How did health officials figure out that the salmonella bacteria that hospitalized more than a dozen people around Nashua and Concord, New Hampshire, this summer came from the same source – dehydrated chicken sold as dog treats?

Science, teamwork and questions. Lots of questions.

“We have a questionnaire we give to every salmonella case that we investigate. It asks about 30 food items,” said Beth Daly, chief of infectious disease surveillance for the New Hampshire Department of Health and Human Services.

“If we know that they are potentially part of a cluster of cases, we give them a much longer questionnaire – 12 pages – that asks about several hundred food items. And from there, once you have a hypothesis, you might need to administer a third questionnaire,” she said.

David Merrill, of Nashua, knows this firsthand.

“The city called me – a nurse, I think. She asked me a series of questions like where did I eat and what foods did I have, did I buy anything from a farmer’s market, have I eaten lettuce, things of that nature,” Merrill said. “Lots of questions.”

Merrill, 76, was hospitalized for five days in late August with salmonellosis caused by bacteria he picked up on his hands while handling Joey’s Jerky treats, dehydrated chicken meat. He had bought the product for more than two months and fed it to the family’s two Havanese dogs, Mulligan and Patches.

“I feed the dogs treats – when my daughter isn’t looking,” joked the retired software developer, who has fully recovered.

The treats were voluntarily recalled Tuesday by their maker, an in-home company called Kritter’s Kitchen Kreations in Loudon, after state health officials found that 21 people who became sick from salmonella had all bought the product. More than half those people were hospitalized.

Tests confirmed Wednesday that the treats were the source of the salmonella.

“The most likely reason is that the jerky was under-processed; the dehydrator didn’t kill the bacteria,” said Beth Daly, chief of infectious disease surveillance for the New Hampshire Department of Health and Human Services.