What is Smallpox and Monkeypox?

Updated October 08, 2022


Smallpox (right) Monkeypox (left) CDC PHIL

Smallpox is an illness caused by the variola virus. This virus belongs to the orthopoxvirus family of viruses. When smallpox was circulating in the environment, there were several forms of the disease, with some more severe and life-threatening than others.1

Variola Major

According to the Centers for Disease Control (CDC), Variola Major was the most common form of the illness when smallpox was circulating. The incubation period of Variola Major ranged from seven to 19 days, but most frequently lasted between 10 and 14 days. Persons infected with the virus were not contagious and had no symptoms of illness during this period.

The first symptoms of illness occurred in the prodrome period, which began immediately following the incubation period, and lasted approximately four days. Symptoms included:

  • Anorexia
  • Vomiting
  • Malaise
  • High fever (between 101°F to 105°F)
  • Chills
  • Headache
  • Backache
  • Severe abdominal pain
  • Pharyngitis
  • Extreme exhaustion
  • A rash, mostly seen in light-skinned individuals.

When the fever resolved, rash lesions would begin to develop and appear in the back of the mouth, behind the oral cavity (oropharynx), followed by the face, arms, legs, and then would have spread to the torso, palms and soles. Rash lesions would develop evenly during the illness and progress from macules to papules to vesicles within four to five days. In another one or two days, the vesicles would evolve to pustules that were round, firm, and found deep in the dermis. Crusting and scabbing of the lesions usually began on the ninth day and the crusts generally began to fall off about 2 weeks after the onset of the rash.2

Complications of smallpox included severe bacterial infections of the skin and organs, sepsis, pneumonia, encephalitis, and keratitis.3

The most common long-term health consequence of smallpox was scarring which occurred all over the body but most often on the face. Additional sequelae included stillbirths and spontaneous abortions, infertility in males, osteomyelitis, encephalitis, and blindness. Persons who recovered from smallpox illness developed long-term immunity.4

Historically, Variola Major was fatal in approximately 30 percent of cases.5

Modified-type Smallpox

When the vaccine failed to prevent illness, previously vaccinated individuals would develop a condition known as modified-type smallpox. Symptoms of illness were usually the same as in persons who developed Variola Major; however, the rash generally resolved within 10 days, instead of 2 weeks, and fever was not always present. There were usually fewer lesions, and lesions were often superficial.6 Modified-type smallpox infections were rarely fatal.7

Hemorrhagic Smallpox

Hemorrhagic smallpox usually occurred in adults but could still develop in children. Additionally, pregnant women were also more at risk of developing this particular form of the disease. Symptoms of hemorrhagic smallpox were similar to Variola Major except that the incubation period was shorter and prodromal symptoms were usually more severe. Additionally, after the onset of illness, skin redness would occur and progress to a petechial rash (small pinpoint purple or red rash) and hemorrhaging of the skin and mucous membranes. Hemorrhagic smallpox was usually fatal by the fifth or sixth day after rash onset, and frequently as a result of multi-system organ failure due to toxemia. Vaccination was ineffective against hemorrhagic smallpox.8

Malignant (Flat-type) Smallpox

Most common among children, flat-type smallpox was rare and identified by skin lesions that developed slowly, merged together, and became soft and flat. Most cases of flat-type smallpox were fatal due to toxemia, however, if a patient survived, the rash would heal without scabbing.9 This type of smallpox was fatal in approximately 97 percent of cases.10

Variola Minor (Alastrim)

Variola Minor was a less severe form of illness and was rarely fatal. Death occurred in less than one percent of cases.11 This form was most common in the U.S. by the early part of the 20th Century.12 By the 1930s and until smallpox was eliminated from the U.S., variola minor, the mildest form of smallpox, had emerged as the predominant strain.13

Additional Orthopoxvirus Diseases

Monkeypox

Monkeypox is an infection caused by the monkeypox virus and like smallpox is also a member of the Orthopoxvirus family.14 The incubation period of monkeypox is usually between 7 and 14 days.15 Symptoms of monkeypox are similar to smallpox but are generally milder. Individuals infected with monkeypox usually present with headache, backache, fever, chills, muscle aches, extreme fatigue and exhaustion. Swelling of the lymph nodes also occurs, which is a symptom not present with smallpox infection.16

