WORLD HEALTH ORGANIZATION VIR/VHF/EC/84.6

ORGANISATION MONDIALE DE LA SANTE ENGLISH ONLY

INDE/ EXPERT COMMITTEE ON VIRAL HAEMORRHAGIC

Geneva, 19-23 March 1984

Draft agenda item 3.1

EPIDEMIOLOGICAL, CLINICAL AND PHYSIOPATHOLOGICAL DESCRIPTION OF IN AFRICA

by ' f^-- c

Dr P. Brès Institut Pasteur, Paris

Although the first epidemic which could reasonably be attributed to yellow fever was described in Senegal in 1778, it is certain that this disease was well established in the African continent before that time, and was particularly feared by European sailors, troops and traders. Several large outbreaks periodically decimated the expatriate populations on the West Coast, but the disease was well known to occur in the Africans as well. In 1972, during one of the periodical outbreaks of the disease, the was isolated simultaneously by the Rockefeller Foundation Mission at Accra, Ghana, from the blood of an African patient named Asibi, and by workers at the Pasteur Institute, Dakar, Senegal, from the blood of a Lebanese, by inoculation of the susceptible Indian rhesus monkey. The strain from the Pasteur Institute was further adapted to mouse brain by M. Theiler and became known as the French Neurotropic Virus. Apart from a different degree in neurotropism, both strains are identical but they show a slight antigenic difference compared with the South-American strains and with the 17D vaccine substrain derived from the Asibi strain and which was not passaged in mouse brain. The molecular structure of yellow fever virus has recently been studied ana found somewhat similar to that of other of the Flavivirus genus of the Togaviridae family.

In spite of similarities with the American virus, the epidemiology of yellow fever is somewhat different in Africa. The clinical pattern of classical severe forms of the disease has not changed since the first descriptions by Dutrouleau in 1858, but mild cases have been more frequently recognized since 1930 owing to the progress in laboratory diagnostic methods. However, there has been little opportunity in Africa recently to add to the classical descriptions and improve the knowledge available ot the physiopathology of the disease.

The issue of this document does not constitute Ce document ne constitue pas une publication. formal publication. It should not b« 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 que articles. leurs auteurs. VIR/VHF/EC/84.6 Page 2

1. EPIDEMIOLOGY

1.1 Geographical Distribution

The endemic, zone for yellow fever defined by the International Health Regulations lies between the 15th parallel North and the 10th South and includes totally or partially 28 countries. This zone has been delimited on the basis of neutralization tests in a serological survey carried out in these countries in the 193ÛS under the Rockefeller Foundation auspices.

1.2 Virus Transmission Chains

The first data on the virus transmission chains in Africa were obtained by the Rockefeller Mission in Bwamba County, Uganda, in the 1930s. It was proved that the virus was transmitted between monkeys in the forest canopy by the Aedes africanus mosquito (sylvatic yellow fever). Monkeys infected in the forest may raid banana plantations in nearby villages and are bitten by Aedes simpsoni mosquitoes which breed in plant axils and afterwards can bite man (jungle yellow fever). In villages, the virus can be transmitted from person to person by Aedes aegypti; if present, or by Aedes simpsoni (so called "urban" yellow fever).

This pattern, typical of East Africa, was observed in Ethiopia during the large epidemic of 1960-1962 which caused about 100,000 cases, of which 30,000 died out of a population of one million inhabitants. The attack rate was lower in the Didessa valley, where man was infected by Aedes africanus on entering the forest, than in the Omo valley where man was infected by Aedes simpsoni in villages.

A slightly different transmission pattern has been found recently in West Africa involving other mosquito species (table 1). Although Aedes simpsoni is also present in Africa west of the Cameroons, it does not bite man.

Studies carried out by the Institut Pasteur and the Oftice de la Recherche scientifique et technique Outre-mer have shown the same enzootic basic maintenance cycle of monkey-to-monkey transmission in the western strip of the Congolese Guinean rain forest by Aedes africanus mosquitoes. This causes exceptionally a few sporadic human cases (sylvatic yellow fever).

