Hundreds of people have been left seriously unwell after branches of the fast food outlet were rocked by the cyclospora bug.
New figures released on Thursday by the US Food and Drug Administration show cases come from 15 different states.
It has led to salads being pulled from 3,000 different stores.
The tiny parasite is normally caused by eating raw fruit and vegetables that have been contaminated by human waste.
Cyclospora cayetanensis is a bowel infection and can cause flu-like symptoms and severe stomach cramps.
Investigators claim the bug, which is linked to human poo, has been linked to people who have bought and consumed contaminated salads.
The initial outbreak began at branches in Illinois and Iowa, where customers complained of being struck down with the vomiting and diarrhoea bug.Other symptoms include appetite and weight loss, intestinal pain and fatigue.
The infection is spread by ingesting food or water contaminated with the microscopic parasite.
It is unclear how McDonald's salads became infected but the parasite has been linked to other outbreaks in the US.
McDonald's is understood to have ditched the supplier, Fresh Express, which had provided the infected salad.
Effects of the illnesses may last from just a few days to more than a month.
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