a1 From the Department of Preventive Medicine and Public Health, University of Leeds
1. The field epidemiology is described of two epidemics of smallpox which occurred in Tripolitania during 1946.
2. Control was effected by the ‘expanding ring’ method instead of mass vaccination of the population.
3. Five hundred cases were admitted to hospital and statistics are given of mortality in relation to age, sex and vaccination state.
4. A new clinical classification is described which enables a reliable prognosis to be made and provides a guide to treatment.
5. Controlled experiments were made on the use of penicillin in treatment. Although it is without effect on the viraemia and in general does not significantly affect mortality, it has the power of suppressing suppuration and reducing the effect of secondary sepsis. It also minimizes the amount of permanent scarring of the skin.
6. Evidence is presented to show that the period of maximum infectivity is the stage of the pre-eruptive fever. The mildest cases are infectious, but only for a short time.
The virus contained in dried scabs is not considered to be an important source of infection; it may, be modified in some way by passage through the skin.
7. Although not protected by successful vaccination many contacts showed a considerable resistance to infection. Amongst the older people this may have been due to previous clinical or subclinical attacks. Amongst the younger persons evidence is produced to show that many were relatively refractory to vaccination and experienced the disease in a different way, suggesting that there is natural partial immunity to smallpox in this particular population.
8. The complex problem of immunity in smallpox is discussed, and it is suggested that three factors operate, an anti-invasion factor, an anti-dissemination factor and a local skin immunity mechanism. A photograph is given of a model made to show the interaction of these three factors in producing the immunity levels which determine the type of disease. It also accounts for the immunity reactions in vaccinia and in smallpox in the vaccinated subject, both when vaccinated successfully during the incubation period of smallpox and when vaccinated successfully some time prior to contact.
It was perhaps unfortunate that this work had to be done under Service conditions without access to literature such as Ricketts & Byles's (1908) classic The Diagnosis of Smallpox, or Marsden's works, particularly his review of vaccination (1946). The lack of authoritative guidance, however, was compensated for by the enthusiasm of all concerned.
I am especially grateful to Lt.-Col. W. McN. Graves-Morris, R.A.M.C., P.M.O. Tripolitania, for allowing me the opportunity to experiment and to use such a large proportion of the territory's supply of penicillin.
I am indebted to Col. W. W. Sharpe, R.A.M.C., D.D.H., M.E.L.F., for enabling me to return to Tripolitania for a short time to complete the records, and to Capt. J. Macbeth, R.A.M.C., Capt. W. L. Forbes, R.A.M.C., Capt. J. Anderson, R.A.M.C. and the late Capt. J. LaFerla, R.A.M.C, who assisted at different stages and in different ways.
(Received September 29 1948)