Budding particles of Nipah virus assemble near the surface of a cell.
The mystery began when pigs on large farms in Malaysia
hacking so loudly that their owners called it a "one-mile cough." Nerve damage was also cropping up in some of the animals. No one knew why. Up to 5 percent of pigs in affected herds were dying, and the illness was spreading like wildfire. Locals named it Nipah, after the place where it was first identified.
More alarming news followed. People, most of them pig handlers, started falling ill. Nipah caused fevers so high that some victims suffered brain inflammation and seizures. Nearly half of those sickened—105 out of 265 cases—died. Through the sale of pigs, the illness continued to spread for several months throughout Malaysia and Singapore.
The history of this disease, which first struck Malaysia in 1999, offers a cautionary tale of how a potential pandemic was averted. Before SARS, before worries about widespread avian flu, the Nipah virus infected humans with surprising ease. Swift action by Malaysian and international public-health authorities, including the Centers for Disease Control, kept the Nipah outbreak under control for several years. But the disease is having a disturbing second act in Bangladesh. Untangling the reasons for Nipah's resurgence exposes the complex threat of new animal-borne diseases and raises difficult questions about our ability to control them.
The Nipah illness is just one of over 40 new diseases that have emerged since the 1970s. Others are Ebola in Africa, hantavirus pulmonary syndrome in North America, Venezuelan equine encephalitis in South America, Hendra disease in Australia, and lethal avian flu in Asia. Why are these diseases appearing now? Three-quarters of new human diseases actually originate in animals. As people move into formerly wild areas, local pathogens increasingly come into contact with new domesticated animal hosts—freshly arrived pigs, chickens, horses—and, eventually, humans.
In the case of the Nipah outbreak, public-health officials acted swiftly and effectively. Singapore banned the import of pigs from Malaysia, pigs were culled, and pig farms were closed or demolished. In January 1999, peninsular Malaysia had 2.4 million pigs and 1,800 pig farms; by August of that year, 1.1 million pigs had been killed, 956 farms had been destroyed, and 48 more had closed. An epidemic was avoided, but at a cost of 36,000 jobs and $120 million in exports.
Stopping the epidemic was only one part of the control strategy, however. The other step was to detect the organism causing the illness. In March 1999, Malaysian researcher Kaw Bing Chua identified it as a member of the paramyxovirus family. This was a major clue. Within the previous four years, two similar viruses had appeared among pigs, horses, and humans in Australia, seemingly out of nowhere. The most likely vector, strange as it seemed, was the flying fox, a large fruit bat of the genus Pteropus. Beautiful creatures with soulful eyes, flying foxes play a crucial role in maintaining tropical forests because they ingest and distribute the seeds of local trees.
As suspected, the fruit bats in the region were the only wildlife to test positive for the virus, making them the pathogen's most likely reservoir. But why would the virus emerge now, when bats and people had shared the territory for millennia?
Looking at the Malaysian landscape and the changes that had taken place in recent years, Chua, along with colleagues Peter Daszak, Jonathan Epstein, and an international team of Malaysian, Australian, and American scientists, considered the possibilities. The proliferation of large-scale pig farming appeared to be a critical factor. Although pig farming is a longtime practice in Malaysia, the industry had recently grown from many small family farms to include a few intensive operations that were large enough to satisfy the demand from non-Muslim communities in Malaysia, Singapore, and other parts of Southeast Asia. The circumstances of the Nipah outbreak suggest that the virus needs a large pig population to maintain itself long enough to have a chance of spreading to humans. The farm on which the Nipah virus emerged had over 30,000 pigs, and the branches of fruit trees hung over the walls of their pens, giving bats and pigs plenty of opportunity to come into close contact.
Fortunately, Nipah virus in Malaysia did not appear to be transmitted between people, and the culling of pigs greatly reduced the odds of transmission from pig to human. Still, Pteropus bats have an extensive range, so researchers began looking for cases in other countries where they live. In the winter of 2001, an outbreak occurred in Bangladesh, and the local species, Pteropus giganteus, was found to harbor Nipah. Worse, in April 2001, a farmer from a village in the Meherpur District came down with the symptoms of Nipah and died six days later. Soon his wife, son, brother, and sister, all sharing the same small household, came down with the illness. Surprisingly, no clusters of sick animals like the coughing pigs in Malaysia turned up.
More outbreaks followed in 2003. Then, between February and April in 2004, 36 people in the Faridpur District became ill with Nipah, and 27 died. The higher death rate could reflect changes in the virus, poorer general health in the victims, less-effective health care, or a combination of factors. More ominously, at least six of the victims developed acute respiratory distress syndrome, a condition in which fluid builds up in the lungs and prevents normal breathing. These symptoms had not been a feature of the disease in humans before.
So far, the human clusters are small in size. The most recent outbreak was in 2005, when 11 of the 12 victims died. Most had consumed raw date-palm juice, which is tapped from cuts made at the tops of trees where clay pots are fastened to collect the juice overnight. Bats roosting near the village naturally sipped from the pots, leaving traces of their Nipah-laden saliva, which people would ingest the next morning.
As the virus circulates and evolves, researchers are racing to identify all the points connecting it with bats and people. The dense populations of humans and flying foxes in a country like Bangladesh may provide the conditions under which the Nipah virus will finally acquire the ability to explode into a pandemic. At that point, it won't matter that there are no flying foxes in America: Nipah will be as close as the next plane from Dhaka.
So while avian flu continues to make headlines, Nipah is also a virus to watch. The good news is that preventing new outbreaks may be as simple as changing the way palm juice is harvested. The larger lesson is that we are in a race everywhere to cut off the pathogen's access to human victims before it establishes a fast track in the local populations.