Malaysia was on the brink of eliminating malaria – then a new parasite swung out of the jungle

A close-up shot of a long-tailed monkey, also known as a crab-eating macaque, sitting on a dead tree branch, spotted beside the Kinabatangan river in the jungle close to the Danau Girang Research Centre, Kinabatangan, Sabah
In the Borneo rainforest, scientists are investigating monkey malaria – a parasitic threat spilling out of the jungle - Mailee Osten-Tan

After hoisting a couple of 32 kilogram gas canisters onto their shoulders, the scientists set off into the Bornean jungle, slicing a path through the dense foliage with a sharp machete.

Above them, a troop of long-tailed macaques swing through the canopy, barking as they leap  from tree to tree. Behind, several colleagues carrying bulky propane-powered “mosquito magnets” are clambering through the undergrowth, completely drenched in sweat.

“I don’t think this is what the designers imagined they’d be used for,” says Dr Kimberly Fornace of the traps, as the team manoeuvres the cumbersome equipment over snake-like roots and under low-hanging vines.

The traps work by emitting carbon dioxide, as humans and other animals do in their breath, attracting and trapping the blood sucking insects in their thousands. “Initially they were designed to clear an area ahead of, say, a garden party,” says Dr Fornace.

Yet this is no summer barbecue. In the depths of Malaysian Borneo, scientists are investigating monkey malaria – a mosquito-borne parasitic threat that’s spilling out of the lush rainforest to infect humans.

Scientists carry bulky propane-powered 'mosquito magnets' through the jungle
Scientists carry bulky propane-powered 'mosquito magnets' through the jungle - Mailee Osten-Tan
Associate Professor Kimberly Fornace, a Spatial Epidemiologist at National University of Singapore, who leads the mosquito project
Kimberly Fornace, who is leading the mosquito project - Mailee Osten-Tan

Malaysia has eliminated ‘normal’ malaria but, since 2011, more than 25,000 people in the region have contracted the simian parasite, which causes intense nausea, malaise, fever and sometimes death. Once rare in humans, cases jumped by 850 per cent between 2008 and 2021.

“It poses a real challenge for malaria elimination moving forward,” says Dr Fornace, an epidemiologist based at the National University of Singapore, pausing in a jungle glade. “It’s really a clear example of what can happen when a disease spills over [from animals] to humans.

“The question now is: how do we deal with zoonotic malaria?”

‘Something different might be going on’

It’s a question experts tracking the steady spread of monkey malaria – officially known as Plasmodium Knowlesi – have been grappling with for more than 20 years.

For Dr Giri Shan Rajahram, now a leading authority on the disease, the issue first burst onto his radar when he was a junior doctor in Kota Kinabalu – a low-key, seafront city in northwest Borneo affectionately referred to as ‘KK’ by locals.

The discovery came as a shock, he recalls. Malaysia was then well on its way to wiping out malaria. Cases had plummeted from 50,000 a year in 1990 to just 6,000 in 2004. With a strong system in place for tracking and treating malaria, eradication of the disease was tantalisingly close.

But then came a landmark study: of 200 samples taken from malaria patients in Borneo’s Sarawak state, scientists found half had monkey malaria.

“I don’t think people recognised that it was a big problem before that,” says Dr Rajahram, sitting in a whitewashed consultation room on the eighth floor of the Queen Elizabeth II Hospital in KK. “That brought to light that something different might be going on.”

Kimberly Fornace, left, and two researchers from her team inspect dead mosquitoes retrieved from the mosquito magnet
The research team inspects dead mosquitoes retrieved from the jungle - Mailee Osten-Tan
Close-up of inserting a mosquito in a tube
Monkey malaria makes people sicker faster, but can be treated with the same therapies as the human strain - Mailee Osten-Tan

There are more than 150 Plasmodium parasites, but only four are endemic to people – the tiny organisms jumped to humans from apes thousands of years ago and have evolved to thrive inside us. Carried between humans by mosquitoes, the parasites cause malaria when they invade our bloodstream and multiply inside it.

