The death of a 6-year-old Massachusetts boy after a mystery E. coli infection continues to stump health officials searching for the source.

Owen Carrignan of Millbury died May 26 after developing hemolytic uremic syndrome, the most serious complication of infection with dangerous strains of E. coli bacteria. The first-grader was infected with E. coli O157:H7, the strain most often associated with illnesses tied to ground beef.

But Owen didn’t appear to have contact with hamburger or other beef before he became ill starting around May 20, said Derek S. Brindisi, the Worcester, Mass., director of public health. In fact, it’s still not at all clear what made Owen sick.

“It’s primarily a foodborne illness pathogen,” noted Brindisi. “But it could be food, it could be a secondary exposure, a cross-contamination or exposure to another animal or person.”

Owen's mother, Michelle Carrignan, told the Worcester Telegram & Gazette that the boy spent the night at a friend's house. His father, Shawn Carrignan, separately said the boy ate a hot dog at a barbecue. But Brindisi said more recent interviews suggested that Owen already had symptoms before those events and that they were unlikely to be the cause. 

After becoming ill, Owen quickly worsened, eventually developing kidney failure caused by HUS.

“It should never happen, you know? A 6-year-old boy full of life,” Todd Carrignan, Owen’s uncle, told WHDH, an NBC affiliate.

Before his death, Owen was a healthy, active boy with a bright smile who loved the outdoors, playing sports and wrestling with his sisters, a family memorial said.

On Monday, a team of local health officials, with advice from state epidemiologists, had expanded their investigation of Owen’s death, Brindisi said.

They’re looking closely at his diet throughout the month of May, not just in the one- to 10-day incubation period for E. coli O157:H7.

Food samples from retail venues, including stores and restaurants, that may have provided food that Owen ate are being examined at state laboratories, said Brindisi, who declined to identify the specific foods being tested.

E. coli O157:H7 causes about 36 percent of the 265,000 infections caused by Shiga toxin-producing E. coli, known as STECs, according to the Centers for Disease Control and Prevention.

Serious complications, such as HUS, are more common in children, the elderly and people with other health problems.

So far, there are no other reports of E. coli infection related to Owen’s, Brindisi said.

In many cases, the source of isolated E. coli infections is never detected. Only about 20 percent of E. coli cases are part of recognized outbreaks. Still Massachusetts health officials plan to exhaust all options.

“Each day, we learn new information,” Brindisi said.

Related stories on Vitals: 

• Testing for new strains of E. coli in beef to begin
• 8-week-old baby sickened by dry dog food, lawsuit claims
• Salmonella by mail? Hatchery sparks 8-year outbreak