The Iowa and Illinois health departments said on Thursday that they were investigating a rise in cyclospora infections linked to salads at McDonald's restaurants.
McDonald's shares fell 1.4 percent in after-hours on Thursday.
The Illinois Department of Public Health said it had seen about 90 cases, and the Iowa Department of Public Health said it had recorded 15 cases.
About one-fourth of Illinois cases reported eating salads from McDonalds in the days before they became ill.
McDonald's did not immediately respond to a request for comment.
The parasite, cyclospora cayetanensis, infects the small intestine, typically causing watery diarrhea and frequent, sometimes explosive bowel movements. It is spread by ingesting food or water contaminated with feces and not directly from one person to another.
Several outbreaks have occurred in the United States in the past several years, especially during the summer months, that had been linked to imported fresh produce including raspberries, basil, snow peas, and lettuce.
The health departments said McDonalds was removing salads from its restaurants and distribution centers and was re-supplying restaurants with salads from other suppliers.
Illinois Health Department director Nirav Shah said the agency would continue to investigate the outbreak.