John Metcalfe was CityLab’s Bay Area bureau chief, covering climate change and the science of cities.
Humans are creating wonderful habitats for disease-spreading robins and sparrows.
People who want to avoid the tick-borne Lyme disease might tread warily in forests and high grass. But research suggests the bacteria responsible for Lyme lives in more-developed conditions, too, such as the meticulously manicured landscape of the suburbs.
The evidence lies in the blood-sucking parasites that infest the feathers and faces of birds. (Never image-search that, by the way.) Several years ago, researchers took on the unenviable job of collecting juvenile ticks from birds around the University of California, Berkeley's extension center. They scraped off 284 ticks and then sampled the birds' blood for Borrelia burgdorferi, the curly pasta-shaped bacteria that causes Lyme. Of the 100 birds that had ticks, 57 tested positive for B. burgdorferi, according to a new study in PLOS ONE.
Infected species include the American robin, golden-crowned sparrow, dark-eyed junco, and oak titmouse—birds that are ubiquitous in California suburbs. And in a revelation for epidemiology, the researchers discovered related bacteria responsible for sickening Europeans with Lyme-like symptoms (such as headaches, joint pain, and chronic fatigue) lurking in the animals. Said coauthor Robert Lane in a press release: "It is worth watching to see if this spirochete expands in this state."
So what does this mean for humans? Will a person who goes out for a Sunday Starbucks and gets pecked by a titmouse contract Lyme disease?
Well, no. A couple things are worth noting:
• The Berkeley extension is located some distance away from major suburban areas (about a two-hour's drive north of San Francisco). If the researchers had tested birds in an area closer to cities the rates of infection might—or might not—have been different, given the changes in habitat and tick density.
• Even if this particular population isn't flying to the 'burbs, it is "well established" that the most suitable bird hosts for Lyme bacteria live in suburban areas, says Erica Newman, a UC Berkeley researcher involved in the study. She explains more via email:
The implication is that humans are generally living among the best hosts for the Lyme disease-causing bacteria, because we create habitat for American Robins, Golden-crowned Sparrows, and Dark-eyed Juncos.
It's similar to the idea that humans might create resources that attract rodents, and those rodents can carry bubonic plague. Not all of them do, but they all have the potential to carry this disease, and if exposed to infected parasites, there can be outbreaks of the disease.
• Birds fly vast distances in migration and could increase the range of Lyme disease more than other Californian hosts, such as wood rats and gray squirrels. (Right now there are about 300,000 new cases diagnosed in the U.S. each year, according to the CDC.) Here's Newman again on the chirping vectors of the sky:
Birds can spread Lyme disease in multiple ways: (1) directly, by carrying infected ticks to new areas where there was no infection previously, or (2) more indirectly, by being abundant in an area, and therefore creating a population of animals that can increase the overall prevalence of the disease in wildlife populations. The number of ticks may increase, but importantly in the case of having infectible hosts, the number of infected ticks may increase in an area. Then when you're walking through an area with a lot of host animals or even sitting on the ground, you or your pet cat or dog are much more likely to encounter an infected tick, which might then bite you and give you the disease.