A rash, which usually appears on the face, begins within one to three days of fever, and spreads throughout the body. The lesions of the rash progress from macules to papules to vesicles before erupting as pustules and scab over. Most individuals recover from monkeypox within two to four weeks.17

Monkeypox is rare and is generally found in Africa, although cases and outbreaks have occurred globally. The virus was first identified among monkeys involved in research in 1958, While the main species that harbors the virus remains unknown, health officials believe that African rodents, monkeys, and other primates likely harbor and transmit the virus. The first known case of human infection was documented in the Democratic Republic of the Congo (DRC) in 1970.18Transmission of monkeypox virus is thought to occur through the respiratory tract, breaks in the skin, or mucous membranes.19

Complications of monkeypox illness include sepsis, encephalitis, eye infections that may result in blindness, bronchopneumonia, and other secondary infections. According the World Health Organization (WHO), death from monkeypox has historically occurred in 0 and 11 percent of cases. More recently, the WHO estimates death rates from monkey pox to be between 3 and 6 percent.20

There are two main clades (types) of monkeypox that have been identified: the West African (WA) and the Congo Basin (CB). The CB clade of monkeypox is considered more easily transmissible and associated with higher rates of mortality. Historically, those most at risk have been individuals hunt, kill, and eat bushmeat.21

IMPORTANT NOTE: NVIC encourages you to become fully informed about smallpox and the smallpox vaccine by reading all sections in the Table of Contents, which contain many links and resources such as the manufacturer product information inserts, and to speak with one or more trusted health care professionals before making a vaccination decision for yourself. This information is for educational purposes only and is not intended as medical advice.


References:

[1] Babkin IV, Babkina IN. The origin of the variola virus. Viruses Mar. 2015;7(3):1100-12.

[2] U.S. Centers for Disease Control and Prevention. Clinical Disease. In: Smallpox. Dec. 5, 2016.

[3] Johns Hopkins Bloomberg School of Public Health Center for Health Security. Variola Virus (Smallpox) Feb. 26, 2014.

[4] U.S. Centers for Disease Control and Prevention. Clinical Disease. In: Smallpox. Dec. 5, 2016.

[5] U.S. Food and Drug Administration. Smallpox Mar. 23, 2018.

[6] U.S. Centers for Disease Control and Prevention. Clinical Disease. In: Smallpox. Dec. 5, 2016.

[7] Johns Hopkins Bloomberg School of Public Health Center for Health Security. Variola Virus (Smallpox). Feb. 26, 2014.

[8] U.S. Centers for Disease Control and Prevention. Clinical Disease. In: Smallpox. Dec. 5, 2016.

[9] U.S. Centers for Disease Control and Prevention. Clinical Disease. In: Smallpox. Dec. 5, 2016.

[10] Johns Hopkins Bloomberg School of Public Health Center for Health Security. Variola Virus (Smallpox). Feb. 26, 2014.

[11] National Organization for Rare Disorders. Smallpox. In: Rare Disease Database. 2009.

[12] Belongia EA, Naleway AL. Smallpox vaccine: the good, the bad, and the ugly. Clin Med Res. Apr. 2003;1(2):87-92. 

[13] Henderson DA. Smallpox eradication--the final battle. J Clin Pathol. Nov. 1975; 28(11):843-9. 

[14] U.S Centers for Disease Control and Prevention. About Monkeypox. In: Monkeypox. Nov. 23, 2021.

[15] Beer EM, Rao VB. A systematic review of the epidemiology of human monkeypox outbreaks and implications for outbreak strategy. PLoS Negl Trop Dis. Oct 16, 2019;13(10):e0007791.

[16] U.S Centers for Disease Control and Prevention. Signs and Symptoms. In: Monkeypox. July 16, 2021.

[17] U.S Centers for Disease Control and Prevention. Signs and Symptoms. In: Monkeypox. July 16, 2021.

[18] U.S. Centers for Disease Control and Prevention. About Monkeypox. In: Monkeypox. Nov. 23, 2021.

[19] U.S. Centers for Disease Control and Prevention. Transmission. In: Monkeypox. July 16, 2021.

[20] World Health Organization. Monkeypox. May 19, 2022.

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