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Abstract & Commentary
Synopsis: The first outbreak of human monkeypox infection in North America has resulted from exposure to infected prairie dogs.Source: http://www.cdc.gov/ncidod/monkeypox/report060903.htm.
In early May, a number of individuals in the Midwestern United States became ill, beginning with a prodrome of fever, headaches, myalgias, chills, and sweats and, in some, a nonproductive cough. Between 1 and 10 days later, they developed a popular rash that typically vesiculated, became pustular, developed a central umbilication, and then crusted over as it resolved. The eruption was generalized in some but typically involved the head, trunk, and extremities. In some, the lesions occurred on the palms and soles. Lesions in different stages of evolution were often simultaneously present.
All 19 patients (17 from Wisconsin and 1 each from Illinois and northwestern Indiana) identified as of June 7, 2003, reported contact with prairie dogs. Most of these prairie dogs suffered from an illness that often began with blepharoconjunctivitis that, in some, progressed to nodular lesions. The infection was lethal in some, but not all, of the animals.
Investigation determined that the prairie dogs had been obtained by a distributor along with a Gambian giant rat that was ill at the time. The prairie dogs were then sold by the distributor to 2 Milwaukee pet shops and were also sold at a pet sale/exchange meeting in northern Wisconsin.
Investigators at the Marshfield Clinic in Wisconsin isolated virus from a patient and a prairie dog that, when examined by electron microscopy, was morphologically consistent with a poxvirus. Preliminary results of serologic testing, polymerase-chain-reaction analysis, and gene sequencing performed at the CDC indicated that the causative agent is monkeypox virus.
Comment by Stan Deresinski, MD, FACP
Monkeypox virus is an orthopoxvirus that can cause human disease resembling smallpox. Genomic analysis has recently confirmed that monkeypox virus represents a distinct species and is not a direct ancestor nor a direct descendant of the agent of smallpox, variola virus.1
Human monkeypox primarily occurs in the rain forest countries of Central and West Africa. Animal species susceptible to monkeypox virus have been known to include nonhuman primates, lagomorphs, and some rodents. Serological surveys of captured animals in the Democratic Republic of Congo suggested that squirrels play a major role as a reservoir of the virus, with infrequent sporadic infection of humans. Human-to-human transmission occurs with an incubation period of 12 days (range, 7-21 days), but transmissibility by this route is too low to sustain continued spread in susceptible populations.2
Human infection results in a vesicular and pustular rash similar to that of smallpox. Limited person-to-person spread of infection has been reported in disease-endemic areas in Africa. Smallpox vaccination provides approximately 85% protection against monkeypox. The secondary attack rate in unvaccinated household members is approximately 9%. Case-fatality rates have ranged from 1-10%.
There is no known proven effective specific treatment of monkeypox infection. The CDC is evaluating the potential role of postexposure use of smallpox vaccine, as well as therapeutic use of the antiviral drug cidofovir.
Suspect cases in animals or humans should immediately be reported to state or local health departments. Physicians should consider monkeypox in persons with fever, cough, headache, myalgias, rash, or lymph node enlargement within 3 weeks after contact with prairie dogs or Gambian giant rats. The CDC recommends a combination of standard, contact, and airborne precautions in management of the patient.3 The following is their recommendation for dealing with a suspect case in the outpatient setting: "Segregate the patient from others in the reception area as soon as possible, preferably in a private room with negative pressure relative to the surrounding area. Place a surgical mask over the patient’s nose and mouth. Care should be taken to cover exposed skin lesions (sheet and/or gown on patient) to prevent contact with infectious material." Exposed health care workers may continue to work but should have twice daily body temperature measurement and be questioned regarding symptoms prior to reporting for duty each day for 21 days following exposure. Specimens for diagnostic testing should be collected and handled in the same way as in testing for vaccinia and smallpox.4
Editor’s note: Excellent electron micrographs and clinical photos can be found at http://research.marshfield clinic.org/crc/monkeypox.asp.
Dr. Derenski is Clinical Professor of Medicine, Stanford; Associate Chief of Infectious Diseases, Santa Clara Valley Medical Center.
1. Shchelkunov SN, et al. Analysis of the monkeypox virus genome. Virology. 2002;297:172-194.
2. Hutin YJF, et al. Outbreak of human monkeypox, Democratic Republic of Congo, 1996 to 1997. Emerg Infect Dis. 2001. http://www.cdc.gov/ncidod/eid/vol7no3/hutin.htm.