How Smallpox Is Diagnosed

Smallpox is caused by the variola virus and has not appeared naturally anywhere in the world since 1977. In fact, many healthcare providers practicing today have never seen an actual case of smallpox in person. Diagnosis is likely to be difficult, especially because any physician seeing the first case of smallpox is bound to assume chickenpox at first. An exam of the symptoms and a blood test will help confirm a diagnosis.

smallpox diagnosis
© Verywell, 2018

Smallpox vs. Chickenpox

Smallpox, like other poxviruses, has lesions that can cover the entire body. Physicians will not likely begin to diagnose smallpox until the lesions become obvious. At that point, healthcare providers will attempt to gain a history of the illness prior to the appearance of the lesions.

To determine the difference between smallpox and chickenpox, the healthcare provider will be looking at the formation of the lesions as the most important sign.

  • Smallpox: Lesions are hard and well-defined. All lesions will develop at the same rate and will be similar in their formation and firmness. Sometimes, the lesions will have a small indentation on their crown, known as an umbilical formation. Smallpox lesions are often preceded up to one to four days by a fever of more than 101 degrees. The lesions will be distributed on the arms and face and appear on palms and soles of the feet.
  • Chickenpox: Lesions are not as well-defined and will be at different stages of development. They are not firm and will be easy to remove. There will not likely be any fever preceding the onset of lesions. The lesions are likely to appear first on the torso rather than the arms and face. They will rarely appear on the palms or soles of the feet.

If you or someone in your family develops lesions that appear to be chickenpox or smallpox, see a healthcare provider. There is no home treatment for either condition and smallpox, although extremely unlikely, would be a major medical emergency.

Minor vs. Major Smallpox

To properly identify smallpox, it's necessary to understand the difference in the disease between infections with major and minor variola viruses. Major smallpox has an overall mortality rate of more than 30 percent while minor smallpox has a mortality of about 1 percent.

  • Major smallpox is characterized by 1 to 4 days of a fever of 101 degrees or more preceding the onset of lesions. The lesions will be deep-seated and hard, possibly with the depression on the crown. All lesions will be the same stage of development and will be distributed all over the body, possibly becoming confluent, meaning that they are placed very close together without clear skin in between. Smallpox lesions that are confluent on the face have a higher mortality than no confluence. Smallpox lesions that are confluent on the trunk and face have the highest mortality.
  • Minor smallpox is not likely to have a fever preceding the onset of lesions. The lesions are more likely to first appear on the arms, face, palms, soles of the feet, and inside the mouth. Like major smallpox (and unlike chickenpox) the lesions will be hard and round. Lesions develop at the same stages but develop slower than during major smallpox, moving between stages (discoloration, raised pimple, hard blister) at a rate of one or two days per stage. A patient of minor smallpox will likely seem listless and have no energy (moribund).

    Healthcare providers are looking for any of the signs of major smallpox or at least four of the signs of minor smallpox in order to have a high suspicion of smallpox as the diagnosis.

    If the patient is suspected of having smallpox, the doctor might order a blood test for the variola virus. If the test is positive, smallpox would be confirmed. If the test is negative, smallpox is not the diagnosis.

    Differential Diagnoses

    Other poxviruses can mimic the way smallpox looks but are significantly less lethal than smallpox. Some of these are closely related to the variola virus.

    Orthopoxviruses

    There are several zoonotic (infects animals and humans) versions of orthopoxvirus, the family of virus that includes variola, which is the cause of smallpox. These often look like smallpox and can be similar. Some can be serious.

    • Cowpox affects both cows and humans. Before clinical vaccination (which comes from the Latin word for cow) was widespread, farmers would develop some inoculation of variola through exposure to cowpox.
    • Vaccinia is another virus that affects cattle and is the base virus for the smallpox vaccine.
    • Monkeypox is the most closely related to smallpox and still naturally infects humans in some African nations. It has a mortality rate of 1 to 8 percent.
    • Camelpox affects camels and can cross over to humans.
    • Buffalopox is closely related to vaccinia and is common in India.

    Because smallpox vaccination was stopped in 1980, human populations have lost immunity to not only smallpox but also to many of these zoonotic poxviruses.

    Varicella and Herpes-Zoster

    Chickenpox is primarily a child illness from the varicella-zoster virus. Kids don't usually have a fever or other signs and symptoms before the pox lesions appear. As mentioned above, chickenpox lesions are less robust than those of smallpox and are very unlikely to appear on the palms or soles of the feet.

    Shingles (herpes-zoster) is a secondary infection from the same varicella virus and appears mostly in elderly patients. Shingles lesions follow major nerve pathways and are nearly always on one side of the body (unilateral). 

    How Smallpox Is Treated
    Was this page helpful?
    Article Sources