Smallpox & Other Orthopoxvirus-Associated Infections
Mary G. Reynolds
Smallpox is caused by variola virus, genus Orthopoxvirus. Other members of this genus that cause infection in humans are vaccinia virus, monkeypox virus, and cowpox virus.
Person to person, principally respiratory; less commonly through contact with infectious skin lesions or scabs.
Person to person, principally respiratory; less commonly through contact with infectious skin lesions or scabs. African rodents and primates may harbor the virus and could infect humans, but the reservoir host is unknown.
Vaccinia virus is the live-virus component of contemporary smallpox vaccines. Rarely, social contacts of people recently vaccinated for smallpox develop infections with vaccinia virus when their skin comes into contact with fluid from the inoculation lesion. Zoonotic infections with vaccinialike viruses have been reported in Brazil and India.
Contact with infected animals; transmission between humans has not been observed.
Eradicated globally. A single confirmed case of smallpox would be the result of an intentional act (bioterrorism) and would be considered an emergency.
Endemic in tropical forested regions of West and Central Africa, notably the Congo Basin. Rodents imported from West Africa were the source of a 2003 outbreak of human monkeypox in the United States.
Infections with wild vaccinialike viruses have been reported among cattle and buffalo herders in India and among dairy workers in southern Brazil.
Human infections with cowpox virus have been reported in Europe and Asia.
Infections present with acute onset of fever >101°F (38.3°C), malaise, head and body aches, and sometimes vomiting. This phase is followed by a rash characterized by firm, deep-seated vesicles or pustules in the same stage of development. Clinically, the most common rash illness likely to be confused with smallpox is varicella (chickenpox). A detailed explanation of the clinical signs and symptoms of smallpox can be found at www.bt.cdc.gov/agent/smallpox/overview/disease-facts.asp.
Clinically identical to smallpox, with fever and widespread vesiculopustular rash involving the palms and soles. One feature distinctive to monkeypox is marked lymphadenopathy.
Vaccinia and Cowpox
Human infections with vaccinia, wild vaccinialike viruses, and cowpox virus are most often self-limited, characterized by localized pustular (and in cowpox, occasionally ulcerative) lesions. Fever and other constitutional symptoms may occur briefly after lesions first appear. Lesions can be painful and can persist for weeks. People who are immunocompromised or who have exfoliative skin conditions (such as eczema or atopic dermatitis) are at higher risk of severe illness or death.
Orthopoxvirus infection is confirmed by PCR or virus isolation. Physicians can refer to the CDC smallpox website (www.bt.cdc.gov/agent/smallpox/diagnosis) for guidance in the application of a clinical algorithm designed to aid in distinguishing orthopoxvirus infections from other disseminated rash illnesses, namely chickenpox.
Mainly supportive, to include hydration, nutritional supplementation, and prevention of secondary infections. Vaccinia lesions should remain covered until the scab detaches to diminish chances of spreading virus to other parts of the body or to another person. Physicians managing orthopoxvirus infection in a patient who is at high risk for severe outcome (such as a patient who is immunocompromised or has an underlying skin condition) should consult with CDC to explore investigational treatment options (770-488-7100). Investigational use of antivirals could be considered.
Smallpox vaccine is not recommended for international travelers. Smallpox vaccination is recommended only for laboratory workers who handle variola virus (the agent of smallpox) or closely related viruses and health care and public health officials who would be designated first responders in the event of an intentional release of variola virus. In addition, members of the US military may be required to receive the vaccine.
For other orthopoxvirus infections, avoid contact with rodents and sick or dead animals, including pets and domestic ruminants (cattle, buffalo). For more information about monkeypox and other orthopoxviruses, contact the CDC Poxvirus Inquiry Line (404-639-4129).
Baxby D, Bennett M, Getty B. Human cowpox 1969–93: a review based on 54 cases. Br J Dermatol. 1994 Nov;131(5):598–607.
Levine RS, Peterson AT, Yorita KL, Carroll D, Damon IK, Reynolds MG. Ecological niche and geographic distribution of human monkeypox in Africa. PLoS One. 2007; 2(1):e176.
Trindade GS, Guedes MI, Drumond BP, Mota BE, Abrahao JS, Lobato ZI, et al. Zoonotic vaccinia virus: clinical and immunological characteristics in a naturally infected patient. Clin Infect Dis. 2009 Feb 1; 48(3):e37–40.