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Smallpox

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Reporting Obligations

Suspect, probable, and confirmed cases should be reported immediately by phone to the Health Unit.

Epidemiology

Aetiologic Agent

Infectious agent is the variola virus, a species of Orthopoxvirus. The virus used in the live smallpox vaccine is known as the vaccinia virus also a member of the genus Orthopoxvirus. In 1979, the World Health Organization declared that smallpox (variola) had been eradicated successfully worldwide however; it does remain as a potential bioterrorism weapon.

Clinical Presentation

Smallpox is a systemic viral disease. The clinical presentation has been described as sudden onset with fever, malaise, headache, prostration, severe backache and occasional abdominal pain and vomiting. After 2–4 days the fever begins to fall, followed by a characteristic eruption of a rash of skin lesions. The rash on any one part of the body is in the same phase of development and progresses through successive stages of macules, papules, vesicles, pustules described as “pearls of pus”, then crusted scabs that fall off 3–4 weeks later.

Modes of Transmission

Infection usually occurred by respiratory tract via droplet spread or by skin inoculation.

Incubation Period

From 7–19 days; commonly 10–14 days from infection to onset of illness (first symptoms/prodrome period), then 2–4 more days to onset of rash.

Period of Communicability

From the time of development of the earliest lesions and until the disappearance of all scabs about 3 weeks after the onset of rash. Risk of transmission appears to be highest at the appearance of the earliest lesions through droplet spread from the oropharyngeal enanthem.

Risk Factors/Susceptibility

Smallpox has been eradicated worldwide. All unvaccinated individuals are susceptible.

Diagnosis and Laboratory Testing

Clinicians are strongly recommended to contact their local medical officer of health prior to collecting specimens on any suspect case of smallpox for laboratory diagnosis. Any testing related to suspected smallpox should be carried out under level 4 containment facilities at National Microbiology Laboratory (NML).

Confirmed Case:

Laboratory confirmation of infection:

  • Isolation of variola virus from an appropriate clinical specimen (e.g., whole blood, lesion (vesicular or pustular) fluid, crust material) OR
  • Detection of variola virus nucleic acid
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Treatment and Case Management

The World Health Organization regards even a single case of smallpox anywhere in the world as a global health emergency. In Ontario, the response would include the activation of the emergency management system.

Additional Resources

Public Health Agency of Canada. “Canadian Immunization Guide, Smallpox Vaccine.”

Ministry of Health and Long-Term Care. “DISEASES- Smallpox.”

Centers for Disease Control and Prevention. “Smallpox.”

World Health Organization. “Frequently asked questions and answers on smallpox.”

References

Ministry of Health and Long-Term Care, Infectious Diseases Protocol, 2019.