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Avian influenza

Introduction to Avian Influenza  



Avian Influenza is a disease of wild birds and domestic poultry caused by many different subtypes of Type A influenza virus. The natural reservoir for Type A influenza viruses is wild water birds such as ducks and geese. New influenza A subtypes are continually emerging in the waterfowl population due to the constant mutation of the virus.  While avian influenza is caused by Type A viruses, seasonal influenza outbreaks in people, which occur almost every winter, are caused by either Type A and Type B influenza viruses. Influenza Type C viruses cause a mild respiratory illness in humans, but are not usually responsible for outbreaks of the flu. Type A viruses are found in both people and animals, whereas Type B viruses are normally only found in humans.

Influenza Subtypes

Influenza subtypes are named for two antigens present on the surface of the virus.  These are:

  • H (hemagglutinin) 
  • N (neuraminidase) 

There are 16 possible H antigens and 9 possible N antigens.  Virus subtypes are named H9N2, H5N1, etc, depending on their combination of antigens

Avian influenza viruses are classified in two ways. One makeup, which determines how it elicits the host's immune response (see more detailed description in box at right). The other is by the severity of the disease they cause in domestic poultry, which is designated as:

  • Low pathogenicity avian influenza (LPAI)
  • High pathogenicity avian influenza (HPAI)

While it is common practice to equate the term "avian influenza" directly with the highly pathogenic form of H5N1 virus in birds (i.e. HPAI H5N1), this is not accurate. There are many different H and N antigenic combinations (see box) that can occur in Type A influenza viruses that cause avian influenza, and the H and N antigens do not, in and of themselves, determine pathogenicity. In other words, there are both HPAI H5N1 and LPH5N1 avian influenza viruses.

The Disease in Birds

LPAI causes no or few clinical disease in chickens and other domestic birds. Mostly it causes reduced egg production and poor growth rates. It may be controlled by quarantine, destruction of affected flocks or, in some circumstances, by vaccination. LPAI caused by H5 and H7 strains can mutate to HPAI, and so control of LPAI is of great concern.

 HPAI causes a severe illness and death among infected birds, both domestic poultry and wild birds. HPAI in wild fowl and domestic poultry must be reported to state and federal authorities. It is controlled by the quarantine and destruction of affected flocks where possible. An outbreak of HPAI is very obvious to the flock owner. Close to 100 percent of the birds in a flock may die in a short period of time.

 Birds with HPAI show one or more of the following signs:

  • Sudden death without clinical signs
  • Lack of energy and appetite
  • Decreased or no egg production
  • Soft–shelled or misshapen eggs
  • Swelling of the head, eyelids, comb, wattles and hocks
  • Purple discoloration of the wattles, combs, and legs
  • Nasal discharge
  • Coughing, sneezing
  • Incoordination
  • Diarrhea

HPAI H5N2 is currently (2015) involved in outbreaks in the US and Canada resulting in the loss (destruction) of millions of turkeys and chickens.   The disease though devastating to the industry, at this time poses no threat to the food supply or human health, thus not the concern raised it  may cause a human pandemic as H5N1 - see below.

The H5N1 Strain of Avian Influenza

 The HPAI H5N1 strain of avian influenza was first identified in Hong Kong in 1997, and behaves as an HPAI strain. Starting in 2003 the virus spread widely among poultry flocks in a number of south-east Asian countries. In 2005, 2006 and 2007 it spread westward into Europe and Africa along the migration paths of wild birds, particularly waterfowl. No specific cause for the February 2007 outbreak of HPAI H5N1 in England has yet been identified, but it has been suggested the outbreak may be linked to frozen uncooked turkey meat imported from Hungary where there had been an outbreak of HPAI H5N1 a month earlier.  The meat did not come from birds who were sick and dying, but from birds thought to have subclinical infections, or birds that were infected, but showed no signs of illness.  Avian influenza virus has been shown to survive in a variety of environmental extremes including frozen lake water, thus the virus may survive in frozen uncooked product.  Some 50 non-domestic species are capable of being infected with the HPAI H5N1 virus, but aquatic birds play the major role in spreading the virus from one country to another.

