Smallpox was an acute disease specific to humans which often led to death within the first two weeks from the appearance of clinical signs. Those victims who survived, however, obtained a lifelong immunity to the disease and neither chronic nor recurring infection followed. Immunity to the disease was not as well guaranteed by vaccination, but in vaccinated individuals who developed the disease, it often expressed a much milder symptomatology and had lower case-fatality rate in its victims (Fenner et al. 1988: 272). The disease could only become endemic within a population when there was a large reservoir of susceptible hosts. This was regularly
achieved by new births and immigration of susceptibles in the larger population regions of the Old World.
Two main clinical-epidemiological varieties of smallpox have been recognized - variola major, the classic type of smallpox, and variola minor (Table 3.1).
Table 3.1 A classification of clinical types of variola virus infection (WHO 1977; Fenner et al. 1988).
WHO Classification
Clinical Manifestation
050.0 Variola major
Ordinary type Modified type
Haemorrhagic (pustular) smallpox Flat type
Variola sine eruptione
050.1 Variola minor
(alastrim)
The two strains of variola virus differed quite significantly in their virulence to humans. Variola minor, as its name suggests, causes mild symptoms in its human host and produces a case-fatality rate in unvaccinated individuals ranging from 0.1 to 2%. The more virulent variola major strain is a life-threatening disease. It often manifests severe symptoms, and produces a case-fatality rate among unvaccinated victims up to and in excess of 50%.
The virus is transmitted from person to person with no other vector involved.
In most cases the virus enters the body via the oropharynx or respiratory tract passed on by respiratory discharges from infected individuals. Other means of transmission
are: direct inoculation, as in variolation; by the skin lesions of patients, by material which has been contaminated by the virus, by placental transmission, and by airborne spread (Benenson 1976:440, 1990:396; Fenner et al. 1988:186). The period of communicability is greatest during the first week of initial infection and continues for approximately 21 days. Susceptibility to the disease is universal but long-term immunity usually follows recovery and second attacks are rare.
There are five main sub-types of variola major (Table 3.1) distinguished by their clinical symptoms and prognoses. Ordinary type variola major is the most common. In typical cases of this type of infection there is an incubation stage that ranges from seven to seventeen days but usually between ten to twelve (Benenson 1990: 396). During this period of infection the virus replicates and spreads throughout the body via the lymphoid organs (spleen, bone marrow and lymph nodes). The onset of clinical symptoms is sudden. Fever, malaise, headache, severe backache, prostration and in some instances abdominal pain are the first to appear. These symptoms can easily be confused with influenza, meningitis and pneumonia (Benenson 1976: 443). After two to four days the fever is reduced and the characteristic stages of the smallpox rash begin. Starting as maculopapules first on the face, hands or forearms and then by centrifugal distribution to the trunk and lower limbs. The lesions are more abundant on the face and extremities than on the trunk.
The papules then become vesicular within a day or two and then form pustules within 48 hours. The fever often returns during this stage. The pustules dry and begin forming scabs within eight to ten days of initial eruption. The matured lesions may be confluent, semi confluent, or discrete. Finally the scabs fall off at the end of the third to fourth week if the patient has survived. This type of variola major has been documented as having case-fatality rates of up to 62% (Rao 1972 cited in Fenner et al.
1988).
The flat type of variola major, distinguished by the lesions which project little, if at all, above the surrounding skin surface, is relatively rare but has a case- fatality rate reaching 98% (Benenson 1976:444; Fenner et al. 1988: 5). Equally as fatal is the haemorrhagic type which causes, either late or early in its development, subconjunctival bleeding, bleeding from the mouth, gums, nose, and blood in the urine (haematuria). In this type of variola major pregnant females are more susceptible than males and non-pregnant females. The modified type and variola sine eruptione are rare and occur principally in vaccinated individuals (Benenson 1976: 444; Fenner et al.
1988: 22-38)
The symptoms of variola minor were similar but much less severe than variola major. The onset was, like variola major, sudden with a fever, headache, backache, and sometimes vomiting. The sequence of development, and distribution of the skin lesions were also similar but their evolution was much more rapid. The eruptions became vesicular on the third day after the appearance of the papular stage and developed into pustules within twenty four hours. The final crusting stage was established on the sixth or seventh day of the rash. The facial lesions were often more sparse than in variola major and rarely reached the confluent stage. Secondary fever was rare and the victims usually remained ambulant and less affected during the course of the disease (Fenner et al. 1988: 38-39).