infectious diseases
Avian Influenza
Introduction
Avian influenza is caused by strains of influenza A that normally infect only wild birds and domestic poultry. Infections due to some of these strains have been detected in humans. Human-to-human transmission is limited, most cases are acquired from animals, typically poultry. (See also Influenza .) Most subtypes of avian influenza that have caused human infections are H5, H7, and H9 viruses. Most cases of avian influenza in humans have been caused by Asian strains H5N1 and H7N9, but other types have also caused some human infections. Avian influenza infections are often asymptomatic in wild birds but may cause highly lethal illness in domestic poultry. Marine mammals can also become infected with avian influenza strains (eg, H10N7 in harbor seals), with subsequent human infection reported. Humans can become infected with avian influenza viruses through inhalation of or direct contact with secretions (saliva, mucous, or feces) from infected birds. 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, avian strains could possibly 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. If these strains acquire the ability to spread efficiently from person to person, an influenza pandemic could result. All cases of human infection with an influenza A subtype other than H1 or H3 must be reported. The first human cases of H5N1 were discovered in Hong Kong in 1997; many patients had severe respiratory symptoms, and the mortality rate was high. 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. One human infection was confirmed in 2021, and it was in India. Over 800 human infections and over 400 deaths have been confirmed since 2003 ( General references ). Since 2014, over 50 human infections with H5N6 have been documented; all occurred in Western Pacific Region, mostly in mainland China ( General references ). In February 2021, the first human infections with H5N8 were reported in 7 poultry workers in Russia; all cases were reported to be mild or asymptomatic. No human-to-human transmission was observed, and the risk to the general public is considered to be very low ( General references ). In early 2013, an extensive outbreak of H7N9 avian influenza in humans occurred in several provinces of southeastern China. About one third of cases were fatal, but significant illness typically occurred only in older patients. 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. Seasonal outbreaks typically correspond with increased poultry traffic and handling for Chinese New Year celebrations. China's 6th wave of H7N9 avian influenza in humans peaked in 2016-2017 with nearly 800 cases, and only sporadic cases have been reported since that time. Worldwide, over 1500 human cases and at least 615 deaths have been reported to the World Health Organization since 2013 ( General references ). 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. Human infection with other avian influenza strains have occurred sporadically, including H7N3 in Canada, H7N7 in the Netherlands, H7N4 and H9N2 mainly in China. Surveillance data indicate that many avian influenza infections may cause mild respiratory symptoms or even be subclinical. However, severe pneumonia with high case fatality rates has been reported in H5N1, H5N6 and H7N9 clusters. 