HPAI H5N1 in the United States

Highly pathogenic Avian Influenza H5N1 virus has not been detected in poultry, wild birds, or humans in the United States or anywhere else in North America. However in the event HPAI should be enter or be detected in the US,  the U.S. Department of Agriculture’s (USDA) Animal and Plant Health Inspection Service (APHIS), in cooperation with Federal partners, have developed a National Response Plan.  This plan allows for a quick and effective response to an outbreak.  

 How would Avian Influenza H5N1 enter the U.S.?

The most likely route of entry for avian influenza into the United States is through wild birds traveling on their seasonal migratory pathways or flyways, and the most likely path would be through Alaska. In this region the East-Asian Flyway connects with at least one of the North American flyways used by migratory birds traveling South.  As with previous outbreaks of avian influenza in the U.S., domestic poultry would be infected by contact with wild birds.  The diagnosis of avian influenza would activate disease control plans by federal and state veterinary authorities, and by the commercial poultry industry, which has contingency plans for dealing with such an outbreak. 

The virus could also enter the U.S. in birds or poultry products smuggled into the US.

If a non-avian H5N1 influenza pandemic develops in humans, then infected humans may bring the virus into the country.


H5N1 and Food Safety

“There is no epidemiological evidence to date that avian influenza can be transmitted to humans through consumption of food, notably poultry and eggs. When one considers the low number of recorded human infections in relation to the high number of people …exposed to H5N1 virus-infected animals, it is clear that a readily accessible portal of entry [into the human body] does not exist.” [However,] there is mounting circumstantial evidence of infection via ingestion so, as a precautionary measure, this mode of transmission cannot be ruled out.

FDA plans to compile a list of foods and dietary supplements at high risk of contamination with avian influenza. Ready-to-eat foods that require little or no cooking might be contaminated with virus by workers who have avian influenza.

Home slaughtering and preparation of sick or dead poultry is extremely hazardous. Under no circumstances should anyone eat a sick bird or a bird that died for an unknown reason. HPAI H5N1 virus can be on the inside and/or outside of eggs laid by infected birds. Therefore, it is important not to consume raw or partially cooked eggs (i.e. no runny yolks).

These food safety rules will protect consumers from both HPAI H5N1 and other food-borne pathogens such as Salmonella:

  • Separate raw meat from cooked foods
  •  Do not use the same chopping block or utensils for raw meat and other foods
  • Do not put cooked meat back on the raw meat platter
  • Wash your hands with soap and hot water after handling raw meat
  • Cook food at or above 165 degrees Fahrenheit (i.e. no pink meat)
  • Fully cook eggs (i.e. no runny yolks)


  • Fully cooked or pasteurized eggs / egg products are safe to eat.
  • HPAI H5N1 is killed by cooking food at or above 165 degrees Fahrenheit (i.e. no pink meat). HPAI H5N1 is not killed by freezing or refrigeration.

Is it safe to eat poultry and eggs? --  World Health Organization


HPAI H5N1 and Hunting

Current circulating strains of HPAI H5N1 avian influenza are becoming more pathogenic to birds. Ducks infected with HPAI H5N1 virus are shedding more virus for longer periods without showing symptoms of illness (subclinical infections). This has implications for the role of ducks in transmitting disease to other birds and possibly to humans as well.    

Each person has to assess the risks for themselves—STAY INFORMED! Gloves always should be worn when handling birds. Wearing gloves prevents germs from entering your body through small cuts and scratches on your hands.

Based on what is known about HPAI H5N1 to date, wearing goggles and an N95-face mask (in addition to gloves) will maximize your protection and minimize the risk of inhaling clumps of virus-laden feces/feather dander or splashing virus-laden fluids (blood, wash water) into your eyes or mouth. This is the same type of protective equipment that officials wear when culling a poultry house infected with H5N1 virus.


