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Avian Flu

Understanding the differences between bird and seasonal human flu and the modes of viral transmission can help determine how real the risk is of an avian flu pandemic.

Part 1 of a two part series
Part Two will discuss the clinical severity of avian flu and similarities to previous flu pandemics.
Public attention and concern about contracting avian influenza has risen dramatically in recent years. This is primarily due to emerging epidemiological, scientific, and clinical evidence documenting the spread and severity of avian influenza, commonly referred to as bird flu. One reason for growing concern is that the data show that airborne diseases, ie, influenza, community-acquired pneumonia, tuberculosis, Legionnaires’ disease, and Severe Acute Respiratory Syndrome (SARS), continue to be commonly reported public health infections.

While a number of bacterial and viral diseases, such as Legionnaires’ Disease, SARS, and West Nile Virus infection, have triggered public health alarms with highly publicized outbreaks, the majority of the general public does not usually view clinical infection with influenza virus as a potentially severe, life-threatening public health menace. Contrary to this commonly held belief, influenza viruses have caused numerous recurrent epidemics and periodic pandemics for centuries throughout the world.1-3 Other health problems are related to the higher incidence of severe illness, hospitalization, and respiratory complications associated with seasonal influenza virus strains, especially in young children and elderly adults with fragile immune defenses. This is exemplified by the figures that show seasonal influenza epidemics are responsible for an average of approximately 36,000 deaths per year in the United States, despite efforts to increase the rates of influenza vaccination among those who are most susceptible to disease and serious sequelae.4,5 Most of the complications resulting from influenza involve secondary bacterial pneumonia and primarily affect children less than 2 years of age, frail elderly persons 65 years of age or older, or immunosuppressed persons.6,7

In the past 3 years, discussion of the potential risk and preparation for an impending influenza pandemic have received high priority by public health agencies, microbiologists, and health care professionals throughout the world. Investigations are focusing on influenza H5N1, the most threatening avian influenza viral subtype currently circulating in Asia. Although at present H5N1 influenza is an avian pandemic in domestic fowl, more than 130 people have been infected in Southeast Asia since 2003, with a fatality rate of more than 50%.5


Seasonal flu is a contagious respiratory disease caused by influenza viruses. In contrast, pandemic flu is a global outbreak that occurs when a new influenza type A virus subtype causes serious human disease and is easily transmitted from person-to-person. These new subtypes have never previously circulated through the human population. Viral subtypes that have not circulated among people for many decades can also trigger a pandemic.


  • All influenza viruses originate in birds, with most remaining there.
  • Natural hosts are wild birds.
  • Avian influenza strains far outnumber human strains.
  • Normally, an influenza strain that infects birds does not attack humans because it is unable to infect and grow in human cells.
  • A small number of avian strains can adapt (mutate) to where they can infect people.
  • Partial immunity from previous exposure to the strain can afford some protection for most people.
  • Occasionally, a strain that only infected birds will cross species relatively intact to cause widespread human infection.

Avian influenza is a contagious disease of animals caused by influenza viruses that normally only infect birds. These viruses are classified as type A influenza and are commonly found in migratory wild birds, most notably wild ducks. Influenza A viruses are perpetuated in wild birds, with hosts functioning as natural reservoirs for specific avian strains. Unlike other susceptible birds like chickens, turkeys, and domestic ducks, wild ducks do not become ill from carriage of the virus in the intestines. See Table 1 for a brief summary of avian influenza viruses.

Many of the avian strains are highly contagious. Infected birds can shed virus in feces, saliva, and nasal secretions. They are spread to other susceptible bird and animal hosts, including pigs and humans, primarily via fecal droppings, which contaminate inanimate surfaces or other environments. Airborne transmission can also occur from bird-to-bird as well as bird-to-other animal hosts from virus present in nasal and oral secretions. Influenza is not considered an eradicable disease. Surveillance, prevention, and infection control strategies are the only realistic public health goals.8


The two forms of disease that develop in birds are classified as either low pathogenic avian influenza (LPAI) or highly pathogenic avian influenza (HPAI), based on the severity of infection. Most avian strains cause mild illness, characterized by affected domestic birds showing ruffled feathers and reduced egg production. An extremely contagious, virulent strain occasionally emerges, causing a severe, rapidly fatal disease in birds. Many animals manifesting HPAI die the same day flu symptoms appear. Dissemination of these virulent viruses can efficiently spread from country to country by migration of carrier birds. In addition, recent outbreaks have been traced back to live poultry markets where viral transmission readily occurs between infected and healthy susceptible fowl that are in prolonged, close contact with each other. These commercial sites also provide excellent conditions for different avian influenza strains to swap genetic material and mutate to form new strains.


