Twelve years ago, the World Health Organization launched the Global Polio Eradication Initiative, with 2000 as its target for eliminating the crippling scourge. At that time, an estimated 350,000 new cases of polio emerged each year. Though the incidence of polio has been dropping dramatically, even in this deadline year, 10 new cases crop up daily somewhere within the 30 countries where the disease remains endemic.
This tally only counts the cases triggered by infection with the virus naturally present in the environment. In addition to this so-called wild-type virus, rivers and sewage host another type—one derived from the very vaccines used to prevent polio. The oral vaccine’s live but attenuated virus in rare cases reverts to the disease-causing form. This vaccine-derived virus turns up in natural waters even in regions now certified free of the wild-type virus, argues virologist Hiromu Yoshida of Japan’s National Institute of Infectious Diseases in Tokyo.
In the Oct. 28 Lancet, Yoshida’s team reports finding infectious polio virus in 29 samples of Japanese sewage and 3 samples of river water. In each case, the researchers note, genetic sequencing of parts of the genome showed that the virus had mutated from live-virus vaccines.
The data confirm, Yoshida says, what had been reported periodically in the past—that an attenuated virus had mutated in the gut of its host, recovering much of its original “virulence.”
Although only trace amounts of such reactivated virus were found, they point to a low, persistent environmental threat, Yoshida told Science News. That threat will continue until injections of inactivated vaccine fully replace live, oral vaccines, he says. At present, Japan uses only live, oral vaccines, though it plans to switch to inactivated vaccine.
Such a switch is almost complete in the United States, notes Roland Sutter of the Centers for Disease Control and Prevention in Atlanta. It began earlier this year, after the American Academy of Pediatrics argued for a phaseout of the live vaccine. That vaccine had been triggering an average of eight cases of polio annually in children. The pediatricians deemed that rate too high now that the wild-type virus has been eradicated in this country, says Sutter.