Clinical History
The patient is a child who developed raised ulcerated lesions on her hand 2 weeks following a prairie
dog bite.
Case 1 - Figure 1 - The biopsy shows ballooning degeneration of the epidermis. There is a mixed inflammatory infiltrate involving the dermis and showing exocytosis. Abundant karryorhexis is present.
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Case 1 - Figure 2 - High-magnification shows ballooning degeneration and inflammation of the epidermis. Several keratinocytes show intracytoplasmic viral inclusions that have a tendency to abut the nucleus.
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Dr. Kurt Reed, Marshfield, Wisconsin, kindly provided this case.
Monkeypox
In 1958, monkeypox was first identified as a pathogen of cynomolugus monkeys. The virus is a member
of the zoonotic orthopoxviruses; others in this group include smallpox, cowpox, camelpox, buffalopox, and
vaccinia. In 1970, the first cases of humans infected with monkeypox were reported in the Democratic
Republic of Congo (Zaire at the time). Over the following decade increasing numbers of sporadic cases
were reported from tropical rain forests of western and central Africa; by 1979, 55 cases had been
reported. Since smallpox vaccination affords protection in most of those who have been vaccinated, the
appearance and increasing numbers of patients infected with monkeypox is, at least partly, attributed to
increasing numbers of unvaccinated people.
Animal-to-human transmission, usually through a bite or exposure to blood or lesions, is thought to
account for the majority of cases. Recent data from Africa, however, raises the possibility that
human-to-human transmission may be more common than previously thought. Human-to-human transmission
through respiratory droplets and disease following exposure to contaminated fomites may also occur.
Increasing deforestation- presumably leading to increased contact between animal reservoirs and humans-
has also been implicated in the epidemic. Animal reservoirs for monkeypox include the Gambian giant
pouched rat (they weigh up to several pounds!), prairie dog, dormice, brush-tailed porcupine, rope
squirrels, tree squirrels and striped mice.
By the mid-eighties several hundred cases of monkeypox had been reported in Africa. In June 2003,
several patients became ill after contact with pet prairie dogs marking the first case of monkeypox
infection of humans in the United States. Electron microscopy showed the virus had characteristic
features of poxvirus. PCR-based assays and gene sequencing established the diagnosis of monkeypox
infection. The number of cases quickly escalated; the CDC reported in July of 2003 that 71 humans had
been infected with monkeypox in the United States, most after having exposure to infected prairie dogs.
These cases were seen in Illinois, Indiana, Kansas, Missouri, Ohio, and Wisconsin. Only two children
developed "serious" illness and recovered. All cases could be tracked to a shipment of mammals to an
animal distributor in Texas in April 2003. These contaminated animals spread the virus to prairie dogs,
who, in turn, infected owners an other contacts. But no human infections have been linked to direct
contact to the animals in the original shipment from Africa. The government has ordered an embargo of
all rodents (living or dead) from Africa. In addition, new regulations have also been passed restricting
sale and transportation, and release into the wild of animals known to carry monkeypox. Unfortunately,
it is not known what mammals may carry the disease and the CDC suggests that, until more is known, it
should be assumed that all mammals are susceptible.
Headache, fever, and backache appear with 2 weeks of infection. Some patients develop a
non-productive cough. One to ten days following these prodromal symptoms, skin lesions develop and last
for 2-3 weeks as they progress from macules and papules to blisters and pustules that may ulcerate and
finally form crust. The lesions are present on the trunk, head, and extremities; they are also commonly
seen on the palms and soles. The duration of illness is 2 to 4 weeks in most patients.
The skin lesions are virtually indistinguishable from those associated with smallpox. In contrast to
smallpox, however, monkeypox is associated with more dramatic lymphadenopathy. Scarring, which can be
disfiguring, is common in smallpox but is but is rare in monkeypox infection.
Approximately 10% of people sharing households with infected patients will also develop the disease.
The mortality rate depends on smallpox vaccination status; the mortality rate is very lower in vaccinated
patients. Overall mortality is 1-10%. (This compares with the oft-quoted mortality rate of 30% for
smallpox. Historical evidence, however, suggests the mortality rate for smallpox was much higher in some
populations such as some Native American populations.) Children are more likely to develop severe or
fatal disease; 15% of children under 5 years old died in one study.
The biopsy findings depend on the duration of the lesion at the time of the biopsy. A vesicle or
pustule may be present. Acute and chronic inflammation of the dermis is seen. Intracytoplasmic
inclusions are seen in the squamous cells of the epidermis.
The CDC recommends smallpox vaccination for people that have had contact with infected animals or
humans.
References
- MMWR Weekly 1997;46:304-307.
- MMWR Weekly 2003;52:642-646.
- Breman G, Henderson DA. Poxvirus dilemmas—monkeypox, smallpox, and biologic terrorism NEJM 1998;339:556-559.
- Stephenson J. Monkeypox outbreak a reminder of emerging infections vulnerabilities. JAMA 2003;290:23-24.
- The CDC has an excellent website: www.cdc.gov
- The AFIP also has an excellent website: www.afip.org
- Wonderful clinical photos are posted on the Marshfield Clinic Wesite: marshfieldclinic.org/crc/prariedog.asp
- See also the World health organization website; www.who.int
