Examples of Human Infections with Avian Influenza Viruses
Human infections with avian influenza viruses are rare and most often occur after people are in contact with an infected bird. However, non-sustained person-to-person spread of avian influenza viruses is thought to have occurred in the past, most notably with Asian-origin avian influenza A (H5N1) viruses reported primarily by 15 countries in Asia, Africa, the Pacific, Europe and the Near East.
- In 1997, when the Asian-origin H5N1 virus first emerged there was evidence of limited transmission of H5N1 virus to health care workers and household contacts of H5N1 patients in Hong Kong, but the virus did not spread further.
- In 2003, in the Netherlands, there was evidence of possible transmission of H7N7 from 2 poultry workers to 3 family members. All 3 family members had conjunctivitis and one also had influenza-like illness. (M Du Ry van Beest Holle, Meijer, et al, 2005. “Human to Human Transmission of Avian Influenza /H7N7, The Netherlands, 2004.”)
- In 2004, in Thailand, there was evidence of probable human-to-human spread in a family cluster. Transmission was associated with prolonged very close contact between an ill child and her mother and her aunt. Transmission did not spread beyond one person. (Ungchusak et al, 2005. “Probable Person-to-Person Transmission of Avian Influenza A (H5N1).”)
- In 2005, in Indonesia, limited person-to-person transmission of Asian-origin H5N1 could not be excluded in two clusters among patients who had no known contact with poultry or other animals. (Kandun et al, 2006. “Three Indonesian Clusters of H5N1 Virus Infection in 2005.”)
- In December 2007, limited, unsustained spread of Asian-origin H5N1 is thought to have occurred between a sick son and his father. (Wang et al, 2008. “Probable limited person-to-person transmission of highly pathogenic avian influenza A (H5N1) virus in China.”)
- Also in 2007, evidence gathered during an outbreak investigation in Pakistan supports the theory of initial transmission of Asian-origin H5N1 from poultry to humans followed by human-to-human transmission involving a third generation. (WHO, Weekly Epidemiological Record, 2008. Human cases of avian influenza A(H5N1) in North-West Frontier Province, Pakistan, October–November 2007
- In 2013, human infections with a new avian influenza A (H7N9) virus were first reported in China. Most of these infections are believed to result from exposure to infected poultry or contaminated environments, as H7N9 viruses have also been found in poultry in China. While some mild illnesses in human H7N9 cases have been seen, most patients have had severe respiratory illness, with about one-third resulting in death. No evidence of sustained person-to-person spread of H7N9 has been found, though some evidence points to limited person-to-person spread in rare circumstances. (Li Q et al, 2013. Epidemiology of human infections with avian influenza A (H7N9) in China.)
Almost all of these cases occurred during unprotected, close and prolonged contact between a caregiver (mostly blood-related family members) and a very ill patient.
Human-to-human transmission ranges along a continuum; from occasional, “dead-end” human-to-human transmission, to efficient and sustained human-to-human transmission. “Dead end” transmission refers to a virus from an animal host infecting a person and then there is subsequent transmission from that infected person to another person that eventually burns out. Efficient and sustained (ongoing) transmission in the community is needed for an influenza pandemic to begin.
- Page last reviewed: February 5, 2015
- Page last updated: March 17, 2015
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