Past U.S. Cases and Outbreaks
In May 2022, an unprecedented global outbreak of mpox (formerly known as monkeypox) was detected. Cases in the United States increased rapidly, peaking in August 2022. This outbreak of mpox is still ongoing and is different from past outbreaks because it is primarily spread through sexual contact. Read more about the status of the 2022 outbreak.
CDC lab experts have found at least two variants of mpox virus circulating in this outbreak. Both variants share common ancestors with strains present in Nigeria since 2017. It is likely that there were at least two separate events where the mpox virus spread from animals to people in Nigeria, and then began to spread person-to-person through close contact.
The Centers for Disease Control and Prevention (CDC) and the Maryland Department of Health confirmed on November 16, 2021 a case of mpox in a U.S. resident who recently returned from Nigeria to the United States. CDC is supporting state and local health officials, airline and travel industry partners, and other stakeholders to identify people who had possible contact with the patient. Because it can take up to 21 days for symptoms to develop after infection, contacts are being asked to monitor their health for that amount of time. CDC will continue to collaborate with partners to ensure the success of this investigation to help prevent additional cases of mpox in the United States.
CDC and the Texas Department of State Health Services confirmed on July 15, 2021 a case of human mpox in a U.S. citizen who traveled from Nigeria to the United States on two commercial flights. CDC supported state and local health officials to identify more than 200 people who had possible contact with the patient. Contacts were asked to monitor their health for 21 days. In early September, 21 days had passed without additional cases identified, and the monitoring period for the remaining contacts ended. Strong collaboration between CDC, state and local health departments, airline and airport partners, and other stakeholders involved in this investigation helped to prevent additional cases of mpox in the U.S. related to this case.
In 2003, forty-seven confirmed and probable cases of mpox were reported from six states—Illinois, Indiana, Kansas, Missouri, Ohio, and Wisconsin. All people infected with mpox in this outbreak became ill after having contact with pet prairie dogs. The pets were infected after being housed near imported small mammals from Ghana. This was the first time that human mpox was reported outside of Africa.
What caused the 2003 U.S. outbreak?
Investigators determined that a shipment of animals from Ghana, imported to Texas in April 2003, introduced mpox virus into the United States. The shipment contained approximately 800 small mammals representing nine different species, including six types of rodents. These rodents included rope squirrels, tree squirrels, African giant pouched rats, brush-tailed porcupines, dormice, and striped mice. CDC laboratory testing showed that two African giant pouched rats, nine dormice, and three rope squirrels were infected with mpox virus. After importation into the United States, some of the infected animals were housed near prairie dogs at the facilities of an Illinois animal vendor. These prairie dogs were sold as pets before they developed signs of infection.
All people infected with mpox became ill after having contact with infected pet prairie dogs. A study conducted after the outbreak suggested that certain activities associated with animals were more likely to lead to mpox infection. These activities included touching a sick animal or receiving a bite or scratch that broke the skin. Another important factor was cleaning the cage or touching the bedding of a sick animal. No instances of mpox infection were attributed exclusively to person-to-person contact.
How was the outbreak contained?
CDC and the public health departments in the affected states, together with the U.S. Department of Agriculture, the Food and Drug Administration, and other agencies, participated in a variety of activities that prevented further spread of mpox. This included extensive laboratory testing; deployment of smallpox vaccine and treatments; development of guidance for patients, healthcare providers, veterinarians, and other animal handlers; tracking potentially infected animals; and investigation into possible human cases. Partners in the response issued an immediate embargo and prohibition on the importation, interstate transportation, sale, and release into the environment of certain species of rodents including prairie dogs. FDA later rescinded the part of the order that restricted the capture, sale, and interstate movement of prairie dogs or domestically-bred African rodents, but CDC’s restriction on the importation of African rodents remains in place.