Although the risk of death from colorectal cancer in the United States has dropped dramatically in recent decades, there are three "hot spots" in Appalachia and the rural South where death rates are "unnecessarily high," researchers said.

The highest colon cancer death rates are in the lower Mississippi Delta, where rates were 40 percent higher than the rest of the country during 2009 to 2011, according to a study published Wednesday in Cancer Epidemiology, Biomarkers & Prevention, a journal of the American Association for Cancer Research.

The other two hot spots are western central Appalachia and eastern Virginia/North Carolina, where rates were 18 percent and 9 percent higher, respectively, than elsewhere in the country for that same time period.

The three regions encompass 238 counties in 12 states.

Excluding skin cancers, colorectal cancer is the third most common cancer diagnosed in both men and women in the United States, according to the American Cancer Society. Colorectal cancer is the third leading cause of cancer-related deaths in the United States when men and women are considered separately, and the second leading cause when both sexes are combined. It is expected to cause about 49,700 deaths during 2015.

Researchers don't know why rates in these three primarily rural regions are so much higher than the rest of the United States. But they tick off several underlying factors: high poverty, unemployment, obesity rates; low education and health literacy levels, poor access to health care, and lower cancer screening rates.

They note that as of March 2015, only six of the 12 states in these high-risk regions had expanded Medicaid coverage under the Affordable Care Act to include more low-income adults.

Most disturbing are the data for black men in the lower Mississippi Delta, said Rebecca Siegel, an epidemiologist with the American Cancer Society and one of the authors. The colon cancer death rate increased by 3.5 percent every year for black men in that region between 1970 and 1990, and has since remained unchanged.

"Everyone else is decreasing -- rates for black women and white men and white women are now declining -- but not in black men," she said. "They should be decreasing, so it's very bad that [the rates] are flat."

The lower Mississippi Delta region is made up of 94 counties that span parts of Arkansas (17), Illinois (16), Kentucky (3), Louisiana (6), Mississippi (27), Missouri (15), and Tennessee (10).

Researchers have previously highlighted disparities in colon cancer death rates among states, but the study published Wednesday looked at county-level data. Having that more precise data makes it easier to come up with targeted interventions, Siegel said.

The quickest fix is increased screening, which she said was the main reason that the death rate fell 27 percent across the United States between 2000 and 2010.

Researchers looked at colon cancer death rates from 1970 through 2011 for all U.S. counties. Before 1990, death rates were highest in the northeast and lowest in the South. By the 2000s, however, rates had evened out everywhere except in these three regions.

"Now that these groups have been identified, there is a moral obligation to do something about it," Siegel said, noting that Delaware put statewide screening in place and essentially eliminated racial disparities in colon cancer death rates in less than a decade.

Regular screening can often find colorectal cancer early, when it is most likely to be curable, or prevent it altogether. Some polyps, or growths, can be found and removed before they have the chance to turn into cancer.

This post has been updated.