Monkeypox Facts

Monkeypox facts written by Charles Patrick Davis, MD, PhD

  • Monkeypox is a rare viral disease, mainly reported in central and western Africa and first discovered in 1958 and has had about 11 outbreaks since then, including one in 2003 in the U.S. There at least two different genetic types.
  • Signs and symptoms of monkeypox begin with fever, headaches, muscle aches, backache, chills, exhaustion, and swollen lymph nodes followed by the development of pox lesions that form scabs and then fall off.
  • In the U.S. outbreak, pets could have monkeypox symptoms and signs that range from minimal to having fever, cough, eye discharge, fatigue, and enlarged lymph nodes that progressed to pox lesions.
  • Monkeypox was first diagnosed by PCR assays from samples taken from a patient with monkeypox symptoms and the patient's pet rodent, a prairie dog.
  • The U. S. stated that in 2003 reported confirmed monkeypox infections were in Illinois, Indiana, Kansas, Missouri, and Wisconsin.
  • Monkeypox first came to the U.S. in a shipment of animals from Ghana that included six different African rodents, several of which were shown to be infected and were housed close to prairie dogs in an Illinois animal pet vendor.
  • Transmission of monkeypox occurs directly or indirectly by contact with an infected human or animal by viral entrance through broken skin, respiratory tract, or mucous membranes. Also, contaminated droplets, bites or scratches, bush meat preparation, and other contaminated items like rodent bedding are other possible ways the virus may be transmitted.
  • Although there are no proven and safe treatments for monkeypox, health care professionals have used smallpox vaccine, antivirals, and VIG (vaccinia immune globulin) to control outbreaks.
  • Prevention and/or risk reduction for monkeypox is possible by avoiding direct and indirect contact with individuals, animals, and possible contaminated items. Health care providers should wear personal protective equipment when caring for patients. Practice good hand hygiene if you may have had contact with any possible contaminated people, animals, or items. Isolate infected patients from others at risk for infection.
  • The CDC and ACIP advised investigators, health care workers, lab workers, and anyone who had close or direct contact with people and animals with monkeypox to be vaccinated with smallpox vaccine (reported to be about 85% cross-protective) up to 14 days after exposure.
  • Health officials in the U.S. (lead by the CDC) in the 2003 monkeypox outbreak eventually contained the infection by activating its Emergency Operations Center, deployed personnel to assist state agencies, conducted lab tests, and issued an immediate embargo/prohibition on the sale of certain rodents and prairie dogs. In addition, health care officials issued multiple guidelines for the use of smallpox vaccine, cidofovir, and vaccinia immune globulin along with guidelines for patient care. Veterinarians, animal control officers, and others were also issued guidelines.
  • The 2003 monkeypox U.S. outbreak was different from most other African outbreaks in that the viral strain introduced to the U.S. was the West African viral type and produces a less severe infection than the Central African monkeypox virus type.

About Monkeypox

Monkeypox is a rare disease that is caused by infection with monkeypox virus. Monkeypox virus belongs to the Orthopoxvirus genus in the family Poxviridae. The Orthopoxvirus genus also includes variola virus (the cause of smallpox), vaccinia virus (used in the smallpox vaccine), and cowpox virus.

Monkeypox was first discovered in 1958 when two outbreaks of a pox-like disease occurred in colonies of monkeys kept for research, hence the name 'monkeypox.' The first human case of monkeypox was recorded in 1970 in the Democratic Republic of Congo during a period of intensified effort to eliminate smallpox. Since then monkeypox has been reported in humans in other central and western African countries (see table below). The 2003 outbreak in the United States is the only time monkeypox infections in humans were documented outside of Africa.

The natural reservoir of monkeypox remains unknown. However, African rodent species are expected to play a role in transmission.

There are two distinct genetic groups (clades) of monkeypox virus -- Central African and West African. West African monkeypox is associated with milder disease, fewer deaths, and limited human-to-human transmission.

