In 2014, researchers with the National Museum of Natural History in Paris working in the northern part of Uganda’s Kibale National Park, noticed something very wrong with the chimpanzees and baboons in the area: their noses were flattened, with nostrils that were abnormally small, or sometimes absent altogether. Their faces were concave in the middle. At the time, researchers estimated that around 10 percent of the chimpanzee population in that part of the park had these facial deformities, otherwise known as dysplasia. Two years later, that estimate jumped up to 25 percent.

Kibale National Park, known as the Sebitoli region, is a protected area about 300 square miles in size and one of the most biodiverse regions in Africa. With wet tropical forests in the north and woodlands and savannahs in the south, the park’s range of ecosystems can play host to hundreds of different species of trees and birds.

There are more than a dozen different species of primates living in the park, ranging from the black-and-white colobus to the L'Hoest’s monkey, and they’ve been studied for over 25 years. Some are observed daily. Until recently, primates with facial deformities were a rarity, spotted only twice before 2014. But by 2016, researchers working in Sebitoli had calculated that 25 percent of the chimpanzees in that area had severe physical deformities, as did 17 percent of the baboons. It was a striking anomaly: just 9 miles away, primates were perfectly healthy.

Along with flat noses and abnormally small nostrils, a number of primates were also missing fingers. Some had patchy, light-colored fur. One female had a cleft lip and some baboons had extra openings near their nostrils. A few of the females appeared to have reproductive problems, not having produced any offspring well into adulthood and not displaying sexual activity or the genital swelling that indicates ovulation. All of the observed primates and their deformities were described in a recent study published in Science of the Total Environment.

A few of the females appeared to have reproductive problems, not having produced any offspring well into adulthood

Some researchers wondered if the primates had succumbed to yaws – a tropical, bacterial infection that can result in similar facial deformities. But that disease typically comes with lesions that are apparent prior to the dysplasia, symptoms not observed on the chimpanzees and baboons in the area. So, they began to look for a cause outside of the forest.

Researchers began to wonder if pesticides in the farmland that surrounds the Sebitoli area were a culprit. “That was one possibility,” says Colin Chapman, an author of the study and a professor in the Department of Anthropology at McGill University, “And it was a possibility we could look into.”

Because chimpanzees are protected, testing the primates themselves wasn’t an option. While primates can be observed and studied in the field, researchers aren’t allowed to physically interfere with them — to take a blood sample, for example — or remove them for observation elsewhere. They can’t even get close enough to touch them. So, researchers have to find ways to collect information around them. Here, the research team tested fresh maize seeds and stems as well as the soil from nearby farms. Soil samples from surrounding fields and river sediments were collected too. The team also tested fish in and outside of the Sebitoli region for traces of pesticide.

Nearly every sample of fresh maize seeds and stems had levels of chlorpyrifos that were higher than authorized. Chlorpyrifos is an insecticide that’s recently been a subject of controversy in the US. Used since the 1960s, chlorpyrifos works on insects by overexciting their nervous system, eventually leading to death. At high enough doses, it has similar effects in humans. Researchers have spent decades telling the Environmental Protection Agency (EPA) that the chemical is dangerous and should be uniformly banned, citing evidence linking chlorpyrifos to neurodevelopmental problems in children, who are more vulnerable to the chemical than adults. A recent EPA report showed the agency had accepted the findings about the chemical’s dangerous impact and appeared to be on track to ban it, but in March, EPA Administrator Scott Pruitt took a meeting with the largest US producer of chlorpyrifos, Dow Chemical, and has since denied the proposed ban.

From 2000 to 2014, gross production values for tea in Uganda more than doubled

Along with chlorpyrifos, the researchers also found DDT and its traces in and around the farms. Chlorpyrifos, a DDT byproduct, and another insecticide – imidacloprid – were also detected in fish near the farms. Fish living deeper in the park didn’t have any detectable levels of pesticides.

The researchers concluded that the pesticides used on farms were a likely culprit behind the facial dysplasia. The primates are affected by the same watershed that impacts the fish and in addition, primates in the area are known to sneak into neighboring farms to eat seeds and crops at night.

While farming in the Sebitoli area isn’t new, pesticide use is. “In general, in Africa there’s not a lot of fertilizer or pesticide use because it’s too expensive,” says Chapman. “In the rural areas, generally most people are subsistence farmers,” says Kenneth Arinaitwe, a researcher at Makerere University in Uganda and an Alexander von Humboldt visiting researcher at the Helmholtz Centre for Environmental Research in Germany. And the subsistence farmers often don’t have enough money to pay for pesticides, says Arinaitwe. But commercial farmers do, and the Sebitoli region has a number of commercial tea plantations. From 2000 to 2014, gross production values for tea in Uganda more than doubled.

