Avian flu is several types of influenza that typically infect birds. The flu that has been making headlines in the United States is a virus called H5N1.
Some influenza viruses carried by birds cause only mild infections and are classified as low pathogenic viruses. Instead, the H5N1 It usually kills birds that contract it, which is why it is classified as highly pathogenic avian flu.
To complicate things though avian flu viruses They mainly affect birds, they can also spread to other animals, including humans. Human infections with bird flu viruses are rare, and are often what scientists call dead-end infections because they are not usually transmitted from person to person. However, the first death from bird flu in the United States has already been reported.
How does bird flu spread?
Birds infected by avian influenza viruses can transmit them to other animals, and possibly to humans, through two different routes of infection:
• Directly from infected birds or environments contaminated with avian influenza A viruses.
• Through an intermediate host organism, such as another animal.
Direct infection can occur through exposure to the saliva, mucous membranes or feces of infected birds. Human infections with avian influenza are rare; However, infections in humans can occur when enough of the virus enters a person’s eyes, nose, or mouth, or when the person inhales it.
People who have prolonged unprotected contact (i.e., without respiratory or eye protection) with infected birds or places contaminated by their mucous membranes, saliva, or feces may be at increased risk of infection with avian influenza virus.
Infection through a host organism is not as direct or likely, but it is possible. Influenza A viruses have eight separate gene segments. The segmented genome allows influenza A viruses from different species to mix genes (reassortant) and create a new virus if influenza A viruses of two different species infect the same person or animal at the same time.
One possible way the viruses could be reassorted could be if a pig was infected with a human influenza A virus and an avian influenza A virus at the same time. The newly replicated viruses would regroup to produce a new influenza A virus with some genes from the human virus and some from the avian virus.
The resulting new virus could infect humans and spread easily from person to person, but it could have proteins on its surface (hemagglutinin or neuraminidase) that are different from those present in current influenza viruses circulating among humans. This could make it look like a flu virus “new” for humans, never before detected.
This type of major change in influenza A viruses is known as ”antigenic variation”. It is called major antigenic variation when a new subtype of influenza A virus that most people have little or no immune protection against infects humans. If this new influenza A virus causes illness in people and spreads easily and steadily among people, then an influenza pandemic occurs.
Although there has not been a “variation” of this type in relation to avian influenza viruses, such a “variation” did occur in the spring of 2009 when an H1N1 virus emerged with genes from North American pigs, Eurasian pigs, humans and birds that infected people and It spread rapidly, causing a pandemic. When these larger variations occur, most people have little or no immunity to the new virus.
It is also possible for the genetic reassortment process to occur in a person with a concomitant infection with a avian influenza virus and a human influenza A virus. The genetic information from these viruses could be reassembled to create a new influenza A virus with a hemagglutinin and/or neuraminidase gene from the avian influenza virus and other genes from the human virus.
Influenza A viruses with a hemagglutinin against which humans have little or no immunity that reassortant with a human influenza virus are more likely to cause sustained human-to-human transmission in addition to having pandemic potential. recovered from humans and animals infected by avian influenza A viruses in order to identify genetic reassortments if they occur.
Although it is not common for people to be directly infected with an influenza A virus spread by animals, sporadic infections in humans and some outbreaks caused by certain avian influenza A viruses and swine influenza A viruses have been reported.
(With information from CDC)
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