How human encroachment drives malaria spread from monkeys

Scientists have suspected for a long time that one of the drivers or emerging diseases, particularly in the tropics, is the conflict between human activities and nature. Where we encroach, there are opportunities for zoonotic - animal-borne - infections to jump the species barrier. But what are the risk factors that make this more likely? One form of malaria - called knowlesi - that is normally carried by monkeys, but occasionally jumps into people and is now increasing in incidence in Asia - has provided Emilia Johnson, from the University of Glasgow, with an opportunity to study why this might be happening…

Emilia - Everyone's familiar with malaria between humans. So human to human passed with mosquitoes, a big, big problem in Africa still. But in Malaysia there's been huge reductions of malaria. So they've managed to nearly eliminate it using bed nets and public health controls. And at the same time there's been this huge rise in monkey malaria, a different type of malaria where you have malaria circulating in non-human primates. And in particular communities which are interacting with the edges of the forest, you start to get spillovers: the mosquitoes that are biting the monkeys also bite the humans and you start to get malaria that way.

Chris - And so what was the unknown, or the question you were trying to probe?

Emilia - This particular type of zoonotic malaria in humans, we knew that that was associated with deforestation and with communities that are living and working at these forest edges. But what we didn't really know was anything about the malaria in the monkeys themselves. You can imagine that if monkey populations have really, really high rates of malaria, you are gonna get a lot of spillover into the humans. Whereas if it's really low, maybe you won't see as much. And actually all the literature had been assuming that all monkeys across Southeast Asia had really high rates. So we wanted to start to untangle whether that was true and also what might be driving that. So if you understand what's driving that, maybe you can start to understand where the risk factors are for people.

Chris - So what was your approach then? How did you begin to try and unpick those questions?

Emilia - Firstly, it was a meta-analysis. So we took all of the literature that we could find, all of the studies where they had selected primates, sort of mainly macaques, anywhere in Southeast Asia, and where they'd tested them from malaria, what those results were. And we pulled them all together and looked across different regions and said, okay, across all of the monkeys, so it's about 6,000 monkeys in 150 different locations, how many monkeys are infected? And then the next thing to do was to look at in the places where there's lots of infection versus the places where there isn't really any, what's the difference? Is it being driven by deforestation? Is it being driven by where there's more forest or less forest, or where there's more humans or less humans?

Chris - And did you see some obvious drivers?

Emilia - For other infectious diseases, in other wildlife hosts, There have already been some links between infection and deforestation, and that's exactly what we saw here. So in areas that are really fragmented, where you chop up the forest into smaller patches, you have higher rates of primate malaria in the non-human primates. And we think this is possibly because where you have a chopped up forest, you have a lot of these forest edges, and that's where the mosquitoes like to live. So where you have more mosquitoes, they're probably passing malaria more readily between the primates. The other thing we found was that it's also associated to where there is more forest. So not only is the forest more chopped up, but there's more of it. So actually there's a bit of a trade off. You need an ideal amount of forest and fragmentation in order to get these really, these areas of high prevalent in the wildlife.

Chris - And as well as the mosquitoes being attracted to the margins are the primates also tended to move towards the margins as well. So it brings a concentration of both the vector, the mosquito, and the animals that can be the source and also be the target of the infection?

Emilia - Yeah, that's exactly right. So the main species that we found had been studied are macaques. And these are really highly attracted to areas where humans live and work. Typically they're nuisance animals because they love to eat leftover food and be in and around villages. So they are quite often living in these margins between urban areas and forested areas. So that's exactly right, where you have more fragmentation, you have both the primates and you have the mosquito vectors and those that both obviously closer to the humans.

Chris - Often when we uncover a mechanism like this, it also inevitably leads us to some solutions or some interventions that can try to reduce risk and so on. So does this point towards some obvious directions we should be moving in? Obviously fewer people, less fragmentation would be ideal, but that's hard to achieve. Mm-Hmm. So what do you think we can do off the back of what you've found?

Emilia - It doesn't necessarily lend itself to a clear solution, but what it does say is that it is absolutely essential to consider both wildlife compartments and landscape composition in public health measures. So this idea of one health or planetary health, everything is so interconnected and so entangled that you can't consider public health without the environment and the wildlife aspects. There is no silver bullet to this. With human malaria, you can use bed nets, but because this is more about how people are interacting during the day in their working life at these forest fringes, that's not applicable here. So what we need is solutions that are community-driven and sustainable and more to do with landscape management in order to reduce risk.