PREVIOUS AND PRESENT STATUS OF YELLOW FEVER IN THE WORLD. by Dr P. Brès Virus Diseases Service

Transcription

1 ^ WORLD HEALTH ORGANIZATION VIR/YF/71.11 ORGANISATION MONDIALE DE LA SANTÉ ORIGINAL: FRENCH EXPERT COMMITTEE ON YELLOW FEVER Entebbe, 915 March 197i PREVIOUS AND PRESENT STATUS OF YELLOW FEVER IN THE WORLD 12 F ' by Dr P. Brès Virus Diseases Service INOEXEO There have been several distinct stages in the development of our situation in relation to yellow fever since the Yucatan epidemic of 1648, the first where the disease can be identified with any certainty. Dutronleau (1858) and Councilman (1890) described the clinical and anatomical characteristics of the disease. Carlos Finlay (1881) and then Walter Reed (1900) established the fact of transmission by Aedes aegypti and the filterability of the pathogen. In 1927, the virus was isolated at Accra by Stokes and at Dakar by Mathis. Theiler (1930) adapted the virus on mouse brain and (1931) developed the protection test with the specific immune serum. In 1932, Sellards proposed a method of vaccination with the neurotropic strain and, in 1937, Theiler described the immunizing properties of strain 17D passaged on tissue culture and in ovo. Epidemiological knowledge advanced considerably when Soper characterized jungle yellow fever in Immediately thereafter (from 1936) intensive investigations were conducted simultaneously in South America and in Africa to discover the transmission cycles of yellow fever among monkeys in the endemic foci of the tropical forests. Abundant documentation on all these stages is contained in the treatise published by Strode (1951) and his coworkers, in which will also be found the considerable body of knowledge acquired since 1936 on the transmission of yellow fever. It seems that the development of an effective and safe vaccine, together with the favourable results of the first Aedes aegypti eradication campaigns in the Americas, produced a mistaken feeling of security which has led investigators, with a few exceptions, to lose interest in yellow fever since However, the events of the past two decades show that yellow fever remains a permanent threat and that there are s t i l l many unknowns, so that there is justification for undertaking further research. Status of yellow fever during the past two decades The cases notified for the period are shown in Table 1. The number of cases notified may be far lower than the number that actually occurred. In the Americas Endemic yellow fever transmitted by Aedes aegypti has disappeared from the Americas since 1934 as a result of the measures taken against this vector with a view to its eradication (Soper, 1967) and since then only jungle yellow fever has continued to be observed. During the period , jungle yellow fever put in an almost regular appearance each year, but with variable incidence, in Bolivia, Brazil, Colombia, Peru and Venezuela. From 1951 to 1953, there was a rather severe recrudescence in southern Brazil. In 1954, 15 cases, three of which were fatal, occurred in Trinity and Tobago, where yellow fever had not been reported since Twelve strains were Isolated from Haemagogus, but three cases were apparently transmitted by Aedes aegypti. The notification at PortofSpain of the infection Tho issue of this document does not constitute formal publication. It should not be reviewed, abstracted or quoted without the agreement of the World Health Organization. Authors alone are responsible for views expressed in signed articles. Ce document ne constitue pas une publication. Il ne doit faire l'objet d'aucun compte rendu ou résumé ni d'aucune citation sans l'autorisation de l'organisation Mondiale de la Santé. Les opinions exprimées dans les articles signés n'engagent que leurs auteurs.

