The Ecology of Lyme-Disease Risk
Complex interactions between seemingly unconnected phenomena determine risk of exposure to this expanding disease
Natural History of Lyme Disease
Lyme disease is mistakenly thought by many to be a new disease that first appeared late in the 1970s. In reality the erythema migrans syndrome—now recognized as indicative of Lyme disease—was described in Europe more than 80 years ago. At the time, it was correctly linked to bites from the tick Ixodes ricinis, although the causative microbial agent had not been established. In 1975 health specialists were called in to investigate a peculiar cluster of childhood arthritis cases in Lyme, Connecticut, and they found that many of the cases had been preceded by EM. Allan Steere at Tufts University and his colleagues suspected a link between the Connecticut EM cases and the earlier European ones, causing them to search for and eventually find highly abundant populations of ticks. At first, it was thought that the ticks represented a newly discovered species, which was named Ixodes dammini, but later evidence indicated that the specimens were simply members of northern populations of the previously described species Ixodes scapularis. Further medical detective work by Willy Burgdorfer at Rocky Mountain Labs and colleagues revealed that both the European and Connecticut ticks played host to B. burgdorferi, which had been previously undescribed.
Even in North America, Lyme disease almost certainly existed for thousands of years before its discovery in the 1970s. Native Americans and European colonists probably experienced the disease in the forested landscapes of the northeastern and north-central United States and the West Coast, but the symptoms went largely unrecognized, perhaps because of the prevalence of more serious, debilitating diseases. Early symptoms of Lyme disease are rather mild compared with those of cholera, typhus, tuberculosis and others that plagued early Americans, and acute Lyme disease is often a brief and self-limiting ailment. However, several weeks or months after initial infection, bacteria may begin dispersing away from the bite to distant sites in the body, to the knees and shoulders or to the central and peripheral nervous systems, where they can wreak havoc on joints and nerves, causing serious arthritic and neurological disorders.
Since 1990, between 9,000 and 14,000 cases of Lyme disease have been reported annually to federal health officials, although the true number of cases is unknown. The Lyme-disease epidemic has caused hysteria in some areas, which probably leads to the false reporting of some cases. But it is also likely that many cases of Lyme disease go unreported each year because health-care workers and patients are unfamiliar with the symptoms. In any event, Lyme disease is by far the most common vector-borne disease in the United States, surpassing malaria, yellow fever, dengue fever, Rocky Mountain spotted fever, plague and others. Although Lyme disease is widespread in the United States, the northeastern seaboard from Maryland to Massachusetts, the upper Midwest and California have the highest numbers of cases.