Smallpox: Clinical diagnosis
Overview
Smallpox has two main forms: variola major and variola minor. The two forms showed similar lesions. The disease followed a milder course in variola minor, which had a case fatality rate of less than 1 per cent. The fatality rate of variola major was around 30%. There are two rare forms of smallpox: haemorrhagic and malignant. In the former, invariable fatal, the rash was accompanied by haemorrhage in to the mucous membranes and the skin. Malignant smallpox was characterized by lesions that did not develop to the pustular stage but remained soft and fat. It was almost invariably fatal.
Smallpox is a disease which can be easily diagnosed by trained health workers without the need for laboratory support. During the eradication campaign, WHO produced training materials designed to help health staff recognize smallpox, distinguish it from chickenpox, and avoid common diagnostic errors. These materials are now electronically available.
- The incubation period of smallpox is usually 12-14 days (range 7-17) during which there is no evidence of viral shedding. During this period the person looks and feels healthy and cannot infect others.
- The incubation period is followed by the sudden onset of influenza-like symptoms including fever, malaise, headache, prostration, severe back pain and, less often, abdominal pain and vomiting. Two to three days later, the temperature falls and the patient feels somewhat better, at which time the characteristic rash appears, first on the face, hands and forearms and then after a few days progressing to the trunk. Lesions also develop in the mucous membranes of the nose and mouth, and ulcerate very soon after their formation, releasing large amounts of virus in to the mouth and throat.
- The centrifugal distribution of lesions, more prominent on the face and extremities than on the trunk, is a distinctive diagnostic feature of smallpox and gives the trained eye cause to suspect the disease. Lesions progress from macules to papules to vesicles to pustules. All lesions in a given area progress together through these stages. From 8- 14 days after the onset of symptoms, the pustules form scabs which leave depressed depigmented scars upon healing.
- In the past, smallpox was sometimes confused with chickenpox, a worldwide infection of children that is seldom lethal. Chickenpox can be distinguished from smallpox by its much more superficial lesions, their presence more on the trunk than on the face and extremities, and by the development of successive crops of lesions in the same area.
WHO instructions for vaccine administration using the bifurcated nedle.