The Lyme Disease Debate

Host Biodiversity and Human Disease Risk

Sharon Levy

Disclosures

Environ Health Perspect. 2013;121(4):a120-a125. 

In This Article

History of Lyme Disease

Lyme disease occurs in Europe and Asia as well as North America, always spread by ticks in the genus Ixodes.[4] Over the last decade, about 20,000–30,000 U.S. cases of Lyme disease have been reported annually by the Centers for Disease Control and Prevention, the majority occurring in the Northeast and the Midwest, where the vector is the black-legged tick, Ixodes scapularis.[5] Average annual numbers of cases in Europe and Asia have been estimated at 65,467 and 3,450, respectively.[4]

The infection's sudden rise in the United States in the 1970s gave the impression that Lyme disease was caused by a newly invading pathogen, but the diaries of early American settlers reported abundant ticks, and the evidence now shows that Bb is an ancient infection in North America. Distinctive Bb genes have been identified in museum collections of ticks from the 1940s[6] and of white-footed mice from the turn of the twentieth century,[7] and studies of genetic variation in separate populations of Bb suggest the pathogen existed across much of the present-day United States many thousands of years before European settlement.[8] Nevertheless, genetic analyses indicate that this genus of bacteria originated in Europe.[9]

Bb is a microbe of forest habitats, and its history is tied to human land use. As European settlers moved west across the United States, they cleared great swaths of forest. Deer, one of the major hosts for black-legged ticks, were overhunted and dwindled to a few small, scattered populations. Populations of white-footed mice, an important reservoir host for Bb, also declined. But in some undisturbed spots in the Northeast and the Midwest, deer, white-footed mice, their tick parasites, and Bb all survived. With the abandonment of most northeastern farm fields in the mid-nineteenth century, forests regenerated, and the microbe traveled with its tick and vertebrate hosts into newly re-expanding habitats.[10]

Lyme disease now appears to be expanding outward from long-time refuges.[10] Migratory birds carry ticks to new habitats, enabling the spread of both ticks and Bb south and north.[11] Some bird species that host ticks are expanding their ranges north, and studies of emerging Lyme disease in Quebec, Canada, suggest that climate change makes it possible for tick vectors to survive in an area that once would have been too cold.[12]

Bb is hitched to the life cycle of its tick vector. Over the course of a life span that lasts at least two years, Ixodes ticks must take a blood meal from a vertebrate host on three separate occasions, dropping off the host after each meal. Tiny larval ticks hatch out on the forest floor in summer and latch onto passing hosts; because the larva waits for a host ("quests") close to the ground, it can attach to an animal of any size, from a rodent, to a bird, to a deer. The blood from this first host will fuel the larva's metamorphosis to the next, nymphal life stage. Nymphal ticks, no larger than a poppy seed, must take another blood meal before molting into adult form. Adult ticks drink blood from a third and final host in order to reproduce. Nymphs and adults sit higher on the vegetation to quest, so they can attach only to larger animals; this is why deer are so important for maintaining tick populations, according to Sarah Randolph, a parasite ecologist at Oxford University.

Adult ticks are large enough to be noticed by any humans they bite within the 24 hours or so it takes to pass along an infection.[13] But nymphs are not as easily detected, and Lyme disease most often arises when a person is bitten by an infected nymph.[14,15] Since Bb is not passed from mother ticks to their offspring, every larva comes into the world uninfected. The natural transmission cycle begins anew when a larval tick feeds on blood from an infected host, typically a mouse, chipmunk, or shrew. Once the larva develops successfully into a nymph, it will seek a new host, putting any passing humans at risk.

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