Madagascar is home to many species of wildlife that are found nowhere else on the planet. The island broke off from India about 88 million years ago and its inhabitants have evolved in isolation ever since, giving rise to the incredible diversity of life found there today.
However, expanding human populations on Madagascar are encroaching on this biodiversity. People and domestic animals are increasingly coming into contact with wildlife– and this provides an opportunity for pathogens to move between them.
Madagascar is famously home to around 100 species of lemur, many of which are critically endangered. Monitoring wild lemurs can tell us about the impact of human activity on their health, but also whether lemur populations carry any diseases that could make the jump to humans. Now, research published in Biology Letters describes a new way to identify unknown pathogens that haven’t been discovered before.
Dr Cathy Williams is a vet at the Duke Lemur Centre, studying the impact of mining on lemur populations in Madagascar. “We would capture lemurs every two years from several different species and do health exams, take blood samples and do a variety of analyses,” she says.
“But one of the limiting things when we started was that there was so little information on the diseases that lemurs are susceptible to, or the pathogens that circulate in wild populations.”
This is a problem because traditional methods for monitoring diseases in wild populations require knowing what to look for. Researchers need this information to design DNA probes and primers that are targeted for specific pathogens.
Discovering new species of pathogens, and analysing the threats they may pose, is therefore very challenging. But a potential solution came about when one of Cathy’s colleagues heard about the work on lemur surveys.
“I saw that the screening methods they were using would be limited when it came to discovering new or unrecognised pathogens,” says Dr Peter Larsen, an evolutionary biologist also based at Duke University. “But I knew from my recent work in genomics that I could sequence blood samples and search for pathogens without needing any information beforehand.”
Peter’s idea was to look at all the genetic material in the lemurs’ blood that was being expressed (switched on). This is done by sequencing the RNA found in a sample – RNA is the genetic messenger which is used by cells to read the genetic code in DNA and turn it into proteins.
“Some of the RNA would be from the host, the lemurs themselves, but some would be from the pathogens also present in that sample,” explains Peter.
With advanced sequencing technology, Peter and Cathy were able to screen the samples brought back from Madagascar, and use algorithms to compare the RNA sequences to databases of known pathogens.
They identified several novel pathogens circulating in wild lemur populations that are spread by ticks and insects, some of which had never been seen in lemurs or even Madagascar before. These include new species of tick-borne Borrelia bacteria and insect-borne Plasmodium parasites, groups that contain the pathogens causing Lyme disease and malaria respectively.
The researchers were also able to use the technology to see whether the pathogens were native to Madagascar and had evolved alongside the lemurs, or were similar to microbes found elsewhere and had been imported to the island. Their analysis suggests that the Plasmodium species were a natural component of the ecosystem, but the tick-borne parasites had been brought over from another country.
The authors don’t know yet whether these pathogens actually cause disease in lemurs, or if they could represent a threat to humans. But the method itself could be a powerful way for researchers to learn more about diseases that might be on the horizon.
“The majority of emerging infectious diseases over the past several decades have been transferred from wildlife to humans,” says Cathy. “This particular procedure could help people working in public health to find out what is being carried in wildlife that has close associations with humans.
“The method could be used in any organism, and we could initiate similar projects anywhere in the world,” adds Peter. “The hope is we can pull together a team of genomicists, vets, doctors and clinicians to sample wildlife and agriculture species, to better inform human health and combat emerging diseases.”
Larsen, P., Hayes, C., Williams, C., Junge, R., Razafindramanana, J., Mass, V., Rakotondrainibe, H., & Yoder, A. (2016). Blood transcriptomes reveal novel parasitic zoonoses circulating in Madagascar’s lemurs Biology Letters, 12 (1) DOI: 10.1098/rsbl.2015.0829
Image credit: The eyes of the Sclater’s Lemur from Tambako the Jaguar on Flickr, under CC BY-ND 2.0, and lemur from Andrey on Flickr under CC BY 2.0