Ottawa Public Health is investigating a lunch catering service they believe caused a salmonella outbreak among children.

Ottawa's chief medical officer of health Dr. Isra Levy sent a letter to local physicians Tuesday warning them of 20 reports of lab-confirmed cases of salmonella and the number is expected to grow.

The reports have come in during the last three days, the letter reads, and CBC News has confirmed it involves children between the ages of 15 months and 14 years old at three schools and one daycare. Four adults are also affected.

There are several cases reported at the Tiny Hoppers Daycare in Kanata, and some at Turnbull School on Fisher Avenue and Steve MacLean Public School and École élémentaire catholique Jean-Paul-II in Gloucester.

Tiny Hoppers confirmed to CBC News it had a lunch caterer named "The Lunch Lady" serving for the past couple months, but this is its last week because the daycare has now hired an in-house chef.

The daycare's director also said health officials were targeting one of the caterer's kitchens as the possible source of the outbreak.

Salmonella causes children to suffer from nausea and vomiting. More serious cases, which have hospitalized three children so far, involve dehydration and possible bloody diarrhea.

The bacterium thrives in uncooked food and unwashed fruits and vegetables. There are usually about 150 cases in the city each year, according to Ottawa Public Health, but they say this kind of outbreak is rare.

Health officials believe a lunch catering business that serves the three schools and daycare is responsible for the outbreak.

Levy is asking local doctors to "increase their clinical vigilance" for salmonella infections.