BANGOR – Hospital officials on Friday upgraded to serious the condition of a Madawaska man and his son who are suspected of having contracted food-borne botulism earlier in the week.

David Pelletier and his son Joshua, 13, remained in the intensive care unit at Eastern Maine Medical Center in Bangor.

The father and son became ill last Sunday. After initially being treated at Northern Maine Medical Center at Fort Kent, they were transferred Tuesday to EMMC, where they had been listed in critical condition.

Meanwhile, officials at the Maine Bureau of Health were still waiting Friday to find out the source of the botulism poisoning. Because the development of symptoms can take from hours to days, samples of food eaten by the father and son over several days have been sent to the Centers for Disease Control and Prevention in Atlanta, Ga., for testing.

“We still don’t have a positive food source,” Geoff Beckett, chief of the infectious disease epidemiology section at the Bureau of Health, said Friday afternoon.

“We’re waiting to get test results back from the CDC,” he said, adding that that could be several more days.

Food-borne botulism is caused by a bacterium found in home-canned foods. The bacteria create a toxin that interferes with the signals transmitted by nerves, causing paralysis of arms, legs and respiratory muscles, which causes difficulty in breathing.

Health officials believe that the poisoning came from something the Pelletiers ate at home. Other common sources of botulism include home-prepared sausage made from wild game meats and some types of smoked fish, according to Beckett.

Neither Pelletier’s wife, Elaine, nor another son, Travis, 8, have been poisoned, and Beckett said there was nothing to suggest that there are any other cases in the region.

The Pelletiers are the first cases of botulism to occur in Maine in at least 20 years.

On Thursday, both David and Joshua were on ventilators to help them breathe. Both also were experiencing various degrees of paralysis. More specific information on their conditions was not available on Friday.

Though rarely fatal, recovery from botulism poising can take weeks and some affects of the disease can last for years.