Troublesome cures
a dose of oral polio vaccine (opv) can relapse and turn into the deadly disease-causing virus, reveals a recent study conducted by researchers from Atlanta-based Centres for Disease Control and Prevention (cdc). The researchers studied a 1999 polio outbreak in Hispaniola, a Caribbean island. The outbreak had killed two children and paralysed a further 19. The researchers analysed genetic sequence of the virus that had affected each of these children. From this they found the origin of the deadly virus. "It came from a single dose of opv given to one child,' says Olen Kew, a virologist from cdc.
Normally, genes crucial to polio's virulence are switched off in opv. But during the Hispaniola outbreak, they had switched back, probably through random mutation. "This is hardly surprising because polio virus is one of the most rapidly evolving viruses we know,' says Bruce Aylward, leader of the World Health Organisation's polio eradication programme ( www.nature.com , March 20, 2002).
In the samples the researchers also found some genes that didn't belong to polio at all. According to the researchers, the offending virus had borrowed the genetic information from related gut pathogens called enteroviruses. This