Flu Hunter by Robert G Webster

Author:Robert G Webster [Webster, Robert G]
Language: eng
Format: epub
ISBN: 9781988531311
Publisher: Otago University Press
Published: 2018-01-15T07:00:00+00:00

Figure 11.2 The H5N1 influenza virus was first detected in geese in Guangdong in 1996 and caused up to 40 per cent of deaths in these birds. Subsequently the virus spread to domestic ducks in many of China’s coastal provinces, where it caused largely inapparent disease.

And the virus wasn’t just appearing among Hong Kong’s local waterfowl suppliers. Farms in coastal Chinese towns in Guangdong, Guangxi, Fujian, Zhejiang and Shanghai provinces were sampled from 1999 to 2002, and their apparently healthy ducks were found to be infected with H5N1 viruses, showing that those viruses were widespread (Figure 11.2).66 The study also clearly showed that the H5N1 viruses continued to evolve over time by obtaining components from other influenza viruses, as suspected, and they were now lethal to mice (tested experimentally). If they could kill mice (mammals), they could potentially infect humans.

In December 2002 there were outbreaks of H5N1 influenza in nature parks in Hong Kong, which killed exotic aquatic birds, including flamingos, as well as ducks and geese. Many parks were affected, indicating that the H5N1 had spread to wild, free-flying migratory birds. This particular virus strain was a lethal one for ducks (as experiments confirmed): infected ducks developed the characteristic head twisting and neurological signs of disease and had to be euthanised.

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During the following Northern Hemisphere winter of 2003–04, the H5N1 influenza virus finally took wing and spread across Asia, almost simultaneously infecting birds in Vietnam, Thailand, Indonesia, South Korea, Japan, Cambodia and Laos. That virus had acquired several new internal components and was designated genotype Z (the mixture of gene segments in an influenza virus can vary with gene segments acquired during reassortment/hybridisation). It still contained the original haemagglutinin protein from geese in Guangzhou but had acquired all of its seven other components, also from aquatic birds in China.67 This H5N1 virus had become entrenched in domestic ducks and then re-infected wild ducks, which contributed to its spread. These H5N1 Z genotype viruses also caused human infections in all of these countries. By 2004, 29 people were infected in Vietnam, and 20 people had died; in Thailand 17 people were infected and 12 people died.

H5N1 remained active in China and spread later in 2004 to Malaysia. All of the H5N1 influenza outbreaks in these different countries could be traced back to the Z genotype from China, but not to the same part of the country. For example, the poultry viruses that infected people in Thailand and Vietnam could be traced back genetically to the H5N1 virus from Hong Kong; the viruses infecting Indonesians could be traced to Yunnan Province in southern China.

So how did these H5N1 viruses that were actively evolving in ducks in China spread nearly concurrently across Asia? The easy explanation is that they were transmitted by wild migratory ducks and other waterfowl. In Hong Kong, the Z strain had been isolated from a dead little egret (Egretta garzetta), two dead grey herons (Ardea cinerea), a black-headed gull (Chroicocephalus ridibundus), a tree sparrow (Passer montanus) and a peregrine falcon (Falco peregrinus).

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