2 VIR/YF/71.11 page 2 transmitted by Aedes aegypti broke a 25 years' silence during which no case had been notified in any port of the Americas. In 1950, the virus crossed the Panama Canal and an epidemic wave of jungle yellow fever progressed towards the north, passing through Costa Rica, Nicaragua and Honduras and finally stopping in Guatemala in 1957, near the Mexican frontier. The high mortality rate among the monkey species Alouatta and Ateles, which are very susceptible to the virus, made it possible to observe the advance of the epizootic front through the forests. But human cases were also observed in regions near Mexico where there are no monkeys, and the possible implication of other mammals such as marsupials or rodents was considered. Brief epidemic outbreaks occurred in Ecuador in 1951 and in northern Argentina in 1966, In addition, two cases were recorded in Guyana in 1961 and one case in Surinam in 1968 and In Africa The situation is one in which regions where the appearance of sporadic cases, very probably of jungle yellow fever, indicate that the disease exists in an endemic state contrast with other regions where sudden epidemic outbreaks with interhuman transmission alternate with periods of quiescence. The zone of endemic prevalence coincides with the zone of the great forests, and the epidemics occur in the savannah regions. We can attribute to endemicity the cases which have been observed from time to time during the past two decades in Ghana, Liberia, Nigeria, Portuguese Guinea, Sierra Leone, Togo and Uganda, In the Democratic Republic of the Congo, against an annual background of endemicity, there occurred in 1958 an epidemic outbreak of jungle yellow fever in the north, where forest merges into savannah (Courtois, 1960; Lebrun, 1963), The epidemic which occurred in Sudan in 1959 in the Upper Nile region was the second to be experienced by the country after the one in the Nuba mountains in 1940, and in both cases it is thought that the transmission was interhuman with Aedes aegypti and other species such as Aedes vittatus acting as vectors (Satti, 1966). Yellow fever was unknown in Ethiopia until the epidemic of 1960 to 1962 which allegedly affected over persons and caused over deaths, which represents the most severe incidence in all of Africa (Sérié, 1968). Depending on the regions and their ecological characteristics, it has been shown that the transmission of the virus was of the jungleyellowfever type with Aedes africanus or interhuman with Aedes simpsoni. A secondary focus was discovered in In Senegal, in 1965, an epidemic with interhuman transmission by Aedes aegypti mainly affected children who had not yet been vaccinated, the adults being protected by previous vaccinations (Brès, 1966). In 1969, epidemics broke out simultaneously in Ghana, Upper Volta and Mali, and also in Nigeria, probably corresponding to two separate active foci. Except in the case of Upper Volta, the epidemiological information is incomplete and, in particular, the number of cases officially reported is certainly less than the true number. The epidemics occurred in savannah regions where the interhuman transmission was effected by Aedes aegypti and by other vectors which play a role whose importance has not been clearly determined. General remarks Both in the Americas and in Africa it has clearly become necessary to resume investigations in order to elucidate the unknown epidemiological factors with which we have been confronted during these recent epidemics: the cycles which ensure the permanent presence of the virus in the enzootic and endemic foci, the factors which govern the exposure of the rural populations to the jungle virus and which determine either the establishment of natural immunity in a proportion of the population or the onset of an epidemic, and lastly, the interaction of other arboviruses which may operate in the reservoirs, the vectors and the receptive subjects, not to mention other factors of whose importance we are s t i l l unaware. The most important objective Is perhaps to determine the position in regard to the risk of propagation of yellow fever in the receptive zone, where large population masses would then be at risk, as in India and other parts of Asia. Vector control can make a major contribution to stopping the propagation of the virus in case of epidemics. Unforeseen difficulties can, however, arise and reduce Its effectiveness

3 VIR/YF/71.11 page 3 in regions where the ecology of the vector species has not been studied. Permanent control of the vectors is also fraught with difficulties. In the Americas, the eradication of Aedes aegypti has proved more difficult to attain or maintain than had been thought. In Africa, potential vectors whose role has not been clearly determined would certainly limit the effectiveness of eradication efforts directed against Aedes aegypti. The 17D vaccine has proved its remarkable effectiveness for protection against yellow fever, but its utilization in mass campaigns in the tropical regions is s t i l l attended by difficulties whose economic implications restrict its use in the developing countries. Mass vaccinations have been carried out in the Americas and in Africa during the actual course of epidemics and with operations limited to the focus and its surroundings. The success of such a policy depends on the vigilance exercised by a fairly elaborate surveillance system. As an alternative, routine vaccination of all populations at risk is not impossible to achieve, and the epidemics in Senegal and Upper Volta have shown that it is effective, but the cost of the campaigns is an obstacle temporary, one may hope to its application. Such appears to be the status of the yellow fever problem in It will be noted that it is not much different from what it was in 1950, and the assessment Soper made of it in 1954 is still fully applicable to present conditions: "Yellow fever is not a disease which has been conquered. It is not a disease which has been eliminated from consideration as a permanent threat. Too many health authorities get alarmed about yellow fever only when it becomes an urban disease, overlooking entirely the fact that, for the jungle populations and for rural workers who have to go into the forest, yellow fever carries the same threat that it previously had for the people in the cities." REFERENCES 1. Brès, P. et al. (1967) In Chambon, L. et al. Bull. Wld Hlth Org., 36, Courtois, Ch. et al. (1960) Ann. Soc, belge Méd. trop., 40, Lebrun, A. J, (1963) Amer. J. trop. Med. Hyg., 12, Satti, M. H, et al. (1966) J. trop. Med. Hyg., 69, Sérié, C. et al. (1968) Bull. Wld Hlth Org., 38, Soper, F. L. (1955) Amer. J. trop. Med., 4, Soper, F. L. (1967) Bull. Wld Hlth Org., 36, Strode, G, K. (1951) Yellow fever, ed, McGrawHill, New York

