02/15/2011 - 05:44

Deforestation and emerging diseases

Laura H. Kahn

Laura H. Kahn

A general internist who began her career in health care as a registered nurse, Kahn works on the research staff of Princeton University's ...

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In the late 1990s a deadly new disease emerged from the tropical forests of Malaysia, spread by fruit bats whose natural habitat had been destroyed by deforestation. The Malaysian government was unprepared for this new disease and subsequently bore high costs from the outbreak, including more than 100 human lives lost as well as an economically devastating collapse of its pig-farming industry. Eventually, the new scourge was identified and named: the Nipah virus.

Forests play a well-known, important role in protecting biodiversity and absorbing carbon dioxide emissions. A less-recognized fact is that maintaining forests can also protect human health by helping prevent the emergence of infectious diseases.

A number of infectious diseases are associated with deforestation, including yellow fever, dengue, Rocky Mountain spotted fever, and Lyme disease. Many of the diseases linked to forest loss are transmitted by insect vectors (mosquitoes that spread yellow fever, for example), but some are spread by direct contact. The emergence of the Nipah virus is a compelling example of how deforestation can contribute to the appearance of a deadly new disease.

A dangerous disease emerges from the forest. In 1997, approximately 5 million hectares (an area approximately twice the size of New Jersey) of tropical forest in Malaysia were slashed and burned to make room for pig farms, and the widespread burning caused a severe haze that blanketed much of the region. The haze coincided with a drought -- a combination of factors believed to have reduced the number of flowering and fruiting trees that the local fruit bat population relied on for survival. With their food sources diminished, the bats sought nourishment in orchards near the pig farms; the pigs in turn ate fruit contaminated with bat urine and saliva, transmitting the virus to the livestock.

The sick pigs developed respiratory illness and neurological abnormalities, and the densely packed pig farms likely contributed to the rapid pig-to-pig transmission of the virus. Farm workers subsequently experienced fever, nausea, vomiting, headache, and dizziness, and more than half of them also developed neurological abnormalities such as reduced consciousness, flaccid muscles, and spasms. Most of the victims had direct contact with the pigs.

Health authorities initially believed that they were dealing with an outbreak of Japanese encephalitis, a mosquito-borne disease that is a leading cause of viral encephalitis in Asia. Yet mosquito-control measures proved ineffective at containing the disease, and autopsy and epidemiologic findings were inconsistent with Japanese encephalitis. Furthermore, when farmers sold their sick pigs across Malaysia, the disease spread -- indicating that livestock, not mosquitoes, were responsible for the disease transmission. By early 1999, the disease had reached Singapore from peninsular Malaysia.

It wasn’t until March 1999 -- when a scientist at the University of Malaya identified the Nipah virus, a previously unknown pathogen -- that the crisis began to come under control. Singapore banned the import of pigs from Malaysia and shuttered its slaughterhouses, and Malaysia took the drastic but effective step of destroying more than 1 million pigs. But by then, the virus had killed 105 people in Malaysia and sickened 160 others; the mortality rate was almost 40 percent.

Crucial economic consequences. The Nipah outbreak devastated the Malaysian pig-farming industry. Before the outbreak, Malaysia had 2.4 million pigs; approximately 1.1 million of them had to be culled, costing about $97 million -- plus $35 million paid by the government in compensation to farmers. In addition, the export of pigs to Hong Kong and Singapore was halted, resulting in a loss of more than $100 million in trade for Malaysia. Pig farming in the region largely collapsed; today, it is allowed only in government-approved areas. In total, the outbreak cost the Malaysian government more than $450 million.

The Malaysian government provided free public health care to patients -- many of whom required intensive care during the initial stages of the illness -- which protected the affected families from destitution. However, most of the pig farms were family-run businesses that provided income and stability for entire communities; the collapse of the industry had a severe negative impact on their livelihoods.

Acknowledge the link between deforestation and disease. More than 10 years after the original outbreak, there is still no cure or vaccine for the Nipah virus. Many of the first survivors continue to suffer from chronic neurological problems. The disease is now known to also be transmittable from human to human, and it has spread to new locations. Since the first Malaysian outbreak, there have been a dozen Nipah virus outbreaks around South Asia. The most recent outbreak, occurring in two villages in Bangladesh, is ongoing and has killed 24 people so far.

New infectious diseases emerging from forests have the power to devastate economies, as the Nipah virus demonstrates. When factoring the consequences of deforestation, environmentalists, policy makers, and others would do well to remember the many costs -- including to human health and to the economy -- associated with emerging infectious diseases. Preserving the world's forests protects not just biodiversity, but also human lives.