Wild ducks and other migratory(迁移的) birds could be important carriers of deadly bird flu, researchers say. Even so, the infectious-disease experts say there is no solid basis for killing wild birds to protect poultry and minimize the risk of human infection. The European team investigating the global spread of the H5N1 strain of avian influenza(禽流感)says certain duck species may be infecting wild bird populations. Geese and wading birds are also possible vectors(带菌者)of the virus, the team says. The team's study was led by Björn Olsen of Umea University in Sweden. Olsen runs Europe's largest wild-bird flu monitoring program. Studies have shown that influenza viruses in lake water, generally passed via bird feces(粪), can stay infectious for up to 30 days. The migration or feeding behavior of dabbling ducks could at least partially explain the spread of the H5N1 strain of bird flu, the researchers add.

This group of duck species includes mallards, teal, pintails, and others that feed at or near the surface, where viruses in water are most likely to be picked up. Perhaps as a result, dabblers have the highest known rates of avian influenza infection, the study says. For instance, nearly 13 percent of mallards tested positive for bird flu. Other species tested include the American black duck (18.1 percent), blue-winged teal (11.5 percent), and northern pintail (11.2 percent). However, bird flu viruses appear to exist in ducks in a low-pathogenic form, meaning infection doesn't usually lead to severe illness and death. "Dabbling ducks are for sure the prime hosts for low pathogenic viruses," said study co-author Ron Fouchier, a virologist at the Erasmus Medical Center in Rotterdam, Netherlands. "But the big question is, how much of our knowledge about these viruses can we translate to high-pathogenic viruses such as the H5N1 strain of bird flu?" In poultry avian viruses can mutate(变异)into more virulent influenza strains, including H5N1. If this mutated virus then finds its way back into wild populations, the birds could then spread the disease through migration. Some scientists have argued that wild birds infected with HN51 would be too ill to migrate. Swans, for instance, appear to be particularly vulnerable to the strain. "Swans apparently drop dead quite easily, but they are unlikely to be the vector because they are not going to fly very far if they are dead," Fouchier said. But the study team says that some birds that have been purposely infected for the sake of research show that wild birds can survive H5N1. "For some reason H5N1 has adapted so it no longer kills dabbling ducks," Fouchier said. This means the ducks may be able to spread the virus over a wide area. The study team says migratory geese may also be vectors, because they often graze in huge flocks, a practice that could encourage transmission. Migrating ducks, the researchers add, "could provide an intercontinental bridge" for bird flu to North America, which has not yet had any known cases of H5N1.

47. According to the author, what may be the possible carriers of bird flu?

48. The main sources of influenza viruses in lake water are ________, which may stay infectious for up to 30 days.

49. By saying "bird flu viruses appear to exist in ducks in a low-pathogenic form" (Para. 6), the author suggests that infection ____.

50. On what condition can the birds spread the influenza through migration?

51. According to the study team, ________ is a practice that can encourage transmission of the bird flu.