Bird Flu: Past, Present, And Future

In this review of the history of avian influenza viruses—from the “Spanish flu” outbreak of 1918 that killed 50 million people to present-day epidemics—the authors describe how bird flu strains have evolved and spread and the measures that have been taken to control them. They suggest that by understanding the patterns of past pandemics, we can make better predictions about future outbreaks.

Influenza viruses have a fast mutation rate, meaning that they evolve quickly, and strains that have evolved in birds are sometimes able to infect humans. For example, the human flu pandemics of 1957 and 1969 had avian origins—and the limited evidence available suggests that the 1918 Spanish flu probably did as well. If you trace the precursor strains of the 2009 H1N1 “swine flu” pandemic back 30 years, they’re of avian origin, too.

Studies have found that the distribution of flu viruses often follows wild birds’ migration routes, although the trade in infected domesticated birds also plays a role. The wild birds most commonly infected with flu viruses are waterfowl—including ducks, geese, swans, gulls, terns, and sandpipers. Direct transmission of flu viruses from wild birds to humans seems to be very rare, likely because of a lack of contact, so transmission via domesticated birds is more common.

In Asia, in particular, commercially bred birds such as ducks may share habitat, water, and food with wild birds, and these farmed birds are thought to play a key role in transmitting flu strains from wild birds to poultry. In this process, viruses can evolve from being “low pathogenic” to “highly pathogenic,” meaning they’re more likely to kill the host.

In the last few decades, avian flu outbreaks have resulted in the deaths of millions of farmed birds, since a large percentage of chickens and other bird species infected with highly pathogenic viruses die or are killed within a few days of infection.

The success of culling measures has varied. During a 1997 outbreak in China, all poultry in Hong Kong were culled—and although that particular form of the virus became extinct there, evolved versions emerged a few years later. In North America, approximately 50 million farmed birds were killed by infection or control measures during a 2015 outbreak, and that time, the cull stopped the virus in its tracks.

Bird flu outbreaks have also been tackled using preventive measures, such as the nationwide bird vaccination program implemented in China in September 2017 in response to a flu outbreak in birds raised for food, and humans. Following vaccination, the rate of immunization among birds was greater than 80%, and cases in humans were massively reduced. 

The authors anticipate that highly pathogenic strains of avian flu will continue to emerge, potentially once or twice per decade on the basis of past trends. To prevent outbreaks among farmed birds that could result in wide-scale culling and lead to human flu pandemics, they suggest that disease-surveillance programs be implemented, biosecurity be increased, and bird vaccination schemes be undertaken. Of course, animal advocates know that the best way to prevent pandemics is to stop raising animals for our consumption. In the meantime, the measures mentioned in this paper can help.