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What is smallpox?

Smallpox is an acute viral disease that is unique to humans.  Smallpox was once the most devastating disease known, killing as many as 30% of those infected.  Beginning in the early 1950s, vaccination programs worked to eradicate this disease.  The World Health Organization certified the world to be free of smallpox in 1980.  Currently the release of smallpox virus as a biological weapon would pose challenges because of the ability of the virus to spread throughout the population unless controlled by vaccination and/or isolation of patients and their contacts.

What are the symptoms?

An infected person develops symptoms 7 – 19 days after exposure.  Early symptoms include high fever (40°C/104°F), headache, extreme malaise, severe backache and sometimes abdominal pain, vomiting and delirium.  After 2 – 4 days, fever begins to fall and a deep seated rash develops, mostly on the face, arms and legs, including the palms and soles and then on the torso.  The rash may also appear in the mouth and throat.  The rash starts with flat red lesions that become pus filled and then begin to crust early in the second week of illness.  Scabs develop and then fall off after about 3 weeks.  The majority of patients with smallpox will recover, but death occurs in up to 30% of cases.  Many smallpox survivors have permanent scars over large areas of their body, especially on their face.  Some are left blind.  

Chickenpox is the disease that is most commonly confused with smallpox.  During the first 2-3 days of the rash, the lesions look the same.  However, all smallpox lesions develop at the same rate, appear to be identical and occur on any part of the body.  Chickenpox lesions are much shallower, develop in crops and tend to be less concentrated on the extremities.

How is it spread?

Smallpox is spread from person to person, by infected aerosols and droplets through face-to-face contact with an infected person, especially if symptoms include coughing.  In addition to nose and throat secretions, fluid from the lesions is also infectious.  There is a lower risk of spread from clothes and bedding that are contaminated with these secretions or by scabs that have come off the skin.

When is smallpox contagious?

Persons with smallpox are infectious from the onset of fever and rash to the disappearance of all scabs, approximately a 4 week period of time.  The risk of transmission is highest during the first week of illness once the rash is present, because that is when the largest amount of virus is present in the saliva. 

If smallpox develops:

Persons diagnosed with smallpox should be isolated from others.  Persons who have or will come into close contact with them should be vaccinated.  Those who are exposed to an infected person will need to check their temperature daily and isolate themselves if fever develops.  Scabs that fall off could contaminate clothing or bed linen.  Precautions need to be taken to ensure that all bedding and clothing is washed appropriately with bleach and hot water.  Disinfectants such as bleach and quaternary ammonia can be used for cleaning contaminated surfaces.  Frequent hand washing is extremely important. 

Vaccination

There is no effective treatment for smallpox other than symptom management, but research on anti-viral medication is ongoing.  Vaccination can be used very effectively to prevent infections from developing if given within four days after the exposure to the virus.  Routine vaccination for smallpox in Canada ended in 1972.  Anyone who was vaccinated against smallpox in the past will likely have some level of protection.  Due to heightened concerns about biological warfare, Canada has increased its supply of smallpox vaccine.  The risk of getting smallpox virus as a result of a deliberate terrorist attack is unknown but it is considered very low.

For more information, please contact Public Health at 519-376-9420 or 1-800-263-3456.