The widespread persistence of H5N1 in poultry populations poses two main risks for human health.
- The first is the risk of direct infection when the virus passes from poultry to humans, resulting in very severe disease.
- Of the few avian influenza viruses that have crossed the species barrier to infect humans, H5N1 has caused the largest number of cases of severe disease and death in humans.
- Unlike normal seasonal influenza, where infection causes only mild respiratory symptoms in most people, the disease caused by H5N1 follows an unusually aggressive clinical course, with rapid deterioration and high fatality.
- Primary viral pneumonia and multi-organ failure are common. In the present outbreak, more than half of those infected with the virus have died.
- Most cases have occurred in previously healthy children and young adults. A second risk, of even greater concern, is that the virus – if given enough opportunities will change into a form that is highly infectious for humans and spreads easily from person to person.
- Such a change could mark the start of a global outbreak (a pandemic). Direct contact with infected poultry, or surfaces and objects contaminated by their faeces, is presently considered the main route of human infection.
- To date, most human cases have occurred in rural or periurban areas where many households keep small poultry flocks, which often roam freely, sometimes entering homes or sharing outdoor areas where children play.
- As infected birds shed large quantities of virus in their faeces, opportunities for exposure to infected droppings or to environments contaminated by the virus are abundant under such conditions.
- Moreover, because many households in Asia depend on poultry for income and food, many families sell or slaughter and consume birds when signs of illness appear in a flock, and this practice has proved difficult to change.
- Exposure is considered most likely during slaughter, defeathering, butchering, and preparation of poultry for cooking.
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