In 1982, a medical entomologist from Switzerland named Willy Burgdorfer stumbled upon a bacterium carried by ticks.
This bacterium was later discovered to be the cause of Lyme disease and was named: Borrelia burgdorferi.
The 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.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
Lyme disease took its name from the Connecticut area where it first drew attention in the 1970s, including the towns of Lyme, Old Lyme and East Haddam. Scores of people in the area, particularly children, had developed rashes, fevers, swollen joints and sometimes more serious symptoms. Early on, it was called Lyme arthritis. It did not take long for scientists to speculate that there was a common link: blacklegged ticks, also called deer ticks.
Many of the children lived and played in wooded areas and developed rashes after being bitten by deer ticks. Cases were more prevalent east of the Connecticut River, where there were more deer. Many laboratory hours were devoted to determining if the deer ticks were spreading a virus. But no virus was detected. There had to be another explanation.
Dr. Burgdorfer, who was born and educated in Basel, Switzerland, moved to Hamilton in 1951 to pursue his distinctive specialty: tick surgery, as he liked to call his meticulous method of dissecting ticks to study the diseases they spread. Hamilton, a small city in the Bitterroot Valley, had been home to a prominent laboratory for decades, after the discovery in 1906 that wood ticks in the region were transmitting Rocky Mountain spotted fever.
LYME DISEASE AND CLIMATE CHANGE
Lyme disease is a forest phenomenon, dependent on complex relationships between its tick vector and an array of host animals that feed ticks and act as reservoirs for infection. Its emergence 30 years ago was linked to major landscape change in the northeastern United States. Regrowth of forests once cleared for farmland enabled a renaissance of the nearly eradicated white-tailed deer, a critical host for the adult life stage of the tick.9
Migratory birds carry infected ticks north into Canada.3,10 Whether the ticks survive there and establish new populations depends, to a great extent, on temperature. Newly hatched I. scapularis larvae need a blood meal from a host in order to progress to their next life stage as a nymph. Nymphs likewise must feed from a host to fuel their transition to adulthood, and adult females require a third blood meal in order to produce eggs.3