‘Explosive diarrhoea’ outbreak remains a mystery as officials struggle to find sources

"While our colleagues at the CDC are working hard to support state partners, we have had to adapt to federal changes," Hope Shuler, a spokesperson for the state's public health department, said.

One reason the parasite is so hard to track is that it takes one to two weeks for people to fall ill after infection, said Jodie Guest, senior vice chair of epidemiology at Emory University's Rollins School of Public Health.

Given the scale of the US outbreak, public health experts said there were likely multiple points of contamination in the food supply - another factor complicating investigations.