Smallpox is a contagious and virulent disease caused by the variola virus. Smallpox is an orthopox virus that causes a contagious, febrile disease characterized by vesicular and pustular eruption that can be severe.
This disease killed approximately 300 million people in the 20th century. Given its infectiousness and its relatively high mortality rate of 30%, experts have publicly stated that smallpox is the most dangerous infectious disease ever, and that more people have been killed by it than all other infectious diseases combined.
A significant international consensus resulted in a global eradication program led by the World Health Organization that successfully eradicated smallpox in the late 1970s. There have been no naturally acquired cases of smallpox since 1977. A fatal laboratory-acquired case occurred in the United Kingdom in 1978. In 1980, the World Health Organization confirmed the global eradication of smallpox.
Following eradication, all countries consolidated their smallpox stocks in two government-controlled laboratories in the United States and Russia. These laboratories still have quantities of the smallpox virus for research purposes. Both have been reviewed by the World Health Organization and found to be secure. Other states, however, may possess clandestine stocks of smallpox. There are reports that the Soviet Union prepared large quantities of bioweapons-grade variola virus in the 1980's, none of which has ever been accounted for in a proper fashion.
Smallpox is spread primarily person-to-person through droplets that are inhaled and usually requires close, face-to-face contact. Fluid from the papules and pustules is also infectious and can infect skin through inoculation or if dried fluid is inhaled. Although rare, contact with contaminated clothing or bed linens will also transmit the infection. Smallpox virus infects through the respiratory tract or skin, and travels to the lymph nodes. The virus invades the oropharyngeal or respiratory mucosa and multiplies in regional lymph nodes, eventually localizing in the small blood vessels of the skin and the oropharyngeal mucosa.
The incubation period is relatively long, averaging 10-14 days, during
which time there is no evidence of the virus. Infectivity begins with the
onset of rash and remains high for 7-14 days. As previously stated smallpox
is most commonly spread from person-to-person by the respiratory route and
usually requires close contact (6-7 feet/2 metres). A person with smallpox
is most infectious from the time the rash begins through day 14. Smallpox
spreads in expanding circles of close contacts with waves of cases every
12-14 days corresponding to the incubation period. A person with smallpox
is infectious from the time the rash first appears (10-22 days after infection)
until scabs fall off about one month later. Infectivity wanes once scabs
have formed over the lesions. Transmission occurs mainly to household and
other close contacts.
10-20 secondary cases may develop from each primary case.
Classic smallpox has a 10-14 day incubation period, followed by a 2-4 day prodrome of fever, headache, backache and extreme malaise, sometimes accompanied by abdominal pain and delirium. After the fever, a maculopapular rash develops on the oropharyngeal mucosa, the face and the arms, spreading shortly thereafter to the trunk and legs. Pustules are denser on the face and extremities. The pustules are round and appear deeply embedded. After 8 or 9 days the pustules become crusted. Mortality is about 30% and usually occurs in the second week. Smallpox may be easily confused with many eruptive diseases, especially chickenpox. Smallpox infection causes pustules concentrated mostly on the extremities, including palms, soles of feet and face. This is the opposite of chickenpox where the lesions are more numerous on the torso and less on the extremities. Again, symptoms include high fever (above 40C, 104F), headache, myalgia (especially in back), abdominal pain, vomiting, and in some patients a blotchy, transient erythematous eruption. Fever and symptoms may decrease, as the rash erupts. A rash first appears inside the mouth, on the face and forearms, spreading to the trunk and legs. Unlike chickenpox where the rash appears mainly on the face and body and the eruptions are in different stages of development, the smallpox rash appears all over the body, and eruptions develop at the same rate on a given body part.
There is no cure or specific treatment for smallpox. Prevention is by vaccination and isolation. Administering smallpox vaccine (vaccinia vaccine) within four days after exposure can ameliorate illness in nearly all cases. The disease is stopped because the immune response to the vaccine is fast enough to stop the virus. Once a person shows symptoms, however, treatment is limited to supportive therapy and antibiotics to treat secondary bacterial infections. As well, a number of antiviral drugs are currently being tested.
Smallpox can be controlled and eliminated, as was successfully done in the past. The Government of Canada has adopted a "search and contain" strategy recommended by public health experts in Canada and around the world, including Canada's National Advisory Committee on Immunization, Canada's Council of Chief Medical Officers of Health, and the World Health Organization. This is the same approach that was used to eliminate smallpox globally in the late 1970s.
"Search and contain" starts immediately upon the confirmation of a case of smallpox. Anyone that may have come into contact with the virus is rapidly identified and vaccinated within the four-day window. Vaccinated individuals are isolated to help ensure containment. This strategy is flexible: find a case, vaccinate and isolate contacts. If required, depending on the dynamics of the spread, vaccination can be extended to an entire community, town, city or region.
On a voluntary basis, a number of "first responders" will be pre-vaccinated for smallpox, beginning in early 2003. First responders, such as laboratory workers and emergency public health workers, among others, would be the first to respond in the unlikely event of smallpox appearing in Canada. Since they would most likely be the first ones sent to the scene of a potential smallpox outbreak they need to be vaccinated, as a precaution, before being exposed to the virus.
In the absence of a case of smallpox, public health experts have stated that the mass vaccination of healthy people, as a preventive measure, is not recommended at this time. Smallpox vaccine itself can have serious side effects. Smallpox vaccination is generally safe, however, in a number of individuals vaccination can result in adverse reactions. Most adverse reactions are treatable. A few rare reactions are extremely serious and can be life-threatening.
The overall risks of serious complications from smallpox vaccine (vaccinia vaccine) are low, and occur more frequently in those receiving their first dose of vaccine, and among young children. Historically, one in every 300,000 vaccinations resulted in serious adverse reactions, while one to four in every million people died from the vaccine.
The most frequent serious complications are encephalitis (brain inflammation), generalized spread of the virus throughout the body, blindness (from scratching the vaccinated area and then touching the eye), and severe infection of skin already affected by eczema or other chronic skin disorders. People who have been previously vaccinated for smallpox will often have a milder reaction to the vaccine if re-vaccinated.
Immunization is contraindicated at this time for people with compromised cellular immunity (e.g., chemotherapy, transplant, HIV and pregnancy).
In the event of a severe reaction, in addition to supportive treatment, Vaccinia Immune Globulin (VIG) may be helpful. VIG is a solution of antibodies produced from the blood of vaccinated persons.
Routine smallpox vaccination of the Canadian population stopped in 1972. Past history indicates that the vaccine offers protection from smallpox for 3-5 years, and sometimes as long as 10 years or more. If an individual is vaccinated again later, the duration of immunity may be even longer. It is important to note, however, that when the smallpox vaccine was used to eradicate the disease in the late 1970s, testing was not as advanced or precise as it is today. As such, there may still be more to learn about the vaccine and its effectiveness and length of protection.
The threat of a smallpox incident is low, but because the consequences are high, a National Smallpox Contingency Plan is in place as a precaution. The Government of Canada is working closely with the provinces and territories to make sure that we would be able to quickly recognize and respond to an outbreak of smallpox.