Monkeypox: catching up with the next viral outbreak

The World Health Organization (WHO) has been tracking an outbreak of monkeypox in 23 non-endemic countries that, as of May 26, included at least 257 laboratory confirmed and 120 suspected cases, including 15 confirmed cases in the U.S. This outbreak is highly unusual because many infected persons do not have a history of travel to an endemic country or contact with infected animals. During the only other large U.S. outbreak (involving 77 individuals) of monkeypox in 2003, patients contracted the virus through contact with infected prairie dogs purchased as pets; a case-control study found that case patients were more likely than controls to have cleaned cages and bedding of a sick animal or touched a sick animal. Although no patients died, 19 were hospitalized. In July 2021, monkeypox was diagnosed in a Dallas, Texas emergency department in a traveler returning from Nigeria. This patient was hospitalized for one month, and no secondary infections developed in any of the 223 identified U.S. contacts.

Monkeypox is a zoonotic double-stranded DNA poxvirus that causes clinical disease in humans that is similar to, but less severe than, smallpox. It is endemic to western and central Africa, particularly the Democratic Republic of Congo. The virus was originally isolated from a monkey in 1958, and the first human case was identified in 1970. African rodents are believed to be the virus's natural reservoir. Transmission occurs through contact with bodily fluids, skin or mucosal wounds, respiratory droplets, or contaminated objects. The usual incubation period is 7-14 days, and symptoms resolve within 14-21 days. Infected persons are considered to be contagious for one day before and 21 days after the onset of symptoms. 

Since routine smallpox vaccination ended in the U.S. in the 1970s, a large proportion of the population is susceptible to monkeypox infection. JYNNEOS, a live, nonreplicating vaccine that is recommended by the Advisory Committee on Immunization Practices for prevention of smallpox and monkeypox in persons at occupational risk aged 18 years or older, may be given for post-exposure prophylaxis within 4 days from the date of exposure to prevent disease and is preferred over the older smallpox vaccine (ACAM2000) due to a lower risk of adverse effects, though 100 million doses of the latter have been stockpiled in the event of a widespread bioterrorist attack.

Early reports from the current outbreak suggest that it has been causing minimal prodromal symptoms (fever, chills, lymphadenopathy) and that the rash is first appearing in patients' genital or perianal areas before progressing to the extremities, rather than more typically beginning in the mouth and face. Although monkeypox is not considered a sexually transmitted infection, many of the initial confirmed case patients are men who have sex with men. It remains unclear if, or to what extent, human-to-human transmission of monkeypox is occurring. "Given the current unfolding outbreak," advised two physicians from the Johns Hopkins Center for Health Security, "clinicians seeing patients with new onset of febrile illness and rash should consider monkeypox, especially if lymphadenopathy is also present."

Along with SARS-CoV-2, monkeypox is another example of the potential of increasingly frequent interactions between humans and wildlife to spread infectious diseases. As this latest viral outbreak continues to evolve, it underlines the importance of physicians and veterinarians taking a One Health approach to optimizing the health and well-being of humans and animals.

Kenny Lin, MD, MPH