Caused by type A strain of influenza. Two types of avian influenza are defined based on their virulence: a highly virulent type that causes fowl plague (highly pathogenic avian influenza, HPAI) and an avirulent type that causes only mild or asymptomatic disease. All HPAI are of the H5 and H7 subtype and responsible for large avian epidemics to date.
2-5 days (range 1-9 days)
No sustained human-to-human transmission documented to date.
Avian influenza viruses spread among susceptible birds through contact with contaminated excretions of other infected birds. They do not normally infect other species with the exception of pigs and horses. Infection of other mammals like cats has also been reported.
Human infection results from close contact with infected poultry (secretions and excrement) or contaminated surfaces.
The virus circulates among birds worldwide.
The first documented human infection with an avian influenza virus occurred in Hong Kong in 1997, when the H5N1 strain caused severe respiratory disease in 18 humans, of whom 6 died. The infection of humans coincided with an epidemic of HPAI, caused by the same strain, in Hong Kong’s poultry population.
The H7N7 subtype was responsible for another outbreak in the Netherlands in 2003 when 83 cases of conjunctivitis and one death from severe respiratory illness were reported.
In the 2004 outbreaks involving H5N1 subtype in Thailand and Vietnam, human cases were associated with severe respiratory disease and high mortality. During the period 2003-2010, a total of 512 cases, including 304 deaths, were reported to WHO, with most of the cases from Indonesia (171 cases with 141 deaths), Vietnam (119 cases with 59 deaths) and Egypt (115 cases with 38 deaths).