Any fan of Laura Ingalls Wilder's beloved Little House books knows how the author's sister Mary went blind: scarlet fever. But turns out that probably wasn't the cause, medical experts say, upending one of the more dramatic elements in the classic stories. An analysis of historical documents, biographical records, and other material suggests another disease, meningoencephalitis, which causes swelling in the brain and upper spinal cord, was the most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Wilder's letters and unpublished memoir, on which the books are based, suggest she was uncertain about her sister's illness, referring to it as "some sort of spinal sickness."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. Meningoencephalitis is easily treated with antibiotics that didn't exist in the 1800s and is no longer considered a serious illness. Blindness can occur if the disease affects the optic nerve.