Alternative names
Bird flu; Influenza virulent in birds; Avian influenza

Definition
Avian influenza (also known as bird flu) is a type of influenza virulent in birds. It was first identified in Italy in the early 1900s and is now known to exist worldwide.

Infection
The causative agent is the avian influenza (AI) virus. AI viruses all belong to the influenza virus A genus of the Orthomyxoviridae family and are negative-stranded, segmented RNA viruses.

Avian influenza spreads in the air and in manure. Wild fowl often act as resistant carriers, spreading it to more susceptible domestic stocks. It can also be transmitted by contaminated feed, water, equipment and clothing; however, there is no evidence that the virus can survive in well cooked meat.

Cats are also thought to be possible infection vectors for H5N1 strains of avian flu (Kuiken et al, 2004).

The incubation period is 3 to 5 days. Symptoms in animals vary, but virulent strains can cause death within a few days.

Avian influenza in humans
Of the 15 subtypes known, only subtypes H5, H7 and H9 are known to be capable of crossing the species barrier from birds to humans. It is feared that if the avian influenza virus undergoes antigenic shift with a human influenza virus, the new subtype created could be both highly contagious and highly lethal in humans. Such a subtype could cause a global influenza pandemic, similar to the Spanish Flu that killed over 20 million people in 1918 (though a variety of sources quote average figures even higher, up to 100 million in some cases). Many health experts are concerned that a virus that mutates to the point where it can cross the species barrier (i.e. from birds to humans) will inevitably mutate to the point where it can be transmitted from human to human. It is at that point that a pandemic becomes likely.

The first cases of avian influenza passed from birds to humans was H5N1 in 1997 in Hong Kong. 16 people were infected, of whom 4 died. The outbreak was limited to Hong Kong. All chickens in the territory were slaughtered.

In January 2004, a major new outbreak of H5N1 avian influenza surfaced in Vietnam and Thailand’s poultry industry, and within weeks spread to ten countries and regions in Asia, including Indonesia, South Korea, Japan and mainland China. Intensive efforts were undertaken to slaughter chickens, ducks and geese (over 40 million chickens alone were slaughtered in high-infection areas), and the outbreak was contained by March, but the total human death toll in Vietnam and Thailand was 23 people. In February 2004, avian influenza virus was detected in pigs in Vietnam, increasing fears of the emergence of new variant strains.

Fresh outbreaks in poultry were confirmed in Ayutthaya and Pathumthani provinces of Thailand, and Chaohu city in Anhui, China, in July 2004.

In North America, the presence of avian influenza was confirmed at several poultry farms in British Columbia in February 2004. As of April 2004, 18 farms have been quarantined to halt the spread of the virus. Two cases of humans with avian influenza have been confirmed in that region.

In August 2004 avian flu was confirmed in Kampung Pasir, Kelantan, Malaysia. Two chickens were confirmed to be carrying H5N1. As a result Singapore has imposed a ban on the importation of chickens and poultry products. Similarly the EU has imposed a ban on Malaysian poultry products. A cull of all poultry has been ordered by the government within a 10km radius of the location of this outbreak.

An outbreak of avian influenza in January 2005 affected 33 out of 64 cities and provinces in Vietnam, leading to the forced killing of nearly 1.2 million poultry. The 2005 outbreak caused 13 deaths in Vietnam and 1 death in Cambodia.

Vietnam and Thailand have seen several isolated cases where human to human transmission of the virus has been suspected. In one case the original carrier, who received the disease from a bird, was held by her mother for roughly 5 days as the young girl died. Shortly afterwards, the mother became ill and perished as well. In March, 2005 it was revealed that a nurse who had cared for an avian flu patient tested positive for the disease.

Prevention and treatment
Avian influenza in humans can be detected with standard influenza tests but these tests have not always proved reliable. In March 2005, the World Health Organization announced that seven Vietnamese who initially tested negative for bird flu were later found to have carried the virus. All seven have since recovered from the disease.

Antiviral drugs are sometimes effective in both preventing and treating the disease, but no virus has ever been really cured in medical history. Vaccines, however, take at least four months to produce and must be prepared for each subtype.

Increasing virulence
In July 2004 researchers, headed by H. Deng of the Harbin Veterinary Research Institute, Harbin, China and Professor Robert Webster of the St Jude Children’s Research Hospital, Memphis, Tennessee, reported results of experiments in which mice had been exposed to 21 isolates of confirmed H5N1 strains obtained from ducks in China between 1999 and 2002. They found “a clear temporal pattern of progressively increasing pathogenicity”.

Symptoms
In humans, it has been found that avian flu causes similar symptoms to other types of flu:

 
• fever  
• cough  
• sore throat  
• muscle aches  
• conjunctivitis  
• in severe cases of avian flu, it can cause severe breathing problems and pneumonia, and can be fatal.

Avian influenza
Also called fowl plague, avian flu, and bird flu. A highly contagious viral disease with up to 100% mortality in domestic fowl. Caused by influenza A virus subtypes H5 and H7. All types of birds are susceptible to the virus but outbreaks occur most often in chickens and turkeys. The infection may be brought by migratory wild birds which can carry the virus but show no signs of disease. Humans are only rarely affected.

Type A influenza viruses can infect several animal species aside from birds, including pigs, horses, seals and whales. Birds are an especially important species because all known subtypes of influenza A viruses circulate among wild birds, which are the natural hosts for influenza A viruses. Avian influenza viruses do not usually directly infect humans or circulate among humans.

Influenza A viruses can be divided into subtypes on the basis of their surface proteins - hemagglutinin (HA) and neuraminidase (NA). There are 15 known H subtypes. While all subtypes can be found in birds, only 3 subtypes of HA (H1, H2 and H3) and two subtypes of NA (N1 and N2) are known to have circulated widely in humans.

The symptoms in humans range from typical influenza-like symptoms (with fever, cough, sore throat and muscle aches) to eye infections, pneumonia, acute respiratory distress, viral pneumonia, and other severe and life-threatening complications.

Studies to date suggest that the prescription medications approved for human influenza strains would be effective in preventing avian influenza infection in humans.

All influenza viruses have the potential to change. It is possible that an avian influenza virus could change so that it could infect humans and could spread easily from person to person. Because these viruses do not commonly infect humans, there is little or no immune protection against them in the human population. If an avian virus were able to infect people and gain the ability to spread easily from person to person, an influenza pandemic, a global outbreak, could begin.

Medical Encyclopedia

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