Highly Pathogenic Avian Influenza A (H5N1) in People
Sporadic Cases of H5N1 Have Occurred in People
Although highly pathogenic avian influenza (HPAI) A (H5N1) virus infection of humans is rare, sporadic cases of human infection have been reported.
More than 600 human HPAI H5N1 cases have been reported to WHO from 15 countries in Asia, Africa, the Pacific, Europe and the Near East since November 2003. Approximately 60% of the cases have died.
Indonesia, Vietnam and Egypt have reported the highest number of human HPAI H5N1 cases to date.
H5N1 Virus Infection Can Cause Severe Illness in People
HPAI H5N1 virus can infect the respiratory tract of humans. When people develop illness from HPAI H5N1 virus infection, severe respiratory illness (e.g. pneumonia and respiratory failure) and death may occur.
The majority of HPAI H5N1 cases have occurred among children and adults younger than 40 years old. Mortality has been highest in people aged 10-19 years old and young adults. Most human HPAI H5N1 cases have presented late in their illness for medical care and have been hospitalized with severe respiratory disease. However, some clinically mild HPAI H5N1 cases have been reported, especially in children.
Of the few avian influenza A viruses that have crossed the species barrier to infect humans, HPAI H5N1 virus has caused the largest number of detected cases of severe disease and death in humans. However, it is possible that the most severely ill people are more likely to be diagnosed and reported, while milder cases are less likely to be detected and reported. Despite the high mortality, human cases of HPAI H5N1 remain rare to date, even among persons exposed to infected sick or dead poultry.
Most Cases of H5N1 in People Have Been Linked to Contact with Infected Poultry
In the majority of cases, the person got HPAI H5N1 virus infection after direct or close contact with sick or dead infected poultry.
Other HPAI H5N1 risk factors include visiting a live poultry market and prolonged, unprotected close contact with a sick HPAI H5N1 patient. For some HPAI H5N1 cases, the source of exposure to HPAI H5N1 virus is unknown.
Seasonality of human cases of HPAI H5N1 has been observed with increases during months at the end and beginning of the year. This seasonality corresponds to the seasonality of HPAI H5N1 virus outbreaks among poultry, which increase during the relatively cooler periods. However, human cases can occur at any time, especially in countries where HPAI H5N1 is endemic in poultry. Currently, HPAI H5N1 virus is considered endemic in poultry in six countries (Bangladesh, China, Egypt, India, Indonesia, and Vietnam), although other counties have experienced poultry outbreaks.
Currently, H5N1 virus does not transmit efficiently from person to person. Some cases of limited, non-sustained human-to-human transmission have likely occurred.
Clusters of human HPAI H5N1 cases, usually 2 cases but ranging from 2-8 cases per cluster, have been identified in several countries. Nearly all of the cluster cases have occurred among blood-related family members, especially those living in the same household. Whether such infections are related to genetic or other factors is currently unknown. While most people in these clusters have been infected with HPAI H5N1 virus through common source exposures such as direct or close contact with sick or dead poultry or wild birds, limited non-sustained human-to-human transmission of HPAI H5N1 virus cannot be excluded and likely occurred in some clusters. In cases where limited human-to-human transmission of HPAI H5N1 virus is thought to have occurred, spread has occurred after a very long period of unprotected close contact (hours in length) with a very sick blood-related family member (e.g. mother-daughter or brother-brother). This has been reported to have occurred in a home and in a hospital room.
For example, a report in 2004 from Thailand, describes probable limited human-to-human HPAI H5N1 virus spread in a family resulting from prolonged and very close contact between an ill child and her mother in a hospital room.1 Limited person-to-person spread of HPAI H5N1 virus from a son to his father in a hospital room was reported in China during 2007.2 In 2006, WHO reported evidence of limited non-sustained human-to-human-to-human HPAI H5N1 virus spread in Indonesia. In this situation, eight people in one family were infected. The first family member is thought to have become ill through contact with infected poultry. This person then infected six family members through unprotected close prolonged contact. One of those six people (a child) then infected another family member (his father). All of the cases were blood-related family members. No further spread outside of the exposed family was identified.3 Similarly, human-to-human transmission of HPAI H5N1 virus was reported in Pakistan among three brothers in 2007.4
- Ungchusak K, Auewarakul P, Dowell SF, et al. Probable person-to-person transmission of avian influenza A (H5N1). N Engl J Med. 2005:352(4):333-40.
- Wang H, Feng Z, Shu Y, et al. Probable limited person-to-person transmission of highly pathogenic avian influenza A (H5N1) virus in China. Lancet. 2008: 371(9622):1427-34.
- World Health Organization. Avian influenza situation in Indonesia update 16. 31 May 2006.
- World Health Organization. Weekly Epidemiological Record [1.1 MB, 8 pages]. 2008:40(83):357-364.