SARS exposed, pandemic influenza lurks.

Sir

The article by J Peiris and colleagues (April 19, p 1319), identifying a novel coronavirus as a possible cause of severe acute respiratory syndrome (SARS), signals the start of a long haul in coming to grips with this emerging disease.

While attention is focused on the SARS crisis, and rightly so, sight should not be lost of the fact that the triad of precursor avian influenza viruses (H5N1, H9N2, and H6N1) involved in the genesis of the highly pathogenic H5N1 virus that caused the incipient influenza pandemic in Hong Kong in 1997 persist variously in poultry in southeastern China—the region from which the SARS coronavirus emerged. In 1997, the world was probably one or two mutational events from a pandemic. The continued circulation of multiple genotypes of H5N1 virus, reassorting with other avian influenza viruses, could result in generation of an H5N1 virus with a gene constellation capable of infecting human beings.

In February, 2003—at the time of early SARS cases—there was a fatal case of H5N1 infection in Hong Kong, suggesting that some of these reassortant influenza viruses are close to causing a new pandemic; important information for baseline influenza pandemic preparedness. Thus, the spectre of a pandemic caused by H5N1 virus arising in the hypothetical pandemic influenza epicentre of southern China threatens; H9N2 and H6N1 viruses seem to be lower order candidates for pandemicity. Failure to capitalise on these gains could be to global detriment.

A pandemic H5N1 virus (or any other subtype of virus) could be seeded globally in a matter of days, spreading at a faster rate than the SARS coronavirus. The similarity of clinical manifestations of recent H5N1 and SARS fatal pneumonia cases (Y Guan, personal communication) could pose problems in differential diagnosis beyond Hong Kong. Health authorities worldwide need to have in place plans to differentiate infection by agent. Rapid, reliable, inexpensive diagnostic reagents need to be developed quickly.

H5N1 virus infection has proven fatal for chicken (and sometimes other poultry) as well as for human beings. With poultry now the major source of meat protein worldwide, this fact has implications for food supply and international trade. It also raises the possibility that human beings and chickens can be cross-infected in the absence of tight biosecurity, the virus potentially spreading in chickens in pandemic mode. Veterinary regulatory authorities need to be aware of the human-avian dimension of H5N1. Furthermore, they and their public-health colleagues might have to consider additional control strategies should the SARS coronavirus be shown to have an animal connection.