White-tailed deer have their own form of malaria

The white-tailed deer, maybe the best-studied wild animal in North America, turns out to carry a malaria parasite that science has overlooked for decades.

The malaria parasite in deer is a completely different species from the ones that cause disease in humans. A report in 1967 based on one deer in Texas had claimed that the parasite existed and a 1980 paper had named it Plasmodium odocoilei. But no one had reported it again until Ellen Martinsen of the Smithsonian Conservation Biology Institute in Washington, D.C., and her colleagues accidentally rediscovered it.

Their find dashes the current belief that no mammals other than people in the Western Hemisphere carry their own native forms of malaria, Martinsen and her colleagues say in the Feb. 5 Science Advances. And the work also challenges the conventional wisdom that no members of the deer family anywhere have their own malaria parasites.

“I feel a bit discombobulated by the paper,” says Penn State evolutionary parasitologist Andrew Read. According to the new paper, the parasite is found in 25 percent of white-tailed deer (Odocoileus virginianus) sampled at some locations. “How could anybody have missed malaria at these levels?” he says. The parasite has so far appeared at very low concentrations in animals’ blood, but Read is now wondering what other forms of malaria biologists have overlooked.

The old report of a deer parasite had been “really just a mystery,” Martinsen says. It came from a leading malaria parasite expert and so was difficult to dismiss lightly. But Martinsen wasn’t even thinking about it when she and Robert Fleischer were using genetic methods to survey for bird malaria parasites in mosquitoes at the Smithsonian’s National Zoo in Washington, D.C.  A peculiar sample of malaria-parasite DNA turned up in a mosquito that had bitten a white-tailed deer (a scenario gleaned from other DNA in blood the mosquito had fed on). Then smears of deer blood checked under a light microscope revealed actual parasites —in the forms they take while reproducing in mammalian hosts.

So far, the parasite appears to be a phenomenon of white-tailed deer mainly in the southeastern United States, with some reports from as far north as Westchester County, New York. The parasite didn’t show up in genetic tests of some blood samples from elk, pronghorn, mule deer, black-tailed deer or even in all of the samples from white-tailed deer. Nor did it show up in mosquitoes tested from San Diego County in California.

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Now what’s needed is large-scale sampling, says Juliane Schaer of the Max Planck Institute for Infection Biology in Berlin. “There is always the chance that other deer, or other mammals, have malaria parasites that just haven’t been detected yet.”

The mosquito Anopheles punctipennis, which could spread human malaria should those parasites return to North America, could be spreading the deer disease, Martinsen suggests.  DNA indicates that the deer parasites, sucked up during a blood meal, can at least make it from this mosquito’s gut to its salivary glands, where they might dribble into the next deer the mosquito bites.

What effects that bite has on the deer could have been overlooked, too, says Steve Demarais of the Mississippi State University Deer Lab. In male sage grouse, for example, no measurable effects show up in health exams, but the birds infected with avian malaria don’t spend time on the breeding grounds as regularly or mate as early and as frequently as uninfected birds do. At first glance, “impacts are not always obvious,” Demarais says, yet behavioral differences matter a lot to an animal’s breeding success.

More information on deer malaria is already on the way. Another research group independently rediscovered malaria parasites in white-tailed deer, says group member Diana Outlaw, also of Mississippi State. She and her colleagues have submitted a paper to a journal and are continuing to check the 30,000 mosquitoes they’ve collected for signs of the parasite and the animals the mosquitoes have bitten.