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From Andrew Maynard - Chair of the University of Michigan Department of Environmental Health Sciences, with help from David Faulkner - 2013 Master of Public Health graduate.





 

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Lyme Disease

What Causes Lyme Disease?

Last modified: 
19/04/2012 - 11:10

Lyme disease is caused by a type of bacterium called a spirochete. The particular species of spirochete that is responsible for Lyme disease is called Borrelia burgdorferi, and it is spread by infected ticks.

In the United States, the tick that carries Lyme disease is commonly known as the deer tick, because in its adult form it usually lives on deer. In its immature form (also called the larval form), it lives primarily on the white-footed mouse. About half of these adult deer ticks are infected with the Lyme bacteria. The average deer carries 200 or more deer ticks at any one time. The life cycle of the tick can be interrupted by removing deer from the pool of host animals. In most parts of the US the deer is the only large mammal capable of providing a blood meal to the adult female tick to enable her to lay eggs. These deer carry the ticks into areas where people live, work, and play.

Ticks become infected with the Lyme disease spirochete by feeding on the blood of animals that have the disease.

The tick usually bites people when it is in the nymph stage, between larval and adult. At this point in its life cycle it is very small, about the size of a poppy seed, and difficult to see. Three-quarters of the people who contract Lyme disease don't even remember being bitten.

When an infected tick bites a person, the spirochetes can enter the skin. This doesn't usually happen until after the tick has been attached for between 36 and 48 hours.

  • The infection causes local symptoms, such as a rash near the bite.
  • It may also travel in the blood to more distant tissues.
  • Additional symptoms can be caused by the body's own reaction to the invader.

Who Is At Higher Risk?

Anyone exposed to ticks is at risk for Lyme disease, including pets. Lyme disease is more common in:

  • Children between the ages of 5 and 14
  • Adults between the ages of 30 to 49
  • Golfers
  • Campers
  • People who work outdoors
  • People who spend a lot of recreation time outdoors

One study has indicated that Lyme disease can pose the most problems for people over age 70.

There is no evidence that people can get Lyme disease from the air, from food or water, through sexual contact, or directly from wild or domestic animals.

According to the Centers for Disease Control, no one has gotten Lyme disease through the bites of mosquitoes, flies, or fleas; or through blood transfusions or other contact with infected blood or urine.

Nice To Know:

Deer in the Neighborhood

Lyme disease has arisen as farms and forests give way to expanding suburban development. Deer habitat has been increasingly crowded out by residential subdivisions. With few predators left to keep down their numbers and the neighborhoods far too congested for hunting, the deer population becomes larger than the diminished woodlands can support.

Deer compete with people for space in grassy parks, public "green space," and even backyard gardens. They feast on carefully tended landscaping. Their close proximity brings deer ticks closer to people, too, and Lyme disease is a result.

For many suburbanites, the novelty of pointing out the deer gamboling on the nearby hillside has worn thin, and control measures are being put into place. These include 9 to 10 ft high fences, culling the deer herd to reduce their numbers below 8 per square mile or moving the animals. The latter is no longer permitted due to the risk of spreading chronic wasting disease. Likewise contraception of deer is only available in a controlled study of isolated herds, such as on an island: such drugs are injected by darts and each animal is tagged to prevent human consumption.

The issue of too many deer has also generated debate among community planners about how development ought to be restricted to limited areas, resulting in proposals for more traditional "cluster communities" with larger blocks of green space around them, so that humans and deer can regard each other from a safe distance.

Contributing Author: Guy Slowik FRCS

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Lyme Disease