Avian Influenza and the Significance of Its Transmission to Humans-Information from WHO

Journal of Environmental Health, October 2005 | Go to article overview

Avian Influenza and the Significance of Its Transmission to Humans-Information from WHO


The Disease in Birds: Impact and Control Measures

Avian influenza is caused by Type A strains of the influenza virus. First identified in Italy more than 100 years ago, the disease occurs worldwide. All birds are thought to be susceptible, although some species are more resistant to infection than others. Infection causes a wide spectrum of symptoms, ranging from mild illness to a highly contagious, rapidly fatal disease resulting in severe epidemics. The latter is known as "highly pathogenic avian influenza."

Fifteen subtypes of influenza virus are known to infect birds, thus providing an extensive reservoir of influenza viruses. To date, all outbreaks of the highly pathogenic form have been caused by Influenza A viruses of subtypes H5 and H7.

Migratory waterfowl--most notably wild ducks--are the natural reservoir of avian influenza viruses, and these birds are also the most resistant to infection. Domestic poultry, including chickens and turkeys, are particularly susceptible. Direct or indirect contact of domestic flocks with wild migratory waterfowl has been implicated as a frequent cause of epidemics. Live-bird markets have also played an important role in epidemics.

Recent research has shown that viruses of low pathogenicity can, after circulating in a poultry population for sometimes short periods, mutate into highly pathogenic viruses. During a 1983-1984 epidemic in the United States, the H5N2 virus initially caused low mortality, but within six months became highly pathogenic, with mortality approaching 90 percent. Control of the outbreak required destruction of more than 17 million birds at a cost of nearly $65 million.

In the absence of prompt control measures backed by good surveillance, epidemics can last for years. An epidemic of H5N2 avian influenza that began in Mexico in 1992 started with low pathogenicity, evolved to the highly fatal form, and was not controlled until 1995.

A Constantly Mutating Virus: Drift and Shift

All Influenza A viruses, including those that cause seasonal epidemics of influenza in humans, are genetically labile and well adapted to elude host defenses. Influenza viruses lack mechanisms for the "proofreading" and repair of errors that occur during replication. As a result, the genetic composition of the viruses changes as they replicate in humans and animals, and the existing strain is replaced with a new antigenic variant. These constant, permanent, and usually small changes in the antigenic composition of Influenza A viruses are known as antigenic "drift."

The tendency of influenza viruses to undergo frequent and permanent antigenic changes necessitates constant monitoring of the global influenza situation and annual adjustments in the composition of influenza vaccines. Both activities have been a cornerstone of the World Health Organization (WHO) Global Influenza Programme since its inception in 1947.

Influenza viruses have a second characteristic of great public health concern: Influenza A viruses, including subtypes from different species, can swap or "reassort" genetic materials and merge. This reassortment process, known as antigenic "shift," results in a novel subtype different from both parent viruses. As populations have no immunity to the new subtype, and as no existing vaccines can confer protection, antigenic shift has historically resulted in highly lethal pandemics.

Conditions favorable for the emergence of antigenic shift have long been thought to involve humans living in close proximity to domestic poultry and pigs. Because pigs are susceptible to infection with both avian and mammalian viruses, including human strains, they can serve as a "mixing vessel" for the scrambling of genetic material from human and avian viruses.

Recent events have identified a second possible mechanism. Evidence is mounting that, for at least some of the 15 avian influenza virus subtypes circulating in bird populations, humans themselves can serve as the "mixing vessel.

The rest of this article is only available to active members of Questia

Sign up now for a free, 1-day trial and receive full access to:

  • Questia's entire collection
  • Automatic bibliography creation
  • More helpful research tools like notes, citations, and highlights
  • Ad-free environment

Already a member? Log in now.

Notes for this article

Add a new note
If you are trying to select text to create highlights or citations, remember that you must now click or tap on the first word, and then click or tap on the last word.
One moment ...
Project items

Items saved from this article

This article has been saved
Highlights (0)
Some of your highlights are legacy items.

Highlights saved before July 30, 2012 will not be displayed on their respective source pages.

You can easily re-create the highlights by opening the book page or article, selecting the text, and clicking “Highlight.”

Citations (0)
Some of your citations are legacy items.

Any citation created before July 30, 2012 will labeled as a “Cited page.” New citations will be saved as cited passages, pages or articles.

We also added the ability to view new citations from your projects or the book or article where you created them.

Notes (0)
Bookmarks (0)

You have no saved items from this article

Project items include:
  • Saved book/article
  • Highlights
  • Quotes/citations
  • Notes
  • Bookmarks
Notes
Cite this article

Cited article

Style
Citations are available only to our active members.
Sign up now to cite pages or passages in MLA, APA and Chicago citation styles.

(Einhorn, 1992, p. 25)

(Einhorn 25)

1

1. Lois J. Einhorn, Abraham Lincoln, the Orator: Penetrating the Lincoln Legend (Westport, CT: Greenwood Press, 1992), 25, http://www.questia.com/read/27419298.

Cited article

Avian Influenza and the Significance of Its Transmission to Humans-Information from WHO
Settings

Settings

Typeface
Text size Smaller Larger
Search within

Search within this article

Look up

Look up a word

  • Dictionary
  • Thesaurus
Please submit a word or phrase above.
Print this page

Print this page

Why can't I print more than one page at a time?

Full screen

matching results for page

Cited passage

Style
Citations are available only to our active members.
Sign up now to cite pages or passages in MLA, APA and Chicago citation styles.

"Portraying himself as an honest, ordinary person helped Lincoln identify with his audiences." (Einhorn, 1992, p. 25).

"Portraying himself as an honest, ordinary person helped Lincoln identify with his audiences." (Einhorn 25)

"Portraying himself as an honest, ordinary person helped Lincoln identify with his audiences."1

1. Lois J. Einhorn, Abraham Lincoln, the Orator: Penetrating the Lincoln Legend (Westport, CT: Greenwood Press, 1992), 25, http://www.questia.com/read/27419298.

Cited passage

Welcome to the new Questia Reader

The Questia Reader has been updated to provide you with an even better online reading experience.  It is now 100% Responsive, which means you can read our books and articles on any sized device you wish.  All of your favorite tools like notes, highlights, and citations are still here, but the way you select text has been updated to be easier to use, especially on touchscreen devices.  Here's how:

1. Click or tap the first word you want to select.
2. Click or tap the last word you want to select.

OK, got it!

Thanks for trying Questia!

Please continue trying out our research tools, but please note, full functionality is available only to our active members.

Your work will be lost once you leave this Web page.

For full access in an ad-free environment, sign up now for a FREE, 1-day trial.

Already a member? Log in now.