February 15, 2012


 Bats increasingly seen as vectors

By Greg Cima

Gerald T. Keusch, MD, said bats are a "largely unknown, underinvestigated, poorly understood, now-definitive vector for new emerging viruses."

He suggested the animals' significance as vectors could make them the "mosquitoes of the 21st century."

Peter Daszak, PhD, said few researchers were examining diseases in bats 20 years ago. But the animals have gained attention and attracted study from numerous groups as having a role in outbreaks of diseases caused by Hendra, Nipah, Lyssa, Menangle, and Ebola viruses and the coronavirus responsible for severe acute respiratory syndrome. 

gray-headed flying fox
Flying foxes such as this gray-headed flying fox and the
spectacled flying fox below are thought to be reservoirs for
Hendra virus in Australia, and Pteropus bats may be a
reservoir for Nipah virus in Malaysia. (Photos courtesy of
the Centers for Disease Control and

Dr. Keusch is a professor of medicine and international health at Boston University and an associate director of the National Emerging Diseases Laboratory. Dr. Daszak is president of EcoHealth Alliance, which conducts research and fieldwork at the intersection of health and conservation. Both are members of the Institute of Medicine Forum on Microbial Threats, and they made their comments during the Dec. 13-14, 2011, meeting "Improving Food Safety through One Health" in Washington, D.C.

A Hendra virus outbreak in 1994 killed one person and 13 horses in Queensland, Australia, Dr. Daszak said. Menangle virus outbreaks among pigs during summer 1997 caused stillbirths and deformities and sickened two workers in Sydney. Nipah virus killed more than 100 people during an outbreak in Malaysia in 1999, and authorities culled about one million pigs in response to the outbreak, he said.

Stephen Luby, MD, an epidemiologist who directs the Centre for Communicable Diseases at the International Centre for Diarrhoeal Disease Research in Bangladesh, described in a presentation at the meeting a series of Nipah virus outbreaks in Malaysia and Bangladesh. In both countries, the virus was likely spread by bats that had fed on plants near human food sources. 

spectacled flying fox
Spectacled flying fox

The Malaysia outbreak started in September 1998 with acute febrile encephalitis and deaths among workers at pig farms, Dr. Luby said. By May 1999, the outbreak had killed 109 of the 283 people sickened.

The virus likely passed from bats to pigs when bats ate fruit from trees near pig housing, dropping fruit contaminated with bat saliva and urine, Dr. Luby said. Pigs likely transmitted the virus to people.

A 2005 Nipah outbreak in Bangladesh killed 11 of 12 people affected, Dr. Luby said. Epidemiologic investigation indicates the outbreak was one of many likely connected with date palm sap collection, which involves shaving segments of date palm trees and collecting running sap in suspended containers.

All of the Nipah illnesses recorded in Bangladesh between 2001 and 2007, for example, occurred between December and May. Date palm sap is typically collected from November to March, Dr. Luby said. Observation in response to a 2008 outbreak found that, among seven trees watched over the course of a week, a mean of 15 bats visited the trees nightly and bats licked sap from the shaved areas an average of 8.4 times nightly. Pteropus bats can shed Nipah virus in their saliva.

Date palm sap is typically cooked into molasses, but raw sap is a delicacy, Dr. Luby said. A child who became infected with Nipah virus and died had drunk raw sap daily, and relatives reported hearing bats in the source trees and finding bat excrement on collection pots.

Because Nipah virus has been shown to infect people in close contact with a patient, Dr. Luby noted that a person could travel while incubating the virus and spread it to other people.

Bangladesh's high population density has reduced food availability for bats and pushed them to feed near humans, and these opportunistic feeders are sharing a food source in a landscape changed by humans, Dr. Luby said. Nipah viruses were probably unrecognized for centuries, and opportunities for human infections increased with encroachment into wilderness.

Dr. Daszak said "wet markets" in China bring together diverse animal species for sale as meat along with pathogens from those animals. In markets where SARS emerged, tests on swab specimens from bats uncovered SARS-like coronoviruses. SARS clearly originated from bats, and the test results suggest the animals are natural reservoirs for SARS-like viruses, he said.

Modernizing food production by trying to farm wildlife, as has been done with many animals sold in wet markets, provides opportunities for new pathogens to spread to people and cause disease, Dr. Daszak said. The risks are compounded by proximity with varied food production systems and connections through global food trade.

Dr. Keusch said difficulties in establishing experimental bat colonies has hindered study of bat transmission of pathogens. In addition, species differences across continents add to the difficulties in simulating natural conditions in controlled environments.

But bats should receive more attention, given their role in transmission of new or recently discovered viruses such as Nipah and the lack of knowledge about what microorganisms are carried by bat species, Dr. Keusch said. He thinks the global community should help establish surveillance and laboratory capacity in areas where bat-source pathogens could emerge, to provide clues before diseases manifest from populations of animals, particularly those humans eat, cultivate, or domesticate.

Dr. Keusch was co-chair of an Institute of Medicine and National Research Council committee that produced the 2009 report "Sustaining Global Surveillance and Response to Emerging Zoonotic Diseases." He expressed concern that, while the report describes a need for global zoonotic disease surveillance and ways international entities could alleviate economic impacts on countries that report outbreaks, targeted surveillance systems are being established for the "emerging infection du jour," as seen with recent influenza virus outbreaks.

The report, he said, "kind of sits on people's shelves, and nobody acts on it."