CountYearsRecorded Human Cases
Central African Republic19846
Democratic Republic of CongoEndemic
Ivory Coast1971
Republic of CongoSporadic
Sierra Leone1970
United States200347


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Signs and Symptoms

In humans, the symptoms of monkeypox are similar to but milder than the symptoms of smallpox. Monkeypox begins with fever, headache, muscle aches, and exhaustion. The main difference between symptoms of smallpox and monkeypox is that monkeypox causes lymph nodes to swell (lymphadenopathy) while smallpox does not. The incubation period (time from infection to symptoms) for monkeypox is usually 7-14 days but can range from 5-21 days.

The illness begins with:

  • Fever
  • Headache
  • Muscle aches
  • Backache
  • Swollen lymph nodes
  • Chills
  • Exhaustion

Within 1 to 3 days (sometimes longer) after the appearance of fever, the patient develops a rash, often beginning on the face then spreading to other parts of the body.

Lesions progress through the following stages before falling off:

  • Macules
  • Papules
  • Vesicles
  • Pustules
  • Scabs

The illness typically lasts for 2-4 weeks. In Africa, monkeypox has been shown to cause death in as many as 1 in 10 persons who contract the disease.

What Signs and Symptoms Were Seen in Pets?

During the U.S. outbreak, illness in animals included fever, cough, discharge from the eyes, and enlarged lymph nodes, accompanied by the development of lesions. Animals that had monkeypox also appeared to be very tired and were not eating or drinking. Some animals had only minimal signs of illness and recovered, while others died.

How Was Monkeypox First Diagnosed in the United States?

The clinical features of the illness in U.S. patients -- fever, headache, muscle aches, and rash -- were consistent with those of monkeypox. Initially, scientists at the Marshfield Clinic in Marshfield, Wisconsin, recovered a virus resembling a poxvirus from one of the first patients and the patient's pet prairie dog. Laboratory tests at CDC -- including several PCR-based assays looking for poxvirus DNA, electron microscopy, and gene sequencing -- confirmed that the agent causing the illnesses was monkeypox virus.

Which States Were Affected by the Outbreak?

Forty-seven confirmed and probable cases of monkeypox were reported from six states -- Illinois, Indiana, Kansas, Missouri, Ohio, and Wisconsin -- during the 2003 U.S. outbreak.

U.S. Monkeypox Cases by State, 2003
StateConfirmed CasesProbable Cases

How Was Monkeypox Virus Introduced into the U.S.?

Investigators determined that a shipment of animals from Ghana, imported to Texas on April 9, 2003, introduced monkeypox virus into the United States. The shipment contained approximately 800 small mammals representing nine different species, including six genera of African rodents. These rodents included rope squirrels (Funiscuirus sp.), tree squirrels (Heliosciurus sp.), African giant pouched rats (Cricetomys sp.), brush-tailed porcupines (Atherurus sp.), dormice (Graphiurus sp.), and striped mice (Lemniscomys sp.). CDC laboratory testing using PCR and virus isolation demonstrated that two African giant pouched rats, nine dormice, and three rope squirrels were infected with monkeypox virus. After importation into the United States some of the infected animals were housed in close proximity to prairie dogs at the facilities of an Illinois animal vendor. These prairie dogs were sold as pets prior to their developing signs of infection.


Transmission of monkeypox virus occurs when a person comes into contact with the virus from an animal, human, or materials contaminated with the virus. The virus enters the body through broken skin (even if not visible), respiratory tract, or the mucous membranes (eyes, nose, or mouth). Animal-to-human transmission may occur by bite or scratch, bush meat preparation, direct contact with body fluids or lesion material, or indirect contact with lesion material, such as through contaminated bedding. Human-to-human transmission is thought to occur primarily through large respiratory droplets. Respiratory droplets generally cannot travel more than a few feet, so prolonged face-to-face contact is required. Other human-to-human methods of transmission include direct contact with body fluids or lesion material, and indirect contact with lesion material, such as through contaminated clothing or linens.

The reservoir host (main disease carrier) of monkeypox is still unknown although African rodents are suspected to play a part in transmission. The virus that causes monkeypox has only been recovered (isolated) twice from an animal in nature. In the first instance (1985), the virus was recovered from an apparently ill African rodent (rope squirrel) in the Equateur Region of the Democratic Republic of Congo. In the second (2012), the virus was recovered from a dead infant mangabey found in the Tai National Park, Cote d'Ivoire.