The current HPAI outbreaks, which began in Southeast Asia in 2003, constitute the largest and most severe poultry pandemic in history, simultaneously involving millions of infected birds in multiple countries. In addition, the etiologic H5N1 influenza subtype is quite resistant to eradication. Despite the death and destruction of an estimated 150 million birds, the virus is already endemic in many areas of Indonesia and Vietnam and in some parts of Cambodia, China, Thailand, and possibly Laos. Government officials and the World Health Organization (WHO) fear that control of avian influenza in poultry may take several more years to accomplish.9

During the investigation of the origins and evolution of H5N1 viruses, no evidence was found before 1997 showing that this type A avian subtype could infect humans and cause fatal disease.10 The precursor of the H5N1 virus that spread to humans was first detected in Guangdong, China, when it caused a number of infections and deaths in geese. Soon thereafter, 18 human cases of H5N1 influenza were diagnosed in Hong Kong, with six deaths attributed to the new subtype.11 Genetic analysis of the infectious goose H5N1 virus acquired genetic segments from a influenza viruses found in quail and a duck virus, before it was spread through a live poultry market in Hong Kong.


Contact with excretions or surfaces that have become contaminated with excretions by susceptible birds often results in viral infection. Movement of poultry and poultry products is the major way in which avian influenza is spread among poultry. In human cases of bird flu, the most likely route of transmission is direct contact with infected poultry or contaminated surfaces. Most of the human cases have occurred in areas where many households keep small poultry flocks, where the birds often run freely sometimes entering homes or sharing living areas with families.9,12

Even though the H5N1 influenza virus has rarely spread to humans, health care professionals all over the world are concerned that continued transmission of HPAI could pose a significant global threat if and when viral dissemination extends beyond countries in Southeast Asia. Of more immediate concern is the fact that when this avian virus has directly infected humans from poultry, resultant infection is severe and often fatal. The current H5N1 subtype has already been responsible for the largest number of severe human disease and deaths of any previous avian influenza virus.9


At the present time, there is no pandemic influenza in the world. Although, many clinicians, scientists, and epidemiologists around the world believe it is only a matter of time until the next flu pandemic develops. However, no one can predict when it will occur. Three influenza pandemics spread around the world in the 20th century subsequent to emergence of new influenza A subtypes.3 Three conditions must be met for pandemic influenza to occur: 1) a new influenza A virus is found to infect humans; 2) infection with this virus must cause serious disease in people; and 3) it can be spread easily from person-to-person on a global scale.13 The lack of any existing immunity within the population from previous exposure to cross-reacting influenza subtypes is a key factor in the onset and spread of disease. All prerequisites must be met before a pandemic can begin.

According to the WHO system for defining the six stages of a pandemic, we are currently in phase three: “human infection(s) with a new subtype, but no human-to-human spread, or at most rare instances of spread to a close contact.” This lethal influenza virus has met two of the prerequisites for progressing to a pandemic, lacking only efficient transmission from person-to-person. Many of those who are studying H5N1 and its geographic spread are concerned that as large numbers of opportunities for human infections continue to be present, the virus will move closer to accomplishing the last required adaptation.

For Additional Information, visit the following websites:


  1. Patterson KD. Pandemic and epidemic influenza, 1830-1848. Soc Sci Med. 1985;21:571-580.
  2. Ghendon Y. Introduction to pandemic influenza through history. Eur J Epidemiol. 1994;10:451-453.
  3. Kilbourne ED. Influenza pandemics of the 20th century. Emerging Infect Dis. 2006;12:9-14.
  4. Thompson WW, Shay DK, Weintraub E, et al. Mortality associated with influenza and respiratory syncytial virus in the United States. JAMA. 2003;289:179-186.
  5. Osterholm MT. Preparing for the next pandemic. New Engl J Med . 2005;352:1839-1840.
  6. Barker WH, Mullooly JP. Impact of epidemic type A influenza in defined adult population. Am J Epidemiol. 1980;112:798-811.
  7. Glezen WP. Serious morbidity and mortality associated with influenza epidemics. Epidemiol Rev. 1982;4:24-44.
  8. Webster RG. Influenza: an emerging disease. Emerging Infect Dis. 1998;4:436-411.
  9. WHO. Avian influenza frequently asked questions. Available at: Accessed February 23, 2006.
  10. Webster RG, Peiris M, Chen H, et al. H5N1 outbreaks and enzootic influenza. Emerging Infect Dis. 2006;12:3-8.
  11. de Jong JC, Claas EC, Osterhaus AD, Webster RG, Lim WL. A pandemic warning? Nature. 1997;389:554.
  12. CDC. Avian Influenza (Bird Flu). Available at: Accessed February 23, 2006.
  13. Treanor JJ. Influenza virus. In: Mandell G, Bennett JE, Dolin R. eds. Principles and Practice of Infectious Disease. 5th ed. Philadelphia: Churchill Livingstone; 2004:1823-1849.

From Dimensions of Dental Hygiene. March 2006;4(3): 14-16.

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