Plague! The word conjures images of horrors past, piles of festering medieval dead overrun by rats. It's not a disease of the past though; the bacteria that causes plague can be found in the United States, and a few cases of human bubonic plague happen every year.

The animals that carry plague in the US are mostly rodents—rats, ground squirrels, and prairie dogs—and their fleas can pass plague bacteria to humans if they decide to bite us. Good news, though: The adorable little owls that live in burrows alongside those plaguey rodents seem to be immune. Although the owls have fleas capable of carrying plague, they are not infected—and their immunity could provide some interesting information to scientists studying the disease.

A Pile of Poo on Every Doorstep

Western burrowing owls (Athene cunicularia) nest underground, often re-using rodent burrows. Working with these predatory birds is not for the faint of heart; even small owls have powerful beaks and claws. Burrowing owls live in areas where you are just as likely to stick your hand in a burrow and and pull out a rattlesnake as an extremely irate little raptor.

Dr. Jim Belthoff of Boise State University has spent years sticking his arm into nest tunnels in search of owls—often past piles of excrement. In addition to being very cute and fluffy, these owls have an endearing habit of decorating their nests with turds. Not their turds, mind you; they collect and bring mammal dung back to their front doorstep. It's thought the poo attracts insects that the owls snack on.

Adding to the grossness, Belthoff noticed burrowing owls seemed to have lots of fleas. "We considered the fleas a nuisance, as they would jump on us as we captured and banded the owls, and they would infest our field vehicles." This made him wonder: Do the fleas affect the owls? And what kind of fleas were they?

The researchers knew burrowing owls used prairie dog and ground squirrel burrows in many portions of their range. Both those rodents host fleas that can carry the plague bacteria Yersinia pestis, maintaining the plague in a cycle of mutual infestation. (Humans play into that cycle, too, when they're bitten by fleas that have abandoned a rodent corpse in search of food.) So Belthoff assembled a team of ecologists, entomologists, microbiologists, public health professionals, and volunteers to investigate how owls, fleas, and plague interacted with that rodent system.

Boise State University

His team collected thousands of fleas from burrowing owl nests during banding and health checks. "We camped for many nights amidst street signs that warned of plague, to avoid contact with sick or dead rodents, and to take other precautions for avoiding exposure," said Belthoff. Nearly all of the 4,750 fleas collected from owls were Pulex irritans, a known plague vector.

Good news, though! They found no evidence of the plague bacteria in owl fleas they collected, or in the blood of the owls. They also did not find antibodies to plague in owl blood, which would be expected if owls were regularly exposed. "There are no public health concerns related to burrowing owls," says Belthoff. "All our test results were negative to plague—in antibodies, in the fleas and in the blood of the owls."

One possibility was that fleas were just hitching a ride on the owls, and not feeding on them. Additional tests determined the fleas did contain burrowing owl DNA in their guts, so they were drinking owl blood. The owls just weren't getting infected with plague.

Poop is an important part of the nesting process

A different research group worked on another question: Does the poop scattered around the nests play a role in the owls' flea infestations? Burrowing owls don't thoroughly clean their nests from year to year, and often re-use burrows.

The scientists wondered if owls increased the number of fleas on their families by re-using nests already infested with fleas. To test this, once owls finished breeding and migrated south for winter, the researchers carefully removed the poop in and around some burrowing owl nests and microwaved it. Then they put the excrement back around and in the nests.

The microwave killed any bacteria and insects that might be present. The researchers also entirely cleared some nests of the doorstep manure, and left others undisturbed, to provide returning owls with a variety of nest sites.

Owls preferred nests with fecal decoration; and the fleas were present on all nests, even ones with microwaved manure or nests cleared of fecal deposits. Somehow the owls are picking up fleas, not from their nests, but possibly from their rodent prey.

Owls Might Be Protecting Us

No owls in the study were infected despite having plenty of fleas. Belthoff thinks fleas biting burrowing owls might be less likely to bite other susceptible hosts. In other words, burrowing owls could take infected fleas out of circulation, thereby slowing infection.

Fleas on owls aren't feeding on ground squirrels, and thus not picking up plague infection or transmitting it. The region is currently undergoing a ground squirrel plague outbreak, which presents a unique opportunity to test if owls actually protect us from plague. Belthoff and his team are at work measuring what effect this major die-off of squirrels due to plague will have on the owls, their fleas, and plague infections in the area.

Burrowing owl populations are declining at an alarming rate. In the last 16 years, the breeding population declined 98.1% in the Southwestern US. They are an endangered species in Canada and a species of conservation concern throughout many western states in the US. The owls readily accept artificial burrows, so maybe adding the knowledge that they might be a buffer between humans and plague might encourage conservation of this species. Beyond, you know, just their cuteness.

Belthoff et al. 2015. Burrowing Owls, Pulex irritans, and Plague. Vector-Borne and Zoonotic Diseases 15(9): 556-564. DOI: :10.1089/vbz.2015.1772

Corey S. Riding & James R. Belthoff. 2015. Removal of old nest material decreases reuse of artificial burrows by burrowing owls. Wildlife Society Bulletin 39(3) 521–528. DOI: 10.1002/wsb.552

K. Cruz-McDonnell & B. Wolf. 2015. Rapid warming and drought negatively impact population size and reproductive dynamics of an avian predator in the arid southwest. Global Change Biology: DOI: 10.1111/gcb.13092

M. Smith & C. Conway. 2011. Collection of Mammal Manure and Other Debris By Nesting Burrowing Owls. Journal of Raptor Research 45(3):220-228. doi: http://dx.doi.org/10.3356/JRR-10-63.1