How the Lyme Disease Epidemic Is Spreading and Why Ticks Are So Hard to Stop

and began to repopulate new forests.

The infected deer ticks on Long Island were only about six miles from Lyme, Connecticut, separated by Long Island Sound. Once they reached the mainland – deer have been seen swimming in Long Island Sound – the infected ticks were able to find an unending supply of reproductive hosts.

By the 1970s, when children in Lyme began getting ill, deer ticks could be found on mice and deer in the area, but they were almost exclusively on the east side of the Connecticut River, which runs from Canada to Long Island Sound.

It was an important clue. Most of the illnesses were on the same side of the river. Scientists began to speculate that the river could be a barrier to Lyme disease’s spread and ticks could be the disease’s carriers. Scientists also noticed that there were no deer ticks on islands that were still without deer. They narrowed in on white-tailed deer as the primary reason for the appearance of the ticks and Lyme disease. Suburbanization, which restricts hunting and provides ornamental plants for deer food, was encouraging deer overpopulation and, with it, the spread of the disease.

Scientists in the early 1980s identified the cause of Lyme disease as a previously unrecognized bacterium, Borrelia burgdorferi, carried by deer ticks and transmitted to humans through their bite.

Today, the deer tick has spread north to Maine and Ontario, Canada, south to Virginia, and as far west as Ohio. Another once-isolated population of ticks in northern Wisconsin has also spread northward into Canada and south to Illinois and Indiana, west as far as Nebraska and east to Michigan.

Continued reforestation and increased deer distribution in Midwestern states will most likely restore the pre-colonial habitat of deer, ticks and Lyme disease.

A Complex Path to Humans
Ironically, the deer that helped the tick population grow and spread do not become infected with the Lyme disease bacterium and cannot cause infection in ticks. But, birds and small mammals, particularly the abundant white-footed mouse, can carry the bacteria and infect immature ticks that feed on them. Infected larvae turn into infected nymphs, the source of infection for larger animals and humans.

Adult ticks hitchhike a ride on the deer, where they mate and feed on the deer’s blood. When they are done, the female then drops off into the leaf litter where deer travel and lays her eggs. Each deer can support hundreds of ticks, and each female tick lays about 2,000 eggs.

Once new tick populations are established by deer that carry them into new areas, infected ticks cause infection in mice, birds and other small mammals. New populations of deer ticks rapidly become infected with the Lyme disease bacterium as soon as they are established.

Lyme Disease Continues to Expand
The rate of Lyme disease spread has been slow compared to mosquito-borne West Nile virus, yet the current epidemic of Lyme disease is steadily increasing. It is estimated to be spreading 30 kilometers per year.

There has been little effort to try to limit the geographic spread of infected ticks. Most control efforts have been focused on managing tick populations where they are already established. Efforts have so far have included area insecticide application, bait stations to treat mice or deer with insecticide and vaccinating mice against the bacterium. All of these methods have had limited success in reducing Lyme disease risk, but none have been employed to limit spread.

From an ecology perspective, the question is not why there are so many ticks, but why are there aren’t more. At least 90% of each tick stage disappears over a single generation and we do not understand what happens to them. How many starve to death before finding a host? How many find hosts, but get removed by grooming before they can feed? How many are eaten by other animals or die from parasites? How does weather affect mortality?

Basic research on tick ecology pales in comparison to that conducted on the bacterium and patients. If we knew what limits tick population growth in nature, we might have better insight on how to manage their spread. For now, Lyme disease will continue to expand unabated.

Durland Fish is Professor Emeritus of Epidemiology (Microbial Diseases), Yale University. This article is published courtesy of The Conversation.