the mystery monkey disease

In January 2014, we saw this male green monkey (Chlorocebus sabaeus) pictured below in Bijilo forest park with a group from Helicon, Netherlands. Photographs were distributed to international zoo vets, who agreed this was an unknown disease. This was a cause for great concern as it was not clear whether the disease could be transmitted to the humans who regularly come into physical contact with the monkeys in the park, especially when feeding them groundnuts (even though this is against the rules). Of course, we were concerned for the monkeys, especially for the endangered Temminck's red colobus (TRC - Piliocolobus badius temminckiimonkeys that share the park with the green monkeys. Unfortunately, our concerns were confirmed when we found the female TRC displaying symptoms (see photograph 2).

In April 2014, we returned with students from a UK university and a UK vet, who, with Gambian vets and the Department of Parks and Wildlife Management staff, collected biological samples from the infected monkeys. The samples were couriered to the Robert Koch Institute in Berlin, Germany, for analysis, which found that the monkeys were infected with a form of Treponema p. pertenue, the bacterium that causes the disease “Yaws” in both primates/humans. Yaws is a debilitating disease that affects thousands of people (mostly children) every year. The World Health Organisation has tried to eradicate it twice but failed both times. Our study contributed to the finding that the disease has a reservoir in non-human primate populations, something that will be vital to future attempts to wipe out this life-changing disease.  

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monkeys

take a look at some of our recent work

 

the missing mother's milk

During our visits to Gambian forests, we’ve always paid special attention to TRC, knowing them to be Endangered and their global distribution to be very limited (making The Gambia critical for their survival). Many of our student projects have focussed on what types of plants they eat (especially at the end of the dry season when food is scarce), and others have looked at their movements and behaviour. TRCs are folivores, i.e., they mostly feed on leaves and flowers, and it has always been thought that their diet provided all the water they need, as drinking had only rarely been observed. Dawn Starin had previously observed nursing mothers licking moisture off leaves etc. but drinking from pools had never been observed.

On two of our dry season field courses, we had observed the sad scene of mother TRCs carrying dead babies around with them for several days. We tried to follow them to retrieve the dead babies to allow post-mortem inspection for signs of diseases etc. but the mothers kept carrying them until after field courses had left. During visits in the dry season the following year, we were happy to see that no mothers were carrying dead babies. Following a discussion with colobus expert Alison Hillyer after fieldwork one day, a possible explanation began to emerge. She was excited to announce that she had just seen TRC drinking from pools in Abuko forest, and we told her we had just seen the same in Bijilo Forest. Sorting through the hundreds of trail cam images from Bijilo that we had collected that year, it became clear that female TRCs, especially nursing mothers, were regularly drinking from the newly-created waterholes (built by the park staff with financial support from the Tour Guides Association and Project Wild Gambia). The image below shows the nursing mothers drinking (you can see a baby's hand clinging to its mother on the female on the right). 

Analysis of rainfall patterns and observations of the monkeys strongly suggest that the mothers were simply not able to provide milk for their babies in previous years due to climate change. The provision of artificial waterholes that were shallow enough for the monkeys to see that no crocodiles were lurking, allowed mothers to get enough water to make milk for their babies. Alison’s and our findings were published in an important scientific paper that we hope will help protect this endangered species from the impact of climate change. Since then, we have also provided a waterhole at Pirang Bonto forest, another key site in the conservation of TRC. 

the missing monkeys

TRC is one of two subspecies of “Western red colobus” and has been re-assessed by the International Union for Conservation of Nature (IUCN) as endangered. Given the geographical extent, urgency and complexity of the threats facing red colobus, a Red Colobus Conservation Action Plan 2019−2021 (ReCAP) was drafted in 2018 to mitigate the risk of extinction (Linder et al. in press).  One of the key priorities for TRC was identified as an urgent need for surveys, as the known global population was only 1,500 individuals. Two studies have addressed this in The Gambia with Mic Mayhew et al. (2020) finding an important population in the Sambel Kunda area and our boat-based surveys of the River Gambia revealing a large, healthy population. Between the two studies, we have added more than 1,000 to the known global population. Our river surveys repeat the survey performed by the British Army Ornithological Society and Makasutu Wildlife Trust in 2003. We aim to repeat this survey regularly as it is the most crucial monitoring scheme for TRC with historical data to track population trends. It is also very valuable for monitoring population trends in other species such as hippos, crocs and birds. 

the future...

The work we have done with monkeys has helped to raise the plight of TRC and identify many of the issues affecting the four species found in The Gambia. We will continue our monitoring and pass on the vital data to the Department of Parks and Wildlife Management and our partners at the Communities for Red Colobus Project. 

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The first green monkey showing symptoms of the new disease

© Anne Kroon/Helicon