Cats and Avian Influenza

Cats can become infected with HPAI H5N1 when they eat wild birds or domestic poultry with HPAI H5N1 avian influenza. For that reason, unusual mortality in cats should raise the suspicion of H5N1 in areas where birds are infected with the HPAI H5N1 virus.

Infected cats develop pneumonia and in some cases, meningitis or encephalitis (inflammation in the brain). Clinical signs include coughing, runny nose / watery eyes, difficulty breathing, sometimes vomiting and diarrhea, and rarely, sudden behavior changes. These cats then spread the infection to other cats and susceptible pets such as ferrets or hamsters. (Dogs can also become infected, presumably from eating an infected bird, but this is rare. To date, there have been no reported cases of a cat infected with HPAI H5N1 passing the infection to a human or a dog.)

The following recommendations will minimize the likelihood of a cat (or their owner) becoming infected with HPAI H5N1:

  • Keep vaccinations up-to-date on all cats, including barn cats
  • Limit contact with outdoor cats until we know more about H5N1
  • Cook poultry products before feeding to pets (e.g. chicken and rice diet)
  • Do not pick up stray cats and take them home

Previous Outbreaks of Avian Influenza in the U.S. and Surveillance for HPAI H5N1 in the U.S.  

The last outbreak of HPAI was an H5N2 outbreak in a 7,000 chicken commercial flock in Texas in 2004.

The previous U.S. outbreak of HPAI was in 1983 in Pennsylvania (also H5N2).  It took two years and the slaughter of 17 million birds to bring the outbreak under control.  No human cases were recorded in either of these outbreaks.

Evidence of LPAI H5N1 has been found in wild birds and domestic poultry in the U.S. in recent years and is not closely related to the more severe HPAI H5N1 circulating overseas.


Avian Influenza outbreaks in the U.S. with transmission to humans are documented by the Centers for Disease Control and Prevention (CDC).


Surveillance for H5N1 in the US


Wild Birds

Two Federal agencies and one state-Federal research partnership conduct surveillance for avian influenza in wild birds. The Department of Interior, U. S. Geological Survey screens wild birds for the presence of H5N1 with a particular concentration on the Alaskan flyway, Atlantic flyway and the Pacific flyway.  Since 2000, thousands of migratory birds have been tested in the Alaskan flyway, with lesser numbers in the Atlantic and Pacific flyways. In that period over twenty avian influenza viruses have been isolated, none of them highly pathogenic. The Wildlife Services division of the USDA Animal and Plant Health Inspection Service also conducts wild bird surveillance. Their surveillance efforts are not restricted to specific flyaways. Also the Avian Influenza Coordfinated Agriculture Project (AICAP), supported through the National Research Initiative and conducted by independent university scientists, has conducted targeted syrveillance of the Pacfic flyway.  The U.S. Interagency Strategic Plan for early detection of avian influenza in migratory wild birds was published in March 2006.


Highly Pathogenic Avian Influenza Early Detection  Data System (HEDDS) reports surveillance data on live birds, hunter-killed birds, sentinel and environmental sampling with over 44,000 samples tested in 2008. Thus far, HPAI H5N1 has not been found.  However, USGS reported October 2008, migratory bird species, including many waterfowl and shorebirds, that frequently carry low pathogenic avian influenza and migrate between continents may carry Asian strains of the virus along their migratory pathways to North America.

  • USGS researchers found that nearly half of influenza viruses isolated from northern pintail ducks in Alaska contained at least one of eight virus genes that were more closely related to Asian than North American strains.  None of the samples contained completely Asian-origin viruses and none were highly pathogenic forms that have caused deaths of domestic poultry and humans.
  • The central location of Alaska in relation to Asian and North American migratory flyways may explain the higher frequency of Asian lineages observed in this study in comparison to more southerly locations in North America.  Thus, continued surveillance for highly pathogenic viruses via sampling of wild birds in Alaska is warranted.

