I^û^^^ WORLD HEALTH ORGANIZATION VIR/RC/74.36 (Arbo)

^^S^ ORGANISATION MONDIALE DE LA SANTÉ ENGLISH ONLY

MEETING OF DIRECTORS OF WHO REFERENCE CENTRES INDEXED FOR ARBOVIRUSES, CHLAMDYDIAE AND RICKETTSIAE

Geneva, 1-5 April 1974

j>y^\ HEO '/^'. YELLOW FEVER IN THE AMERICAS

by 1 2 7 Thomas P. Monath, M.D. Chief, Vector-Borne Diseases Division Center for Disease Control Ft. Collins, Colorado

Efforts to eradicate Aedes aegypti during the first 30 years of this century resulted in the virtual disappearance of urban epidemics of yellow fever but uncovered the underlying sylvan transmission cycle. Since 1930, only 14 epidemics of urban yellow fever have occurred; each followed introduction of the virus from a juxtaposed jungle focus, and none resulted in a continuing endemic presence. The last outbreak was recorded in Port of Spain, Trinidad in 1954.

Since 1930, the annual incidence of human cases of jungle yellow fever has been low and fairly constant. Periodically an increase in virus activity is manifested by cases clustered in time and space within the endemic zone. A few epidemics have broached the boundaries of the endemic wet forests of South America, extending into Argentina to the south or into Central America to the north.

Yellow fever virus is endemic in Brazil, , Peru, and Colombia, in forested and sparsely-populated areas under limited cultivation, drained by tributaries of the Amazon and Orinoco Rivers. The incidence of cases in the last five years, reflecting only the activity of yellow fever in its sylvan cycle, is shown in Table 1.

Brazil. Yellow fever cases are sporadically recognized in the states of Para, Amazonas, Acre, Mato Grosso, Goias, and in the territories of Roraima and Rondoma. No discreet out­ breaks occurred from 1969 to December, 1972 when six cases were confirmed in four municipalities of south-central Goias State. Another 31 cases were recorded from this region in 1973, the last in the month of April.•'• At least 30 counties of Goias State were affected;2 scattered cases were also recognized in contiguous areas of Minais Gérais. A. aegypti was reportedly absent from the area. The outbreak occurred very near the city of Brasilia.

Bolivia. Yellow fever is recognized annually in an area extending westward from the low- lying Amazonian forests of eastern Bolivia to the Cordillera Oriental. The departments of Santa Cruz, Cochabamba, and are generally affected. As in Brazil, the incidence of cases was low from 1969 to 1972. In January, 1972 a minor outbreak (eight confirmed cases) occurred in Province in eastern La Paz Department. In December, einother small epidemic began in north-eastern Cochabamba and ended in March, 1973; 24 cases were recognized. Another 30 cases were officially reported in eastern La Paz Department in May, 1973.

The issue of this document does not constitute Ce document ne constitue pas une publication. formal publication. It should not be reviewed, Il ne doit faire l'objet d'aucun compte rendu ou abstracted or quoted without the agreement of résumé ni d'aucune citation sans l'autorisation de the World Health Organization. Authors alone l'Organisation Mondiale de la Santé. Les opinions are responsible for views expressed in signed exprimées dans les articles signés n'engagent articles. que leurs auteurs. VIR/RC/74.36 (Arbo) page 2

Peru. From December, 1969 to April, 1970 an outbreak of jungle yellow fever occurred in Junin Department, with a total of 79 cases recorded. The affected region lies in the endemic zone of Central Peru, bounded to the west by the Andes and to the east by Amazonas and Acre states of Brazil. No further cases were recognized in 1971, but in i;)72 and 1973, increased virus activity was reflected by a total of 39 recorded cases from Huanaco and Puno departmen-fcs^

Colombia. A steady dribble of cases are recognized annually. Although in the past, yellow fever has frequently appeared in the Magdalena River valley, in recent years the llanos of eastern Colombia drained by tributaries of the Orinoco and Amazon rivers have been affected.

Venezuela. In May, 1972 yellow fever was recognized for the first time since 1966. total of 22 cases were reported from Lara, Apure, Barinas and Portuguesa states in the south­ western part of the country. This area, drained by the Orinoco river, is contiguous with endemic regions of Colombia. A. aegypti was either absent or present at a very low level. 3

Other areas. Yellow fever has not been reported from Ecuador since 1967. Prior to 1969, scattered cases were recognized annually along the borders of Venezuela and Guyana; this region has subsequently remained silent. Sporadic cases have been confirmed in Surinam in 1969 and 1972. i

Panama has periodically experienced epizootics of yellow fever in Darien province, manifested by illness and deaths among wild howler monkeys and by an increased prevalence of yellow fever antibodies in spider monkeys."* The most recent virus activity probably occurred near the Colombian border during the rainy season of 1970. Studies conducted in 1973 showed no evidence for westward movement of the virus toward populated areas of Panama recently discovered to be re-infested by A. aegypti.^

Surveillance of yellow fever

Yellow fever is maintained in nature by "wandering epizootics" in which the virus slowly passes between territorial bands of monkeys which are decimated or immunized and subsequently repleted with susceptibles. Because the endemic areas have few medical services and are distant from diagnostic laboratories, surveillance of human cases is at best difficult, and the number of reported cases vastly underestimated.

