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HPAI A H5 Virus Background and Clinical Illness

The HPAI H5 viruses that have infected birds in the United States have been caused by three subtypes to date: H5N1, H5N2, and H5N8 viruses. To date, no human infections with HPAI H5 viruses have been identified in the United States. Most poultry outbreaks in the United States have been caused by HPAI (H5N2) viruses.

  • The HPAI (H5N8) viruses detected in the United States are similar to viruses that were first reported on duck farms in China in 2009-2010. During 2014, similar HPAI (H5N8) viruses were found in wild birds and poultry in Korea and Japan. In November 2014, HPAI (H5N8) viruses in poultry and wild birds was reported in England, the Netherlands, Germany and Italy. No human cases of infection have been associated with these HPAI H5N8 viruses.
  • The HPAI (H5N2) viruses detected in the United States are similar to HPAI (H5N2) viruses first detected in early December 2014 on poultry farms in British Columbia province, Canada. This HPAI (H5N2) is a reassortant virus that combines genes from Eurasian H5 viruses and North American N2 viruses. No human cases of infection have been associated with either the North American or the Eurasian lineages of HPAI (H5N2) viruses.
  • The HPAI (H5N1) virus detected in the United States is a reassortant virus with genes from HPAI H5 Eurasian viruses and low pathogenic North American viruses.

HPAI H5 viruses infect the respiratory and gastrointestinal tracts of birds, can spread rapidly, and cause high mortality in infected poultry. Many birds have died and millions of chickens and turkeys in several states have been culled to try to control the outbreaks. Signs of HPAI in poultry can include sudden death; lack of energy, appetite, and coordination; purple discoloration or swelling of various body parts; diarrhea; nasal discharge; coughing; sneezing; and reduced egg production, or soft-shelled or misshapen eggs. For more information on avian influenza viruses in birds, please refer to the resources at this USDA website.

To date, no human infections with HPAI H5 viruses have been identified in the United States. Preliminary laboratory studies suggest that the HPAI H5 viruses causing the poultry outbreaks are not well-adapted to humans. However, sporadic cases of human respiratory illness with high mortality from infections with other closely related HPAI H5 viruses (e.g., H5N1, H5N6) have occurred in other countries. Most human infections with HPAI (H5N1) or (H5N6) viruses have occurred in persons not using appropriate PPE who had exposures consisting of either 1) direct physical contact with infected birds or surfaces contaminated by the viruses; 2) being in close proximity (e.g., within about 6 feet) to infected birds; or 3) visiting a live poultry market. Human infection with avian influenza viruses does not occur from eating properly cooked poultry or poultry products. Although the public health risk of avian-to-human transmission of HPAI H5 viruses circulating among birds in the United States is thought to be low, direct or close (e.g., within about 6 feet) contact to infected poultry or virus-contaminated environments without wearing PPE may increase the risk of human infection.

HPAI H5 virus infection of humans can start with signs and symptoms of uncomplicated seasonal influenza (e.g. fever, upper respiratory tract symptoms, myalgia) and progress to lower respiratory tract illness. Atypical presentations of fever and diarrhea preceding pneumonia have been reported. Severe pneumonia, multi-organ failure, encephalitis, and septic shock have been reported with HPAI (H5N1) and (H5N6) virus infections in other countries.  While a rare sign of seasonal influenza, conjunctivitis has been reported as a sign of avian influenza virus infection.

Exposed persons, including those who used personal protective equipment (PPE), should monitor their health starting on the first day of exposure and for ten days after the last exposure, and report any illness signs or symptoms to a physician and the local and state public health department as soon as possible.  Signs and symptoms to watch out for include fever or feeling feverish, cough, runny nose, sore throat, headache, muscle aches, eye redness, difficulty breathing, shortness of breath, and diarrhea. Consultation and close coordination with public health departments are recommended.