H5N1 highly pathogenic avian influenza virus in wild birds

Abstract

Mortality associated with H5N1 highly pathogenic avian influenza (HPAI) viruses has been reported in a diversity of wild avian species, but it is unknown whether these viruses can be maintained or become established in free-living avian populations. In this research, the potential for H5N1 HPAI viruses to be transmitted and maintained in these populations was determined by evaluating both host and environmental factors that could affect transmission. The two H5N1 HPAI viruses evaluated in this study generally had a shorter duration of persistence in water than the low pathogenic avian influenza viruses that naturally circulate in wild avian populations. This suggests that, at some point in the evolution of H5N1 HPAI, these viruses have lost some degree of environmental fitness. Several species of anseriforms and charadriiforms were experimentally infected with a H5N1 HPAI virus and the data from these studies indicate that specific wild waterfowl species (primarily the swans, geese, and gulls) are highly susceptible to this virus, as evidenced by high mortality, and these species excrete high concentrations of virus for several days. Additionally, most duck species in our study were resistant to H5N1 HPAI infection and viral shedding in these species was minimal. Taken together, our data suggests that H5N1 HPAI virus epidemics in wild birds may be dependent on the presence of highly susceptible species that may represent only a small component of the total avian population. This is consistent with the field data from the European outbreaks of 2005/2006, in which the majority of wild bird mortality occurred in a relatively few anseriform species. Additionally, our findings suggest some waterfowl species transmit virus asymptomatically for several days and, consequently, have the potential to spread H5N1 HPAI virus between limited geographic regions. Although this information provides some insight into H5N1 HPAI virus transmission within a waterfowl population, the data are not sufficient to indicate whether H5N1 HPAI viruses can or cannot be maintained wild avian populations under natural conditions, which would depend heavily on biology of the each individual wild bird species, the environmental conditions, and the species composition of the avian population