What Nesting Songbirds Reveal About Lyme Disease in Canada

Migratory birds are increasingly recognized as an important factor in the spread of Lyme disease–carrying ticks across North America. Researchers estimate that migratory birds disperse between 50 million and 175 million Ixodes scapularis ticks across Canada each spring.

The blacklegged tick (Ixodes scapularis), commonly called the deer tick, is the primary vector of Lyme disease in eastern North America.

“We estimate that migratory birds disperse 50 million to 175 million I. scapularis ticks across Canada each spring, implicating migratory birds as possibly significant in I. scapularis range expansion in Canada,” wrote Ogden and colleagues.¹

A study by Scott and colleagues found that migratory songbirds can disperse ticks into new areas hundreds of kilometers away from their original source.²

“This indicates that many I. scapularis-carrying migratory birds in spring have destinations far north in Canada, including some farther north than the current region of climatic suitability for I. scapularis,” the authors explain.

Because ticks spread Lyme disease to humans, understanding how wildlife spreads ticks helps explain patterns of Lyme disease risk.

Scott and colleagues studied nesting songbirds to determine whether established populations of deer ticks existed in Canada. Nesting birds stay close to their nests during June and July, making them useful indicators of local tick populations.

READ MORE: Lack of knowledge about Lyme disease in Canada

“Short runs ensure that the eggs in the nest stay warm and protected from predators,” Scott writes in the article, “Monitoring of Nesting Songbirds Detects Established Population of Blacklegged Ticks and Associated Lyme Disease Endemic Area in Canada.”

“Additionally, during the fledging period, ground-foraging songbirds make hasty flights near the nest and promptly return to feed their young.”

How Researchers Identified Established Tick Populations

Researchers examined deer ticks collected from nesting songbirds. To demonstrate an established population of ticks in an area, they needed to identify:

• At least six ticks collected from a specific location
• Ticks from at least two of the three life stages (larva, nymph, or adult)

The investigators identified 63 blacklegged nymphal ticks and 1 larval tick. They also found multiple tick life stages on a single bird during the same year.

Finding both larval and nymphal ticks in the same area suggests the presence of an established local tick population.

Ticks Collected from Songbirds Were Frequently Infected

Nearly half of the deer ticks collected were infected with Borrelia burgdorferi sensu stricto, the bacterium responsible for Lyme disease.

According to the authors, “14 (42%) of 33 blacklegged tick nymphs tested were positive.”

This high infection rate suggests that nesting songbirds may help reveal previously unrecognized Lyme disease risk areas.

A Novel Approach to Tracking Lyme Disease Spread

The authors described their approach as a novel way to understand the relationship between birds and Lyme disease. Over a two-month monitoring period, researchers were able to pinpoint areas where tick populations had likely become established.

The growing number of infected ticks found in expanding geographic regions raises concerns about public health.

“The westward expansion of canine Borrelia burgdorferi seroprevalence from Minnesota into North Dakota mirrors recent reports that Lyme disease is poised to become a significant human public health concern in North Dakota,” Watson notes. [1]

As ticks expand into new regions carried by wildlife, the risk of Lyme disease in humans may also increase.

Recommendations from the Study

The authors made three recommendations in their discussion of birds and Lyme disease:

1. “Individuals frequenting this area should take extra precautions to avoid tick bites and perform full-body tick checks at the end of the day.”
2. “Because Borrelia burgdorferi sensu stricto is pathogenic to humans, anyone bitten by a tick or experiencing Lyme disease symptoms should seek medical attention.”
3. “Since chronic Lyme disease can be a debilitating infection, healthcare practitioners should carefully evaluate symptomatic patients for possible Lyme disease.”

Monitoring wildlife—particularly migratory and nesting birds—may help researchers detect emerging Lyme disease risk areas before human cases increase.

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Dr. Daniel Cameron, MD, MPH
Lyme disease clinician with over 30 years of experience and past president of ILADS.

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