NEW YORK >> Genetic analysis of recent monkeypox cases suggests there are two distinct strains in the U.S., health officials said today, raising the possibility that the virus has been circulating undetected for some time.

Many of the U.S. cases were caused by the same strain as recent cases in Europe, but a few samples show a different strain, federal health officials said. Each strain had been seen in U.S. cases last year, before the recent international outbreak was identified.

Analysis from many more patients will be needed to determine how long monkeypox has been circulating in the U.S. and elsewhere, said Jennifer McQuiston of the Centers for Disease Control and Prevention.

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“I think it’s certainly possible that there could have been monkeypox cases in the United States that went under the radar previously, but not to any great degree,” she told reporters today.

Monkeypox typically begins with a flu-like illness and swelling of the lymph nodes, followed by a rash on the face and body. The disease is endemic in parts of Africa, where people have been infected through bites from rodents or small animals. It does not usually spread easily among people.

Last month, cases began emerging in Europe and the United States. Many — but not all — of those who contracted the virus had traveled internationally, and health officials in a growing number of countries are investigating.

As of today, the U.S. had identified at least 20 cases in 11 states. Hundreds of other cases have been found in other countries, many apparently tied to sexual activity at two recent raves in Europe.