Influenza A: Asian Horses Escape Epizootic Influenza Outbreaks

In Mongolia, horses are particularly vulnerable to avian influenza viruses circulating among wild birds. However, in Central Asia, influenza A does not cause epizootic cases of equine influenza because the viruses have not acquired the genetic mutations necessary for effective interspecies transmission, according to a study by the University of Glasgow. The authors sought to determine the causes of these barriers to cross-species infection using avian influenza and equine influenza viruses to better understand the viral adaptation mechanisms underlying the emergence of disease in mammals. Emerging viral diseases pose a constant threat to humans and animals. Sometimes, certain viruses establish themselves in new hosts, often with devastating consequences. It is not known what enables a virus to infect and spread within a new host species, but viral ecology and evolution play an important role in this process. A study sought to determine why no epizootic outbreaks caused by avian influenza viruses have affected horses in Central Asia over the past 25 years . Influenza A viruses are the quintessential example of emerging viruses: their primary natural reservoir consists of wild birds, but they are also established in humans, horses, pigs, seals, dogs, and poultry . To better understand how influenza A viruses circulate in nature and the ecological and evolutionary factors responsible for their emergence, Scottish researchers sequenced more than twenty avian influenza viruses collected from wild birds in Mongolia. This country has ecological characteristics conducive to the emergence of equine influenza: it has a large horse population (approximately 3.3 million) and a high number of migratory and waterfowl. By combining seroepidemiological analyses, phylogenetics, and experimental infections, the authors have documented, with unprecedented detail, the stages leading up to the initial outbreak of the disease during its emergence in nature. The results show that avian influenza viruses are genetically related to a virus that caused an equine influenza epizootic in Asia between 1989 and 1990. Experimental infections have shown that most avian influenza viruses replicate in the respiratory tract of horses without causing tissue damage , which explains the absence of an epizootic. Thus, Mongolian horses, despite being regularly exposed to these influenza viruses—which circulate across a vast geographic area—do not develop clinical signs of the disease. The results also suggest that equine infections with avian viruses capable of replicating in horses are more common than initially thought , and that the inability of avian viruses to acquire the genetic mutations required to adapt to horses is the main barrier to the emergence of avian influenza. According to the authors, surveillance for this contagious viral disease should focus on the presence of adaptive mutations in equine influenza viruses derived from birds and on conducting additional serological surveys to determine exposure levels among horses.