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World Health Organization, Virus Diseases, 1211 Geneva 27, Switzerland.

The sudden appearance of a "new" communicable disease, such as Ebola haemorrhagic fever which manifested itself in two outbreaks in 1976, was a challenge for those who tried to understand its nature and to carry out appropriate measures under the pressure of current events. Four questions were raised at that time : (1) what was the agent; (2) how did it appear; (3) why were the outbreaks so extensive and (4) what should be done ?

1. What was the agent ? The first agent which came to mind to explain the haemorrhagic syndrome with diarrhoea was Salmonella typhi but the haemocultures were negative. Yellow fever virus was the second possibility since Sudan and Zaire are located in the epidemic zone and the way an epidemic of yellow fever is propagated by Aedes aegypti may to some extent evoke a pattern of man-toman transmission. However, the deaths of three missionaries who had been vaccinated invalidated this hypothesis. We had then to envisage viruses which had already caused haemorrhagic diseases in Africa: Lassa, Marburg, and Crimean-Congo haemorrhagic fever viruses. Rift valley fever virus was not taken into account at that time but has since shown itself to be a possibility even outside its traditional focus although its clinical-pattern is more often like dengue than a haemorrhagic fever. The high attack rate in hospital staff clearly indicated that specimens ought to be sent to high security laboratories. In collecting samples it was recalled that the viraemia of such viruses as Lassa virus may be prolonged and the rise of antibodies may be postponed up to two weeks after onset of the disease. In addition, we now know that electron-microscopy is indispensable to detect a new virus for which the existing batteries of reference antigens and antisera will not give any positive response. The first samples from the Sudan and Zaire were received on 5 October 1976. Ten days later it was found that the agent which had caused the two outbreaks was a new virus morphologically similar to Marburg virus but antigenically distinct. This discovery was simultaneously announced on 15 October by the three laboratories which had participated in the isolation of the virus (1). On 16 October, two epidemiological teams were sent to the Sudan and Zaire, to reinforce consultants already operating in both countries, within the framework of the WHO Emergency Aid Scheme against viral diseases. The outbreaks being exceptional by the high risk of man-to-man transmission the teams had to take appropriate measures to halt the spread of the epidemic to the entire affected countries and to other countries.

2. Why a new virus ? Is the number of viruses in the world finite ? Influenza virus shows that recombinants may occur frequently in natural conditions. The plasticity of arboviruses is well known. However, certain viruses such as measles virus, although very widespread, show only a single type. Did Ebola virus exist before September 1976 or was it a new agent ? We have the same problem with two new Enteroviruses 70 and 71. Hopefully, retrospective serological surveys will provide an answer to this question.

3. Why extensive outbreaks ? Was the introduction of Ebola virus by a single person, or a single animal, in a community all that was needed to start an outbreak ? If so, why did it not occur earlier ? There is some evidence that a single carrier can start an outbreak of in fluenza, measles, viral hepatitis and several other diseases. Is a single person infected with yellow fever virus, or a single infected mosquito able to start an epidemic ? On the contrary, is it necessary to have simultaneously a certain number "X" of abortive cycles of transmission for one of them to be more successful than the others ? If so, epidemiologists could develop a mathematical model of silent infections with amplication indices (Ai) of each element of the chain of transmission, as in figure 1, and fix a threshold above which such abortive, silent cycles are successful in cre ating an outbreak. This is in fact analogous to what Gorgas in 1908 called the "critical number" below which the population of Aedes aegypti is unable to transmit yellow fever. Seroepidemiological surveys outside the two foci, or in children born after 1976 within the foci, will tell us whether silent infections are common with Ebola virus and in which geographical zones they occur. Epidemiological situations are so often unforeseeable that it will probably be impossible to establish with confidence the above-mentioned threshold for Ebola virus. It is perhaps more realistic to say that some exceptional epidemiological condition, such as nosocomial transmission, can on rare occa sions enable a silent virus to manifest itself. The way the Ebola virus outbreaks started in the Sudan and Zaire in 1976 can be compared to the way the outbreak started in Germany in 1967. Another hypothesis would be a change in the virulence of the virus.

4. What to do in the face of such outbreaks ? In September 1976, the lack of previous experience with such sudden outbreaks and of such magnitude inevitably meant a good deal of improvisation. Responsible persons had to find a way between insufficient and excessive measures. Political implications often interfered with epidemiological decisions. Communications and transport raised many problems. Logistics were quite a challenge in organizing the mobilization of personnel case finding expeditions, the collection of specimens, their shipment and investigation, the isolation of patients, the procurement of supportive treatment, the preparation of immune plasma, its testing, and the protection of hospital personnel. This last problem was perhaps the least foreseen and the most important.


The Ebola virus outbreaks in 1976 created an exceptional situation which the two affected countries, Sudan and Zaire, met so admirably by mobilizing all possible resources. The amplitude of this public health problem called for unprecedented international cooperation. The experience thus acquired will be most valuable should a "new" virus again appear and create such a dramatic situation.

1. WHO Wkly. Epidem. Rec. No.42, 15 October 1976, p. 327.

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