MIAMI BEACH — Investigators at the Centers for Disease Control and Prevention are still perplexed as to why a monkeypox outbreak in the United States was less virulent than a simultaneous outbreak in Africa. But genetic differences in the pox strains may provide an answer.
“Early in our outbreak, we noted that monkeypox in the United States appeared to be milder than what we expected,” Anna M. Likos, M.D., a researcher in the Epidemic Intelligence Service at the CDC, said at the annual meeting of the American Society of Tropical Medicine and Hygiene.
The first human monkeypox infection reported in the United States stemmed from a shipment of infected rodents that arrived from Ghana in June 2003 (FAMILY PRACTICE NEWS, July 1, 2003, p. 6). A pet distributor in Illinois acquired the rodents and other small mammals and then sent them on to a number of distributors in Iowa and Texas. By August 2003, there were 37 confirmed human infections.
Dr. Likos and her colleagues compared 266 African cases (260 from the Democratic Republic of the Congo and 6 from West Africa) that had direct evidence of monkeypox infection with the cases in the United States. Infected Americans had significantly fewer lesions, lower rates of hospitalization, and no deaths. Higher complication rates in the Congo could have been attributed to differences in hygiene or socioeconomic status, but the investigators decided to look further.
“It is interesting to note that the only country with deaths was the Democratic Republic of the Congo. No deaths were reported in the United States,” Dr. Likos said.
Researchers sequenced the genomes of the different strains. They found that the U.S. and West African strains were essentially the same, which makes sense because the rodents were imported from Ghana. A strain with different protein and amino acid sequences caused the outbreak in the Congo. These genomic differences may explain the differences in disease manifestations, Dr. Likos explained, “although more work needs to be done.”
CDC investigators visited the Illinois animal distributor facility and found it to be clean. The animals were kept in close proximity, but “the mode of transmission among the animals is unknown,” said Christina Hutson, guest researcher at the CDC's Poxvirus Program.
“Introduction of monkeypox to North America may pose a very strong potential threat to the health of native rodent species and perhaps to humans,” Ms. Hutson said.
Of particular concern is transmission of monkeypox to North American prairie dogs, which are proving to be an especially suitable reservoir. “High levels of virus in some of these animals may explain why they could transmit the virus to humans … prairie dogs are exceptional vectors to humans and other animals,” she explained.
The investigative work is ongoing. CDC researchers plan to do a complete diagnostic evaluation of existing specimens and to continue the genomic comparisons to obtain additional clues, in case monkeypox reemerges in the United States.
“These were all observational infections—so we have a lot of questions remaining,” Ms. Hutson said.