many unknowns about the ecology of influenza virus in the wild bird reservoir. This thesis includes five articles where data are presented that add new knowledge on this subject. We add proof that wild ducks are indeed the host for most influenza A virus subtypes by presenting data from a meta-analysis on all published screening data from wild birds and by presenting data from a four year screening of migratory ducks that were caught and sampled at Ottenby Bird Observatory. Our investigations have shown that the prevalence of influenza virus in the wild duck population of western Eurasia shows temporal differences in comparison to the results found in studies in North America. The prevalence in western Eurasian ducks is high during the period August to December and also rises in the spring. These findings are of importance for the understanding of how influenza virus is perpetuated in nature. During the course of the study only low pathogenic subtypes were isolated. Of concern is the high frequency of isolation of virus strains of the H5 and H7 subtypes that are prone to change into highly pathogenic variants in poultry. Many of the strains isolated in our study are similar to the ones that have caused influenza outbreaks in poultry in Europe during the last seven years. This indicates that wild bird surveillance for influenza A virus can be of major value as a sentinel system to prevent outbreaks in domestic poultry.Studies on Black-headed Gulls (Larus ridibundus) revealed a previously unknown subtype, H16. This finding widened the spectra of known influenza A virus subtypes in nature.Influenza A virus was also isolated in samples from Guillemots (Uria aalge) in the Baltic Sea. This was the first time influenza A virus was isolated from this species in Europe. The isolated virus strains contained a mix of genes, some of which must have been derived from influenza A virus strains present in the North American bird population. This finding proves that limited exchanges between the virus strains present on the American and the Eurasian continents exist, which is of concern for evaluating the risk of spread of highly pathogenic virus strains by wild birds to the Americas.