"Many Health and biologist researchers suggests that fragmentation of forest habitat plays an important role in facilitating the spread of Lyme disease".......... "Diaries of early American settlers reported abundant ticks, and the evidence now shows that Bb(lyme spyrochete) is an ancient infection in North America.that did not morph into a scourge impacting humans due to the equilibrium created via carnivores(Wolves/Pumas/Black Bears/Marten/Fishers/Bobcats/Lynx/Coyotes, Red and Gray Foxes/Eagles/Hawks et al) preying on tic hosts(Deer/Elk/Caribou/white footed mice/skunks/raccoons/chipmunks/voles et al.)......................... "Distinctive Bb genes have been identified in museum collections of ticks from the 1940s and of white-footed mice from the turn of the twentieth century"........... "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".................... "Nevertheless, genetic analyses indicate that this genus of bacteria originated in Europe"..............."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, major host 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, bringing on the scourge of Lyme we now face as a nation"

http://ehp.niehs.nih.gov/121-a120/

The Lyme Disease Debate: Host Biodiversity and Human Disease Risk

Sharon Levy

Sharon Levy, based in Humboldt County, CA, has covered ecology, evolution, and environmental science since 1993. She is the author of Once and Future Giants: What Ice Age Extinctions Tell Us about the Fate of Earth’s Largest Animals.

History of Lyme Disease

Lyme disease occurs in Europe and Asia as well as North America, always spread by ticks in the genus Ixodes. 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. Average annual numbers of cases in Europe and Asia have been estimated at 65,467 and 3,450, respectively.

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 and of white-footed mice from the turn of the twentieth century, 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. Nevertheless, genetic analyses indicate that this genus of bacteria originated in Europe.

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.

Lyme disease now appears to be expanding outward from long-time refuges. Migratory birds carry ticks to new habitats, enabling the spread of both ticks and Bb south and north. 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.

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.1But nymphs are not as easily detected, and Lyme disease most often arises when a person is bitten by an infected nymph. 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.

Lyme Hosts

Blame for the emergence of both black-legged ticks and Lyme disease has typically focused on deer, which have abundantly repopulated the northeastern and midwestern United States over the last few decades. Yet deer turn out to be immune to infection with Bb; even though they’re an important host for ticks, especially in the adult life phase, they don’t transmit Lyme disease.

Early research tested the assumption that reducing deer populations would lower the risk of human infection by reducing numbers of infected nymphal ticks searching for a host. The results were mixed. Some studies showed a strong relationship between deer abundance and tick density. Others, however, reported that tick density was tightly linked with numbers of white-footed mice or small mammalian predators, not deer. Experiments in the Italian Alps reported an increased density of questing nymphs in habitat patches where deer had been fenced out.

In assessing such findings, it is essential to take into account the time scale. “We all know that tick abundance will increase at first in the absence of hosts; they accumulate on the vegetation with no hosts to attach to,” she explains. “But later the abundance declines fast as the ticks die and are not replaced through natural reproduction—no hosts to feed adult ticks, no eggs.”

A number of studies in Europe and the United States have shown that while some species are competent reservoir hosts for Bb (that is, they’re likely to pass Bb along to the ticks that bite them), others are not.  In 1990 Durland Fish, an epidemiologist at Yale School of Public Health, coauthored a study in which wild raccoons, striped skunks, opossums, and white-footed mice were held in cages over water pans that collected all the engorged larval ticks that dropped off. In the laboratory, the larval ticks were incubated, and the researchers tracked the numbers that developed successfully into nymphs. They then tallied the percentage of nymphs that carried Bb. Forty percent of the nymphal ticks that had fed on white-footed mice as larvae were infected. The figures for ticks that had fed on raccoons and skunks were much lower. (In the jargon of zoonoses, such animals may be “dilution hosts,” meaning they tend to make infection less prevalent in the tick population.) None of the nymphs from larvae that had fed on opossums survived long enough to be tested.

