There will not be a bird flu pandemic

23-05-2006 | |

Avian influenza (H5N1) infection is predominantly a disease of commercial and subsistence poultry and some free-living bird species. This fact is demonstrated by records of only 208 diagnosed human cases in ten nations since 1997, albeit with 115 fatalities. By Simon Shane

Avian influenza (H5N1) infection is predominantly a disease of commercial and subsistence poultry and some free-living bird species. This fact is demonstrated by records of only 208 diagnosed human cases in ten nations since 1997, albeit with 115 fatalities.

Despite the high level of mortality, among the few unfortunate victims, the attack rate is infinitesimally small given the potential exposure of rural residents, bird-handlers, farmers and consumers purchasing live poultry in Asia, Eurasia and West Africa.

Since the first human cases in Hong Kong in 1997, doomsayers have forecast mutation of the avian virus to become infectious to humans, initiating a worldwide pandemic. Dire warnings were issued by national and international public health authorities.

David Nabarro, formerly Crisis Coordinator for the WHO, initially predicted as many as 50 to 150 million deaths. His pessimistic views were amplified by the media which has generated an unjustifiable anxiety reflected in decreased consumption of eggs and poultry meat.

The cost to poultry producers, particularly in affected countries, has been severe with over 250 million chickens and ducks culled or dead as a result of infection. It is estimated that farmers in Egypt have lost between $500 million to $1 billion as a result of decreased sales and the destruction of 20 million birds. Malaysia recently experienced a 20% drop in demand and a disproportionate 50% decrease in selling price of live poultry, responsible for a loss of $400 million. Farmers in India and Pakistan have experienced up to a 50% decrease in demand in some regions with a corresponding impact on selling prices. This is reflected through the entire chain of production from feed milling to breeding through growing and extending to distribution and live-bird sales.

Fear which has been generated by the scientifically unfounded and irresponsible statements by ‘concerned’ scientists has impacted the financial return and image of the poultry industry in many nations to a greater degree than the virus. The mid-May screening of a highly dramatised depiction of a hypothetical influenza epidemic in humans in the United States, failed to generate viewers despite extensive pre-show publicity. Apparently US families tuned to alternative channels when confronted with depictions of plastic-shrouded bodies being dumped in trenches and National Guard troops shooting citizens to enforce quarantines.

Responsible scientists reporting on the molecular biology and pathogenesis of H5N1 virus have provided encouraging opinions as to the remoteness of a human pandemic. Dr Anthony Fauci, Director of the US National Institute of Allergy and Infectious Diseases, predicted that the remote possibility of a diagnosis of H5N1 influenza in migratory birds or limited and controlled outbreaks in commercial poultry in North America would not represent a danger to consumers in the USA. He stated that it is not possible at the present time to predict whether an H5N1 pandemic would occur. This contrasts with statements from the WHO in 2004 warning of the ‘imminence’ and ‘inevitability’ of a pandemic.

Dr Julie Gerberding, Head of the US Centres for Disease Control and Prevention, stated that there was no evidence that H5N1 avian influenza would be the causal agent in the next influenza pandemic. Dr Mark Siegel, author of False Alarm: The Truth About Epidemic of Fear, who is affiliated to the New York University School of Medicine, emphasises the distinction between birds and humans.

Studies published in the April 21st edition of Science documents that the cells susceptible to attachment of H5N1 virus in humans are represented by the pneumocytes of the alveoli situated in the deepest part of the respiratory tract. In contrast, avian species are infected through the superficial epithelial lining of the nasal passages and trachea and are therefore more susceptible to infection by the aerogenous route. This study headed by a group of scientists at the Erasmus Medical Center, Rotterdam, by the aptly named Professor Thijs Kuiken, explains why severe pneumonia occurs in the small number of humans infected with H5N1.

A concurrent investigation of the pathogenicity of H5N1 virus has shown that the small number of victims carried receptor sites for the avian virus (alpha 2-3 sialic acid) as a result of a genetic predisposition. The family ‘clusters’ observed in Asia involving one parent and children are attributed to a combination of hereditary predisposition and intimate contact with the virus following contact with excreta or blood of infected chickens or ducks.

Dr Jeremy Farrar, an infectious disease specialist stationed in Vietnam who has the most experience in dealing with human cases of H5N1, considers that the virus is unlikely to be transmitted from poultry to humans. It is noted that in Vietnam, raw ducks’ blood is consumed at family gatherings as a delicacy and until the advent of the infection in poultry, virtually all ducks and chickens were sold live in wet markets in cities and regional centres.

Proponents of the pandemic scenario have pointed to the inevitability of worldwide spread of virus by migratory birds. Although there is evidence of transmission from some ducks and swans to free-ranging poultry, the problem is not as extensive as originally envisaged. To date, 455 individual birds have been diagnosed with avian influenza in 16 EU nations. Outbreaks among commercial poultry have been restricted to a few free-ranging flocks in Germany and France.

Ward Hagemeijer of the Netherlands-based Wetlands International Organisation has reported that an FAO surveillance study of 7,500 birds flying north to Europe from their wintering habitats in Africa confirmed freedom from H5N1 virus. It is now accepted that illegal movement of raw poultry products and live birds coupled with deficiencies in biosecurity are involved in cross-border transmission of AI, especially in Asia.

Outbreaks of highly-pathogenic avian influenza attributable to H5N1 can be effectively controlled by application of rapid diagnosis, elimination of foci of infection and extensive administration of inactivated vaccines. It is noted that China has the capacity to produce up to 40 billion doses of inactivated AI vaccine annually and has largely contained outbreaks in a population of 2 billion laying hens and a broiler industry which produces 10 million metric tons of processed meat annually.

Appropriate and considered concern regarding the possibility of H5N1 extending to human populations is however justified. Recent studies reported by Stevens, Tumpey and Taubenberger have indicated that a remixing event involving H1, H2 or H3 genes would be required to establish an affinity for alpha 2-6 sialic acid receptors which predominate on human respiratory epithelium.

It is conceivable that this could occur if humans infected with H1 or H3 influenza came into contact with affected poultry flocks. Collaborative studies conducted in Holland, the National Institutes of Health in the USA and a sister agency in the UK have clearly demonstrated the host-species barriers to infection with H5N1 virus. The sequences of mutations which may be required to successively adapt an influenza virus to a new host are complex and belie the simplistic predictions of the ‘inevitability’ of a pandemic.

Initiatives to improve the availability and effectiveness of influenza vaccines for human populations together with contingency plans to respond to a pandemic will benefit humanity faced with the prospect of a newly emerging influenza strain.

There is concern that by ‘crying wolf’ over SARS and the NIPAH virus, in addition to the dire predictions of outbreaks of cholera and other infections following the 2004 tsunami, WHO and regional health authorities may be losing credibility. At best their warnings should be evaluated and appropriate preventive measures should be taken. At worst, their warnings may be rejected as manifestations of scientific blackmail to generate funding and power.

From the perspective of mid-2006, it is unlikely that H5N1 avian influenza will become the ‘next human pandemic’. It is also clear that accepted principles of epidemiology can be applied successfully to the control and prevention of a severe pandemic. Application of emerging scientific knowledge can dispel fear and avert the precipitous decline in consumption of poultry meat and eggs in the event of future outbreaks of disease affecting free-living, subsistence and commercial poultry.

By: Simon Shane

Photo source: Winnipeg Free Press, March 23rd, 2006