Currently, there is no proven, safe treatment for monkeypox virus infection. For purposes of controlling a monkeypox outbreak in the United States, smallpox vaccine, antivirals, and vaccinia immune globulin (VIG) can be used. Learn more about smallpox vaccine, antivirals, and VIG treatments.


There are number of measures that can be taken to prevent infection with monkeypox virus:

  • Avoid contact with animals that could harbor the virus (including animals that are sick or that have been found dead in areas where monkeypox occurs).
  • Avoid contact with any materials, such as bedding, that has been in contact with a sick animal.
  • Isolate infected patients from others who could be at risk for infection.
  • Practice good hand hygiene after contact with infected animals or humans. For example, washing your hands with soap and water or using an alcohol-based hand sanitizer.
  • Use personal protective equipment (PPE) when caring for patients.

Vaccine Distribution During the 2003 U.S. Outbreak

During the 2003 U.S. monkeypox outbreak CDC, along with the Advisory Committee for cidofovir Practices (ACIP) advised the following people to get the smallpox vaccine:
  • People who investigated animal or human monkeypox cases (e.g., public health and animal control workers).
  • Any healthcare worker who was in close contact with a monkeypox patient. (Vaccination was considered up to 14 days after exposure to a monkeypox case.)
  • Anyone who had close contact with someone who was infected with. (Vaccination was considered up to 14 days after exposure to a monkeypox case.)
  • Anyone (including veterinarians and veterinary technicians) who had direct physical contact within 4 days of exposure with a confirmed infected animal. (Vaccination was considered up to 14 days after exposure.)
  • Lab workers who handled specimens that could have contained monkeypox virus.


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How Was the Outbreak Contained?

CDC and the public health departments in the affected states, together with the U.S. Department of Agriculture, the Food and Drug Administration, and other agencies, participated in a variety of activities that prevented further spread of monkeypox. To assist with the investigation and outbreak response, CDC took the following steps:

  • Activated its Emergency Operations Center.
  • Deployed teams of medical officers, epidemiologists, and other experts to several states to assist with the investigation.
  • Conducted extensive laboratory testing on specimens from humans and animals thought to have been exposed to monkeypox.
  • Issued interim U.S. case definitions for human monkeypox and for animal monkeypox.
  • Issued interim guidelines on infection control and exposure management for patients in the health care and community settings.
  • Issued an immediate embargo and prohibition on the importation, interstate transportation, sale, and release into the environment of certain rodents and prairie dogs.
  • Provided ongoing assistance to state and local health departments in investigating possible cases of monkeypox in both humans and animals the United States.
  • Worked with state and federal agencies to trace the origin and distribution of potentially infected animals.
  • Issued an interim guidance on the use of smallpox vaccine, cidofovir, and vaccinia immune globulin in the setting of an outbreak of monkeypox.
  • Issued interim guidelines for veterinarians.
  • Issued interim guidance for persons who have frequent contact with animals, including pet owners, pet shop employees, animal handlers, and animal control officers.

How Was the U.S. Monkeypox Outbreak Different From Outbreaks That Have Occurred in Africa?

Studies of monkeypox virus suggest that there are at least 2 different genetic types (clades) of the virus. Virus clades segregate based upon geographic separation, with one type being found in West Africa and the other in Central Africa. The strain introduced into the U.S. came from Ghana, located in West Africa. Human infections with Central African monkeypox virus are typically more severe than infections with the West African virus type. Person-to-person spread of monkeypox viruses occurs, and has been well-documented for Central African type of virus.

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Monkeypox Sign

Swollen Lymph Nodes

Lymph nodes (erroneously called lymph glands) are a part of the lymphatic system, a component of the body's immune system. Swollen lymph nodes may signal an infection.

There are several groups of lymph nodes, which are small, bean-shaped, soft nodules of tissue. The ones most frequently enlarged or swollen are found in the neck (a chain of lymph nodes is located in the front of the neck, the sides of the neck, and the back of the neck behind the ears), under the chin, in the armpits, and in the groin. There is also a large group of lymph nodes in the chest and abdomen, which are sometimes found to be enlarged on X-rays or CT scans.


United States. Centers for Disease Control and Prevention. "About Monkeypox." May 11, 2015. <>.

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