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How did these people become infected with monkeypox virus?
On the basis of preliminary investigations, it appears that most of the patients became ill after having close contact with infected prairie dogs that had been purchased as pets. Some patients may have been infected though contact with other infected animals, including a Gambian giant rat (purchased as an exotic pet) and a rabbit. In addition, the possibility of human-to-human transmission in some cases cannot be excluded at this time.
What evidence is there that monkeypox virus causes these illnesses?
The clinical features of the illness in U.S. patients - fever, headache, muscle aches, and rash - are consistent with those of monkeypox. There is also strong laboratory evidence of monkeypox. Scientists at the Marshfield Clinic in Marshfield, Wisconsin, recovered viral isolates from a patient and a prairie dog. Using an electron microscope, they found that the virus had the size and appearance of a poxvirus. Laboratory tests at CDC - including several PCR-based assays, serologic tests, electron microscopy, immunohistochemistry, and gene sequencing - confirmed these results and showed that the virus is Monkeypox virus.
Has monkeypox previously been reported in the United States?
No. Prior to the recent report of the disease in the United States, community-acquired monkeypox had never been reported outside of Africa.
How was monkeypox introduced in the United States?
There is not enough information to determine with certainty how monkeypox arrived in the United States. One hypothesis is that the prairie dogs contracted the virus from infected Gambian giant rats, which were housed in the same animal-holding facility or pet shop as the prairie dogs. The Gambian giant rats were likely imported into the United States from Africa. Studies have shown that Gambian rats and other rodents in Africa have evidence of monkeypox virus infection.