Plasmodium Knowlesi, or monkey malaria, is not one of those strains that are endemic in humans – yet.

First identified almost a century ago, its host organism for the moment at least is macaques. Experts have long known it could jump to humans (the first documented case was in 1965, a US Army surveyor who fell sick after a stint in the Malaysian jungle), but it was not common.

But as Malaysia has beaten back human malaria – virtually eliminating the threat in 2019 – Plasmodium knowlesi has flourished. Thousands of cases are reported each year, including 2,505 in 2022, thwarting Malaysia’s bid to gain ‘malaria free’ status from the World Health Organization (WHO).

“There should be recognition of the work that has been achieved to eliminate human blood species. But zoonotic malaria does present unique challenges,” says Dr Rajahram. You can’t destroy a pathogen when its reservoir is an animal as it has too many places to hide, he adds.

“I wonder if we’re just going to see this constant grumbling of cases.”

Monkey malaria makes people sicker faster, but can be treated with the same therapies as human malaria – if caught in time.

The spread of the disease to humans is driven by deforestation. Malaysia has lost roughly a third of its total tree cover since 2000, pushing humans, monkeys and mosquitoes into ever closer proximity. It’s a classic spillover risk, experts say.

Yet there are still huge knowledge gaps. Have cases also risen because the drop in human malaria reduced our natural immunity to the parasite, making us more susceptible? How prevalent is the disease in macaques? And – most worrying of all – will monkey malaria adapt to better spread between people?

Rats of the rainforest

Some 220 miles south of KK on the meandering, crocodile-infested Kinabatangan river, the boat ferrying Dr Fornace’s team tilts left as it tackles a hairpin bend.

Later, the researchers will return to their jungle traps to harvest the mosquitoes so they can be frozen and sent to a lab in KK for analysis. But this morning is all about the monkeys.

“Some people here call the macaques the rats of the rainforest,” says Dr Fornace, peering at a consul showing live infrared footage from a drone soaring high above the trees. “But I’m never bored of the monkeys, they’re pretty entertaining.”

But the team didn’t wake up at 4am for a wildlife safari. Before the macaques wake up and join the dawn chorus, they want to test a new toy: a drone with an inbuilt thermal camera.

“When I started working for the monkey malaria project I was doing a primate census, but they’re really hard to count when they’re moving,” says Amaziasizamoria Jumail, a research officer at the Danau Girang Field Centre, a remote outpost on the Kinabatangan river. “It’s easier when they’re sleeping as they’re still. But then it’s dark, so we need thermal cameras.

“It’s really important to know the population of the monkeys for conservation. But also, if you know the population, you can link it with the spread of diseases,” she adds.

Associate Professor Kimberly Fornace and members of her research team travel by boat along the Kinabatangan river
The research team travelling by boat along the Kinabatangan river - Mailee Osten-Tan
An over-the-shoulder shot of a researcher from the mosquito project gazing at the remote control screen of a thermal imaging drone camera he is flying over the jungle, close to the Danau Girang Research Centre, Kinabatangan, Sabah
A researcher observes thermal imagery on the screen of his remote control, taken by the drone camera flying over the jungle - Mailee Osten-Tan

The work is part of a project – launched two years ago and led by Dr Fornace – to more thoroughly map the spread and risk of monkey malaria.

“In order to understand the risk to humans, you have to understand the situation in the landscape, the prevalence in monkeys, the prevalence in mosquitoes etc,” says Dr Fornace. “All of these are different parts of the puzzle to look at long term risk.”

For instance, infections in people are most frequent at the edge of forests, where monkeys, humans and mosquitoes collide. Higher rainfall and flooding also create the ideal breeding sites for the insects.

Now, Dr Fornace’s team are working with the Danau Girang Field Centre’s Regrow Borneo programme to track if reforestation can put the spread of the disease into reverse.

“We want to know if restoration is a good way to stop [monkey] malaria,” says Dr Fornace. “We think that for a few years transmission will rise as there are more mosquito breeding sites. But at a certain point, when the trees get taller, mosquito density will fall”.

