Avian influenza is caused by strains of influenza A that normally infect only wild birds and domestic poultry (and sometimes pigs). Infections due to these strains have recently been detected in humans.
Most cases of avian influenza in humans have been caused by strains of avian influenza type A H5N1 and, most recently, by type A H7N9, but types H7N7, H7N3, H9N2, and H10N8 have also caused some human infections. Infections with these strains are asymptomatic in wild birds but may cause highly lethal illness in domestic poultry.
The first human cases of H5N1 were discovered in Hong Kong in 1997. Spread to humans was contained by culling domestic bird populations. However, in 2003 and 2004, H5N1 infections in humans reappeared, and occasional cases continue to be reported, primarily in Asia and the Middle East. Human infections with other avian influenza strains have also been reported in Asia (H9N2), Canada (H7N3), the Netherlands (H7N7), and China (H10N8). At the end of 2013, the first 2 human cases of avian H10N8 influenza infection were diagnosed in southeastern China. One patient, an elderly woman with a compromised immune system, died. All her contacts remained asymptomatic. Although most cases of avian influenza occurred through exposure to infected birds, some person-to-person transmission probably occurred in the Netherlands and in Asia.
In early 2013, an extensive outbreak of H7N9 avian influenza occurred in several provinces of southeastern China. About one third of cases were fatal, but significant illness typically occurred only in the elderly. Sustained human-to-human transmission did not occur, although there is some evidence of limited human-to human transmission. Human infection appeared to result from direct exposure to infected birds in live (wet) poultry markets, where birds are purchased for subsequent consumption at home. The outbreak peaked in late spring of 2013, subsided (partly because the markets were closed down), but then reappeared in early autumn. China is currently experiencing its 5th epidemic of Asian H7N9 avian influenza in humans; as of August 2017, 759 people have been infected in this epidemic, bringing the total cumulative number of people with Asian lineage H7N9 avian influenza to 1557. Some cases of Asian H7N9 avian influenza have been reported outside of mainland China, but most occurred in people who had traveled to mainland China before becoming ill.
It is likely that avian influenza viruses of any antigenic specificity can cause influenza in humans whenever the virus acquires mutations enabling it to attach to human-specific receptor sites in the respiratory tract. Because all influenza viruses are capable of rapid genetic change, there is a possibility that avian strains could acquire the ability to spread more easily from person to person via direct mutation or via reassortment of genome subunits with human strains during replication in a human, animal or, avian host. Many experts are concerned that if these strains acquire the ability to spread efficiently from person to person, an influenza pandemic could result.
Human infection with avian influenza H5N1 strains can cause severe respiratory symptoms. Mortality was 33% in the 1997 outbreak and has been > 60% in subsequent infections. Infection with the H7 strains most commonly causes conjunctivitis, although in the Netherlands outbreak, a few patients had flu-like symptoms and one patient (of 83) died.
An appropriate clinical syndrome in a patient exposed to a person known to be infected or to birds in an area with an ongoing avian influenza outbreak should prompt consideration of this infection. History of recent travel to regions with ongoing transmission of virus from domestic poultry to humans (eg, for H5N1, Egypt, Indonesia, and Vietnam) plus exposure to birds or infected people should prompt testing for influenza A by RT–PCR. Culture of the organism should not be attempted.
Suspected and confirmed cases are reported to the Centers for Disease Control and Prevention (CDC).