Arinaitwe says that this increase in the horticultural sector of Uganda has added to the pesticide use, but long-term farming is a problem as well. Omeja Patrick, a senior researcher at Makerere University, says that the quality of the soil has become an issue. “The soils before were much better, more fertile than now,” he says. “Also, we have had situations where many pests and diseases have come up. When you do not spray your crops, there is a very high likelihood of you losing so much crops to these pests.” That’s a particularly pressing problem when poor soil is already limiting crop growth. According to data from the Food and Agriculture Organization of the United Nations, the value of pesticide imports into Uganda increased by over 500 percent between 2000 and 2014.

Like most countries with extensive agrarian sectors, Uganda has had a complicated relationship with pesticides. Take DDT, for example: DDT was heavily used as a pesticide around the world in the 1940s and 1950s, and is credited with the eradication of diseases like malaria and typhus in Europe and the US. But its many dangerous health and environmental effects led the US to ban the chemical in the 1970s.

The WHO reports that 100 percent of the Ugandan population is at risk of contracting malaria

In 2004, Uganda agreed to uphold the Stockholm Convention, a UN treaty aiming to eliminate the use of persistent toxic chemicals like DDT. Under the agreement, DDT is only allowed for use in the case of controlling malaria, an exception Uganda and at least eight other signatories have opted into. Malaria continues to be a very real threat in Uganda: in 2015, 90 percent of malaria cases and 92 percent of malaria deaths occurred in Africa, and the World Health Organization (WHO) reports that 100 percent of the Ugandan population is at risk of contracting the disease. WHO’s 2016 malaria report noted that Uganda carried 18 percent of the malaria burden in southern and eastern Africa. It’s the leading cause of death in the country and accounts for over 28 percent of hospital admissions.

But since 2000, Uganda’s malaria cases and deaths have dropped significantly, by about 50 percent. Part of that is due to increased use of insecticide-treated mosquito nets and part of it is due to indoor pesticide spraying, which included DDT for a brief period from 2008 to 2010.

Uganda is now faced with the challenge of effectively managing dangerous pesticide use: In the country, pesticides are largely controlled by the Agricultural Chemicals Board, which is under the Ministry of Agriculture, Animal Industry, and Fisheries. But a handful of other agencies also play some sort of role in managing pesticides and it’s not always clear who has authority over certain situations.

Under the Agricultural Chemicals Control Act, only registered and labelled pesticides can be imported or sold. But enforcement isn’t easy. Even though the law mandates that pesticide retailers must register with the Ministry, many don’t. “The retail sector is not really regulated so much,” says Arinaitwe. “It’s like anyone can wake up and start a business selling inputs including pesticides.” And the research team found proof of this in the farms around Sebitoli. Some of the maize farmers were using a hybrid seed produced by Kenya Seed Company that was coated with a red substance that neither the farmers nor the retailers could describe. There was also no indication on the package of what the substance was. After testing, the researchers found that the substance was imidacloprid, the insecticide they detected in the fish living in the Sebitoli area. The Verge reached out to Kenya Seed Company, but it didn’t respond to multiple requests for comment.

According to Omeja, patrols along Uganda’s border are supposed to be on the lookout for imports that don’t meet standards, like unlabeled pesticides. He says that the National Bureau of Standards, the National Drug Authority, and the Uganda Revenue Authority all work to make sure the right products come in and that they do so following protocol. “But we have our porous borders,” he says. “So, some goods are smuggled into the country and the smuggled imports are hard to track and regulate.”

Arinaitwe thinks the issue goes beyond regulating the borders. “There’s also a problem that we have which is poor regulation of pesticide trade, handling, and application,” he says. He believes the country could benefit from more publicity and training on the dangers of chemical use for both farmers and retailers. And the Ministry needs to be equipped with the necessary tools, he adds.

But Arinaitwe admits that resources are hard to come by. “I’m not saying there’s no effort by government — it’s there,” he says, pointing to work done by groups like the National Agricultural Research Organization. But he notes that the kind of training and enforcement that the country may need requires a lot more. “It’s challenging for a developing country with a poor economy where you have a whole lot of competing demands,” he says. “So, the resources that are left have stiff competition, but that’s where I would say we need to put in more effort. How? I have no clue yet.”

“There’s also a problem that we have which is poor regulation of pesticide trade, handling, and application.”

For their part, the researchers notified the Uganda Wildlife Authority about the dysplasia in the Kibale primate population. “The Uganda Wildlife Authority does an amazing job and they’re pretty proactive,” says Chapman. The Wildlife Authority then passed the information along to the Ministry of Health, “Because they really want to find out what’s happening,” he adds.

As for next steps, the research team is looking into which pesticides might be the biggest culprits and trying to find direct evidence that the pesticides are what’s affecting the primates. They’ll start by collecting the primates’ urine, which they’ll need a lot of in order to test for chemicals. That process requires researchers with containers to wait beneath treetops for the primates to urinate who will hopefully snag some of the liquid as it falls to the ground. Chapman also wants to expand their focus to humans as well. If large numbers of primates living near farmlands are developing such severe deformities that could be due to pesticide exposure, what about all of the people working on those farms, living beside them, and coming in contact with pesticides on a daily basis?

“If the chimpanzees are getting this mostly from runoff and eating a little bit of the seeds and crops,” says Chapman, “what about all of the people?”