Guano could be the answer.

“When the world is facing problems with climate change,” says Thi, “guano can restore soil quality and keep soil healthy.” That’s because, unlike chemical fertilizers, the guano contains essential micronutrients as her research on the effect of guano on plants found.

Neil Furey, who has studied bats in Southeast Asia for over 20 years, adds that “bat guano increases soil organic matter and water retention.”

What’s more, the bats responsible for this fecund fertilizer eat up the mosquitoes responsible for spreading deadly diseases, such as dengue fever. Once a neglected disease found only in tropical countries like Cambodia, dengue fever is emerging as a serious global health risk. To date, epidemics have been recorded in over 100 countries, according to the World Health Organization (WHO), placing over half the world’s population at risk.

Again, climate change is exacerbating the threat: rising temperatures allow dengue-carrying mosquitoes to set up home in parts of the world that were previously uninhabitable, including subtropical and even temperate regions like Europe and North America.

What’s more, according to new research published in the scientific journal Nature, even a small rise in temperature could spell disaster. A moderate increase in heat-trapping carbon dioxide could put 2 billion more people at risk of the deadly disease in 2080 compared to 2015, bringing the total to 60% of the world’s population, the study found.

Cambodia has a history with dengue fever as one of only nine countries that experienced a severe endemic before 1970. Between 2000 and 2011, the number of cases per year was around 13,000, says Dr. Vibol Chan, Climate Change and Health Coordinator at the national WHO office.

Now, things are going from bad to worse. According to data from Cambodia’s Ministry of Health, there’s been a dramatic spike in the number of cases in 2019. Six thousand cases were recorded in the first eight months of 2018. This year, 38,000 cases were recorded over the same period. The number now stands at over 62,000.

As mosquitoes push into new territories around the world, so too in Cambodia.

“This year,” Dr. Chan explains, “dengue fever has spread from urban to rural areas,” where over three-quarters of the population live, increasing the number of people at risk from 3.5 million to almost 11 million, says WHO-Cambodia.

When it comes to countries like Cambodia where dengue already has a foothold, new research suggests that elevated temperatures can intensify the virus through faster mosquito reproduction and biting rates. What’s more, once mosquitoes hatch out, higher temperatures can improve their odds of survival, leading to “longer transmission seasons and a greater number of human infections, more of which are expected to be severe.”

As with malaria, dengue fever affects children under five and pregnant women the most. Also, at risk are the elderly and “sick people whose immune systems are weak,” Dr. Chan explains. For them, the initial flu-like symptoms of high fever and nausea can quickly turn into a severe hemorrhagic form, which can kill within a few hours, according to the US-based Center for Disease Control and Prevention.

Unlike malaria, there is no commercially available cure for dengue fever. However, as Dr. Chan points out, both diseases are preventable. But the time for current disease controls may be running out as resistance to insecticides is emerging around Cambodia. The WHO is testing a new biological control: guppy fish, which can live in water storage containers and eat mosquito larvae, dramatically reducing their numbers.

Seng has his own mosquito gobbling force: bats.

Seng’s tens of thousands of bats eat tens of thousands of insects every night.

According to some studies, certain bat species can eat their own weight in insects, with a single bat consuming 200g (about half a pound) of insects in one night, says Chhin. By Furey’s calculations, just 1,000 of the bats living in Seng’s trees could eat between 9 kg (nearly 20 pounds) and 26 kg (57 pounds) of insects per night. Seng counts 70,000 bats in his trees, putting the number closer to 2 tones per night.

While specific research linking the presence of bats with the incidence of mosquito-borne diseases like dengue fever has not yet been conducted, as Chhin points out, “bats could reduce the disease burden, it’s one of the ecosystem services that they may provide.”

The value of bats’ insect control service is phenomenal: in Ratchaburi province in neighboring Thailand, the voracious appetites of cave-dwelling bats save local rice farmers at least $300,000 a year in pesticides. This ecosystem service could outweigh even the high value of guano sales, says the founder of Merlin Tuttle’s Bat Conservation, Merlin Tuttle, “the benefits from pest control may substantially exceed the economic value of guano fertilizer sales.”

Nonetheless, through guano, “people will come to know that bats are very important for the economy, and for agriculture,” says Chhin. “So, the people will try to conserve the bat,” in what he describes as a win-win situation.

For Seng, bats changed everything. “Farming bats has made my life much better,” he says. It’s something he’s eager to share with others.

“I am always happy to share my bat farming experience,” Seng smiles. “I want all people in Cambodia to start doing it.”

For now, bat farming is limited to a handful of provinces in Cambodia and a small area of neighboring Vietnam.

But the lesser Asiatic yellow house bat (Scotophilus kuhlii), the species found in Cambodian bat farms is common throughout the region, says Neil Furey, so there’s no reason why the practice couldn’t work elsewhere in Southeast Asia, and beyond. “I suspect the idea just hasn’t occurred elsewhere yet.”

For Tuttle, the practice offers great potential for promoting both bat conservation and local livelihoods.

Seng is ready to teach anyone interested in learning bat farming. “If they have any questions, they can ask,” he says, “or I will go and advise them on how to protect the bats because I love bats so much.”

Most of all, he wants his children to continue bat farming and be mindful of the bats’ contribution.

“One of my sons, who studied engineering, helped me build this house,” Seng says fondly. “I told him I wanted to build a bat symbol,” he says, gesturing towards his larger than life bat light feature. “This is like a souvenir for me.”

But Seng has even bigger plans. “We should erect a bat statue as a family symbol,” he says, “to remind them that they could come this far… thanks to bats.”

This story was made with support from Internews’ Earth Journalism Network.

Clarification: The name of Merlin Tuttle’s organization was updated January 9, 2019 to “Merlin Tuttle’s Bat Conservation.”

Citations

• Messina, J. P., Brady, O. J., Golding, N., Kraemer, M. U. G., Wint, G. R. W., Ray, S. E., … Hay, S. I. (2019). The current and future global distribution and population at risk of dengue. Nature Microbiology, 4(9), 1508–1515. doi: 10.1038/s41564-019-0476-8
• Ryan, S. J., Carlson, C. J., Mordecai, E. A., & Johnson, L. R. (2019). Global expansion and redistribution of Aedes-borne virus transmission risk with climate change. PLOS Neglected Tropical Diseases, 13(3). doi: 10.1371/journal.pntd.0007213