Bird CSI: A Loon, a Swollen Spleen, and a Mysterious Disease

On a hot summer day, in July of last year, Inga Sidor found herself gazing down at a dead Common Loon that lay sprawled across her necropsy table in the New Hampshire Veterinary Diagnostic Laboratory at the University of New Hampshire.

In her decades-long career as a veterinary pathologist, Sidor has performed necropsies on hundreds of loons. In fact, she first learned and practiced her trade on the large waterbirds, unlike most others in her field. Even so, this case struck her as unusual. The dead bird looked otherwise healthy: From the outside, it showed no signs of injury from boat strikes or gunshots, and an X-ray revealed no lead fishing weights in its gizzard. Right away, Sidor had ruled out the most common causes of loon deaths in New England.

It was also rare to have such a fresh carcass on the table. Thanks to a vast network of researchers and volunteers, Sidor sees nearly every dead loon found in the state, but they typically arrive half-decomposed and freezer-burned from weeks in storage, making it nearly impossible to piece together a complete postmortem. This time though, a camper at Umbagog Lake National Wildlife Refuge—on the border of New Hampshire and Maine—had alerted the network soon after the bird died, giving Sidor the opportunity to study and examine the mystery from start to finish.

She took a deep breath, made her first incision, and got to work.

As Sidor delved into the necropsy, she noticed deformities in the bird’s organs. The loon’s heart showed signs of leaking fluid, and its liver and spleen were enlarged—the spleen was three times bigger than it should have been—and unusually fragile.

Taken together, these clues suggested avian malaria. As malaria parasites move between the bloodstream and blood vessels, they burst through tissues, making a mess along the way. The spleen works overtime to filter the tainted blood and becomes engorged in process, Sidor says. “Spleen enlargements in particular are a malaria thing.”

But most birds can survive with infection; the parasite needs a living host. And so Sidor dug further: Under the microscope, she examined tissues from various parts of the loon’s body and found clusters of small malaria parasites. In the brain, they blocked much of the blood flow, and in the heart, they caused inflammation. For whatever reason, the infection had become fatal in this case. “Either of those two things could have killed this loon,” Sidor says.

At that point Sidor was confident that her specimen had fallen victim to malaria, but she couldn’t yet be certain. “We pathologists are pretty cagey as a lot,” she says. “You don’t say something is a thing unless you’ve got it confirmed six different ways.” Plus, her diagnosis would be a first: There were no other documented cases of death-by-malaria in a New England loon.

Sidor sent tissue samples to her long-time collaborator Ellen Martinsen, who studies wildlife parasites at the Smithsonian Conservation Biology Institute. Martinsen soon confirmed Sidor’s suspicions through DNA analysis: The infection had overwhelmed the bird. She also discovered the loon was harboring not one, but two malaria parasites in its body—a shocking and deadly double whammy.

The parasites, it turns out, are a mystery of their own. Both are typically found in Western birds from southern climes—not waterbirds in the northeastern United States. Martinsen speculates that they could be moving farther north as the climate changes, causing warmer temperatures at higher latitudes. Because New England loons spend their winters off the coast of Cape Cod, their range may now overlap with the mosquitoes that carry and spread the parasites.

Avian malaria itself isn’t unusual: It can be found in species on every continent. Furthermore, scientists have been tracking the spread of the disease in live, wild loons in the Midwest since the ‘90s. Typically, the pathogen results in a stunted lifespan and fewer offspring for its hosts; but loons could be especially vulnerable. Like many northern-latitude birds, they haven’t evolved with malaria, so it’s possible that they don’t have the immunological tools to survive the infection.

Being that this is the first confirmed kill in the region, Sidor and her collaborators still aren’t sure if they’ve found a bellwether for a more worrisome trend. It could be that the loon’s swift death was an anomaly—or it could be the first sign of an epidemic.

Mark Pokras, a retired professor at the Cummings School of Veterinary Medicine at Tufts University, and a key figure in the New England loon network, has mulled that question. “Is this one bird that became immune-compromised, got the parasite, and died?” he says. “Or is this the tip of the iceberg, where this year we see one dead loon, next year we see 10, and the next year we see 500? We don’t know.”