The family Orthomyxoviridae includes the influenza A viruses (Figure 3.20). The virions have two species of surface glycoprotein: a haemagglutinin (H) and a neuraminidase (N). There are 16 types of H antigen and nine types of N antigen, and there are many subtypes of each type. From time to time a virus emerges with a new combination of H and N genes formed by reassortment, and causes a pandemic (see Chapter 20).
The hosts of influenza A viruses are principally birds that frequent aquatic habitats. The birds (e.g. ducks, geese, gulls) acquire infections by ingestion or inhalation and the viruses infect their intestinal and/or respiratory tracts. Infection with most virus strains results in few or no signs of disease, but some strains are highly pathogenic and can kill their avian hosts. The viruses can be spread to new areas when the birds migrate.
Some influenza A viruses infect mammalian species including pigs, horses and humans; the respiratory tract is the main site of virus replication. Normally humans are infected only with viruses that have H type 1, 2 or 3 and N type 1 or 2. Patients are commonly very ill and some die, either as a direct result of the virus infection, or indirectly from secondary pathogens, which are able to infect as a result of damage to the respiratory epithelium.
The previous two paragraphs describe the normal situation, but exceptions occur: some avian strains of influenza A virus can be highly pathogenic in birds, can be transmitted from wild birds to domestic poultry and can be transmitted to humans (Figure 21.11). This was the situation in Hong Kong in 1997, when an H5N1 virus caused an outbreak of serious disease in poultry. Eighteen people also became infected with the virus and six of them died. In order to bring the outbreak to an end all poultry in Hong Kong were slaughtered.
Figure 21.11 Transmission of avian influenza A viruses. The viruses are present in wild birds from which they may be transmitted to poultry. Sometimes there is transmission to humans who have close contact with poultry.
H5N1 viruses appeared in a number of Asian countries in 2003, in Europe in 2005 and in Africa in 2006. Millions of ducks, chickens and turkeys died from the disease or were slaughtered; H5N1 viruses also infected humans, causing severe respiratory disease, culminating in death in many cases. At about the same time H9N2 viruses emerged in Asia, causing disease in poultry with some transmission to humans.
Most, if not all, of the human cases of 'bird flu' caused by H5N1 and H9N2 viruses were in people who work with poultry and who presumably became infected as a result of direct contact with virus on the birds themselves, or in their faeces. Like many of the animal coronaviruses that have infected humans (Section 21.5.1), these avian influenza viruses appeared to have little or no propensity for human-to-human transmission. H5N1 viruses have also infected other mammalian species including domestic cats; tigers and leopards at a zoo in Thailand died as result of infections that they acquired from eating virus-contaminated chickens.
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