Smallpox Workup

Updated: Jul 28, 2020
  • Author: Aneela Naureen Hussain, MD, MBBS, FAAFM; Chief Editor: John L Brusch, MD, FACP  more...
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Approach Considerations

Suitably vaccinated and trained personnel should obtain viral swabs of the patient's pharynx and/or open skin lesions (eg, pustule contents, material from the base of the scab).

Under biosafety level 4 (BSL4) laboratory conditions, these samples can be examined for the presence of virions by using electron microscopy, PCR assay, or immunohistochemical analysis or by growing the virus on live cell cultures. [13, 14]

Serologic testing can be performed to detect neutralizing antibodies, but the results cannot be used to differentiate Orthopoxvirus species.

Although smallpox and all other viruses in the Orthopoxvirus genus exhibit identically appearing brick-shaped virions, the clinical aspects of these diseases generally suffice for distinguishing cowpox and vaccinia from smallpox.

Monkeypox virions may also be indistinguishable from smallpox virions, but naturally occurring monkeypox is typically limited to tropical rain forest areas of Africa.

Smallpox infection may be confirmed based on the presence of brick-shaped virions viewed with electron microscopy examination of vesicular or pustular fluid or scabs. PCR assay and electron microscopy can be used to examine inactivated samples and therefore do not require such high levels of isolation and can be performed in local laboratories.

However, although electron microscopy can help to identify the virus as a member of the Orthopoxvirus genus, it cannot help to determine the exact species.

PCR assay can be used to identify the species and can even distinguish minor genetic variations in the different strains. PCR assay has been used to identify variola only twice previously, and never in a clinical situation. PCR assay can amplify small and specific lengths of DNA and can accurately differentiate variola virus DNA from other species in the genus. The sensitivity is 5-10 copies of DNA. PCR assay can be useful in distinguishing between chickenpox and smallpox.

Cell culture is seldom used, because it is not as effective as the other methods and because it requires the use of live virus, which, in turn, requires the use of a BSL-4 laboratory.

Depending on the presenting clinical symptoms, other diseases, such as meningococcemia, leukemia, herpes viruses, and drug eruptions, must be ruled out. A meticulous drug history should be obtained. Tests likely to be performed include the following:

  • Tzanck preparations

  • Direct fluorescent antibody testing for herpes viruses

  • Blood tests

  • Skin biopsy

  • Lumbar puncture


Specimen Collection and Handling

A smallpox skin specimen should be collected with precautions in place. Gloves should be worn during collection; fluid from lesions can be harvested on a cotton swab. Prior to shipping specimens, state and local health department laboratories should be contacted for specific instructions.

The CDC recommends the following procedures for handling specimens obtained from a patient thought to be infected with the smallpox virus:

  • Specimens should be collected by someone who has recently been vaccinated (or who has been vaccinated that day) and who is wearing gloves and a mask

  • To obtain vesicular or pustular fluid, the lesions may need to be opened with the blunt edge of a scalpel; the fluid can then be harvested on a cotton swab; scabs can be picked off with forceps

  • Specimens should be deposited in a Vacutainer tube; the tube should be sealed with adhesive tape at the juncture of the stopper and the tube, and this tube, in turn, should be enclosed in a second durable and watertight container

  • State or local health department laboratories should be contacted immediately for proper specimen shipping protocols.

Laboratory examination should be performed only in designated BSL-4 laboratories. Once it has been established that an epidemic is being caused by the smallpox virus, clinically similar cases do not require further laboratory testing.