At present surveillance depends upon the histopathologic diagnosis of viscerotomy specimens collected from fatal cases in rural areas. The governments of Brazil and Colombia in collaboration with WHO, maintain two reference centres for the pathologic diagnosis of ' yellow fever. Interestingly, although there has been a decline in emphasis of viscerotomy services since the 1930's and 1940's, the incidence of reported cases in the last decade approximates that of the viscerotomy era.

One may reasonably question the propriety, in 1974, of such a passive, loosely- constructed surveillance system. Improved surveillance is necessary for the following reasons :

1. The true morbidity of jungle yellow fever has never been accurately defined in the endemic zones. What fraction of the whole "Iceberg" is represented by viscerotomy monitoring? The priorities of surveillance activities and vaccination campaigns would undoubtedly be changed by information of this sort. The selection of areas for intensified surveillance is relatively simple. In the Ucayali-Apurimac valleys of central Peru, for example, sporadic and clustered cases are recorded annually.

2. Amplification of virus activity should be early detected near densely populated areas infested with A. aegypti. There is universal agreement^-'' that the present confinement of yellow fever to its sylvan cycle represents an unstable condition. Recent epidemics of VIR/RC/74.36 (Arbo) page 3 dengue in the Caribbean and northern South America have underscored historical fears of potential urban yellow fever outbreaks. The risks are easily assessed in terms of the reported presence of the urban vector, A. aegypti.

In Surinam, for example, where A. aegypti indices are high, yellow fever has been reported in 1969 and 1972. In Colombia, dengue spread inland in 1971 from Barranquilla on the coast towards Medellin, affecting approximately 416 000 persons. A. aegypti populations had become re-established in this region following curtailment of eradication efforts in 1969, This area is contiguous with the Magdalena Valley, where yellow fever has been periodically active and may be endemic,

3, It is necessary to monitor natural corridors through which yellow fever may spread from its customary bounds. Examples of such corridors are the Darien Province of eastern Panama and the Misiones Province of Argentina.

What epidemiological methods can be adapted to effect intensified surveillance of yellow fever? Although admittedly difficult to apply, techniques are available for the diagnosis of non-fatal as well as fatal cases. These include the collection of paired serological samples and of acute bloods for virus isolation attempts. Preservation and shipment of the latter is always a problem. Liquid nitrogen can be used; alternatively, Dr Downs has suggested the use of tube cultures of mosquito cell cultures, which can be maintained at ambient temperatures for prolonged periods in the field. Serological surveys, both pros­ pective and retrospective are useful for defining infection rates in a given area. On the basis of age-specific antibody rates, populations repeatedly infected can be defined. The results of serological surveys are more easily interpreted than in West Africa, since the provalence of antibodies to other group B arboviruses is relatively low.

Studies of monkey populations and of vector mosquitos provides ecological information of predictive value. In endemic areas, examination of incidence tables clearly demonstrates that virus activity is high in certain years, low in others. Can a build-up of activity be detected early? Does the virus "wander" down predictable pathways, river valley corridors?

Surveillance of the kind suggested requires collaborative efforts of health ministries and laboratories or institutes versed in the techniques of arbovirus research. There are fewer such institutes, fewer arbovirologists, and fewer funds than in former years. Limited programmes with specific goals might nevertheless be supported by concerned govern­ ments and PAHO/WHO.

Yellow fever control. The prevention of urban yellow fever outbreaks is one of the agreed goals of the A. aegypti eradication programme. In 1947 and again in 1972 the Directing Council of PAHO resolved to pursue eradication in areas of the hemisphere infested and re- infested by A. aegypti.

Jungle yellow fever can be prevented only by vaccination. Large scale vaccination campaigns have been conducted during and following outbreaks of the disease, for example, in Goias State in 1972. Efforts are generally made to vaccinate immigrants, labourers, and tourists entering endemic regions. Several limited permanent vaccination programmes are also in effect. Vaccination of all susceptibles is not, however, an accepted or organized goal in most of South America. VIR/RC/74.36 (Arbo) page 4

REFERENCES

1. PAHO Weekly Epidemiological Report (1973) XLV(45), 263

2. F. Pinheiro (unpublished)

3. WHO Weekly Epidemiological Record (1973) No. 35, p. 345

4. Galindo, P. &. Srihongse, S. (1967) Evidence of recent jungle yellow fever in eastern

Panama, Bull. Wld Hlth Org., 36, 151-161

5. Gorgas Memorial Laboratory and Middle America Research Unit (unpublished information)

6. Reeves, W. C, (1972) Recrudescence of arthropod-borne virus diseases in the Americas, PAHO Sci. Pub., 238, pp. 3-14 7. Downs, W. G. (1970) The problem of yellow fever in the Americas. In: International Conference on the Application of Vaccines against Viral, Rickettsial, and Bacterial Diseases of Man, PAHO/WHO, pp. 29-30 ~

8. PAHO Weekly Epidemiological Report, (1972) XLIV, 62

TABLE 1. ANNUAL INCIDENCE OF YELLOW FEVER CASES

1969 1970 1971 1972 1973* Total

Argentina - 2 - - - 2 Bolivia 8 2 8 9 75 102 Brazil 4 2 11 12 40 69

Colombia 7 7 9 3 4 30 Ecuador ------Guyana ------

Peru 28 75 - 7 32 142

Surinam 1 - - 1 - 2

Venezuela - - - 22 3 25

Total 48 88 28 54 154 372

* January-September.