1st fatal H5N5 human case unlikely to spark pandemic: Experts

Taipei, Nov. 29 (CNA) The world's first fatal human infection with the H5N5 avian influenza strain, reported in the United States earlier this month, appears to be an isolated case with a low risk of wider transmission, Taiwanese experts said this week.

Chao Day-yu (趙黛瑜), a professor of the Graduate Institute of Microbiology and Public Health at National Chung Hsing University (NCHU), said the infection was likely the result of a spillover pathway from wild birds to poultry and finally to humans.

Sharing her thoughts with Taiwan's Science Media Center (SMC) on Thursday, Chao said such a single, sporadic event carries a low probability of escalating into a pandemic.

Washington state health authorities confirmed on Nov. 21 that an elderly resident had died after becoming infected with the H5N5 virus -- a strain not previously known to infect humans.

Chao noted that historically, only a few avian influenza subtypes have caused significant human infections, including H5N1, which triggered an outbreak in Hong Kong in 1997, and H7N9, which has been confined to China. Other subtypes have occasionally been detected in people but have shown no signs of human-to-human transmission.

Chao cautioned, however, that avian influenza viruses are increasingly spilling over into a wider range of animal hosts. These include carnivorous and omnivorous wildlife, and even marine mammals -- a trend she said warrants heightened vigilance and preventive action.

Chao's colleague, NCHU distinguished professor Chang Poa-chun (張伯俊), added that seasonal human influenza viruses do not simply transform into avian influenza strains if passed to birds.

Such a change would require long-term, multistep evolutionary processes rather than a direct or immediate shift.

Avian influenza viruses fall under the category of influenza A, a designation that also includes other animal-origin influenza A viruses circulating in poultry and livestock such as cattle and pigs.

Any human infection with these non-seasonal strains is reported as a novel influenza A case, which is classified in Taiwan as a Category 5 notifiable infectious disease.