Pigs in the middle

日期:2017-06-12 04:01:31 作者:司城蜀 阅读:

By Jonathan Knight STRANGE mixtures of human and avian viruses have spawned some of the century’s worst pandemics, and researchers have long suspected that a third animal acts as carrier. Now a team of virologists think they know how pigs could be a mixing bowl for flu: their throats have receptors for viruses from humans and birds. Although new strains of influenza circulate each winter, they are usually just variants of existing human viruses. A rare exception was the chicken virus that infected some 18 people in Hong Kong last year, killing six of them. At least two 20th-century flu pandemics were caused by viruses that were part human and part avian. One of these, which caused the Hong Kong flu of 1968, was mostly human with two bird virus genes. The other, the cause of the Asian pandemic of 1957, had three bird genes. Such viruses can be more deadly than the annual variants because they are so different from anything the immune system has previously handled. Since bird viruses replicate poorly in humans and vice versa, researchers have long suspected that two strains must combine in a third animal before infecting people. Circumstantial evidence, such as frequent contact with both people and birds, pointed to the pig. “It was a great concept, but many people didn’t believe it in the beginning,” says Yoshihiro Kawaoka, a microbiologist at the University of Wisconsin in Madison. Kawaoka, Toshihiro Ito of Hokkaido University in Sapporo, Japan, and their colleagues tested epithelial cells from pig tracheas for flu virus receptors. Influenza particles start an infection by sticking to a glycoprotein on the surface of these cells. Bird viruses attach to one called alpha-2,3, whereas human viruses prefer alpha-2,6. The researchers found that the swine cells had both glycoproteins on their surface, meaning that both human and bird viruses can infect the pig. The team then examined isolates of a bird virus that had entered the European pig population in 1979. Viruses taken from European pigs before 1984 could recognise both the alpha-2,3 and the alpha-2,6 receptor. But those isolated after 1985 could only bind to alpha-2,6. “It now looks a lot more like the human virus,” says Kawaoka. In this month’s Journal of Virology (vol 72, p 7367), the researchers say the bird viruses may have exchanged genes with human viruses infecting the same animal, or they may have developed the ability to bind the alpha-2,6 receptor by random mutation. Ian Brown, an expert on pig flu viruses at the Central Veterinary Laboratory in Surrey, says, “Evidence is building in favour of a role for the pig in pandemic influenza.” He adds that it is not yet certain whether pigs were the source of the flu pandemics in 1957 and 1968 because there is no evidence of these strains in pigs prior to their appearance in humans. However, such evidence could easily have been missed, he says. The new study, together with recent concern that the world is due for another influenza pandemic it is ill prepared to fight (Focus, 31 January, p 18), suggests that swine flu should be closely monitored. “Clearly the pig presents a real danger,” Brown says. Kawaoka agrees. “The ideal thing would be to eradicate swine influenza,” he says. But according to European veterinary experts, modern high-density pig breeding practices make such a goal seem increasingly unrealistic (see p 18,