From the forest cycle, the virus can irradiate periodically along riverine forests which penetrate into the surrounding savanna zone and is transmitted by different vector mosquito species according to the type of savanna, humid (Guinean) or dry (Sundanese): this is the "emergency zone" (table 1). Sporadic human cases reflect an enzootic situation, or circumscribed outbreaks reflect an epizootic situation in monkeys. Man-to-man transmission may also be caused by these mosquitoes, which play the same role as Aedes simpsoni in the East (jungle yellow fever). It has been shown that the density of mosquitoes in riverine forests increases during the second half of the rainy season, first for Aedes luteocephalus and soon after Aedes furciier and Aedes taylori. The proportion of infected mosquitoes also increases rapidly. Male mosquitoes have been found naturally infected with yellow fever virus which proves the "vertical" (transovarial) transmission through eggs which persist VIR/VHF/EC/8A.6 Page 3

between two rainy seasons. Although this mechanism may transmit the virus through the dry season only to a low percentage of the Fl generation, it may perpetuate the virus locally for two or three years if the virus circulation is enhanced each year by of non-immune monkeys which can be a source of virus to infect many more mosquitoes. R.M. Taylor stated in 1951 that "from the aspect of the time element and the reservoir of the virus, the mosquito qualities better as a host than does the vertebrate, for it is in the mosquito that the virus finds a more permanent abode" (1). In order to determine the risk for nearby villages, the enzootic or epizootic activity of the virus in the riverine forest can be monitored by capture of mosquitoes to find the presence of vector species and monkeys to find serological evidence of recent infection.

At long intervals, the virus may progress further during the rainy season towards villages in the semi-arid zone where Aedes aegypti colonizes in domestic breeding sites, or in towns, where most of the human population no longer has any immunity since the last epidemic or vaccination campaign, and this may cause severe and extended epidemics of urban yellow fever. The virus may be transported from the jungle cycle to distant places by infected mosquitoes or patients.

Incidence in the Population

There is no ethnic difference in susceptibility to . Maternal immunity, if strong enough, may protect a child until about six months of age. It seems that repeated infections with certain other flaviviruses closely related antigenically to yellow fever virus such as Zika, Wesselsbron, Uganda S, might afford some degree of cross-protection by the age of 15 years. Experimentally, the viraemia after challenge with yellow fever virus is lower if the monkey has been immunized with dengue, Zika and Wesselsbron virus. However, whether dengue would result in cross-protection of man against yellow fever is not certain, knowing that yellow fever vaccination does not protect against dengue.

During urban outbreaks of yellow fever in the past, it was observed that the epidemic "burned itself out" when about 60-80% of the population had been infected. These figures can be taken as targets for vaccination coverage.

Epidemics which occurred in Africa during the period 1965-1983 are indicated in table 2. The numbers of notified cases are frequently under-estimated. By comparison with the regular pattern of yearly occurrence of yellow fever in the Americas it may be seen that outbreaks occur at less regular intervals in Africa.

Sporadic cases of jungle yellow fever are rarely detected in Africa because of the scarcity of entomological surveys and lack of laboratory capabilities for virus isolation. Even outbreaks reaching some importance may also not be reported to health authorities. Situations have been documented where an important epizootic activity was going on in riverine forests and sero conversions occurred in the child population or nearby villages without any evidence of clinical outbreaks. This may be interpreted as a low virulence of the virus in the sylvatic cycle, whereas it can be enhanced after some man-to-man transmission cycles. VIR/VHF/EC/84.6 Page 4

2. CLINICAL FEATURES

Classical severe torms of yellow fever can easily be recognized by their clinical features. The incubation period is 3-6 days after the bite of an infected mosquito. The onset is abrupt with lumbosacral and generalized muscular pains, intense headache, conjunctival infection, flushed face, and . Faget's sign (slow pulse in relation to the fever) is typical. After a short remission of fever the toxic and haemorrhagic conditions develop; epistaxis, gum bleeding, black, spots (as flies) in vomiting which announce the massive black vomit of digested blood, the ascending albuminuria (a good sign but sometimes of late appearance and not constantly found), uremia, the deterioration of central nervous system functions, coma and terminal shock syndrome. is in general moderate and may even not be visible before death. The case fatality ratio varies between 20-80^. of hospitalized cases.

In Ethiopia, the majority of cases during the 1959 epidemic showed only of the central nervous system (2), similar to cases which were observed during the Nuba mountain epidemic in Sudan in 1940. In Ethiopia again, during the 1960-1962 epidemic, fulminant forms without any hepatic or renal signs, and death occurring in two to three days were observed in a certain region, whereas only the classical form was seen in others (3).

