Smallpox and other Orthopoxviruses

Please see Monkeypox and Other Orthopoxviruses for updated information on Monkeypox


Reporting Obligations

Suspected cases must be reported immediately by phone to the Thunder Bay District Health Unit at 625-8318 or toll-free 1-888-294-6630, ext. 8318 (Monday-Friday, 8:30am to 4:30pm). After hours and on weekends/holidays call Thunder Bay Answering Service at (807) 624-1280.



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; followed by a characteristic skin eruption after 2-4 days when the fever begins to fall. A rash progresses through successive stages of macules, papules, vesicles, pustules, then crusted scabs that fall off 3 - 4 weeks later.

Modes of transmission:

Smallpox is spread most commonly in droplets from the oropharynx of infected individuals. Rare transmission from aerosol and direct contact with infected lesions, clothing or bedding has been reported.

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 to disappearance of all scabs, about 3 weeks. The risk of transmission appears to have been highest at the appearance of the earliest lesions through droplet spread from the oropharyngeal enanthem.


Risk Factors/Susceptibility

Smallpox has been eradicated worldwide. The virus is only approved to exist in two main research centers, one in the United States and the other in the Russian Federation.

• Deliberate use (e.g. bioterrorism)

• All unvaccinated individuals are susceptible


Diagnosis & 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 with clinically compatible signs and symptoms:

• Detection of variola virus nucleic acid


• Isolation of variola virus from an appropriate clinical specimen (e.g., blood, vesicular fluid, scabs) followed by confirmation through detection of variola virus nucleic acid OR

• Detection of poxvirus particles in a clinical specimen by electron microscopy followed by confirmation through detection of variola virus nucleic acid

Clinical Evidence:

• Acute onset of fever of > 38.3o C followed by a rash involving vesicles or firm pustules in the same stage of development without other apparent cause

• Major distinguishing features include a febrile prodrome with a temperature of > 38.9o C and systemic symptoms (prostration, severe headache, backache, abdominal pain, or vomiting) 1-4 days before rash onset; lesions are deep, firm, well-circumscribed pustules (may be confluent or umbilicated).

• Other distinguishing features include rash concentrated on face and extremities; rash in same stage of evolution on any one part of the body; first lesions on oral mucosa/palate followed by centrifugal rash on face or forearm; and lesions on palms and soles (seen in > 50% of cases); lesions evolve from papule to pustule in days, illness lasts 14-21 days.

• A typical presentations of smallpox include a) hemorrhagic lesions or b) at velvety lesions not appearing as typical vesicles or not progressing to pustules


Testing Information & Requisition


Treatment & 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.


Patient Information

Patient Fact Sheet



1. Ministry of Health and Long Term Care, Infectious Diseases Protocol - Appendix 1 (2022) Smallpox and other Orthopoxviruses.

2. Heymann, D.L., Control of Communicable Disease Manual (20th Ed.), Washington, American Public Health Association, 2015


Additional Resources

Ministry of Health and Long Term Care. "DISEASES- Smallpox"

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

CDC. "Smallpox Disease Overview"

WHO. "Frequently Asked Questions and Answers on Smallpox."

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