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Excerpts have been reprinted with permission from the APHA’s Control of Communicable Diseases Manual (CCDM). Please refer to the CCDM for more complete information.

SMALLPOX ICD-9 050; ICD-10 B03

The last naturally acquired case of smallpox in the world occurred in October 1977 in Somalia; global eradication was certified 2 years later (1979) by WHO and sanctioned by the World Health Assembly (WHA) in May 1980. Except for a laboratory-associated smallpox death at the University of Birmingham, England, in 1978, no further cases have been identified. All known variola virus stocks are held under security at CDC, Atlanta GA, USA, or the State Research Centre of Virology and Biotechnol­ogy, Koltsovo, Novosibirsk Region, the Russian Federation.

1. Identification – Smallpox was a systemic viral disease generally presenting with a characteristic skin eruption. Preceding the appearance of the rash was a prodrome of sudden onset, with high fever (40°C/104°F), malaise, headache, prostration, severe backache and occasional abdominal pain and vomiting; a clinical picture that resembled influenza. After 2-4 days, the fever began to fall and a deep-seated rash developed in which individual lesions containing infectious virus progressed through succes­sive stages of macules, papules, vesicles, pustules, then crusted scabs that fell off 3-4 weeks after the appearance of the rash. The lesions first appeared on the face and extremities, including the palms and soles, and subsequently on the trunk-the so-called centrifugal rash distribution ­and were at the same stage of development in a given area.
Two types of smallpox were recognized during the 20th century: variola minor (alastrim), which had a case fatality rate of less than 1 % and variola major with a fatality rate among unvaccinated populations of 20-50% or more. Fatalities normally occurred between the fifth and seventh day, occasionally as late as the second week. Fewer than 3% of variola major cases experienced a fulminant course, characterized by a severe prodrome, prostration, and bleeding into the skin and mucous membranes; such hemorrhagic cases were rapidly fatal. The usual vesicu­lar rash did not appear and the disease might have been confused with severe leukaemia, meningococcaemia or idiopathic thrombocytopenic purpura. The rash of smallpox could also be significantly modified in previously vaccinated persons, to the extent that only a few highly atypical lesions might be seen. In such cases, prodromal illness was not modified but the maturation of lesions was accelerated with crusting by the tenth day.

Smallpox was most frequently confused with chickenpox, in which skin lesions commonly occur in successive crops with several stages of maturity at the same time. The chickenpox rash is more abundant on covered than on exposed parts of the body; the rash is centripetal rather than centrifugal. Smallpox was indicated by a clear-cut prodromal illness; by the more or less simultaneous appearance of all lesions when the fever broke; by the similarity of appearance of all lesions in a given area rather than successive crops; and by more deep-seated lesions, often involving sebaceous glands and scarring of the pitted lesions (chickenpox lesions are superficial and chickenpox rash is usually pruritic) …
Laboratory confirmation used isolation of the virus on chorioallantoic membranes or tissue culture from the scrapings of lesions, from vesicular or pustular fluid, from crusts, and sometimes from blood during the febrile prodrome … Molecular methods, such as PCR, are now available for rapid diagnosis …

2. Infectious agent – Variola virus, a species of Orthopoxvirus.

3. Occurrence – Formerly a worldwide disease; no known human cases since 1978.

4. Reservoir – Smallpox was exclusively a human disease, with no known animal or environmental reservoir.

5. Mode of transmission – Infection usually occurred via the respira­tory tract (droplet spread) or skin inoculation …

6. Incubation period – From 7-19 days; commonly 10-14 days to onset of illness and 2-4 days more to onset of rash.

7. Period of communicability – From the time of development of the earliest lesions to disappearance of all scabs; about 3 weeks … through droplet spread from the oropharyngeal enanthem.

8. Susceptibility – Susceptibility among the unvaccinated is universal.

9. Methods of control – … Because of the relatively long period of incubation for smallpox, vaccination within a 4-day period after exposure can prevent or attenuate clinical illness.

VACCINIA ICD-9 051.0; ICD-10 B08.0

Vaccinia virus, the immunizing agent used to eradicate smallpox, has been genetically engineered … with low potential for spread to nonimmune contacts. Vaccination with licensed smallpox vaccine is recommended for all laboratory workers at high risk of contracting infection … WHO does not recommend vaccination in the general public because the risk of death (1 per 1 000 000 doses) or serious side-effects is greater than the known risk of infection with smallpox. Vaccination is contraindicated in persons with deficient immune systems; persons with eczema or certain other dermatitis disorders; and pregnant women. Vaccine immune globulin can be obtained for laboratory workers …