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Severe malaria cases in rescued orangutans raises concerns for wild populations

An orangutan under treatment.

Malaria in wild orangutans could pose a greater threat to infants, though further studies are required to shed light on the issue, say researchers. Image courtesy of IAR Indonesia and the Indonesian Ministry of Environment and Forestry.

  • Researchers in West Kalimantan, Indonesia, sampled orangutans and found a high prevalence of malaria.
  • In some cases, malaria resulted in severe illness among the great apes, causing concern about the conservation implications for the animals.
  • On the plus side, all of the orangutans inflicted with severe illness responded well to treatment and recovered.
  • Some experts says these findings underline the need for a precautionary principle when translocating or reintroducing orangutans that test positive for malaria.

Rahayu, a juvenile female orangutan, arrived at a rehabilitation center on the Indonesian part of Borneo island feverish, unresponsive and almost comatose, recounts Karmele Llano Sanchez, program director at International Animal Rescue’s Indonesia affiliate. Based on the symptoms, veterinarians suspected a severe case of cerebral malaria and began treatment. Though Rahayu made a full recovery, she never regained her eyesight.

Researchers recently reported Rahayu’s case among other findings that confirm malaria can cause severe illness in orangutans, published in the Malaria Journal. “There [previously] was no scientific evidence that malaria was really an illness in orangutans,” Llano Sanchez, lead author of the paper, told Mongabay.

Between 2017 and 2021, Llano Sanchez and her team sampled critically endangered Bornean orangutans (Pongo pygmaeus) at the IAR’s wildlife rescue and rehabilitation center in West Kalimantan province. Eighty-nine of the 131 orangutans they sampled tested positive for malaria, caused by the parasite Plasmodium pitheci.

Malaria resulted in “moderate to severe illness” in 14% of the orangutans, confirming that serious illness can occur as a result of infection.

Drawing blood from an orangutan.
Researchers found that 89 out of 131 orangutans tested positive for malaria, caused by the parasite Plasmodium pitheci. The research project was carried out in collaboration with the Indonesian Ministry of Environment and Forestry and the National Research and Innovation Agency. Image courtesy of IAR Indonesia and the Indonesian Ministry of Environment and Forestry.

Rahayu’s case was somewhat of an outlier, Llano Sanchez said, and lies at the extreme of what severe illness can look like in orangutans. All of the infected orangutans that developed clinical illness responded well to anti-malarial treatments, an important finding in itself, she added.

“I think it’s very likely that if they went without treatment, they could die,” Llano Sanchez said. “I think we should really consider that this illness could potentially be fatal for orangutans.”

Particular concern lies around the potential impact on juveniles, like Rahayu, but also pregnant females in the wild. At the moment, though, there’s no indication of levels of mortality in wild populations from malaria. “Is it a conservation threat to the orangutans or not? Well, we don’t know,” Llano Sanchez said. “But I think this paper is opening the question that we should try to know more, to study [malaria] more.”

There is no evidence that the parasite afflicting orangutans is transmissible to humans, she added, nor that malaria prevalent in humans made the jump to orangutans.

This is good news as it’s not the case for other great apes, such as chimpanzees, said Juan Lapuente, head of the Comoé Chimpanzee Conservation Project in Côte d’Ivoire. “On the other hand, the high prevalence and the fact that many infected [orangutans] derived an illness, some of them severe, suggest that this infection poses a risk for the conservation of these populations in similar [rescue rehabilitation centers],” Lapuente wrote in an email.

An orangutan.
Climate change, habitat loss and disturbance are some of the factors that are likely to change the way pathogens and diseases affect wildlife like orangutans, say experts. Image courtesy of Darren Pearce via Flickr (CC BY-NC 2.0).

Though the study sheds light on the potential severity of illness, it also raises further questions: What lies behind the vastly different severity of cases; how might habitat disturbance impact levels of immunity; and how orangutans in rehabilitation centers that test positive for malaria should be managed.

