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Transcript
Trees, Ticks, and Spirochetes: The Story of the Lyme Disease Epidemic “Lyme disease is an accident by all accounts.”--Alan G. Barbour, author of Lyme Disease: The Cause, the Cure, the Controversy. Lyme disease is very rarely fatal, but it can make people very sick and it’s sharply on the rise in certain areas of the United States. It’s a good example of an outbreak that is environmentally related because occurrence and location are so strongly linked. In nature, wild animal populations are reservoirs for the bacteria which cause Lyme disease. Lyme disease is common only where deer are abundant and where humans have encroached on their habitat, which is exactly what’s happening in many suburban areas. Named after Old Lyme, Connecticut, Lyme disease, a bacterial infection is transmitted by the bite of a tick--though it took many years to figure that out. The infection itself is caused by a spirochete bacterium called Borrelia burgdorferi after its discoverer Dr. Willy Burgdorfer. In the northeastern U.S. in the 1970’s, a particular set of biological and environmental circumstances converged to create the perfect conditions for a Lyme disease outbreak. Current estimates range as high as fifty to sixty thousand cases of Lyme disease per year. “Physicians report thousands of cases each year, in a steadily increasing pattern that promises to continue,” writes Andrew Spielman, Professor of Tropical Public Health at the Harvard School of Public Health. What’s a Vector? In epidemiology, a vector is an organism that transmits the disease-causing agent between one organism and another. (If an infectious agent can’t be transmitted by being swallowed, breathed in, or through direct contact with an infected person, chances are the disease in question is vector-borne--unlike, say, cancer or heart disease, which are not infectious.) The Troublesome Tick Ticks are parasites that feed by latching on to an animal host and sucking its blood. Their life cycle has four stages: egg, larva, nymph, and adult. Larvae and nymphs each need a blood meal in order to proceed to the next stage, and adult females need a blood meal before laying their eggs. These meals may be months, or even years, apart. Because ticks feed on multiple individuals in the course of their life cycles, they are ideal vectors for infectious microbes. Nine diseases are known to be tick-borne in the U.S. alone, including Rocky Mountain spotted fever, relapsing fever, and tularemia. The bacterium that causes Lyme disease isn’t a new species; Borrelia burgdorferi is thought to have been in the U.S. for at least a hundred years. It may have been brought over in ships from Europe by rats or livestock, or infected ticks may have hitched rides on migrating birds. “What’s happened during the last twenty years is that the range of tick that carries the bacteria that causes the disease has expanded,” explains Professor Tom Daniels, of the Vector Ecology Lab at the Louis Calder Center of Fordham University. In the United States, the disease is concentrated in three areas: the Northeast, the upper Midwest (Minnesota and Wisconsin), and northern California and Oregon. People also get Lyme disease in temperate zones in Europe, Russia, China, Japan, and Korea. Figuring Out the Where and Why of the Disease Understanding and preventing any infectious illness--not just Lyme disease--requires an understanding of its ecology: the interactions between the disease agent or pathogen, any vectors, its host(s) or reservoir(s), and human behavior. The Lyme disease pathogen is the spirochete; the vector is Ixodes scapularis, commonly known as the “deer tick,” primarily in its larval and nymphal stages; the major reservoir is the white-footed mouse (which is also the preferred intermediate host for the nymphal and larval ticks); the preferred host for the adult tick is the deer; and the relevant behaviors are deforestation, reforestation, and suburban development of deer/tick habitat. Other smaller mammals, some birds, and humans, can also serve as hosts for the tick. More Woods + More Deer The North American white-tailed deer population has been exploding since the mid-1900’s because of human-induced environmental changes. Forest has returned to much of the land cleared in the last century for farming, and suburban neighborhoods have developed. There’s more forest in many places than any time in the previous hundred years. This secondary forest occurs patchily around homes, creating “edge habitat,” which is ideal for deer, but not for the predators that used to control their numbers. The deer’s only remaining significant predator is man, and hunting is restricted in suburban areas. The white-tailed deer is an important host for an adult tick because big hosts provide a better meal. A tick will stay on for a week or so, turning into what Daniels cheerfully describes as “a Raisinet with legs. She converts that blood meal into between 2500 and 3000 eggs, every one of which hatches into a larval tick.” More Woods + More Deer + More Ticks + More People The tick population has expanded along with that of the deer, and Americans first became exposed to the related infections in the mid-1960s. Since ticks don’t travel far, “the only way you can become infected is to go to where the tick is, and that’s what has happened in many suburbs,” explains Daniels. “We’ve not only moved into the deer/tick habitat, but we have been very busy making it even more attractive to the deer by doing ornamental planting, which, as any gardener can tell you, the deer love to eat. So, we’ve gone where the bugs are and we’ve done things that make it easier for the bugs to make a living.” A Bum Rap for Deer? Unlike many typical host-parasite relationships, which have evolved over time to be quite specific, I. Scapularis parasitize different animals through various stages of its life. “In fact,” write Daniels and entomologist Richard C. Falco in Natural History magazine, “the immature deer tick’s catholic taste in