Diseases show healthy geographical disparity
By Jo Ann LeQuang-Correspondent
Published November 19, 2006

Epidemiology is the study of where diseases occur and how they spread.

For example, there is an AIDS epidemic right now in Africa. In some areas of Africa, as much as 25 percent of the population is infected.

But sometimes diseases have unique geographical patterns that defy normal disease distribution. Understanding these geographical distinctions can provide valuable clues about the disease itself.

One such pattern is the “stroke belt,” an 11-state area in the southeastern United States. People who live in these states are at 50 percent greater risk for stroke than other Americans. Not only is there a higher incidence of stroke, there is a 10 percent higher death rate from stroke.

The 11 states just miss Texas. They are Alabama, Arkansas, Georgia, Indiana, Kentucky, Louisiana, Mississippi, North Carolina, South Carolina, Tennessee and Virginia.

The “buckle” of the stroke belt is an area from the Blue Ridge Mountains to the Atlantic, right around the Carolina coast. Stroke rates are twice as high there as for the rest of the stroke belt.

Why does the stroke belt exist? There are many theories, but no real answers. Some researchers point to smoking rates, a high-fat diet and the fact that there is more high blood pressure here than other parts of the country.

Others wonder if there is not some environmental factor that exposes residents of these states to a higher risk of stroke.

Breast cancer is another disease with some odd “hot spots.” Residents of Cape Cod have twice the incidence of breast cancer than residents of other parts of Massachusetts. When this first came to light, other “hot spots” were pointed out: Long Island, N.Y., Marin County, Calif., Puget Sound, Wash., and Brownsville all have much higher incidences of breast cancer than the surrounding areas.

One of the most intriguing geographical puzzles for disease is multiple sclerosis. The lowest incidence of MS occurs at or near the equator, where the disease affects roughly 1 in 100,000 people. The farther away from the equator you move, the higher the incidence of the disease.

In the northern part of the United States, the incidence of MS is 30 to 80 in 100,000. The same applies for Europe and other parts of the world.

What’s more, with MS, your risk changes if you move before you’re 15 years old. A Northerner who moves closer to the equator before age 15 has the same risks as a person born near the equator. Postpone that move until after age 15, and you can’t alter the risk even if you move to a place exactly on the equator.

Another geographical riddle is schizophrenia, a neurological disorder that affects cognitive function. The incidence of schizophrenia is higher in large urban areas (populations more than 1 million) than in smaller towns. The geography does not seem to matter as much as the environment: big cities have proportionally more schizophrenics.

What’s more, if a schizophrenic parent has a child, that child is twice as likely to develop schizophrenia if raised in a big city than a small town