Mild cases are more frequent than was formerly thought and even subclinical infections are not rare. Both can be confirmed by an increase of IgG between an early and a late blood specimen or by early detection of specific IgM.

Whereas there is no great difficulty during an epidemic, sporadic cases may be confused with or influenza when they are mild, or with other haemorrhagic when they are severe. Ebola virus infections were confirmed serologically during the yellow fever epidemic in the Gambia in 1978 and there has been a possibility that Orungo virus could cause haemorrhagic signs during a yellow fever outbreak. Malaria is perhaps the most frequent cause of error for mila forms and viral hepatitis for icteric forms. Erroneous diagnosis of infectious hepatits can be suspected when lethality in the outbreak of jaundice, which occurs usually at the beginning of the rainy season, (as does yellow fever) exceeds 1%. However, this late differential diagnosis provides ample time for the yellow fever virus to spread and produce many more cases.

3, PHYSIOPATHOLOGICAL FEATURES

There is no specific feature differentiating yellow fever in Africa from the same disease in the Americas. The pathological characteristics of the disease have been known since the description by Councilman (1890) of the lesions and the isolation of the virus in 1927 enabled their origin to be ascertained by further studies in rhesus monkeys. Practically nothing could be added since. In contrast, little is known on the diathesis aspect of the disease and its immunopathology.

3,1 Pathological Characteristics of Yellow Fever

Lesions of the liver mainly attract attention because of their diagnostic significance, but lesions which are also conspicuous in other organs tend to be forgotten and could be of interest for the further understanding of the physiopathological process. They are summarized hereunder from J.G. Bugher (4). VIR/VHF/EC/84.6 Page 5

As a constant characteristic, there is a mild, exceptionally intense, icteric tinting of organs and no inflammatory response. Nothing is in favour of a necrotizing activity of the virus.

(a) General appearance

- The general appearance is unimpressive and most of the time the icterus is mild. Submucosal and subepithelial petechial haemorrhages are common and the gums and mucous membrane of the mouth show persistent haemorrhage. Subconjunctival haemorrhages may be found.

Organs show little gross change exept an icteric tinct which is generally light, but microscopic changes are important.

(b) Liver

There is little change in the volume and aspect.

- Microscopic lesions are characteristic but hepatic failure may not be the primary cause of death.

A diffuse disorganization of the parenchyma is most marked in the midzone of the lobule: discontinuous coagulative necrosis in the cords of liver cells, beginning by turgescence of the cell, formation of small and large droplets of fat within the nucleus and cytoplasm, followed by migration of chromatin to the nuclear membrane, cytoplasmatic eosinophilic coagulation resulting in a hyaline mass forming the Councilman body from which the chromatin will disappear. A collar of surviving cells around the central vein of the lobule is the difference with other forms of hepatitis.

Haemorrhage is not characteristic of the liver lesion.

Kupfer cells are increased in number, enlarged, contain small amounts ot bile and iron pigments. Nucleus pyknosis and karyorrhexis are seen in fulminant forms of the disease.

Depletion of glycogen has been demonstrated in the rhesus monkey.

In average, 60% of the lobule is affected (S-lOO/i) with sometimes marked difference between the left and right lobes.

There is an outstanding absence of significant inflammatory reaction.

Torres intranuclear eosinophilic bodies have been described in the nuclei of the surviving liver cells, somewhat similar to the of herpes virus infection, but they are less frequent in humans than in the rhesus monkey. Nothing is known ot their virus content. When death is delayed, Councilman bodies cannot be found in most specimens. Ocher-coloured bodies (Villela bodies ) are visible in the midzone which represent an intermediate stage in the complete disintegration of the Councilman bodies. There is no inflammatory reaction to the necrotic cells.

When the patient survives, regeneration of liver cells is rapid and complete by the end of convalescence without any proliferation of stroma.

Intestinal Tract

- An impressive quantity of partially digested blood is found within the cavity of the stomach.

The mucosa shows a large number of petechial haemorrhages, surrounded by polymorphonuclear leukocytes and lymphocytes.

Some haemorrhage may be found in the duodenum, but none occurs in the small intestine.

- There is a proliferation of large round cells with a diminution of mature lymphocytes in the lymphoid tissues of the intestinal tract.

