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London School of Hygiene and Tropical Medicine, Keppel Street (Gower Street), London WC1E 7HT, England.

During the past 20 years or more, there have been a number of severe outbreaks of viral disease apparently "new" to medicine. Good examples are the initial outbreaks of Kyasanur Forest disease, O'nyong nyong fever, Bolivian haemorrhagic fever, Marburg and Ebola fevers. To some extent their recognition can be accounted for by improved communications, both general and medical (through WHO and other international channels), but in almost every case where an explanation has been advanced, these outbreaks have been attributed to intrusion into or interference with previously little frequented areas, because of population pressure and/or agricultural developments. With population throughout most of the world continuing to increase markedly despite all efforts at control, and with an estimated 460 million people in the world with insufficient to eat for good health (F.A.O., 1975), these pressures are unlikely to diminish during the remainder of this century at least - and it seems reasonable to expect further outbreaks of severe viral diseases (some of them "new").

Were these diseases really new ? It seems unlikely. If not, why were they recognized when they were ? The 1956 outbreak of Kyasanur Forest Disease was recognized mainly because of attention drawn to it by a high monkey mortality, but investigated and identified primarily because of the proximity of the Virus Research Institute, Poona. Similarly, O'nyong nyong fever, except perhaps for the scale of the epidemic, might well have been dismissed as yet another outbreak of dengue-like disease without serious consequences, had not the Virus Research Institute, Entebbe, been ready and able to investigate it. The original Bolivian haemorrhagic fever outbreak was brought to notice (and to investigation by the Middle America Research Unit) because of its severity. Marburg fever received the fullest investigation from the start because it first occurred in highly developed countries, and affected laboratory workers. The Ebola outbreaks were brought to notice because of their great severity, because they were spreading rapidly among hospital staff, because the whole health services of the epidemic areas appeared to be in jeopardy, and because there appeared to be risk (widely reported in the local and world Press) of much wider spread.

We can conclude that unless "new diseases" which occur in relatively remote areas cause large or severe epidemics, or affect hospital staffs, or occur in the "parish" of a virus research institute, they are unlikely to be investigated or their cause discovered. There are probably a number of candidate viruses for "new" diseases in the International Catalogue of Arboviruses.

The emergence of a previously latent zoonosis is probably usually due to a change in the ecology of its maintenance cycle or to changes in the ecology of neighbouring areas. New contacts with man, or changes in virulence, may be induced by increases in the population of maintenance hosts or related species and/or by establishment of the infection in a new maintenance host. The larger the scale of man-made environmental changes and the more they involve areas little frequented by man, the greater must be the probability of emergence of a zoonosis ("old" or "new"). Intrusion of agriculture, particularly of food crops attractive to rodents, into previously underdeveloped areas obviously increases the hazard of rodent maintained infections; extensive food storage inadequately rodent-proofed has a similar effect; irrigation or other water developments (including those that reduce the salinity of surface waters, e.g. the emergence of West Nile fever in the Camargue due to a vast increase in Culex modestus following rice growing) increases the hazard of mosquito-borne infections; and the introduction of large domestic mammals (especially cattle) into new territory may enhance the risk of tick-borne infections.

Clearly the first priority for early recognition of potentially dangerous outbreaks must be to educate the health and administrative authorities, particularly in the tropics, of the need for some form of surveillance and reporting of outbreaks of acute febrile disease (particularly in hospital personnel) in all new agricultural ventures involving intrusion into underdeveloped territory. This need not be elaborate and need not involve expensively trained staff-policemen, foremen, teachers, or villagers can be given simple but clear instructions. A small but well-trained team with limited investigational facilities should keep reporting under review, assess and advise on communicable disease problems as they become apparent and, when an incident requires investigation, it should be able to go to it with minimum delay carrying all necessary equipment. The team should have a first call on any available microbiological laboratory resources. It should be equipped, well trained and disciplined to collect specimens safely from cases or corpses of dangerous infectious diseases, trained to make an epidemiological assessment, able to institute emergency control measures with such local support as is available and to advise on seeking appropriate assistance from within or outside the country when necessary. It must be capable of sending specimens properly refrigerated, packed according to international regulations and with adequate prior arrangements, to a reference laboratory. The receiving laboratory must be furnished with the fullest details of the outbreak, and of the patient(s) from which the material was collected. The fullest co-operation of airline staff, customs officials, etc., has to be arranged in advance, by telephone or telex if necessary. It is therefore wise if accurate information about the outbreak is issued officially at the earliest possible moment to minimize the otherwise inevitably inaccurate press reporting which may greatly increase the difficulties of sending and handling specimens and cause unnecessary public alarm.

Outside expert assistance may be needed in the control and investigation of such outbreaks. The nature of the help required will vary with circumstances but if the infection is a highly dangerous one, only a well-equipped and welltrained team should be sent. The team must not only be expert and well trained but needs an able, experienced and tactful leader; and it must be selfsufficient in terms of immediate medical care for its members, equipment (including protective clothing, containers, field sterilizers, etc.), materials, and camping equipment, electricity generators, fuel, etc., if necessary; and it must carry adequate supplies to enable it to effectively equip the local hospital and health authorities to control the outbreak. This implies formidable logistic problems, the most important of which are communications, transport and the dissemination of information.

The Ebola epidemics exposed many of these problems and we have learned a great deal from them and will learn even more at this meeting. I hope that when the next serious epidemic ("new" or "old") occurs, we will be able to show that we have profited from these lessons.

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