An analysis of historical documents, biographical records and other material suggests another disease that causes swelling in the brain and upper spinal cord was the most likely culprit. It was known as "brain fever" in the late 1800s, the setting for the mostly true stories about Wilder's pioneer family.
Scarlet fever was rampant and feared at the time, and it was likely often misdiagnosed for other illnesses that cause fever, the researchers said.
Wilder's letters and unpublished memoir, on which the books are based, suggest she was uncertain about her sister's illness, referring to it as "some sort of spinal sickness." And a registry at an Iowa college for blind students that Mary attended says "brain fever" caused her to lose her eyesight, the researchers said.
They found no mention that Mary Ingalls had a red rash that is a hallmark sign of scarlet fever. It's caused by the same germ that causes strep throat. It is easily treated with antibiotics that didn't exist in the 1800s and is no longer considered a serious illness.
Doctors used to think blindness was among the complications, but that's probably because they misdiagnosed scarlet fever in children who had other diseases, said study author Dr. Beth Tarini, a pediatrician and researcher at the University of Michigan.
Her study appears online Monday in Pediatrics.
It's the latest study offering a modern diagnosis for a historical figure. Others subjected to revisionists' microscope include Soviet leader Vladimir Lenin, composer Wolfgang Mozart and Abraham Lincoln.
Tarini said as a girl she was a fan of the "Little House" books and wanted to research Mary Ingalls' blindness ever since scarlet fever came up during a medical school discussion.
"I raised my hand and said, 'Scarlet fever can make you go blind, right?'" The instructor hesitated and responded, "I don't think so."
The disease that Mary Ingalls probably had is called meningoencephalitis (muh-NING-go-en-sef-ah-LY-tis). It can be caused by bacteria and treated with antibiotics, but Tarini said it's likely she had the viral kind, which can be spread by mosquitoes and ticks.
The viral disease is fairly common today, particularly in summer months and can cause fever, headaches and sometimes seizures, said Dr. Buddy Creech, an infectious disease specialist at Vanderbilt University. Affected children typically require hospitalization but lasting effects are uncommon, Creech said.
Still, blindness can occur if the disease affects the optic nerve, and it's entirely possible that Mary Ingalls had the condition, he said.
Historian William Anderson, author of Laura Ingalls Wilder biography, said various theories about Mary Ingalls' blindness have been floating around for years. The new analysis provides credible evidence that it was caused by something other than scarlet fever, but it does nothing to discredit the books, Anderson said.
"From a literary standpoint, scarlet fever just seemed to be the most convenient way" to describe Mary's illness, he said.
AP Medical Writer Lindsey Tanner can be reached at http://www.Twitter.com/LindseyTanner