Monkeypox virus represents a double-stranded DNA virus of the genus Orthopoxvirus, genetically distinct from other viruses in this group - including variola virus (the cause of smallpox) and vaccinia virus (the virus used for the smallpox vaccine).

Human monkeypox was initially identified in the Democratic Republic of Congo (Zaire) in 1970, near the end of smallpox eradication efforts in Africa. Transmission of the disease is primarily by large droplets or direct contact, and the symptoms are similar to those of smallpox, albeit usually milder.

The outbreak of monkeypox in the US Midwest during June 2003 was identified as the first documented human infection in the Western Hemisphere. In concordance with the outbreaks in Africa, a majority of cases were presented with febrile rash illness.

The development of the outbreak

On April 9, 2003, a shipment of 812 small mammals from the city Accra in Ghana was received by a Texas animal distributor. This included rope squirrels (Funisciurus pyrropus), sun squirrels (Heliosciurus gambianus), African dormice (Graphariurus spp.) and giant Gambian rats (Cricetomys spp.).

Twelve days later, an Illinois vendor received the animals from the Texas distributor. An infected Gambian rat was housed together with captive prairie dogs (Cynomys spp.) that this vendor later distributed to six different states in the US. These animals were considered to be a primary source of this outbreak, as most of the infected people became sick after contact with pet prairie dogs.

On May 15, physicians reports started to emerge from the Midwest, describing people who were exhibiting symptoms similar to smallpox, although less contagious and in milder form. Affected individuals became ill with a fever, respiratory symptoms and swollen lymph nodes. A rash developed with progression into vesicles and spread across the body, with eventual formation of crusts and scabs. The illness lasted between 2 and 4 weeks.

Over the next couple of weeks, outbreaks of monkeypox were identified in Wisconsin, Indiana and Illinois, i.e. in the countries where the Texas distributor has sent prairie dogs to pet dealers. Although the route of transmission to humans was from close contact with infected mammalian pets, human-to-human transmission may have also occurred. The last case was reported on June 20.

The impact of the outbreak

Of 72 reported cases, 37 human cases were laboratory confirmed as human monkeypox during 2003 US outbreak. Electron microscopy and serologic studies were methods used to confirm the correct diagnosis. Most cases were associated with a mild, self-limited febrile rash illness and it is important to note that there were no deaths.

Of the patients from whom the data was available, 18 were hospitalized, although some due to isolation precautions only. Two patients (both children) had serious clinical illness – the first child presented with severe encephalitis, while the second child exhibited profound and painful cervical and tonsilar lymphadenopathy with diffuse pox lesions.

Introduction of exotic species (such as rodents from Africa) still represents a significant public health threat because of the potential infection with monkeypox virus, but also with other nonindigenous pathogens. Therefore all health-care providers, veterinarians and public health officials who suspect monkeypox in animals or humans are obliged to report such cases to their state or local health departments.