Urinary tract

The kidney function is profoundly affected and plays an important part in causing death. Albuminuria usually makes its appearance soon after the onset of fever and rapidly increases in intensity. Anuria is one of the signs of gravest pronossis. Uremia may cause part of the observed cerebral symptoms.

The tubular epithelium shows more severe changes than the glomeruli, especially in the convoluted tubules where a discontinuous degenerative fatty infiltration and necrosis process is generally observed.

The lumina of most tubules are filled with granular debris derived from the epithelial cell necrosis and calcareous masses are frequently encountered.

Renal congestion is part of the general manifestation of the terminal vascular collapse.

As in the liver, there is a striking lack of stromal reaction, no subsequent fibrosis and the kidney may return to a normal histologic appearance during recovery.

Submucosal haemorrhages are found in the bladder which contains a small amount of blood-stained urine. VIR/VHF/EC/84.6 Page 7

(f) Brain

Yellow fever is characterized by the relative mental clarity of the patient even though he may actually be near or at the point of death.

Small perivascular haemorrhages are consistently seen in spaces around small arteries, veins and around capillaries and the involved vessels show cuffs of red cells, with moderate oedema and no significant cellular infiltration.

Although the small haemorrhages are widespread, they are present in concentration in the mammillary bodies and the optic thalamus.

The brain involvement is related to the general haemorrhagic tendancy and there is no true encephalitis, but the perivascular haemorrhage alone may cause death.

(g) Spleen

The early lesions are characterized by the appearance of large mononuclear cells in and about the follicle and progressive diminution in the number of lymphocytes (a constant lymphocytopenia is encountered in yellow fever).

They are followed by a diminution of mononuclears and marked reticuloendothelial hyperplasia.

(h) Pancreas

There is considerable bile staining of the organ and pacchy haemorrage.

(i) Cardiovascular tract

There is a cloudy swelling and degenerative infiltration of the heart muscle fibers, most severe beneath the endocardium. The nuclei often show hydropic changes leading to actual necrosis with karyolysis and vacuolization of the cytoplasm.

These lesions occur also in the sinoauriculo node, the bundle of Hiss, and the auriculo-ventricular bundle, which are in accordance with the observed bradycardia and the alterations in the electrocardiogram.

The involvement of the conduction system explains the bradycardia during convalescence, myocardial insufficiency and sudden death occurring during exertion.

Degenerative changes of the major blood vessels have been described with no mention of capillaries.

The lung, as well as other organs, is congested to a degree dependant upon the extent of the cardiac damage. VIR/VHF/EC/84.6 Page 8

(J) Summary of Lesions

- In whatever organ it is present, the yellow fever lesion consists in a selective necrobiosis that attacks only highly specialized epithelial or myocardial cells. The changes are fundamentally toxic in character, the stroma cells are not involved and there is no inflammatory response to necrosis. The most significantly involved organs are the liver and kidneys. During convalescence the necrotic material is slowly absorbed and there is a complete replacement of the lost tissue by direct regeneration.

(k) Further findings in Africa

Out of 60 liver specimens taken post-mortem from children who died during the 1965 outbreak of yellow fever in Senegal and which were considered as suspect, 14 (23%) had a jaundice caused by other agents and malaria was associated with yellow fever in 80% of children (5). Any physiopathological study of yellow fever should first look at the purity of the case. Similar findings were made during the 1969 epidemic in Upper Volta C6). During the 1969 epidemic in Nigeria, it was found that as the disease progresses towards convalescence the histological features begin to resemble those of a nonspecific hepatitis (7).

3.2 Evidence for Disseminated intravascular Coagulation

Today, little evidence of this phenomenon has been obtained in human yellow fever (8). The occurrence of antigen-antibody (IgM) complexes around the fifth-sixth day alter onset has been shown in man and monkey. The monkey might be a model for study. In one study, either the initial antibody response was correlated with rapid clearance of viraemia, the limitation of transaminase elevation and the limitation of C'3 depression, or initial antibodies did not get rid of viraemia and these parameters aggravated until death. VIR/VHF/EC/84.6 Page 9

References

Taylor, R.M. Epidemiology in Yellow fever, G.K. Strode, McGraw-Hill New York. 426-538, 1951

Berdonneau, R. et al. Sur l'épidémie de fiève jaune de l'année 1959 en Ethiopie (frontière Soudano-Ethiopienne). Bulletin de la Société de Pathologie extotique. 5^, 276-283, 1963.

