Tanapox virus disease, also known as riverine smallpox or boutons de Boma, is a zoonosis endemic to equatorial Africa. It is transmitted to humans from monkeys by an unknown mechanism, although culicine mosquitoes are the suspected vectors. Occasionally the transmission occurs during direct contact with infected monkeys.

The disease was first described in Kenya along the Tana River. Patients will present with fever (58%), lethargy, headache, arthralgias, backache, and 1-3 pox lesions (78% have a single lesion) on their extremities, lower torso, or head. The lesions usually appear 1-3 days after fever begins. Rarely patients may present with up to 10 lesions. Regional lymphadenopathy commonly occurs. This disease is benign and self-limited, unlike other poxvirus diseases such as variola (smallpox).

Most patients are adults (average age: 20 years), and cases in nonendemic areas have occurred in animal handlers who sustained monkey scratches. Outbreaks have occurred in endemic areas during floods where both human and nonhuman primate populations were confined to a small area.

The incubation period is unknown but found to be 3-5 days in animal and human experimental infections.

The pox lesions begin as pruritus without a specific lesion, then small papules erupt and evolve into vesiculopapules, which then umbilicate, and then ulcerate with a surrounding raised border and finally crust over. A red, indurated halo of several millimeters to several centimeters may surround the pox lesions. The pox lesions slowly evolve over 6-8 weeks, ultimately resulting in a scar.