Endemic Monkeypox, Fear of U.S. Animal Reservoir
By Gary Evans
The United States continues to report the most cases of monkeypox (MPXV) of any country in world, with 27,884 cases and six deaths as of Oct. 21. However, the outbreak is receding compared to earlier spikes in cases, as that case count represents only a small increase over the 26,049 reported Sept. 30.1
Similar trends are occurring internationally, but the CDC has conceded some endemic level of MPXV will continue in the United States. “While the most likely scenario is that monkeypox cases will fall significantly in the next few months, transmission in the United States is unlikely to be eliminated in the near future,” the CDC reported. “We note that low-level transmission could continue indefinitely.”1
That means even if the epidemic tide recedes, healthcare workers will have to remain alert for an incoming case of monkeypox. “Personally, I think it is unlikely we will be able to eliminate this infection now that it has been out there spreading for some time in our population,” says William Schaffner, MD, a CDC advisor and medical professor at Vanderbilt University. “I think it will now be added to the list of infections that can be transmitted through sexual activity. The longer it is here, it will spread from the predominantly affected population — MSM [men who have sex with men] — into the heterosexual communities.”
To some degree, that already is happening. Most infections have been transmitted among MSM, but the CDC reports the current outbreak has included heterosexual sexual transmission, transmission to children through close nonsexual skin-to-skin contact with a caregiver, and through body piercing and tattooing. Mother-to-newborn transmission and miscarriages have been found in past outbreaks, and breastfeeding is a risk because of the close contact involved.2
In a disturbing scenario, the CDC raised the possibility MPXV could establish an animal reservoir in the United States. “Several animal species in North America, both wild and domestic, may be susceptible to monkeypox infection and may be able to transmit the virus to other animals or species,” the CDC reported.
That certainly raises the stakes of any ongoing endemic transmission.
“That would be depressing because anyone who had contact with some of those animals, sometimes quite casually, could then acquire the infection, not recognize for what it is, and then spread it to others,” Schaffner says. “If we had MPXV in an animal reservoir here in the United States, there is no telling how widely geographically it would spread. That would add a substantial layer of complexity in dealing with this.”
MPXV transmission has occurred between infected humans and household pets. “That has been well documented in a handful of cases,” Schaffner says. “The interactions we have between our dogs and cats can be extremely intimate. Some people kiss their pets and they sleep with them in the bed.”
As part of research in this area, the CDC is investigating companion animals of current MPXV cases. “CDC is working with partners in states and other jurisdictions to collect samples from animals that are present in the home while their owners are isolating due to monkeypox,” the authors noted. “A standard questionnaire is administered to examine risk factors for human-to-animal transmission and to assess if behavioral modifications following monkeypox diagnosis are associated with reduced likelihood of animal infection in the household setting. The findings will help create evidence-based guidance for public health and animal health professionals.”
An orthopox virus akin to smallpox, MPXV was so named after a colony of the primates became sick in a research lab in Denmark in 1958.3 However, the name is a misnomer since monkeys are not the animal reservoir for MPXV. Scientists believe the natural reservoir for MPXV is in African squirrels and rodents.
In 2003, in the first MPXV outbreak beyond Africa, 47 people in six states developed suspected or confirmed MPXV after contact with newly acquired pet prairie dogs. The pets were infected while housed near imported squirrels and rodents from Ghana. Importation of African rodents was banned after the outbreak, but some states still allow large Gambian pouch rats as pets under varying requirements.
This does not seem particularly prudent during a MPXV outbreak, given the Gambian rats were one of the sources of the prairie dog infections. Moreover, as the world’s largest rat at up to nine pounds, the pouch rat has been hunted for food in Africa. The authors of one study theorized this is how MPXV jumped into humans, as “they are hunted for consumption, and it is possible for MPXV-infected pouched rats to shed infectious virus without displaying overt clinical signs.”3 The first case of human MPXV was diagnosed in 1970 in the Democratic Republic of Congo.
- Centers for Disease Control and Prevention. Technical Report 3: Multi-national monkeypox outbreak, United States, 2022. Updated Sept. 29, 2022.
- Centers for Disease Control and Prevention. Science Brief: Detection and transmission of monkeypox virus. Updated Oct. 18, 2022.
- Falendysz EA, Lopera JG, Lorenzson F, et al. Further assessment of monkeypox virus infection in Gambian pouched rats (Cricetomys gambianus) using in vivo bioluminescent imaging. PLoS Negl Trop Dis 2015;9:e0004130.
The United States continues to report the most cases of monkeypox of any country in world, with 27,884 cases and six deaths as of Oct. 21. However, the outbreak is receding compared to earlier spikes in cases, as that case count represents only a small increase over the 26,049 reported Sept. 30. In a disturbing scenario, the CDC raised the possibility monkeypox could establish an animal reservoir in the United States.
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