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Australian Journal of Pharmacy : October 2006
pharmacy ed u cdi drug news • approved CPE The current concerns are now that the virus may mutate to an extent where it will be able to have a human-to-human possibility of infection whereas, at the moment, it remains only a bird-to-human infective virus. The current bird flu virus typing In May 1997, an H5N1 virus was isolated from a Hong Kong three-year old boy who later died of extensive pneumonia.11 By the end of 1997, a total of 18 people had become infected, six of whom subsequently died.1 The clinical features were typically those of influenza—fever and upper respiratory tract infection.1 However, some patients had severe complications including pneumonia, gastrointestinal symptoms, elevated liver enzymes and renal failure.1 In general, children fared better than adults.1 Studies suggested that the infection was spread by direct contact with birds and human-to-human transmission was limited to only a few cases, indicating that, at that time, the virus had not truly adapted to its human hosts.1 The slaughter of all poultry in Hong Kong successfully stopped a major outbreak of this infection.1 These human isolates from the Hong Kong outbreak were not reassortants. ‘Reassortants’ means that the virus can take components of both human and bird viruses which can allow it to be transmitted from human to human. In contrast, the 1957 and 1968 pandemics showed a combination of both bird and human viruses, although it was a strain which wholly originated from a Eurasian bird virus.12 From 1997 to 2002, H5N1 viruses continued to circulate in birds in south eastern China.13 In 2002, another H5N1 virus emerged which was highly pathogenic in ducks and other aquatic birds, a feature not seen previously in wild birds.14 This virus was now showing signs of antigenic drift. Antigenic drift It is important to distinguish an influenza pandemic from an influenza epidemic because the preparations for both these outbreaks are entirely different. Influenza epidemics are caused every few years by a phenomenon known as ‘antigenic drift’. This refers to minor changes in the virus usually at the haemagglutinin (H) site and the neuraminidase (N) site.16 Because the change is slight, there is enough residual immunity in the population to the original strain to result in partial immunity to the new, but similar, strain of virus. However, this resultant strain is sufficiently different to the original strain to need a new vaccine to be developed in order to be an appropriate preventive therapy. Although there is change to the virus, the change is not sufficient to create a significant increase in mortality or morbidity numbers.16 In influenza cases associated with antigenic drift, the epidemic influenza strains are identified early enough to create a sufficient 84 ? THE AUSTRALIAN JOURNAL OF PHARMACY, VOl.87 OCTOBER 2006 supply of vaccines to cope with the flu season.16 Mortality in epidemics usually occurs in the very old or very young where immunity in these populations may not be as strong as young healthy adults.16 Antigenic shift An antigenic shift is a much more serious change in a virus. This refers to a change in the virus to such an extent that there is no immunity in the general population.16 As a result, mortality figures are much higher and young adults may have mortality figures as high as, or higher than, the very young and the very old.16 This was reflected in the pandemic of 1918 where adults in the 20–40 year age cohort had the highest mortality rates.7 A pandemic strain which can be transmitted from human to human is likely to appear suddenly.16 Until this transformation takes place, researchers cannot totally forecast the change in the virus and a vaccine production from the time of appearance of the changed virus can take six-to-nine months and, in that time, the virus can spread across the world.17 Incidence of avian influenza The bird flu influenza outbreak has increased fears that a worldwide pandemic catastrophe is imminent. The bird flu epidemic first appeared in south- east Asia.16 Between January 2004 and April 2005 people have become infected in Thailand, Cambodia, Hong Kong and Vietnam.16 Although numbers infected during this period in south east Asia were small (79 reports), there was a 62 per cent mortality rate.16 In late 2005 and early 2006, there were reports that deaths had occurred in the rural areas of eastern Turkey. This was thought to be caused by migratory birds infecting households who cultivated domestic fowls for their own use and who had come in close contact with their own birds. There is still no evidence of transformation of the virus such that human-to- human infection occurs. Thus, H5N1 still has not achieved pandemic status. However, the expanding geographic distribution of bird flu is alarming, with recent outbreaks being reported in Kazakhstan, Mongolia, Russia and Turkey. While Australia is also at risk, since large colonies of wild birds migrate to Australia to breed, our country does not have farming communities close to wild breeding grounds where domestically raised birds could become infected. Indeed, most of the domestic chickens and ducks consumed by Australians are raised within enclosed sheds and, as such, are not exposed to infection by migratory wild birds. Transmission of influenza Human influenza is transmitted by persons inhaling infected light weight droplets, by direct touch contact from one person to