What is smallpox?
Smallpox is an infectious disease of the past that was eliminated by vaccination. The disease was caused by the variola virus, which spread from person to person. Affected people became very ill with a high fever and a characteristic rash. Up to one-third of people with smallpox died. Although the disease has been wiped out, samples of the virus still exist in high-security laboratories in the United States and Russia. This has led to concerns about use of the virus in biological warfare. For this reason, some military personnel are still vaccinated against smallpox.
What is the history of smallpox?
Smallpox is thought to have existed for more than 12,000 years. Evidence of infection can be found in mummies from ancient Egypt, including the mummy of Ramses V. Smallpox entered the New World in the 16th century, carried by European explorers and conquistadors. Because the aboriginal inhabitants had no immunity to the disease, smallpox often decimated native populations. There are even reports where infected blankets were used to intentionally infect Native American populations in the 18th
century -- one of the early examples of biological warfare. During the 20th century, it is estimated that there were 300 million to 500 million deaths from smallpox worldwide, compared to 100 million from tuberculosis.
It was not until the end of the 18th century that an effective method of vaccination was developed. It was an English scientist named Edward Jenner who discovered it. Jenner observed that milkmaids often got a mild disease called cowpox and that this seemed to make them immune to smallpox. His vaccination strategy involved transferring the blister fluid from a person with cowpox to a person who had not yet had smallpox. This gave the susceptible person a cowpox infection (which was usually mild) and conveyed protection from smallpox. After a time, a virus similar to cowpox, called vaccinia, was substituted in the vaccine.
The last naturally occurring case of smallpox was in Somalia in 1977. In 1980, the World Health Organization (WHO) certified that the world was finally free of smallpox. Thus, smallpox was the first disease to be entirely eradicated. Campaigns are now under way to try to eliminate other diseases such as polio and measles.
The WHO has encouraged all member nations to destroy any remaining laboratory cultures of the virus. However, the rise of biological warfare technology led to concerns that smallpox could be weaponized and used in bioterrorism. Both the U.S. and Russia decided to retain their stockpiles in case they were needed to produce novel vaccines against a biological agent. This has understandably stirred up controversy, and the WHO is expected to renew its request to eliminate stockpiles in 2011. Supporters of retaining the cultures note that existing stocks of the virus have been used to develop and test new treatments and vaccines. The entire viral genome has been sequenced, leading to concerns that the virus may be recreated even if current stocks are destroyed.
Smallpox is a deadly disease and is on the list of potential biological weapons that are considered to pose the greatest threat to public health. Other agents on this list include anthrax, plague, smallpox, botulism, tularemia, and the viral hemorrhagic fevers, including Ebola and Marburg viruses.
What causes smallpox?
Smallpox is caused by a poxvirus called variola (
Poxviridiae family of viruses). Variola is a relatively large virus that contains double-stranded DNA. The virus can be found in large numbers in many organs (skin, kidneys, spleen, liver, and other organs). Death occurs because of overwhelming toxemia, thought to be due to immune complexes trying to react to the large number of viral particles. Variola infection only occurs in humans, which was helpful in eradicating the disease. There are two strains called respectively variola major and variola minor (also known as alastrim). As implied by the names, variola major is more likely to cause serious disease and death than variola minor.
How is smallpox transmitted?
Most transmission of smallpox is directly from person to person. Large, infectious droplets of saliva are expelled during coughing or sneezing and then inadvertently inhaled by another person. This usually requires close face-to-face contact and is similar to the way that mumps, measles, and influenza are spread. On average, a single individual would infect approximately 60% of their household contacts. Infected objects, such as used silverware or heavily contaminated bedding, may carry sufficient numbers of organisms to infect another person if improperly handled, although this route of transmission is much less common.
What are smallpox symptoms and signs?
Symptoms develop seven to 17 days after exposure. Fever is the most common initial symptom and can be quite high. This is accompanied by body aches. Often, the patient is too unwell to get out of bed. Within 24 to 48 hours, a rash begins to appear everywhere on the body but especially on the legs, arms, mouth, and face. The eyes may also be affected, leading to potential blindness among survivors. Symptoms in children are similar to adults. The rash also appears on the palms and soles and goes through stages as the disease progresses. At the beginning, the rash consists of red dots that become raised. The lesions (see Figure 1) rapidly fill with fluid resembling a blister or cold sore and are known as "vesicles." The fluid in the vesicles may turn yellow, resembling pus. Rarely, the rash may start to fill with blood (hemorrhagic smallpox), which is a poor prognostic sign. After one to two weeks, the lesions scab over and eventually fall off, leaving deep scars. One of the defining features of smallpox is that all the lesions on the body are always at the same stage of development. This is in contrast to chickenpox where new lesions form while old ones are healing.
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| Figure 1: Picture of smallpox in a young child. SOURCE: Dr. Michael Schwartz/CDC |
Approximately one-third of people with smallpox died from the infection. People who had an extensive rash were at higher risk to die. People who had only a few lesions or a milder rash had a lower risk of death. Infections caused by the variola minor strain were less severe and death occurred in only approximately 1% of cases.