potential hosts insures that virtually all warm-blooded wildlife in the wooded areas of Westchester County will eventually be infected.” Indeed, a Harvard University study demonstrated that the deer population would have to be virtually eliminated in order to have any effect on tick numbers. Though deer are an important host, the name “deer tick” is something of a misnomer, and the role played by deer has been exaggerated. The Plot Still Thickens: Meet the White-Footed Mouse When they hatch, 99% of the larval ticks are born uninfected and harmless. That’s where mice come in. “The larval ticks need to feed on a competent reservoir,” explains Daniels, “and the best one is the white-footed mouse, which is very good at maintaining the spirochete in its body.” An individual white-footed mouse is capable of infecting nearly 100% of the larval ticks that feed on it, though typically the infection rate is around 40%. In the southern United States, by comparison, immature Ixodes scapularis ticks feed mainly on lizards, which can’t be infected by the spirochete. A similar tick that carries the disease on the Pacific coast, Ixodes pacificus, also feeds on species that are not as highly infected. Enter the Acorn Another factor in the ecology of Lyme disease is acorns: both deer and mice love them. Every three to five years--called “mast years”--oak trees produce bumper crops. And an abundance of acorns in the fall has been statistically correlated to peak population densities of white-footed mice, larval ticks, and nymphs the following summer--the tick stage most likely to infect humans. In 1994, scientists documented a mast year in Duchess County, NY. In 1996 the Center for Disease Control in Atlanta recorded the highest number of Lyme disease cases on record, 30 to 40% higher than much of the preceding decade. This correlation doesn’t prove the theory, of course. The point, as Richard S. Ostfeld of the Institute of Ecosystem Studies puts it, “is that understanding the diversity of interactions in ecological systems may be beneficial for predicting, and therefore avoiding, the times and places at which we are at highest disease risk.” Then There’s Climate--Another Reason the Northeast Is Hit So Hard Because of northeastern winters when it’s too cold for ticks to be active, I. scapularis has a twoyear life cycle. “That’s critically important to maintaining this disease in nature,” explains Daniels, “because the seasonal activity pattern allows an overlap of generations.” Nymphs, the next-to-last life stage of a tick, become active in the spring and infect all the wildlife “just a month or so ahead of the next generation of larvae, hatching out in the summertime,” explains the scientist. “The larvae will feed on all kinds of wildlife, but all of it has just been reinfected by nymphs. This system allows the spirochete to be very efficiently transmitted form one generation to another.” In the milder climate of the Southwest, by comparison, the life cycle of the tick has been compressed to one year, so the likelihood of spirochete transmission from one generation to the next is greatly reduced. Nymphal infection rates are around 5%, compared to 25-30% in the northeast. Nymphs, harder to spot than adults, are responsible for nearly 90% of Lyme disease cases caused by deer ticks. The range of ticks also seems to be expanding. “Ticks that carry the infection have also been found in some areas, such as northern Sweden, that used to be too cold for ticks to survive in, a finding that suggests a regional, if not a global, change in climate,” notes Dr. Barbour. And Last But Not Least, Human Behavior “Superimpose these variables with the edge forest that deer like, put yourself in harm’s way . . . ” Tom Daniels’ sentence ends in a shrug. What appears to be an ideal residential environment-free-standing houses on large, wooded lots--can be extremely dangerous, an example of the law of unintended consequences. The risk of contracting Lyme disease is highest in densely populated suburban areas like New York’s Westchester County or the suburbs of Vienna, Austria--not on a hike in the wilderness. But people like to live near wildlife. Scientists are at work on models that predict a higher incidence of Lyme disease in certain areas at certain times, so that people can be warned to take extra preventive measures. The Bacterium Hasn’t Changed, But the Environment Has Nobody’s actually looked at whether the pattern of suburbanization and reforestation is in place everywhere that Lyme disease has increased, “but I would suspect it is,” says Daniels. “There are more deer across the country (an estimated 25-30 million, a historic high as far as we can tell), more forest (of the fragmented variety, which is more deer-friendly and more peoplefriendly), and more development,” which puts humans into contact with the disease vectors. The irony, as Daniels points out, “is that normally we think of these changes in the environment--re-planting forests and so forth--as good things.” It’s deforestation that’s implicated in the spread of many infectious diseases. “In general, reforestation results in better health, not an increase in disease,” corroborates Dr. Durland Fish of the Yale University School of Public Health. “Nobody really understands the patterns of environmental change that cause disease.” BioBulletin by the American Museum of National History http://www.amnh.org/sciencebulletins/biobulletin/biobulletin/story986.html 1. Lyme disease is caused by what organism? 2. Cases of the disease have increased due to humans doing what? 3. What is a vector? 4. Why are ticks ideal vectors for infectious microbes? 5. Adult ticks like to feed on what animal? 6. The larval ticks like to feed on what animal? 7. What is a reservoir? 8. How does the presence of acorns affect the number of cases of Lyme disease? 9. As climate changes, how have the ticks been able to adapt? 10.What are 3 factors that have increased human contact with the disease vector? 11.It’s _____ that’s implicated in the spread of many infectious diseases.