Consider Lyme disease, identified in 1975 and named for Lyme, Connecticut, U.S.A., where it was first observed. The bacterium that causes Lyme disease may have come to North America a hundred years ago with rats or livestock on ships from Europe. After a tiny Ixodes tick ingests the blood of an infected animal, the bacteria remain in the tick’s gut for the rest of its life. When the tick later bites another animal or a human, it can transmit the bacteria to the victim’s bloodstream.

In the northeastern United States, Lyme disease is endemic​—it has been present there for a long time. The main local reservoir of Lyme disease bacteria is the white-footed mouse. The mice also serve as hosts for the ticks, particularly ticks in their developing stages. Adult ticks prefer to make their home on deer, where they feed and mate. Once engorged with blood, the adult female tick drops to the ground to lay her eggs, from which larvae soon emerge to begin the cycle anew.

A Shift in Circumstances

Pathogens have coexisted with animals and insects for many years without causing disease in humans. But a change in circumstances can turn an endemic disease into an epidemic​—a disease affecting many people in a community. What changed in the case of Lyme disease?

In the past, predator animals helped to limit contact between deer ticks and man by controlling the deer population. When early European settlers cleared the forests to farm, the deer population dwindled even further and the deer’s predators also moved on. But during the mid-1800’s, many farms were abandoned as agriculture moved westward, and the forest began to reclaim the land. The deer came back, but their natural predators did not. Thus, the deer population rebounded explosively, and so did the tick population.

Some time later, the Lyme disease bacterium arrived and took up residence for decades before emerging as a threat to humans. However, when suburbs began to be built at the forest’s edge, children and adults in far greater numbers began to enter the ticks’ domain. The ticks found humans to attach themselves to, and the humans got Lyme disease.

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The deer tick (shown enlarged at right) spreads Lyme disease to humans

Left to right: Adult female, adult male, and nymph, all shown actual size

[Credit Line]

All ticks: CDC