Returning rehabilitated orangutans to the wild and translocating them to other areas to bolster populations is a key element underpinning conservation of the critically endangered species.

Managing the potential health implications of these activities is a “conservation priority” and conducting pathogen and disease surveillance is “critically important, said Marc Ancrenaz, a wildlife veterinarian and science director of Hutan, a conservation NGO based in Malaysian Borneo.

This, he told Mongabay, includes adopting a precautionary principle: testing animals prior to release or translocation and basing a decision on the results. “[O]rangutans with this pathogen should not be considered candidates for translocation into natural habitats, as this disease may recur or be chronic, and could have severe consequences for wild orangutans,” he wrote in an email.

A past study also indicated that prevalence of malaria in rehabilitation centers can be far greater than in the wild, due to higher concentrations of orangutans. In another study, released last year, researchers, including Ancrenaz, warned that without health screening, the translocation of captive and wild orangutans could pose “considerable infectious disease transmission and species conservation risks” to wild populations.

Taking the risk of transmission into account before reintroducing or translocating orangutans from an area with high prevalence of malaria to locations with healthy wild or semi-free populations is essential, Lapuente agreed. “This is something that represents a major concern in management of captive and semi-free Great Apes in general,” he told Mongabay.

Llano Sanchez, however, said that until further research is conducted on malaria and wild populations, it’s too soon to draw potential management conclusions from this work. “This is a very important finding, and it’s a good step towards knowing more about the epidemiology of the disease,” she said. “But I don’t feel in a position to give any strict recommendations about rehabilitation or reintroduction efforts regarding malaria.”

An orangutan under treatment.
Wildlife veterinarian Marc Ancrenaz says a precautionary principle should be applied when translocating or releasing captive orangutans to minimize risks to wild populations. Image courtesy of Jorge Franganillo via Flickr (CC BY 2.0).

Though researchers disagree on management approaches to take in light of these findings, there is agreement that current and future disturbance to orangutan habitat will have likely epidemiological consequences.

“[I]t is to be feared that this type of disease will happen more and more frequently with the drastic changes in the environment where wild orangutans are found in Borneo and Sumatra,” Ancrenaz wrote. “These new landscapes will increase the risk of exposure of orangutans to emerging diseases. It is thus essential to try detecting what diseases are susceptible to striking these populations.”

Climate change, forest fragmentation, biodiversity loss — all these impacts on the natural environment can drive changes not just in the behavior of wild species such as orangutans, but also in the epidemiology of diseases like malaria, said Llano Sanchez. How mosquitoes, which transmit the malaria parasites, respond to these changes and the corresponding consequences for wildlife remains an open question.

“I think is very clear that veterinarians now have a responsibility to really start doing more studies of wildlife pathogens and wildlife diseases and their importance in conservation,” Llano Sanchez said.

Banner image: Malaria in wild orangutans could pose a greater threat to infants, though further studies are required to shed light on the issue, say researchers. Image courtesy of IAR Indonesia and the Indonesian Ministry of Environment and Forestry.


Llano Sanchez, K., Greenwood, A. D., Nielsen, A., Nugraha, R. T., Prameswari, W., Nurillah, A., … Baird, J. K. (2022). Plasmodium pitheci malaria in Bornean orang-utans at a rehabilitation centre in West Kalimantan, Indonesia. Malaria Journal, 21(1). doi:10.1186/s12936-022-04290-8

Wolfe, N. D., Karesh, W. B., Kilbourn, A. M., Cox-Singh, J., Bosi, E. J., Rahman, H. A., … Spielman, A. (2002). The impact of ecological conditions on the prevalence of malaria among orangutans. Vector-Borne and Zoonotic Diseases, 2(2), 97-103. doi:10.1089/153036602321131896

Sherman, J., Unwin, S., Travis, D. A., Oram, F., Wich, S. A., Jaya, R. L., … Ancrenaz, M. (2021). Disease risk and conservation implications of orangutan translocations. Frontiers in Veterinary Science, 8. doi:10.3389/fvets.2021.749547

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