The good news is that the death rate from heart disease and stroke in the United States declined 50 percent over the last 35 years, according to a new study in the Journal of the American Medical Association. The bad news is that there are stunning differences in the death rates across the country — and doctors don’t fully understand what’s responsible for the huge disparities.

The differences in death rates from stroke and ischemic heart disease were especially acute in certain counties. An individual in Franklin Parish, Louisiana, is 13 times more likely to die from ischemic heart disease than someone from Pitkin County, Colorado. Meanwhile, a person in Angelina County, Texas, is nine times more likely to have a fatal stroke that an individual in Summit County, Colorado.

“I was most surprised by how large the differences in the death rates were,” said Gregory Roth, M.D., an assistant professor in the cardiology division of the University of Washington, who was the lead researcher of the study. “There are counties that are just getting left behind and not seeing the benefits we’ve made in treating heart disease.”

The study examined death certificates from people in 3,110 counties and found that from 1980 to 2014 the death rate from cardiovascular diseases fell from 507 deaths per 100,000 people to 253 deaths per 100,000 people — a relative decline of 50 percent.

Yet, in some counties, the death rate from ischemic heart disease was double that of counties with the lowest rates: 236 deaths compared with 119 deaths per 100,000 Americans. For stroke, death rates ranged from 40 deaths per 100,000 people in counties with the lowest rate to 68 deaths per 100,000 in counties with the highest.

Other conditions had even greater gaps, at least in relative terms. There was a four-fold difference between some counties in deaths from high blood pressure-related heart disease, for example. However, that disease had lower overall death rates. At its worst, hypertensive heart disease caused 18 deaths per 100,000 people. In contrast, in the counties where death from stroke was most extreme, the disease caused 135 fatalities per 100,000 people.

Overall, the counties with the largest concentration of deaths from cardiovascular diseases extended from southeastern Oklahoma along the Mississippi River Valley to eastern Kentucky. Tennessee, Arkansas, Louisiana, Georgia and Texas were also home to numerous counties with high cardiovascular disease death rates. The regions with the lowest death rates from cardiovascular diseases included the San Francisco area, central Colorado, northern Nebraska and central Minnesota.

Roth said the study wasn’t designed to determine the reasons for the disparity, so further research is needed to understand regional variations in three main areas: behavior and environmental risks, treatment regimens and delivery of emergency services, and acute care after a cardiovascular event.

Cardiovascular diseases, which include heart disease and stroke, accounted for about one in every three deaths in the United States in 2014, according to American Heart Association statistics.

An editorial about the study, also published in JAMA, noted that geographic variation in social determinants of cardiovascular disease, such as smoking, diet, education and alcohol use, make a compelling explanation for the differences. It said the mortality rates for heart disease, stroke, rheumatic heart disease and peripheral artery disease are similar to the pattern of poverty reported by the U.S. Census Bureau, as well as the pattern of high blood pressure, diabetes and smoking reported in the large, federally funded REGARDS study that looked at geographic and racial differences in stroke.

Patient behavior is likely a bigger contributor to the difference than the healthcare delivery system, said David Goff, M.D., Ph.D., director of the Division of Cardiovascular Sciences at the National Heart, Lung, and Blood Institute and an author of the editorial. He said that doctors have long advised their patients that they can cut their risk of heart disease and stroke by making changes to their lifestyles.

“We send a message of health that’s crowded out by other messages,” said Goff. “What you get on TV all day, every day are messages of products and lifestyles that are not as health-promoting.”

Goff doesn’t believe that moving to a healthier county will change someone’s health outcome as much as changing their own behavior. “You can stop smoking and start eating a healthier diet where you live now,” he said.

Roth agrees that better prevention could substantially cut the disparities and said it is crucial that government and society eliminate the barriers to living a healthy lifestyle. It’s critical, he said, that all people have places to buy affordable and healthy food, have access to safe places to exercise and proper preventive care.

Yet, he said it is essential to examine how health care is delivered to end the disparities. For example, he said authorities need to understand what happens in emergency rooms in counties with good outcomes that doesn’t happen in those that need improvement.

“We need to further focus not just on prevention, but on high-quality treatment,” said Roth. He added some states and counties need to do a better job of collecting and sharing information to ensure everyone is using best practices.

“These disparities are a serious and worsening problem,” said Roth.