Lyme disease correlates with the waning fox population, not an increase in deer.
A twist in Lyme disease research. I’m Bob Hirshon and this is Science Update.
Coyotes and foxes, not deer, may be the key animals involved in Lyme disease. This according to wildlife ecologist Taal Levi, now at the Cary Institute for Ecosystem Studies. He explains that the ticks that transmit Lyme disease are called deer ticks, but they typically acquire the bacteria from small mammals like mice and chipmunks. Foxes prey heavily on those mammals, but in many places, foxes are losing territory to coyotes.
While at the University of California at Santa Cruz, Levi and his colleagues studied the relationship between the fox-coyote balance and Lyme disease in the Northeast and Midwest.
And those spatial patterns of coyote abundance and fox rarity really effectively predicted Lyme disease incidence.
Levi says the prevalence of deer, on the other hand, was unrelated to human disease rates. I’m Bob Hirshon for AAAS, the Science Society.
Making Sense of the Research
If you haven't heard of Lyme disease, you should probably learn about it. It's an illness caused by one of three different bacteria, transmitted to humans through tick bites. Early symptoms include fever, tiredness, headache, and a distinctive circular skin rash. The disease can be treated with antibiotics, if it's caught early enough. Left untreated, it can cause long-term disabilities. The disease often goes unnoticed because many of its symptoms are common, and because people often aren't aware of tick bites.
Lyme disease is found in many parts of the world, including North and South America, Europe, and the British Isles. In the U.S., it's most common in the Northeast and parts of the Upper Midwest, and to a lesser extent, in the Southeast and West Coast. The bacteria that cause Lyme disease live in different kinds of woodland mammals, and it's transmitted to humans through ticks that bite those animals as well as people. The tick most responsible for transmission on the U.S. East Coast is known as the deer tick, and indeed, it does bite deer. That's given deer a bad reputation for spreading Lyme disease, and some communities encourage deer hunting in order to keep deer populations down.
However, this study suggests we may be blaming the wrong animal. One of the key findings of Levi's research is that deer populations did not correlate with incidence of Lyme disease in the U.S., at least among areas where Lyme disease is common. In other words, the rates of human Lyme disease didn't have anything to do, statistically, with deer populations.
What Lyme disease rates did correlate with was the balance between red foxes and coyotes in a given area. The higher the fox population compared with coyotes, the lower the rates of Lyme disease. The reason for this provides a good lesson in ecology and the food web. Levi explains that small rodents, like mice and chipmunks, are really the major carriers of Lyme disease bacteria. In many areas, red foxes prey on these rodents and keep their populations in check.
However, coyotes have recently begun taking over fox territories. Some of these coyotes have even cross-bred with wolves to create a new kind of top predator. These coyotes attack foxes that get in their way, since foxes compete for some of the same prey. Studies show that once coyotes move into an area, foxes steer clear of it.
But replacing foxes with coyotes isn't an equal exchange. For one thing, coyotes live in much less dense populations than foxes do. That means fewer predators over the same size area, which allows more of the prey animals to survive. What's more, although coyotes certainly eat small mammals, the ones that carry Lyme bacteria don't make up as large a percentage of their diet as they do in the foxes' diet. That also allows small mammals to thrive. Finally, the foxes themselves sometimes serve as a host for the Lyme bacteria, but they don't transmit it to ticks as efficiently as mice and chipmunks do.
As a result, with coyotes you get fewer predators, with less of a taste for the Lyme disease carriers, and fewer alternate carriers that don't affect humans as much. The most striking comparison in the study was between Eastern and Western New York state. In the east, where coyotes have invaded, human Lyme disease rates are much higher than in the west, where foxes still rule. The research shows that changes in an ecosystem can have far-reaching effects that almost always reach people.
Now try and answer these questions:
- What is Lyme disease? How is it transmitted?
- How do woodland mammals contribute to Lyme disease in humans?
- What are some of the ways in which surging coyote populations and dwindling fox populations have affected Lyme disease rates?
- How does this change perceptions of how the disease might be controlled?
You may want to check out the July 27, 2012, Science Update Podcast to hear further information about this Science Update and the other programs for that week. This podcast's topics include: Naked mole-rats defy old age, vampire spiders attack Franken-mosquitoes, dogs protect babies against asthma, what the genes of parrots reveal about speech, and the relationship between fox populations and Lyme disease.
Hear about other consequences of removing predators in the Science Update Predators and Plants.
In the National Geographic Xpeditions lesson Parasites and Disease, students can research some parasite-borne diseases like Lyme disease, and report on how parasites infect their hosts and how people are trying to reduce infection rates.