Monkeypox virus

Monkeypox is a rare viral disease that usually found in central and western Africa but can infect rats, mice, rabbits, and prairie dogs. Infected pets may appear healthy with no clinical signs of disease or may develop fever, cough, conjunctivitis, and lymphadenopathy, followed by a nodular rash.

Although the virus is not currently reported in the United States, people can become infected if they are bitten or come in contact with an affected animal’s urine, blood, or rash. Signs and symptoms of monkeypox initially include fever, headache, muscle aches, and lymph nodes swelling. A rash that progresses to fluid-filled bumps may develop days later as the disease spreads.


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.


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.


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.

2003 U.S. outbreak

Monkeypox in the United States

In 2003, the U.S. experienced an outbreak of monkeypox. This was the first time human monkeypox was reported outside of Africa.

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.

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.

How was monkeypox virus transmitted to humans?

All people infected with monkeypox became ill after having contact with infected prairie dogs purchased as pets. A study conducted after the outbreak suggested that certain activities associated with animals were more likely to lead to monkeypox. These included touching a sick animal or receiving a bite or scratch that broke the skin. Another important factor was cleaning the cage or touching the bedding of a sick animal. No instances of monkeypox were attributed exclusively to person-to-person contact.

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.

Vaccine distribution during the 2003 U.S. outbreak

During the 2003 U.S. monkeypox outbreak CDC, along with the Advisory Committee for Immunization 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.

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. There have been no human deaths recorded attributable to infection with West African-type monkeypox virus, nor have there been any documented instances of person-to-person spread of disease with West African viruses. Person-to-person spread of monkeypox viruses from Central Africa is well documented.


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.