 Domestic Poultry

The National Avian Influenza Surveillance System (NAISS) is based on the Surveillance and Data Standards for USDA/APHIS/Veterinary Services (VS). These standards are designed to facilitate the collection, collation, validation, and analysis of accurate and representative surveillance data for a comprehensive surveillance program. Well-planned surveillance and data management at a national level will help ensure that the necessary data is efficiently collected and made available. This surveillance plan divides the domestic poultry population in the U.S. into four categories: the large-volume commercial poultry industry; the small-volume, but high-value, commercial poultry industry; the Live-Bird Marketing System (LBMS); and backyard poultry flocks.


Commercial Poultry Flock Surveillance Program: The program in commercial poultry is administered through the National Poultry Improvement Plan (NPIP) and includes monitoring of poultry production facilities and random testing of poultry flocks, including testing all birds that appear ill. As part of the program, USDA has worked with states to develop state response and containment plans. April 2009, the USDA Animal and Plant Health Inspection Service amended the sampling and testing procedures of NPIP to keep the provisions of the plan current with advancements in the poultry industry.


Under this surveillance plan to date, more than 245,000 flocks of meat-type poultry have been tested and none found positive for HPAI H5N1.

Backyard Flocks: Through the "Biosecurity for the Birds" program, USDA continues to encourage backyard and small poultry producers to strengthen biosecurity practices in order to prevent the introduction of avian influenza into their flocks and to report sick birds.

Live Bird Markets:  A federal control and prevention program targeting the live bird marketing system involves regular monitoring and surveillance of all facilities in the voluntarily participating states, which are California, Delaware, Florida, Georgia, Illinois, Indiana, Kentucky, Maine, Maryland, Massachusetts, Minnesota, Missouri, New Jersey, New York, North Carolina, Ohio, Pennsylvania, South Carolina, Texas, Virginia, and Vermont. Those states with live bird markets that do not participate in the federal program have a state poultry surveillance program in place.


Advice to Flock Owners and the Public  

What to do with dead wild birds

If you find dead migratory geese, ducks, swans or shorebirds do not pick up the birds for testing.  Call your local state or federal wildlife agency for instructions.  Dead wild birds should not be handled with bare hands.  If you do need to dispose of a dead bird, use gloves or a plastic bag turned inside out over your hand to pick up the bird, double bag it, and either bury it or dispose of it in the trash.

 Biosecurity Measures on the Farm

  • Keep an "all–in, all–out" philosophy of flock management.
  • Protect poultry flocks from coming into contact with wild or migratory birds.
  • Keep poultry away from any source of water that may have been contaminated by wild birds.
  • Permit only essential workers and vehicles to enter the farm.
  • Provide clean clothing and disinfection facilities for employees.
  • Thoroughly clean and disinfect equipment and vehicles (including tires and undercarriage) entering and leaving the farm.
  • Do not loan to, or borrow equipment or vehicles from, other farms.
  • Avoid visiting other poultry farms. If you do visit another farm or live bird market, change footwear and clothing before working with your own flock.
  • Do not bring birds from slaughter channels, especially live bird markets, back to the farm.

Tips for Small Flock Owners and 4-H

  • Learn the symptoms of Avian Influenza
  • Observe common-sense flock biosecurity
  • Restrict contact with your birds by people who have birds of their own
  • Protect your birds from contact with wild birds
  • Protect feed and feeders from wild birds
  • Use good hygiene: clean and disinfect equipment
  • Don't haul disease home: quarantine birds that have been at exhibitions
  • Buy birds from reputable sources, and quarantine them on arrival 
  • Don't borrow equipment from neighbors or other flock owners
  • Report sick or dead birds
  • Read "Biosecurity for the Birds"

Biosecurity Measures at Live Bird Markets:

  • Use plastic instead of wooden crates for easier cleaning.
  • Keep scales and floors clean of manure, feathers, and other debris.
  • Clean and disinfect all equipment, crates, and vehicles before returning them to the farm.
  • Keep incoming poultry separate from unsold birds, especially if birds are from different lots.
  • Clean and disinfect the marketplace after every day of sale.
  • Do not return unsold birds to the farm.

