What is Bird Flu (H5N1) and How is it Evolving?

In 1878 Dr. Eduardo Perroncito, an Italian veterinarian who specialized in pathology, noticed something strange. Poultry in a few villages began to become sick. These were symptoms he had never observed before in chickens, turkeys and other birds. The bright red combs on the tops of their heads and the wattles that dangle below their chins took on a purple discoloration, and so did their legs. The birds were having a hard time standing upright, their appendages swollen, appetite diminished, and egg production low. They were coughing and sneezing; some would even drop dead without exhibiting any symptoms at all. He described it as a ‘malattia gravissima’, a most severe disease.

Today it is known as avian influenza (bird flu) and comes in two strains. H5N1, which is a strain with a high pathogenic potential that makes birds as visibly sick as described above but there is also a less obvious strain H7N9 where birds generally don’t exhibit symptoms, maybe a few ruffled feathers, but it’s hard to tell that they are actually infected. In 2013 the first case of zoonotic avian influenza (the disease spreading to humans) was reported in China.  Several hundred people are believed to have died in China due to this outbreak.

How did this bird disease make its way to humans? It’s about evolution. Throughout history bird flu has been controlled by killing the birds. Over 150 million birds have been killed in efforts to eradicate this disease. Well viruses, just like animals and plants, possess genetic variation and this means that natural selection can act upon them. Viruses also reproduce very quickly, and their rate of mutation is higher than that of plants and animals. These are the perfect conditions for evolution. Since viruses have many genetic variants, the odds that one of them will carry a useful mutation that natural selection can select favorably (such as surviving in humans) is high.

One worry comes from how easy it is for viruses to share genetic material. Regular, non-lethal human influenza is easily recognized by our immune systems as a threat and attacked. Being as though our immune systems have no experience with avian influenza, it won’t know to attack these strains. The avian influenza virus could acquire the genes from our human influenza virus and mix them together in one lethal package. This would result in the virus having a combination of human transmissible genes and the deadly qualities of the avian virus. Every time avian influenza infects a human, it is another opportunity for the virus to adapt to human hosts by random mutation or acquiring genes from viruses within the host.  If a strain of the virus succeeds in human-to-human transmission and reproduces rapidly enough, we might have a pandemic on our hands.

In more recent studies, it has been suggested that the avian influenza virus historically may have swapped genes with a Eurasian strain of the swine influenza virus which began the bird-to-mammal transmissibility.  A separate strain of swine influenza proceeded to trade genes with human influenza, which allowed this swine influenza to infect humans. Researchers suggest that these two different forms of swine influenza provided the bridge to connect bird flu to humans.

The cases of humans infected by bird flu have gone down in recent years as the outbreaks have been consolidated, but the best way to prevent the spread of avian influenza to humans is to reduce interaction with wild/domesticated birds. If you own domesticated birds, keep their quarters very clean and routinely disinfect your footwear and wash your hands. And lastly, receive your annual flu shot.

Additional Readings:

Asian Lineage Avian Influenza A(H7N9) Virus – CDC

The History of Avian Influenza –ScienceDirect

Information to prevent human exposure to disease agents associated with wildlife—U.S. Geological Survey

Why Vaccinate? – Ricochet Science

Article by Tatiana Eaves


  • H5N1: CDC/ Courtesy of Cynthia Goldsmith; Jacqueline Katz; Sherif R. Zaki
  • Turkeys: Andrew Filer from Seattle (ex-Minneapolis) [CC BY-SA 2.0 (https://creativecommons.org/licenses/by-sa/2.0)]

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