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Interim Guidelines for Smallpox Response and Management in Scotland in the Post-Eradication era
IV Epidemiology
Incubation period
For smallpox, this is usually defined as the time between exposure and onset of fever. The range given by most authorities is 7 to 17 days,
usually 10 to 16 days, with a median of 12 days. The typical vesicular rash appears 4 to 7 days later.
Transmission
There is no known animal reservoir or vector for the smallpox virus. The most frequent mode of transmission is person-to-person spread via direct inoculation of infective droplets onto the oral, nasopharyngeal or respiratory mucosa during close contact with an infectious individual. From the mucosa the virus is transferred to local lymphoid tissue where replication occurs.
Patients are not infectious during the asymptomatic incubation period. They become infectious with the onset of fever. Infectiousness then increases until the onset of vesicular rash and remains high for the next 7 days. As a precaution, for the purpose of contact tracing, patients should be regarded as infectious from 24 hours prior to the time when fever was first recognised.
Patients remain infectious until the last scabs fall off. As a precaution, WHO isolation policy during the eradication campaign required that patients remain in isolation, in hospital or at home, until the last scab had separated. However, the
virus shed from the skin is not highly infectious and exposure to patients in the late stages of the disease is unlikely to produce infection in susceptible contacts.
The most efficient transmission of smallpox occurred during
close contact with infected persons. Household contact produces the highest attack rate, and contact in an open ward was a major cause of spread. In outbreaks in Asia and Africa, the attack rate in households varied from 37% to 96%, with some of the variation probably related to different living conditions and crowding, as well as to strain variation among variola viruses.
Casual contact, such as working in the same building, is much less likely to result in infection, although airborne spread of virus in draughts or air conditioning systems is known to cause transmission. Contaminated clothing or bed linen can also spread the virus.
Organism survival
In normal environmental conditions (ambient temperature, ordinary levels of humidity and exposure to sunlight) the virus is very unlikely to survive for more than 48 hours.
Depending on the conditions, variola viruses can survive for long periods in dry scabs (13 years has been documented), however this is not considered to represent an infectious threat.
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