What plant caused milk sickness in 19th-century American settlers?

What plant caused milk sickness in 19th-century American settlers? White snakeroot, or Ageratina altissima, caused milk sickness when livestock ate the plant and contaminated their milk and meat with its toxin tremetol. People did not usually become ill by eating the plant directly; they were poisoned indirectly through products from affected animals.

Milk sickness was especially deadly in the early 19th century, when European-American settlers moved into the Midwest and Upper South and were unfamiliar with the local vegetation. Cattle grazing in wooded pastures could consume white snakeroot, and the resulting contaminated milk could spread illness through entire households and communities. Symptoms included trembling, vomiting, severe intestinal pain, weakness, and sometimes death.

The cause took decades to establish. Dr. Anna Pierce Hobbs Bixby is credited with recognizing the plant’s connection to the disease in the 1830s, reportedly after learning from a Shawnee woman. Nancy Hanks Lincoln, Abraham Lincoln’s mother, is often identified as a possible victim. Modern pasture management and milk pooling have made the disease extremely rare.