Every new insight is important, not least because some of the tools used to fight human malaria don’t work with the simian variety. Bed nets, for instance, are less useful because knowlesi-spreading mosquito bites all day, rather than at night.

Palm tree tops
A bird's-eye view of the treetops in a palm oil plantation - Mailee Osten-Tan
An aerial view of the Kinabatangan river, seen from close to the Danau Girang Research Centre, Kinabatangan, Sabah
The spread of the disease to humans is driven by deforestation - Mailee Osten-Tan

Dr Fornance and others experts warn that unless the root causes of transmission are addressed  – especially the destruction of macaque habitats – it will be challenging to curb monkey malaria.

“Unfortunately if you deforest the natural environment for the long tail macaques, they’re only going to go one way: towards human conurbations,” says Dr Stephen Woolley, an honorary clinical researcher and malaria specialist at the Liverpool School of Tropical Medicine.

While Malaysian Borneo has emerged as the world’s monkey malaria hotspot – with 90 per cent of all cases worldwide in 2022 – it’s not the only country grappling with the parasite.

Neighbouring Thailand reported 176 cases that year while Indonesia saw 71, according to Dr Elkhan Gasimov, head of the malaria elimination unit at the WHO. To date, infections have been detected in every southeast Asian country bar Timor-Leste.

“It is also important to note that more and more countries are reporting imported knowlesi cases, so the problem of zoonotic malaria is not limited to southeast Asia,” Dr Gasimov says, adding that infected travellers have returned to places including the UK, Spain, Sri Lanka and Japan with the parasite.

Of particular concern to the WHO is whether monkey malaria will adapt to better spread between people, which would allow the parasite to thrive in areas without macaques. While modelling studies have suggested this is not currently happening on a large scale, it’s far from out of the question; a study in 1967 showed it was possible in a controlled setting.

“In my point of view, the question is if/when sustainable human-mosquito-human transmission will be established,” says Dr Gasimov. “It is difficult to predict when exactly this will happen.”

In the meantime, there are signs that other species of zoonotic malaria could follow Plasmodium knowlesi’s lead.

“We’re starting to see spillover in South America as well, with two species: Plasmodium brasilianum and Plasmodium simium,” says Dr Woolley. They’re spreading where urban conurbations have expanded into the jungle.

“The monkeys have got nowhere else to go. And therefore humans then get caught in the crossfire.”

A researcher carries a Mosquito Magnet, which attracts and captures mosquitoes, through the jungle over his shoulder in the late morning, Kinabatangan, Sabah
The traps work by emitting carbon dioxide, attracting and trapping the blood sucking insects in their thousands - Mailee Osten-Tan
At dawn, researchers from the Danau Girang Research Centre remove a net of mosquitos caught inside a Mosquito Magnet which had been set up in the jungle the night before, Kinabatangan, Sabah
Researchers return to the traps at dawn, collecting nets of mosquitoes - Mailee Osten-Tan

Back at the Danau Girang Field Centre, Dr Fornace’s team are peering at their haul of frozen mosquitoes under a bright light when the power cuts out. The centre is made up of a series of wooden buildings which stand on stilts in the jungle, accessible only via boat.

“I never really get bitten by mosquitoes,” muses Dr Fornace as we wait for the back-up generator to kick in. “I always get attacked by leeches … my friend who’s an entomologist says I’m just not very attractive.”

The American laughs before returning to the matter at hand. She’s spent more than a decade returning to this patch of Borneo’s rainforest, studying the dynamics of mosquitoes, macaques and parasites, and she’s convinced that monkey malaria isn’t going anywhere fast.

“It’s certainly possible that it can be transmitted [between people], it was shown in the lab,” Dr Fornace says. “It’s certainly possible if you had the right conditions… but I actually think zoonotic spillover is more important.

“My feeling is that it’s similar to Lyme disease, where you have constant spillovers which make it really challenging to control,” she adds. “And I think they’ll be an increased burden around the region, especially as more countries start to eliminate other malaria species”.

And with that, the electricity kicks in and the scientist returns to work.

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