Introduction

Monkeypox is from the family of zoonotic orthopoxviruses, which also include: buffalopox (India, Egypt, Indonesia); camelpox (Africa, Asia); cowpox (Europe, western Asia); vaccinia (worldwide) and smallpox (eradicated). The first cases of human monkeypox were identified in the Democratic Republic of the Congo (DRC, formerly Zaire) in 1970. By 1979, 55 sporadic cases were reported in Central and West Africa. Because vaccination against smallpox gives greater than 80% protection against orthopoxviruses, the incidence of monkeypox continues to increase with a larger unvaccinated (susceptible) population. Nonlaboratory disease had never been reported outside Central and West Africa prior to the current outbreak in Central United States.

Monkeypox has a clinical appearance nearly indistinguishable from smallpox, which led the World Health Organization to conduct intense studies in the DRC from 1980 to 1986. As a result of the studies, an additional 346 cases were identified. Most were young boys living in rural areas adjacent to tropical rain forests, and forest squirrels rather than primates were identified as the reservoir. Hunting and deforestation for agriculture play important roles in transmission. Monkeypox has a significantly lower fatality rate than smallpox. The 10% attack rate in nonvaccinated household contacts is significantly less than the 25-40% for smallpox and human-to-human transmission is considered less common than primary contact with an infected animal. A 1996-1997 outbreak of monkeypox, again in the DRC, reported a much higher incidence of person-to-person transmission than previous studies (70% vs less than 30%). Animal studies now indicate that antiretrovirals are effective in treating monkeypox.