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CDC - Highly Pathogenic Avian Influenza A (H5N1) in People Information

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  • CDC - Highly Pathogenic Avian Influenza A (H5N1) in People Information

    Highly Pathogenic Avian Influenza A (H5N1) in People

    Current Summary

    Currently, highly pathogenic avian influenza A (HPAI) (H5N1) viruses are widespread among poultry in several countries in Asia and the Middle East. Infection in wild birds has also been reported in many countries. Poultry outbreaks of HPAI H5N1 virus in Europe, and Africa have also been reported. Although HPAI H5N1 virus infection of humans is rare, sporadic cases of human infection have been reported in 15 countries. When humans develop illness from HPAI H5N1 virus infection, severe respiratory illness (e.g. pneumonia and respiratory failure) and death may occur. Sporadic human cases have occurred after recent direct or close contact with infected poultry that were sick or dead, and nearly 600 human H5N1 cases have been reported in 15 countries to WHO since November 2003, resulting in approximately 60% mortality. Other H5N1 risk factors include visiting a live poultry market and prolonged, unprotected close contact with a sick H5N1 patient. For some H5N1 cases, the source of exposure to H5N1 virus is unknown. Indonesia, Vietnam and Egypt have reported the highest number of H5N1 cases to date. The majority of H5N1 cases have occurred among children and adults younger than 40 years old. Mortality has been highest in cases aged 10-19 years old. Most human H5N1 cases have presented late to medical care and have been hospitalized late in their illness with severe respiratory disease. However, some clinically mild H5N1 cases have been reported, especially in children. Clusters of human H5N1 cases ranging from 2-8 cases per cluster have been identified in several countries. Nearly all of the cluster cases have occurred among blood-related family members, especially those living in the same household. Whether such clusters are related to genetic or other factors is currently unknown. While most people in these clusters have been infected with H5N1 virus through direct or close contact with sick or dead poultry or wild birds, limited non sustained human-to-human transmission of H5N1 virus cannot be excluded and likely occurred in some clusters.

    The current cumulative number of confirmed human cases of highly pathogenic avian influenza A (H5N1) is available on the WHO Avian Influenza website. Despite the high mortality, human cases of H5N1 remain rare to date.


    Evolution of H5N1 Viruses

    Like all influenza A viruses, HPAI H5N1 viruses continue to evolve. The Centers for Disease Control and Prevention (CDC), the World Health Organization (WHO), World Organization for Animal Health (OIE), and Food and Agriculture Organization of the United Nations (FAO) conduct routine surveillance to monitor influenza viruses, including H5N1 viruses, for changes that may have implications for animal and public health. CDC and WHO surveillance efforts are geared toward human health. FAO and OIE are concerned with issues affecting food and agriculture.

    Changes in H5N1 viruses that WHO and CDC look for specifically are those that could increase the threat to human health. Over time, H5N1 viruses have evolved into different groups, called “clades.” Since early 2008, 20 different clades of H5N1 viruses have been identified, but 13 clades have not been detected since at least 2008. There has been no indication that these viruses are becoming more transmissible or more capable of causing serious illness in humans. While H5N1 viruses are spreading more widely among poultry and wild birds and this increases the possibility of human exposures to infected birds or poultry, it has not increased the ability of H5N1 viruses to infect and transmit between people.


    Antivirals for Treatment

    Genetic sequencing of some avian influenza A (H5N1) viruses isolated from human cases in Vietnam, Thailand, and Indonesia shows resistance to the antiviral medications amantadine and rimantadine, and these drugs are not recommended. This leaves two remaining antiviral medications (oseltamivir and zanamivir) that are still effective against currently circulating strains of H5N1 viruses. A small number of oseltamivir-resistant H5N1 virus infections of humans have been reported. Antiviral treatment is most beneficial when treatment is started as soon as possible after illness onset, but may still be beneficial when started later.

    Vaccine for Prevention

    As part of the U.S. government’s pandemic preparedness activities, H5N1 vaccine virus candidates have been prepared and some H5N1 vaccine has been manufactured and put in the U.S. strategic national stockpile (SNS) to protect humans against this virus if it were to change to infect humans and spread easily from person to person.

    Current seasonal influenza vaccines are not expected to provide protection against human infection with avian influenza A viruses, including H5N1 viruses. However, reducing seasonal influenza risk through influenza vaccination of persons who might be exposed to avian influenza viruses might reduce the theoretic risk for recombination of influenza A viruses of animal origin and human influenza A viruses by preventing seasonal influenza A virus infection within a human host.


    Assessment of Public Health Threat

    As the highly pathogenic avian influenza A (H5N1) epizootic (animal outbreak) is not expected to diminish significantly in the short term, sporadic human infections with H5N1 virus, resulting from direct or close contact with infected sick or dead poultry, are expected to continue to occur, including fatal cases. So far, the spread of H5N1 virus from person-to-person has been very rare, limited, and not sustained.

    To date, there is no evidence of genetic reassortment between human influenza A and H5N1 viruses. However, influenza A viruses circulating among poultry have the potential to recombine with human influenza A viruses.

    Therefore, this H5N1 epizootic continues to pose an important public health threat. There is little pre-existing natural immunity to H5N1 virus infection in the human population. If H5N1 viruses gain the ability for efficient and sustained transmission among humans, an influenza pandemic could result, with potentially high rates of illness and death worldwide.

    Human infection with novel or nonhuman influenza A viruses, including influenza A viruses of animal origin, is a nationally notifiable disease in the United States. Human infections with nonhuman or novel human influenza A virus should be identified quickly and investigated to determine possible sources of exposure, identify additional cases, and to evaluate the possibility of human-to-human transmission.

    Page last reviewed: November 25, 2011
    Page last updated: November 25, 2011
    Content source: Centers for Disease Control and Prevention, National Center for Immunization and Respiratory Diseases (NCIRD)
    Last edited by sharon sanders; December 6, 2011, 12:54 PM. Reason: added