The World Health Organization has reported two small, but extremely deadly, human outbreaks in Bangladesh of the Nipah virus -- a rare bug that first emerged in 1998 with devastating effect on Malaysia's pig-farming industry.

In its Weekly Epidemiological Record published April 23, the WHO said the virus was discovered in six Bangladeshi districts between Jan. 17 and April 14. Of the 53 people identified as having contracted the virus -- which typically causes extremely high fever, seizures and encephalitis -- 35 died, the Wall Street Jounral reported last week.

Nipah's high mortality rate -- 66% in this latest outbreak -- makes it one of the world's most lethal viruses for humans and has earned it a spot on a U.S. list of potential bioterrorism agents. However, the virus isn't known to be transmitted effectively between humans, suggesting that a widespread human outbreak in Bangladesh or elsewhere isn't likely. Smaller Nipah outbreaks in Bangladesh were recorded in 2001 and 2003.

Unlike the first appearance of Nipah in Malaysia in 1998-99, pigs don't appear to have been involved in transmitting the disease to humans in the most recent outbreak, the WHO said. Field investigations "failed to establish any link between these cases and sick pigs or other types of sick mammals," it said.

The WHO added, however, that some of the young Bangladeshi men killed by the virus had been collecting and eating fruit picked from trees before dawn. "This observation gave rise to the hypothesis that contamination could have occurred while eating the same fruits that fruit bats fed on during the night," the WHO said.

The WHO said preliminary studies of so-called "flying fox" bats in Bangladesh's Rajbari district "confirm that several Pteropus flying foxes have evidence of Nipah infection." Flying foxes, native to South and Southeast Asia, are so named because of their large size; one species has a 1½-meter wingspan.

The ultimate source of Nipah in nature -- what scientists call its "natural reservoir" -- has yet to be conclusively identified.

The latest findings in Bangladesh, corroborated by preliminary studies by the U.S. Centers for Disease Control and Prevention in Atlanta, are relevant to the work of scientists conducting studies of flying fox bats near the center of the first Nipah outbreak in Ipoh, Malaysia.

There, scientists working under a grant from the National Institutes of Health in the U.S. are seeking to prove a complex, yet compelling, theory: Humans contracted the virus from pigs; the pigs, in turn, contracted the virus from saliva on half-eaten fruit dropped into their pens by nocturnal flying fox bats.

Those scientists believe that Nipah -- which doesn't cause sickness in the bats -- emerged only because flying foxes were forced to forage for food near human settlements owing to the gradual eradication of their forest habitats.

The Nipah outbreak in Malaysia led to a mass slaughter of pigs in that country and killed 40% of the 257 people who contracted the virus.