International travelers are returning home with potentially deadly cargo -- a multidrug-resistant form of the stomach bug shigella that defies a long list of antibiotics, including the first-choice drug Cipro.

There is no evidence that the bug is on Long Island, public health officials say, but experts emphasize that most forms of shigella now carry some degree of antibiotic resistance and the newly discovered form is the worst.

One case of the multidrug-resistant strain has been documented in New York City, according to the city Department of Health.

The menace was announced Thursday by the Centers for Disease Control and Prevention, which found cases turning up individually and in clusters. While investigating clustered cases, federal microbe hunters found people carrying the Cipro-resistant strain.

The bacteria, they say, have learned how to neutralize what is thought to be the most effective drug.

Dr. Aaron Glatt, a Long Island specialist in infectious diseases and a spokesman for the Infectious Diseases Society of America, said shigella causes a relatively common diarrheal disease called shigellosis.

Shigella is a fecal bacterium, Glatt added, and shigellosis can occur following ingestion of food or water contaminated with the bacteria.

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"You see it frequently in army barracks, day care centers and other places where people are in close contact," Glatt said, adding that most people are free of infection within a few days of following a course of antibiotics.

"But I am very concerned," Glatt said, "when I hear that shigella is resistant to Cipro."

The CDC has identified 243 infections to date; clusters have emerged in Massachusetts, Pennsylvania and California.

All told, individuals have been affected in 32 states and Puerto Rico.

CDC director Dr. Thomas Frieden said multidrug-resistant shigella first emerged in India and the Dominican Republic.

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"Drug-resistant infections are harder to treat and because shigella spreads so easily between people, the potential for more -- and larger -- outbreaks is a real concern," Frieden said in a statement Thursday.

Frieden said 90 percent of people tested in the current string of cases were infected with Cipro-resistant bugs.

Drug resistance occurs, Glatt said, when people overuse or misuse antibiotics, causing bacteria to develop the genetic capacity to spurn them.

In this country, most shigella is already resistant to the antibiotics ampicillin and trimethoprim/sulfamethoxazole, according to the CDC.

But worldwide, shigella resistance to Cipro is on the rise. It is often prescribed to people who travel internationally, just in case they encounter the bacteria while out of the United States.

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On Thursday, Frieden said more study is needed to determine whether Cipro prescriptions for travelers have played a role in the current crisis.