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Contributors: Edith Lederman MD, Noah Craft MD, PhD
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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.


B08.71 – Tanapox virus disease

58544000 – Tanapox

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Differential Diagnosis & Pitfalls

  • Variola minor (see variola) commonly occurs on the volar surfaces and presents with numerous lesions, whereas tanapox almost never presents on the volar surfaces, and most cases present with a total of only 1-3 lesions.
  • Mpox induces antibodies that cross-react with vaccinia / variola antibodies while tanapox does not. In addition, mpox lesions evolve into pustules whereas tanapox lesions do not.
  • Rickettsialpox lesions are smaller and evolve more quickly than tanapox lesions. Rickettsialpox is contracted in urban areas whereas tanapox occurs in rural East Africa.
  • Orf lesions occur from contact with livestock and, therefore, usually occur on the hand, whereas tanapox lesions occur most often on the lower extremities. Orf is endemic to Europe and lesions heal without scarring.
  • The lesions of disseminated herpes simplex virus (HSV) or herpes gladiatorum are smaller and more delicate vesicles, which classically occur in crops.
  • Molluscum contagiosum lesions do not ulcerate or crust over and are not accompanied by constitutional symptoms.
  • The lesions of hand-foot-and-mouth disease (HFMD) appear on the oral mucosa, unlike the lesions of tanapox. In addition, the lesions of HFMD rarely appear on the volar surfaces.
  • Vaccinia vaccination or dissemination
  • Varicella (minor case or immediately post-vaccination)
  • Smallpox has been eradicated but remains a bioterrorism threat.

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Last Updated:09/05/2023
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A medical illustration showing key findings of Tanapox : Fever, Headache, Monkey exposure, Regional lymphadenopathy, Umbilicated papule, Umbilicated vesicle, Arthralgia, Pruritus
Clinical image of Tanapox - imageId=2237387. Click to open in gallery.  caption: 'A crusted pink and purpuric plaque on the upper arm.'
A crusted pink and purpuric plaque on the upper arm.
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