Bird flu, or avian influenza, is an infectious disease that in most cases affects only birds. But a subtype of the virus called H5N1 can migrate to humans and is sometimes fatal.

H5N1 is one of several hundred subtypes of the Type A influenza virus and is not the only one to have infected humans. The H7N7 and H9N2 subtypes have also been found in humans, but it is a highly pathogenic strain of H5N1 that showed up in Asia in 1996 and caught the attention of public health authorities around the world.

Human cases of H5N1 "remain rare and sporadic events, occurring mostly in areas where H5N1 viruses circulate regularly in poultry," according to the World Health Organization (WHO). But a recent controversial case involving H5N1 research has again raised fears about the virus.

U.S. and Dutch scientists managed to mutate the H5N1 virus to make it more transmissible in ferrets, raising concerns that similar mutations could make the virus easily transmissible in humans.

The highly pathogenic H5N1 subtype was first isolated in farmed goose in Guangdong province in China and within a year it had made its way to poultry farms and live-animal markets in Hong Kong.

This strain distinguished itself from other bird flu viruses, which are common in wild ducks and geese but are mostly "low pathogenic," meaning they don't usually cause illness or kill large numbers, because it could jump from wild waterfowl to domesticated poultry and kill large numbers of birds immediately.

The first human infections of H5N1 occurred in 1997 in Hong Kong from the same strain of the virus that caused the bout of bird flu in Hong Kong poultry. Eighteen people developed severe respiratory disease caused by the virus, and six died.

Symptoms

The symptoms of avian flu in humans are similar to other types of flu and include:

  • Fever
  • Fatigue
  • Cough
  • Sore throat
  • Eye infections
  • Muscle aches

Health officials determined that close contact with live infected poultry was the source of human infection in Hong Kong. It was also the first evidence that the H5N1 virus had been transmitted directly from birds to humans.

In the wake of the infections, officials ordered the destruction of Hong Kong's poultry population. More than 1.5 million birds were killed over three days.

Since then, more than 300 people have died from H5N1 and almost 600 are known to have been infected with the virus. According to the World Health Organization, human cases of H5N1 "remain rare and sporadic events, occurring mostly in areas where H5N1 viruses circulate regularly in poultry."

Humans usually contract H5N1 by touching sick or dead domestic poultry infected with the virus.

For a more complete history, see the WHO's timeline of H5N1 events.

Since it was first identified, H5N1 has migrated to at least 15 countries, with human cases showing up throughout Asia and Southeast Asia and also in Iraq, Turkey, Egypt, Azerbaijan, Djibouti and Nigeria.

Bird cases have spread even more widely, and the number of reported outbreaks of H5N1 in poultry and wild birds has increased since mid-2008, mostly in Asia, according to the WHO.

In November 2005, a less pathogenic strain of H5N1 was found in wild ducks in Manitoba and in a duck on a commercial farm in Chilliwack, B.C. Another case was reported in P.E.I. in June 2006. 

There have also been outbreaks of other types of avian flu in Canada that have attracted more than the usual attention because of H5N1 fears.

In February 2004, an outbreak of the H7N3 virus swept through B.C.'s poultry industry. The province ordered more than 17 million birds killed, and it was six months before B.C. was declared free of avian flu.

In 2010, a flu outbreak on a Manitoba turkey farm raised alarms about the possible spread of H5N1 and resulted in the cull of 8,200 turkeys. It turned out to be a different subtype of bird flu called H5N2.

Confirmed human cases of H5N1

CountryCases (as of Dec. 15, 2011)Deaths  (as of Dec. 15, 2011)
Azerbaijan85
Bangladesh30
Cambodia1816
China4026
Djibouti10
Egypt15553
Indonesia182150
Iraq32
Laos22
Burma (Myanmar)10
Nigeria11
Pakistan31
Thailand2517
Turkey124
Vietnam11959
Total573336

Source: World Health Organization

Meanwhile, as human cases of H5N1 continued to turn up around the world, the WHO began warning of a possible pandemic.

On Jan. 22, 2007, the agency's executive board opened its meeting with a warning about bird flu and said that as long as the virus continues to circulate among birds, the threat of a human pandemic will persist. The WHO also said that the world was still years away from effectively controlling the illness in the agricultural sector.

Since then, the agency has dialed back its warnings and classified H5N1 as being only in the third phase of the WHO's six-phase pandemic alert system. Phase 3 means that the virus has "caused sporadic cases or small clusters of disease in people but has not resulted in human-to-human transmission sufficient to sustain community-level outbreaks."

To date, 2006 has been the worst year in terms of absolute number of human deaths from bird flu (79 out of a total 115 human cases, a death rate of 69 per cent), but 2008 had the highest death rate (33 deaths out of a total 44 cases — a 75 per cent death rate).

In 2011, 53 per cent of people who contracted the virus died from it.

Since it was first spotted, the virus has also been found in animals other than birds. In December 2003 and January 2004, two tigers and two leopards at a Thai zoo that had fed on chicken carcasses died from H5N1. It has also mutated into different strains, which is normal for a virus, whose characteristics change rapidly and often. That makes development of vaccines to stop the spread more difficult.

No bird flu vaccine has been approved for commercial sale, but in 2007 and 2008, some European countries and the U.S. — but not Canada — purchased H5N1 vaccines developed by GlaxoSmithKline and Sanofi Pasteur for use in the event of a pandemic.

However, there were doubts about how effective such stockpiling would be as the vaccines were based on strains existing at the time, which would not necessarily be the strains circulating during a pandemic. Generally, it takes at least four to six months to develop vaccines to combat new strains of flu that emerge.

The WHO says influenza pandemics can be expected to occur three or four times each century, when new virus subtypes emerge and are readily transmitted from person to person. The last great pandemic occurred in 1918-19, when the Spanish flu swept the world, killing 40-50 million people, including more than 50,000 in Canada.

But while experts agree that another pandemic is inevitable, whether it'll be caused by bird flu remains an open question.