Forest Fragmentation and Biodiversity

Ostfeld suggests that fragmentation of forest habitat plays an important role in facilitating the spread of Lyme disease. His argument is based on the notion of nested biodiversity: Large swaths of habitat house diverse animal communities, and as forests are cleared for human use, species disappear from the remaining isolated scraps of habitat in a predictable sequence. This pattern has been documented on oceanic islands and other isolated habitats. But whether it applies to the forests of the northeastern and midwestern United States, where Lyme disease is most prevalent, remains a contentious issue.
16th century forest

 settlers clearing forest

Ostfeld’s work shows that the white-footed mouse, a powerful amplifier of Lyme disease risk, persists in small fragments of forest after other species disappear. He argues that hosts resistant to tick infestation and Bb infection are far more sensitive to human disturbance. Yet raccoons and opossums, which appear to be among the most effective dilution hosts for Bb, are common in urban and suburban areas. Studies from Illinois and California showed these animals thrived in remnants of forest and moved easily across farm fields. The California study noted that opossums prefer intensely disturbed habitats.

“If you fragment the forest, you still have all the main hosts for Bb,” says Maria Diuk-Wasser, a disease ecologist at Yale School of Public Health. “The major hosts are all human-adapted. Raccoons and opossums are present in people’s backyards.”

Diuk-Wasser is now collaborating with Fish on a study that tests the hypothetical link between biodiversity and human risk of Bb infection in new ways. Among the human residents of Block Island, off the coast of Rhode Island, Lyme disease is a common affliction. The island has low mammalian biodiversity; the only tick hosts present there are deer, white-footed mice, and birds. The researchers are trapping mice, collecting the ticks that infest them, and testing them for Bb infection. They’re cooperating with colleagues who have been collecting data on human cases of Lyme disease for years. The results from Block Island will be compared with those from a site on the Connecticut mainland, where a full complement of vertebrate tick hosts is present—and Lyme disease is also endemic.

If the dilution hypothesis holds, the number of infected nymphal ticks should be much higher on Block Island than on the mainland. The Yale investigators are also collecting ticks from backyards to directly examine the interface between humans and vectors of Bb. Fish, a critic of Ostfeld’s model of Bb ecology, does not expect to find simple correlations. “Community composition does affect Lyme disease ecology, but it’s not a rule of thumb that more biodiversity means less risk to people,” he says.

Both Ostfeld and Fish have coauthored studies that found a correlation between the size of forest habitats and the risk of Lyme disease. In surveys of 14 forest fragments ranging in size from 0.7 to 7.6 hectares, Ostfeld’s team found that white-footed mice were abundant in small forest patches and that the density of infected nymphal ticks was highest in the smallest patches (less than 1.2 hectares, comparable to the area inside an athletic track).Fish and his colleagues found a similar pattern in woodland habitats near Lyme, Connecticut, but noted that despite the higher number of infected ticks in fragmented habitats, the rate of human infections was lower there. This was so, the group concluded, because as woods were cleared for suburban development, the remaining habitat patches became few and far between, so that most people in the area never got near enough to a forest fragment to contact an infected tick.

The relationship between forest fragmentation, biodiversity, and human risk of Lyme disease is still under debate—at least two research groups have shown that the density of Bb-infected ticks increased as habitat size shrank, yet one study showed that human infection rates went down at the same time. The role of urban-adapted “dilution hosts” such as raccoons and opossums remains unclear.
Houses: © Steve & Dave Maslowski/Science Source; raccoons: © Paul A. Souders/Corbis

Download larger image (TIF File)

It boils down to a numbers game. The tick population depends on the presence of hosts to provide blood meals. If a Bb-resistant host species feeds enough larval ticks to lower the density of infected nymphs at the next life stage, it’s also likely to boost the overall tick population. That means more larvae will be around to feed on hosts that do pass along the disease, explains Randolph; the proportion of infected ticks may decline even as their abundance increases.

That phenomenon was illustrated in a recent experiment on Lyme disease ecology in California. The western fence lizard is an important host for disease-bearing ticks there but is resistant to Bb. Its immune response to the bacterium is so powerful that a lizard can actually clear the infection from the midgut of ticks feeding on it. This ability would seem to make the fence lizard the ultimate dilution host, and when researchers removed the lizards from test plots in oak woodlands, they expected the numbers of infected nymphal ticks to increase as a result. But the opposite occurred. The density of infected nymphs—and thus the potential risk of human infection—decreased in plots where lizards had been removed. The study, coauthored by Ostfeld, concluded that “the California Lyme disease system behaves differently than that in New York.”

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Ticks carrying Lyme disease in almost half of US counties

By Reuters

January 18, 2016 

http://nypost.com/2016/01/18/ticks-carrying-lyme-disease-in-almost-half-of-us-counties/

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