4 TABLE 1 Yellow fever cases notified during the period 195C1959 " ^ 1^ T) TABLEAU 1 Nombre de cas de la fièvre Jaune notifiés au cours de la période o < B H, oq 50 Country Pays I j> AFRICA AFRIQUE Central African Republic C 1 République centrafricaine D 1 Congo, Democr. Rep. of C Congo, Rép. dém. du D Ethiopia 10 Ethiopie D 6 Ghana C _ D " 2 Guinea C 1 Guinée D 1 Liberia C 5 Libéria D 3 Mali C D Nigeria C A Nigeria D io Portuguese Guinea C 6 Guinée portugaise D D Senegal C 2 ' A Sénégal D 2 2lD Sierra Leone C D Sudan C _ J12O Soudïm D 88 Togo C D Uganda C 1 1 Ouganda D 1 1

5 Country Pays i AFRICA AFRIQUE Upper Volta C ^ 5 Haute Volta D AMERICA AMERIQUE Argentina B C Argentlne D _ 2 l 6 l Bolivia Bolivie B C^27 S 1 I8 H 6 I I Brazil B C ^4 ^ 5C ^220 ^39 ^ ^10 ^ ^14 ^ Brésil D , Colombia B C*12 *24 * : ^2 ^ Colombie D, ^6 ^ *9 *2 ' Costa Rica BC D, ' Ecuador BC 42 Equateur D * 9 ^1 ^2 ^3., Guyana B C 2 Guyane D 2 Honduras BD_ Nicaragua B C 7 8 D 7 8 Panama B C l 4. _ Peru Pérou B C I

6 Country Pays ^ 1959 I I l9o2 196? D 19D7 I9bt 19^9 3^ AMERICA AMERIQUE Trinidad and Tobago B C 15 Trinité et Tobago Venezuela B C I 2 ^ 5 ^ 5 D B Jungle yellow fever C Cases notified to health authorities D Deaths registered to health authorities D Deaths registered by the Central Statistical Services on the basis of death certificates * Preliminary, approximate or estimated data.. Data not available + Data not yet available Nil or magnitude negligible 8 Suspected case 1 Confirmed cases 2 Estimated data 3 Including 199 suspected cases and 172 suspected deaths 1* Including one suspected case 5 Deaths medically certified 6 Confirmed cases only 7 Data for the period from 1 July 1959 to 30 June 196O. B Fièvre Jaune de brousse C Cas déclarés aux autorités sanitaires D Décès enregistrés aux autorités sanitaires D_ Décès enregistrés par le Service central de statistique d'après les certificats de décès ' Donnée préliminaire, approximative ou estiiriatlve.. Donnée non disponible + Donnée non encore dispor.ible Zéro ou quantité négligeable s Cas suspect 1 _ Cas confirmés 2 Données estimées 5 Y compris 199 cas et 172 décès suspects 4 Y compris 1 cas suspect 5 Décès certifiés médicalement 6 Cas confirmés seulement 7 Données pour la période du 1 juillet 1959 au 30 juin I90O.