Malaria infects around 200 million people worldwide each year, but it is also a major parasite in other animals around the globe. The mammalian version of the parasite has been found in a range of species, from monkeys to buffalo, but was not thought to be native to the Americas. That was until researchers from the Smithsonian National Zoo discovered the parasite living right under their noses, not in a rare and hard to study species, but instead in one of the most common North American deer species.
“People intensively study white-tailed deer and their pathogens, so it was surprising to find two malaria parasites,” explains Rob Fleischer, who co-authored the paper published in Science Advances, along with Ellen Martinsen, who made the original discovery. “It's quite surprising to discover something about a species in our backyards, much less so about the rare and endangered species in faraway places.”
Map showing the distribution of the parasite (Plasmodium odocoilei) in the U.S. Red dots denote where the parasite has been confirmed, while the red star shows where it was detected in 1967. Martinsen et al. 2016
While there are four species of malaria that are known to infect humans, there are thought to be over 100 that are found in other animals, ranging from birds and mammals to reptiles. It was actually while trying to discover the origin of a malaria species found to be infecting birds at the Smithsonian National Zoo that researchers stumbled on the finding that the parasite was also infecting the local deer. The scientists were sampling mosquitoes in the zoo’s ground and chanced upon one that contained a malarial parasite with DNA unlike any they had seen before.
After sequencing the blood to find out what species it belonged to, they were amazed to find the mosquito had recently fed on white-tailed deer. Interestingly, this isn’t actually the first time that the malaria parasite had been found in deer, though this is now the first time it can be unequivocally confirmed. There was one case reported by a biologist in 1967, in which a single deer in Texas was found to be infected, but as it was just one report it was widely dismissed. “It was like the guy was reporting he saw Big Foot,” says Joseph Schall, a malaria expert from the University of Wisconsin–Milwaukee, in a statement.
When the researchers found that the mosquito had fed on the deer, they then screened over 300 white-tailed deer from 17 states for the parasite. In addition, they looked at over 150 samples from a further three species of deer found in the region. They discovered that between 18 and 25 percent of the white-tailed deer had low levels of the malaria parasite, while none of the other species came up positive.
There was some suggestion that maybe the parasites found in the local deer were due to introductions by people or exotic species recently brought into the country. Further analysis of the microorganisms' DNA, however, suggests that this is probably not true, as they found two distinct genetic lineages, implying that they could actually be looking at two separate species of the parasite. “We can date the evolutionary split between those two lineages,” explains Martinsen, who found that the split was probably 2.3 to 6 million years ago. This means it probably arrived in the Americas when deer crossed the Bering Land Bridge around 5 million years ago.
The species discovered in the deer is not thought to be a threat to humans, and while tests on domestic cattle in the region came back negative, there is the small possibility that the parasite could make the jump. The researchers, however, next want to understand whether or not low-level infection by the parasite has caused disease in the white-tailed deer that has so far gone undetected.