2 monkeypox strains in U.S. suggest possible undetected spread

NEW YORK — Genetic analysis of recent monkeypox cases suggests there are two distinct strains in the U.S., health officials said Friday, 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.

“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 Friday. However, she added, “there could be community level transmission that is happening” in parts of U.S. where the virus has not yet been identified.

The findings mean the outbreak likely will be difficult to contain, said Dr. Angela Rasmussen, a virologist at the University of Saskatchewan.

It’s not clear how long infections have been happening, and where. Some infections may have been misdiagnosed as something else.

“We don’t really have a good sense of how many cases there are out there,” Rasmussen said.

Monkeypox 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.