Merck Manual

Please confirm that you are a health care professional

honeypot link

Avian Influenza

(Bird Flu)


Brenda L. Tesini

, MD, University of Rochester School of Medicine and Dentistry

Last full review/revision Sep 2021| Content last modified Sep 2021
Click here for Patient Education
Topic Resources

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.

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.

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; several cases were reported in Egypt in 2017. One human infection was confirmed in 2019 in Nepal. Over 800 human infections have been confirmed since 2003.

Since 2014, 24 human infections with H5N6 have been documented; all occurred in mainland China. One human infection has been confirmed in 2019.

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; worldwide, over 1500 human cases and at least 615 deaths have been reported to the World Health Organization since 2013 (1 General reference 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... read more ). 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. There was only 1 case confirmed in mainland China in 2019.

Human infection with other avian influenza strains have occurred sporadically, including H7N3 in Canada, H7N7 in the Netherlands, H7N4 and H9N2 in China, and in January 2019 a first case of H9N2 in India.

Marine mammals can also become infected with avian influenza strains (eg, H10N7 in harbor seals), with subsequent human infections reported.

China has active poultry vaccination campaigns for H5 and H7 influenza viruses to help prevent the spread from wild to domestic birds, which are more likely to come in contact with and spread the virus to humans.

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.

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 (25 to 69%) has been reported in H5N1, H5N6 and H7N9 clusters.

General reference

  • 1. World Health Organization: Influenza at the human-animal interface: Summary and assessment, 26 November 2019 to 20 January 2020. Accessed 2/28/2020.

Symptoms and Signs of Avian Influenza

Manifestations of avian influenza are the same as those of seasonal influenza Symptoms and Signs Influenza is a viral respiratory infection causing fever, coryza, cough, headache, and malaise. Mortality is possible during seasonal epidemics, particularly among high-risk patients (eg, those... read more ; however, disease severity and case fatality rate tend to be higher, although with wide variation depending on the viral strain.

Diagnosis of Avian Influenza

  • Reverse transcriptase–polymerase chain reaction (RT-PCR)

An appropriate clinical syndrome in a patient exposed to a person known to be infected or exposed 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 using a nasal or throat swab. Patients with lower respiratory tract illness can have samples taken from sputum, endotracheal aspirate, or bronchoalveolar lavage fluid. Culture of the organism should not be attempted because special precautions are required for these highly pathogenic viruses.

Suspected and confirmed cases are reported to the Centers for Disease Control and Prevention.

Treatment of Avian Influenza

  • A neuraminidase inhibitor

Treatment with oseltamivir or zanamivir (neuraminidase inhibitors) at usual doses is indicated.

The H7N9 and H5N1 viruses are resistant to the earlier antiviral drugs amantadine and rimantadine; resistance or reduced susceptibility to oseltamivir has also been reported.

The antiviral agent, baloxavir, has also shown in vitro activity against various avian influenza viruses, but clinical data are currently lacking.

Key Points

  • Avian influenza affects mainly birds, but several strains of avian influenza virus have caused severe respiratory illness and death in humans.

  • Human infection is typically acquired from infected birds although human-to-human transmission has occurred.

  • Because all influenza viruses are capable of rapid genetic change, it is possible that avian strains could acquire the ability to spread more easily from person-to-person and cause a serious pandemic.

  • These viruses should not be cultured because they are highly pathogenic and special precautions are required.

  • Treat with oseltamivir or zanamivir at usual doses.

Click here for Patient Education
NOTE: This is the Professional Version. CONSUMERS: Click here for the Consumer Version
Professionals also read
Test your knowledge
Human Papillomavirus (HPV) Virus
Human papillomavirus (HPV) infection is the most common sexually transmitted infection and can cause skin warts, genital warts, or certain cancers. Of the many different strains of HPV, which of the following is most likely to cause visible genital warts?
Download the Manuals App iOS ANDROID
Download the Manuals App iOS ANDROID
Download the Manuals App iOS ANDROID

Also of Interest