On September 22, the Nigeria CDC was notified of a suspected case monkeypox in Bayelsa State, and the initial investigation identified 11 additional cases in the same state. On October 9, the Nigeria CDC issued an update, reporting 31 suspected cases spread across 7 states—including the capital city, Lagos. Several local news articles published today cite the the Minister of Health in reporting 33 suspected cases. If these cases are confirmed, this would already be the second largest monkeypox outbreak in recorded history, based on US CDC data. The previous largest outbreaks were in the United States in 2003 (47 cases) and Sudan in 2005 (19 cases). Nigeria’s only previous experiences with monkeypox totaled 3 cases—2 in 1971 and 1 in 1978. Authorities caution, however, that they expect many of these suspected cases to be something other than monkeypox and explicitly state that none of the cases have yet been confirmed.

A rare disease in humans, monkeypox was discovered in 1958 in colonies of monkeys used for scientific research, and the first recorded human cases occurred in the Democratic Republic of Congo in 1970. Despite its namesake, monkeypox can be transmitted by numerous species of mammals. Rodents, such as rats and squirrels, are believed to be its primary natural reservoir, while monkeys are secondary carriers. However, both primary and secondary carriers can transmit the infection to humans.

Monkeypox virus is an Orthopoxvirus, the same family as chickenpox, smallpox and vaccinia, used in the smallpox vaccine. Monkeypox has similar symptomology to smallpox—fever, muscle aches, and characteristic pox rash—but it is a much less severe disease. Case fatality can be as high as 10%, compared to approximately 30% for smallpox.

There are two types of monkeypox, Central African (or Congo Basin) and West African. West African monkeypox is typically less severe and less transmissible than Central African monkeypox. The US CDC notes that person-to-person spread of Central African monkeypox is well documented, whereas no instances of human-to-human transmission of the West African strain has been recorded. Additionally, a model based on prairie dogs—another carrier—found limited respiratory transmission of Central African monkeypox and no respiratory transmission of West African monkeypox.

Unanswered Questions

The size and geographic spread of the outbreak raise a number of questions regarding 1) the source of the outbreak and 2) the mode of transmission. The answer to these questions can help to inform public health interventions.

A first question concerns the source of the outbreak. Although the Nigeria CDC has not identified the source, a single source of exposure is certainly possible; however, it may be difficult to identify, especially if there are multiple nearby animal reservoirs. One report speculates that recent flooding has resulted in displaced animal populations entering cities, where they can transmit the infection to humans via direct contact. Exposure to multiple animals across a broad geographic area is certainly possible, as this was the cause of the 2003 US outbreak. Multiple exposures to infected pet prairie dogs (who in turn contracted the disease from imported African rodents) resulted in cases across 5 states: Illinois, Indiana, Kansas, Missouri, and Washington.

A second question concerns the mode of transmission. The large number of cases — if they are confirmed to be monkeypox — raise the possibility of human-to-human transmission, which has only been documented for the Central African strain. However, according to one phylogeographic analysis, the distribution of the two strains does not appear to have any geographic overlap, with major rivers serving as a physical boundary between animal populations that harbor each strain. While we cannot rule out the possibility of a Central African strain, it seems likely that the outbreak is of the West African variety. So, unless the West African strain of the virus has mutated to enable or improve respiratory transmission, it is unlikely that person-to-person spread is the primary driver of the outbreak.

One potential factor contributing to the large number of cases is the decreased immunity to Orthopoxviruses. Since the eradication of smallpox and the subsequent end of smallpox vaccination programs, the number of orthopox cases globally has risen. One theory is that the immunity to smallpox (via the vaccinia-based vaccine) conferred some immunity to other Orthopoxviruses as well. The waning immunity to smallpox worldwide is resulting in a growing naive population such that other Orthopoxviruses like monkeypox are able to infect more hosts.

In response to the outbreak, the Nigeria CDC activated its emergency operations center to coordinate investigation and response activities at the state level. They have also initiated a communication and awareness campaign to reassure the public and provide them with information to protect themselves from infection. The principal recommendations from the Nigeria CDC include not eating “bush meat” (meat from wildlife), avoiding contact with infected individuals, and proper hand hygiene. Several locally published articles are reporting a statement by the Minister of Health that the Nigeria CDC expects to have confirmatory laboratory diagnostic test results in the very near future . Future updates are worth paying attention to as Nigerian health officials continue their investigation and fully characterize the illness in the remaining suspected cases.


Outbreak Observatory aims to collect information on challenges and solutions associated with outbreak response and share it broadly in near-real time to allow others to learn from these experiences in order to improve global outbreak response capabilities.

Photo: CDC official holding Gambian rats during an investigation into the 1996-97 monkeypox outbreak in the Democratic Republic of the Congo (formerly Zaire).
Photo courtesy of CDC/Brian W.J. Mahy, BSc, MA, PhD, ScD, DSc