How to Report Sick or Dying Poultry:

Do one of the following:

  • Call your veterinarian
  • Call the State Veterinarian's office in your state
  • Call USDA Veterinary Services at 1-866-536-7593
  • Call your County Extension Educator

After exposure to wild birds, bird feathers, feces or other materials, wash hands with soap and water. Remove clothing and clean shoes immediately. Do not drink, eat, or use tobacco products before washing hands. For more information:

Occupational Risks of Becoming Infected With High Path HPAI H5N1

People who may routinely be exposed to HPAI H5N1 (Asian strain) Influenza virus include:

  • Veterinarians
  • Poultry producers / industry workers
  • Backyard hobby farmers
  • Live bird market employees
  • Bird fighting groups
  • Employees involved in disease control and eradication activities
  • Medical professionals treating HPAI H5N1-infected humans

For information about protecting yourself from occupational exposure to H5N1, see:


Avian Influenza vs. Bird Flu vs. Pandemic Flu  

Avian Influenza

 As noted before, avian influenza is a disease of birds caused by one of many different subtypes of avian influenza virus, including H5N1. Avian influenza infections in birds may be high pathogenicity (HPAI) or low pathogenicity (LPAI). Some types of the H5N1 virus behave as high pathogenicity strains.


Bird Flu

 The term "Bird Flu" has been used to refer to the disease in birds, but lately it's also been used to describe H5N1 infections in humans. Although hundreds of millions of wild birds and poultry in over 60 countries have been infected with HPAI H5N1, infection in humans is quite rare. When human infection does occur it is through close contact with sick or dead poultry, and the disease in humans is often fatal. Worldwide, there have been 628 cases of confirmed HPAI H5N1 infections in humans and 374 people have died of the disease. The World Health Organization keeps an up-to-date count of human infections and deaths caused by HPAI H5N1.   Indonesia and Viet Nam have had the highest infection and death rates from HPAI H5N1. 

 So far, the HPAI H5N1 virus has shown little or no ability to be transmitted from one human to another. Almost all the investigations of human cases have shown that infection occurred through direct handling of infected poultry, consumption of uncooked poultry products, or contact with virus-contaminated surfaces or materials including feathers. Occasionally more than one person in a household has been infected with HPAI H5N1 and, in those cases, it is possible that person-to-person transmission may have occurred.

Pandemic Flu

 A pandemic is an outbreak of a disease that occurs in many different countries at the same time. An epidemic is an infectious disease that spreads very rapidly. Pandemic influenza is a global epidemic of influenza that occurs when a new influenza A virus appears in humans and then spreads easily and rapidly from person-to-person worldwide.

 The current outbreak of avian flu among poultry in Asia, parts of Europe, the Middle East, and Africa is a source of growing concern among public health officials. The HPAI H5N1 virus infection in humans has characteristics similar to the HPAI H1N1 virus that caused the 1918 "Spanish Flu" pandemic.

 The current strain of HPAI H5N1 virus has not acquired the ability to transmit easily from human to human. If the virus changes in such a way that it can transmit easily from person to person, and retains the ability to cause serious disease in humans, it may cause an influenza pandemic. In that case, Pandemic Flu would become a reality.

At this time, there is no way to predict whether HPAI H5N1 will ever become a human pandemic strain.

 For a full discussion of Pandemic Influenza see the EDEN Pandemic Influenza Issues Page.


Last Updated:3/9/2017 6:56 AM
EDEN Avian Influenza Response Team

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