Sérié, C. et al. Etudes sur la fiève jaune en Ethiopie Introduction. Symptomatologie clinique amarile. Bulletin of the World Health Organization. 38, 835-841, 1968.

Bugher, J.C.; The of yellow fever. Yellow Fever, McGraw-Hill, New York. 137-163, 1951.

Group Study - Une épidémie de fiève jaune au Sénégal en J965. Bulletin of the World Health Organization. 36, 113-150, 19b/.

Ricossé. J.-H. et al. Le diagnostic anatome - pathologique de la fiève jaune. Annales d'Anatomie Pathologique. IT.' 21-3», l^'^

Francis T. I. et al. A clinicopathological study of ^u^^" yello«/"«ver. Bulletin of the World Health Organization. 46, b59-t)b/, iv//

8. Johnson, K. Personal communication T3 < 0) M Table 1. Epidemiological Features of Yellow Fever in West Africa CWest ot Cameroons) < O ^ PI n MOSQUITO VECTORS TRANSMISSION EPIDEMIOLOGICAL MODE OF TYPE OF ECOLOGICAL ZONE 00 CYCLES ZONE OCCURRENCE YELLOW FEVER

Epidemic Urban yellow fever Wooded steppe (Sahel) Ae. aegypti man-to-man Invasion zone (may occur in vil lages)

Undifferentiated (Sudan Ae. aegypti man-to-man savanna of dry savanna) type

( Different iated (Northern Ae. furcifer ( savanna Guinea Ae. taylori monkey-to-man savanna) Ae. luteocephc Endemic ( CU Emergency (sporadic u ( or circum­ Jungle yellow fever o Undifferentiated (Southern Ae. africanus zone !^ (may occur savanna of Guinea * man-to-man scribed Q) outbreaks in villages) C moist type savanna) Ae. opok •i-l u during 0) epizootics) > Forest (Derived Ae. africanus OS savanna savanna) (Ae. simpsoni mosaic East Africa)

Moist (rain Ae. africanus monkey-to-man Maintenance Sporadic Sylvatic yellow Forest forest) zone (enzootic) fever

Note; Aedes Aegypti transmitted yellow fever may occur in any urbanized area where this vector is present, whatever the geographical location. Table 2. Yellow Fever in Africa and America during the period 1965 - 1983

1965 1966 1967 1968 1969 1970 1971 1972 1973 1974 1975 1976 1977 1978 1979 1980 1981 1982 1983 1984

Angola 65 Equatorial Guinea Guinée équatoriale 4 Ethiopia - Ethiopie .... 350 Gambia - Gambie 30 Ghana 5 12 3 5 5 1 2 2 110 219 494 8 4 6 315 Ivory Coast - C6te d'ivoire 25 Liberia - Libéria 5 21 Nigeria - Nigéria 208 4 2 25 11 1 Senegal - Sénégal 243 3 3 Sierra Leone 130 Tongo I 2 United Rep. of Cameroon Rép.-Unie du Cameroun ... 1 21121 7 Upper Volta - Haute-Volta . 87 b 356 Zaire - Zaïre 2

TOTAL 243 350 5 322 23 70 7 8 27 134 3 110 249 508 16 7 31 671

1965 1966 1967 1968 1969 1970 1971 1972 1973 1974 1975 1976 1977 1978 1979 1980 1981 1982 1983 1984

Argentina - Argentine . . 2 51 1 Bolivia - Bolivie .... 19 69 27 8 2 8 9 86 12 151 19 2 11 10 45 1U2 95 11 Brazil - Brésil 14 167 2 2 4 2 11 12 70 13 1 1 9 27 12 27 22 24 5 Colombia - Colombie . . . 2 3 5 11 7 7 9 3 16 36 12 22 9 105 51 11 6 2 1 Ecuador - Equateur 1 31 1 14 22 5 Guyana - Guyane l Panama 4 Paraguay 9 Peru - Pérou 45 9 3 5 28 75 7 33 2 1 1 82 93 97 30 98 19 25 Suriname 11 2 n < Trinidad and Tobago !* Trinit -et-Tobago 18 < Venezuela 55 27 33 4 1=:^

TOTAL 87 304 12 47 48 86 28 55 212 76 168 44 102 240 205 119 237 140 47 " OO *-