or animals, wild or domestic, were Nipah’s reservoir? Knowing what animal or animals a virus lives in prior to infecting humans helps us respond to it. Depending on the reservoir, we may have the potential to simply change farming practices or modify human behavior to avoid the critical contact that leads to viral exchanges, effectively cutting off the virus’s ability to enter humans.
Knowing that a microbe has the capacity to maintain itself in an animal reservoir also changes the way that we think about public health strategies. Microbes can jump in both directions, so while novel human microbes like Nipah originate in animals, established microbes also have the potential to cross back into animals. Animal reservoirs for established human bugs can potentially derail control efforts. In effect, if we eliminate a bug in humans in a particular region, but it lives on in animals, the microbe may have the potential to reemerge with deadly consequences. In order to truly eradicate a human pathogen, we must know if it can also live outside of humans.
When Nipah emerged in 1999, the scientists studying it moved quickly to home in on its reservoir. Over the years that followed, an intricate relationship among wild animals, domesticated animals, and plants revealed itself, a story that emphasizes the complex ways that domestication can provide new avenues for bugs to pass into people.
The Malaysian piggeries that Nipah entered are not small-scale affairs. They house thousands of pigs at very high densities, creating a ripe environment for viral spread. The farmers who raise the pigs work hard to maximize their income both from the pigs themselves, but also from the surrounding land. One of the practices in this area of southern Malaysia is to grow mango trees in and around piggeries, providing a second source of income to increase the viability of the farming enterprise.
In addition to producing delicious fruit for the farmers to sell, the mango trees attract the flying fox, a large and appropriately named bat with the scientific name Pteropus . This bat was the unexpected Nipah reservoir, the virus’s link to the wild. Remarkably, it now appears that the Pteropus bats, while consuming their mango suppers, urinate and drop partially eaten mango into the pig pens. The omnivorous pigs consume the Nipah-infected bat saliva and urine as they eat the mango. The virus then spreads quickly in the dense pig populations, which, because the animals are sometimes shipped from place to place, infect new piggeries and occasionally infect their human handlers. 5
Wahlberg’s Epauletted Fruit Bat ( Epomophorus wahlbergi ) eating mango. ( Dr. Merlin D. Tuttle / Bat Conservation International / Photo Researchers, Inc. )
Emerging thousands of years after the advent of domestication, Nipah illustrates the impact that domestication had on our relationship with microbes. The larger and more sedentary populations of humans that emerged following the domestication revolution were susceptible to outbreaks in ways that our predomestic ancestors never were. In the small mobile communities that dominated human life prior to agriculture, novel microbes that entered these communities from animals would often sweep through, killing certain individuals and leaving the rest of the small populations immune. At that point the viruses would effectively die out; a virus without a susceptible host is unable to survive.
As villages and towns formed around agricultural centers, they did not do so in isolation. Communities were connected, at first with footpaths, then roads. While we might think that these towns were separate functional entities, from the perspective of a microbe, they represented a single larger community. As this interconnected community of towns grew, it provided the first opportunity in human history for an acute virus to persist permanently in the human species.
* * *
Chronic viruses that live permanently within their hosts,
Knowing that a microbe has the capacity to maintain itself in an animal reservoir also changes the way that we think about public health strategies. Microbes can jump in both directions, so while novel human microbes like Nipah originate in animals, established microbes also have the potential to cross back into animals. Animal reservoirs for established human bugs can potentially derail control efforts. In effect, if we eliminate a bug in humans in a particular region, but it lives on in animals, the microbe may have the potential to reemerge with deadly consequences. In order to truly eradicate a human pathogen, we must know if it can also live outside of humans.
When Nipah emerged in 1999, the scientists studying it moved quickly to home in on its reservoir. Over the years that followed, an intricate relationship among wild animals, domesticated animals, and plants revealed itself, a story that emphasizes the complex ways that domestication can provide new avenues for bugs to pass into people.
The Malaysian piggeries that Nipah entered are not small-scale affairs. They house thousands of pigs at very high densities, creating a ripe environment for viral spread. The farmers who raise the pigs work hard to maximize their income both from the pigs themselves, but also from the surrounding land. One of the practices in this area of southern Malaysia is to grow mango trees in and around piggeries, providing a second source of income to increase the viability of the farming enterprise.
In addition to producing delicious fruit for the farmers to sell, the mango trees attract the flying fox, a large and appropriately named bat with the scientific name Pteropus . This bat was the unexpected Nipah reservoir, the virus’s link to the wild. Remarkably, it now appears that the Pteropus bats, while consuming their mango suppers, urinate and drop partially eaten mango into the pig pens. The omnivorous pigs consume the Nipah-infected bat saliva and urine as they eat the mango. The virus then spreads quickly in the dense pig populations, which, because the animals are sometimes shipped from place to place, infect new piggeries and occasionally infect their human handlers. 5
Wahlberg’s Epauletted Fruit Bat ( Epomophorus wahlbergi ) eating mango. ( Dr. Merlin D. Tuttle / Bat Conservation International / Photo Researchers, Inc. )
Emerging thousands of years after the advent of domestication, Nipah illustrates the impact that domestication had on our relationship with microbes. The larger and more sedentary populations of humans that emerged following the domestication revolution were susceptible to outbreaks in ways that our predomestic ancestors never were. In the small mobile communities that dominated human life prior to agriculture, novel microbes that entered these communities from animals would often sweep through, killing certain individuals and leaving the rest of the small populations immune. At that point the viruses would effectively die out; a virus without a susceptible host is unable to survive.
As villages and towns formed around agricultural centers, they did not do so in isolation. Communities were connected, at first with footpaths, then roads. While we might think that these towns were separate functional entities, from the perspective of a microbe, they represented a single larger community. As this interconnected community of towns grew, it provided the first opportunity in human history for an acute virus to persist permanently in the human species.
* * *
Chronic viruses that live permanently within their hosts,
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