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Monkeypox: Spread in Nonendemic Countries

Amesh A. Adalja, MD, FACP, FACEP, FIDSA, April 13, 2018

Of all the poxviruses, smallpox was the most feared, the most deadly, and the most disruptive of society, until it was eradicated in 1980 in a campaign led by D. A. Henderson. Several other poxviruses, however, are capable of infecting humans; these include orf, molloscum contagiosum, cowpox, Akhmeta, and monkeypox. Of these, the zoonotic monkeypox is likely the most significant and has arguably become an emerging threat to human health. Monkeypox, which is an orthopox virus like smallpox, has largely been confined to the Democratic Republic of Congo, but in recent years cases in other African countries – and even importation to the United States in 2003 – have heightened concern. A new report published in Emerging Infectious Disease describes the reemergence of monkeypox in Nigeria after a nearly 40-year absence.[1] 


2 Lineages with 2 Prognoses

Monkeypox is a disease that was first described in humans in the 1970s after it was initially discovered in monkeys; rodents are thought to be its natural host. It produces a disease clinically indistinguishable from smallpox (with the exception of its ability to cause lymphadenopathy). The disease has been facilitated by the ending of routine smallpox vaccination, as the vaccine is also protective against monkeypox.[1]

The monkeypox virus is divided into 2 clades: West and Central African. The West African clade is noted for having lower mortality and less human-to-human transmission. The Central African strain, by contrast, has a mortality rate that is approximately 10%.[2] 


Nigerian Outbreak

In September 2017, the Nigeria Centre for Disease Control (NCDC) was notified of an 11-year-old boy with monkeypox. The source of the disease was unclear, though the boy and 2 of his siblings had played with a neighbor’s monkey (which was not ill). A total of 5 other members of the index patient’s household developed similar symptoms.[1] 

The discovery of these cases led NCDC to enhance its emergency operations to investigate the extent of the outbreak and resulted in its finding that a larger outbreak was occurring in Nigeria. This outbreak encompassed over 60% of the states of the country and amounted to 146 cases. Most patients were adult males, and only 1 fatality – in an HIV-positive individual – occurred.[1] 

There were clusters in 31 families, with a secondary attack rate of 71% in 1 family. However, most case patients had not had contact with each other, suggesting limited human-to-human transmission.[1] 

A monkeypox outbreak in an area where the disease is not endemic suggests that the geographical breadth of the virus is large and zoonotic sources are more widespread. As time passes since smallpox vaccination was routine, the number of immune naïve people will increase, creating more and more population vulnerability. As the virus, thus far, has not been able to spread efficiently between humans, sporadic outbreaks that are extinguishable may become more common – as evidenced by new outbreaks in Liberia [3] and in other nations.[2] However, if the virus becomes more adapted to humans, zoonotic spillovers with longer and longer chains of human transmission could occur. The efficacy of smallpox vaccines against monkeypox, however, could provide a means to halt these outbreaks if warranted. The increasing recognition of monkeypox outbreaks, especially in nonendemic countries, underscores the need for better understanding of its epidemiology, host species, and ability to transmit between humans. 



  1. Yinka-Ogunleye A, Aruna O, Ogoina D, et al. Reemergence of monkeypox in Nigeria, 2017. Emerg Infect Dis 2018;24:6. Accessed April 12, 2018.
  2. Durski KN, McCollum AM, Nakazawa Y, et al. Emergence of monkeypox—West and Central Africa, 1970-2017. MMWR Morb Mortal Wkly Rep 2018;67(10);306-310.
  3. MonkeypoxAfrica: Liberia. ProMED Mail April 11, 2018. Accessed April 12, 2018.