History and Origin

Avian influenza viruses have been around for a long time and have caused disease in many bird species for over 100 years. The earliest known major outbreak of bird flu in poultry occurred in Italy and Egypt in the early 1900s. New subtypes were identified during outbreaks in chickens in Scotland in 1959 and Pakistan in 1994. The H5N1 strain that is currently of greatest human health concern first emerged in chickens in China in 1996 and then spread to multiple countries. This led to major control efforts to limit its spread in poultry and protect human health.

While most avian influenza viruses do not infect humans, some subtypes such as H5N1, H7N9 and H9N2 have caused human infections and illness. It was initially thought that these bird flu viruses did not easily infect humans. However, since 1997, there have been over 800 confirmed human cases of H5N1 infection, resulting in over 450 deaths. In 2013, a new flu strain, H7N9 emerged from China which has caused over 1,500 human infections and over 600 deaths. More recently, Pakistan has also reported human cases of H9N2 infection since 2020.

Signs and Symptoms in Humans

Human infections with Bird Flu viruses can cause a spectrum of illness from very mild to severe and life-threatening. Common symptoms of infection include fever, cough, sore throat, muscle aches and fatigue. However, some people may also experience more severe symptoms such as acute respiratory distress, pneumonia, multi-organ failure and in some cases death. Time from exposure to symptoms appearing is usually 2-5 days but can range from 1-7 days.

The severity of disease appears to be related to the immune response of the infected individual and subtype virus. Younger age group, people with weak immune systems and those with underlying medical conditions are at highest risk of severe illness and death from virus infections. Overall mortality rates among reported human cases have been variable but highest for H5N1 infections at around 60%. For H7N9 infections, the current mortality rate is estimated to be around 40%. However, asymptomatic and mild infections may go undetected hence the actual risk could be lower.

Spread of Flu Between Animals and Humans

These viruses primarily spread through contact with infected live or dead poultry such as chickens, ducks and turkeys. However, human-to-human transmission of bird flu viruses is very rare. Most human infections have been the result of direct contact with infected animals or contaminated environments. Activities such as slaughtering poultry, culling operations, as well as visiting live animal markets and farms have been associated with human infections.

Close physical contact remains important for bird flu viruses to spread between humans. However, some virus subtypes like H5N1 have shown potential for limited human-to-human transmission on rare occasions particularly among family clusters. This ability to transmit between humans is still very limited but remains a concern that needs close monitoring. While evidence is still limited, it is possible that some subtypes could acquire mutations over time that make them effectively transmissible among humans, which could potentially trigger a pandemic.
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