Avian Influenza - How Serious is the Threat to Human Health?

For the past several months, news reports have centered on a specific strain of avian influenza, H5N1, and its role in the deaths of 32 people in Vietnam and Thailand and tens of millions of birds in Asia.

Coupled with widely reported comments by Dr. Shigeru Omi, regional director for Asia and the Pacific with the World Health Organization (WHO), concern has mounted that a possible global pandemic of avian influenza could emerge, threatening the lives of millions.

A December 8, 2004 statement on the WHO's web site ominously reinforced the concern:

"Influenza pandemics are recurring and unpredictable calamities. WHO and influenza experts worldwide are concerned that the recent appearance and widespread distribution of an avian influenza virus, Influenza A/H5N1, has the potential to ignite the next pandemic."

What is Avian Influenza?

First identified 100 years ago in Italy, avian influenza is an infectious disease found mainly in chickens, ducks and other birds. There are 15 virus subtypes. Most influenza viruses cause mild infection in birds yet the range of symptoms in birds varies greatly depending on the strain of virus. Infection with certain avian influenza A viruses, H5 and H7 strains, can cause widespread disease and death among some species of wild and especially domestic birds.

Yet what alarms health officials is how these viruses mutate into highly pathogenic viruses. In fact, influenza A viruses H5 and H7 have been identified as being responsible for several outbreaks due to their highly pathogenic nature. Two epidemics in the last 20 years illustrate the virus's potency and its impact on the bird population.

 

Years	Virus Strain	Epidemic Length	Results
1983-1984 H5N2 6 months 17 million birds dead or destroyed 1999-2001 H7N1 9 months 13 million birds dead or destroyed

Avian Influenza, H5N1 and Humans

Typically, avian influenza viruses do not infect species other than birds and pigs but in the last ten years there have been three documented outbreaks of human infection. All of the human cases have involved the H5N1 strain.

The H5N1 strain mutates rapidly and is capable of acquiring genes from viruses, thereby infecting other animal species. As a result, the spread of infection in birds increase the opportunities for direct infection to humans. Symptoms of avian influenza in humans have ranged from typical influenza-like symptoms (e.g., fever, cough, sore throat and muscle aches) to eye infections, pneumonia, acute respiratory distress, viral pneumonia, and other severe and life-threatening complications.

The first confirmation of the avian influenza H5N1 strain infecting humans was cited in Hong Kong in 1997. In this particular instance, the H5N1 strain caused severe respiratory disease in 18 humans, resulting in six deaths. Health officials believed that the infection in humans was propelled by a concurrent epidemic of a highly pathogenic avian influenza in Hong Kong's poultry population.

An investigation revealed that the source of the outbreak and subsequent human infection was the close contact between live infected poultry and humans. This link was confirmed by studies at the genetic level, which showed that the virus had been transmitted directly from birds to humans. Many experts believed that a pandemic was fortunately averted, by the rapid destruction - within three days - of Hong Kong's entire poultry population, estimated at around 1.5 million birds.

To global health officials this was a seismic event. For the first time it was realized that the deadly avian influenza virus had the capability to wreak its havoc on humans. A second far more minor H5N1 avian influenza outbreak took place in February 2003 resulting in the death of one person in Southern China.

While anecdotal evidence of human infection by avian influenza was supported by the two aforementioned outbreaks, laboratory research also showed that isolates from this virus had caused severe disease in humans because of the virulent composition of their pathogens. In addition, birds that survived infection and excreted the virus for at least 10 days, either orally or through their feces, facilitated spread of the virus.

In mid-December of 2003 a third epidemic of the highly pathogenic avian influenza of H5N1, was identified in the Republic of Korea. This epidemic has spread to other Asian countries, causing considerable concern among world health officials.

What is the Threat of Human to Human Transmission?

H5N1 variants have demonstrated a capacity to directly infect humans. Of the 32 human deaths in 2004 attributed to this most recent epidemic, only one has been verified as stemming from human-to-human transmission, according to data from the WHO (the only instance of human-to-human transmission of the 32 human deaths in 2004 involved a mother who had cradled her dying daughter all night).

With the human death toll from this latest epidemic showing no signs of abating, agreement exists in the global health community of the risk for further casualties. Dr. Omi's recent comment that a global pandemic is "very, very likely" marked the first time a health official had publicly voiced such concern.

While Dr. Omi is not alone in his worry about a possible pandemic, some scientists are not as prepared to publicly sound alarm and question the potential ease with which an outbreak could take place. Dr. Mark Peiris, a top influenza researcher at Hong Kong University, told the New York Times last month that he wondered whether the disease could maintain its lethalness and ability to readily transmit from human to human.

Health officials believe that there are several reasons why the virus could be spread from human to human.

• The virus has proved quite adept at mixing genetic material with other viruses
• The disease has begun to survive in domesticated ducks making it harder for farmers to identify which animals to destroy
• If more humans become infected over time, the likelihood also increases that humans, if concurrently infected with human and avian influenza strains, could serve as the "mixing vessel" for the emergence of a novel subtype with sufficient human genes to be easily transmitted from person to person.

As the year 2004 comes to a conclusion, disconcerting signs continue. Just two weeks ago, a dead gray heron was found near Hong Kong's border with China and tested positive for avian influenza. Health officials are bracing themselves for an uncertain year ahead.

Given the current threat, WHO has urged all countries to develop or update their influenza pandemic preparedness plans for responding to the widespread socioeconomic disruptions that would result from having large numbers of people unwell or dying.

For further information from the WHO regarding avian influenza, please go to: http://www.who.int/csr/disease/avian_influenza/en/