DALLAS - FDA inspectors are currently in Mexico and Florida, trying to trace the source of tainted tomatoes.

More than 550 cases of salmonella have been linked to tomatoes.

Here in North Texas, there has been a spike in salmonella, that may not be linked to tomatoes at all.

One North Texan man knows he was infected by salmonella.

He kept the proof -- a tomato and tuna salad.

"They give me a lot of IV fluid in my arm so they can keep me from being dehydrated," he said.

After federal authorities issued a nationwide warning about tainted tomatoes, patients started showing up in droves at emergency rooms.

At Presbyterian Hospital Dallas, doctors have diagnosed four salmonella cases in recent weeks. Normally they would have none.

"I think there's an increased sensitivity, so I think they're testing more. And anytime for test for stuff more, you tend to find more," said Dr. Mark Till.

None of the cases at Presbyterian are tied to the tomatoes in question.

So far in North Texas, Dallas County has confirmed 12 cases of salmonella linked to tomatoes.

There are 13, so far, in Tarrant County.

In fact, food poisoning is so common, doctors rarely test for it.

One symptom sets salmonella apart, says Dr. Mark Till.

"If you have blood diarrhea, I would go see someone," he said.

Until the source of tainted tomatoes is confirmed, people are being warned to stay away from round, roma, and plum tomatoes from Mexico and Florida.

E-mail j.stjames@wfaa.com.