3 November 2008

The role of bats in disease transmission

Laura H. Kahn

Bats are considered mysterious creatures and often generate fear. Specifically, South American vampire bats feed on animal blood and possess a legendary lore. But more importantly, bats are the host species for deadly diseases such as rabies, Nipah and Hendra viruses, and SARS. There's also evidence that they continue to serve as sources of novel emerging viruses. Some statistics: While bats make up, on average, less than 2 percent of the natural reservoir for all human pathogens, they harbor about 6 percent of the emerging viruses. Yet despite the disease risks, bats are extremely important for public health and agriculture because they eat harmful insects and pollinate crops.

Unfortunately, fear has trumped understanding, and people are killing so many bats that many bat species have become endangered. Bat Conservation International has had some success at stemming the dwindling numbers of bats. But in addition to saving them, we need to do a better job of understanding their ecology, biology, and immunology, which will help protect fragile ecosystems, prevent further zoonotic disease transmission, and understand why bats harbor so many deadly viruses.

In Bangladesh, Nipah virus outbreaks have occurred after people drank infected raw date palm sap. Bats like the sap, too, and they often drink from the clay pots used to collect it from trees."

Not many scientists today study bats--probably a reason why so little is known about them. Part of the problem is that it's dangerous work. Researchers must capture and handle them, exposing themselves to potentially infectious urine, feces, saliva, blood, scratches, and bites. (Of course, this could be true of work with any mammal.) Fortunately, one study of bat biologists found that there wasn't any evidence of SARS transmission from the bats to the researchers. Another issue is that bat habitats can be difficult to reach. While they roost in natural settings such as caves, rock crevices, and tree cavities, they also roost in human-made structures such as bridges, mines, and tombs.

Overall, bats constitute about 20 percent of the 4,600 known species of mammals. This diversity of bat species is reflected in the different foods they eat--small animals, insects, plants, and blood. Most bats are nocturnal, but a few are diurnal. They're social creatures and live in colonies ranging in size from less than 10 individuals to more than 200,000 individuals. (See "Bats as a Continuing Source of Emerging Infections in Humans.")

How do they transmit disease? In the case of rabies, they do so through direct bites or from contact with saliva. In the cases of Ebola and Nipah virus, transmission can occur after eating the regurgitated stones of fruit. For example, apes might become infected with Ebola after consuming partially eaten fruits that bats contaminated with infected secretions. In Bangladesh, Nipah virus outbreaks have occurred after people drank infected raw date palm sap, a local delicacy. Bats like date palm sap, too, and they often drink from the clay pots used to collect the sap from trees, leaving behind both excrement and carcasses

We don't know much, however, about how bats' immune systems work. Because bat bones are hollow--allowing them to be light enough to fly--they're unable to produce B cells needed for immune function in their bone marrow like other mammals. Thus, we don't know where they make their B cells or how they're able to evade the deadly effects of the viruses they harbor.

Unfortunately, we're not going to find these answers any time soon because the vast majority of biomedical research is conducted on mice. In 1999, commercial breeders sold an estimated $200 million in rodents for research. Many of these mice are genetically altered in order to mimic human diseases. This strategy might be helpful in studying inherited and chronic diseases such as cancer, diabetes, and asthma, but it won't help us understand how and why animals such as bats harbor and transmit deadly diseases. Neither will simply chronicling new viruses--an activity akin to stamp collecting.

To actually prevent potentially deadly diseases from causing more outbreaks, we should invest more resources into understanding the animals that transmit them. Bats would be an excellent place to start.