Leishmaniasis in the Sudan Republic

30. Final Epidemiologic Report

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  • United States Naval Medical Research Unit Number Three, Hooper Foundation, University of California, San Francisco Medical Center, Cairo, Egypt

VII. Summary

Findings in the Paloich-Malakal Area


Kala azar was first found in the Sudan in 1904(65) only a year after the disease was discovered and described in Algeria.(64) Shortly afterward, sporadic outbreaks were reported along the Ethiopian frontier of Blue Nile Province.(67–78) Increasingly intense outbreaks during the ensuing 20 years afflicted resident tribespeople, new settlers, and military patrols passing through uninhabited areas.(33)

Although these sporadic, geographically localized outbreaks indicated an endemic zoonosis, regional spread and involvement of greater numbers suggested epidemic extension of the disease among nonimmune persons in new areas. In 1940, a serious outbreak with characteristics of a true epidemic was reported from Melut, Upper Nile Province.(79) The infection apparently was brought to the Southern Fung District about 1956 by Arab nomads who had traveled here for the first time in half a century or more. The disease was fatal in most cases, rapidly disseminated in the population, and marked by cutaneous and nasopharyngeal symptoms, short incubation, and a tendency to attack several family members or an entire village within a short period.(85) The pattern suggested local, perhaps direct, transmission. Epidemic episodes increased in frequency and severity during the 1950s, with peak outbreaks in Melut-Paloich (1952), Wadega-Kurmuk (1956), Paloich, and Southern Fung (1958).

Kirk and Lewis considered Phlebotomus orientalis of the subgenus Larroussius, to be the probable vector, and that Phlebotomus (S.) clydei might or might not be a vector.(36,41,53,62) The former species was implicated on circumstantial grounds, the latter from a limited feeding experiment.(53) No information was available on the ecology, seasonal dynamics, flight habits, or other pertinent biological attributes of these possible vectors. No reservoir hosts were known, and prevention of kala azar transmission by insecticide spraying programs in the villages was fruitless.

The seriously worried Minister of Public Health, Dr. Mohammed Ahmed Ali, urgently requested help in solving this growing problem, which not only afflicted whole tribal groups and villages but also endangered the entire plan for economic and agricultural development of the central Sudan. Of immediate concern was the program for resettling numerous nonimmune persons within the kala azar endemic region.

Following an on-site tour of infected areas by the Minister of Public Health and the NAMRU-3 Director, Dr. John R. Seal, an agreement was reached for prompt epidemiologic research to discover the transmission pattern and source of infection and to suggest practical control measures.

The main NAMRU-3 kala azar research laboratory was established in Malakal early in 1960, and the Paloich “Forward Field Laboratory” was opened near the end of the 1962 dry season. Full-scale efforts continued through the 1964 dry season. Work during the 1962 and 1963 rains contributed to knowledge of sandfly biology during the little-studied summer period.

By the onset of the 1964 rains, the major epidemiologic and ecologic factors had been blocked out and confirmed by repeated studies, the vector had been established and proved experimentally, and several reservoir host species had been demonstrated. Leishmania donovani strains were isolated from three species of urban and wild rodents and from two species of wild carnivores, from Sudanese and American patients suffering from cutaneous and visceral manifestations, and from Phlebotomus orientalis captured as they bit man. The W.H.O. Leishmaniasis Reference Center later proved all these isolates to be identical to each other, serologically indistinguishable from the known agents of kala azar in the Sudan.

Vector Studies

Man-biting sandflies

After we had searched at length in various biotopes, we first found sandflies biting man in a forest environment near Paloich, 160 km north of Malakal, a region long endemic for kala azar. Man-biting sandflies in this area fell into two epidemiologically distinct groups: one exclusively in Acacia-Balanites forests and in villages within or adjacent to these forests; the other largely restricted to human communities in grasslands and forests.

The forest man-biting species were chiefly P. orientalis and Phlebotomus heischi. The latter was collected only in restricted portions of Acacia-Balanites forests, and its numbers were much less constant than those of P. orientalis. P. heischi adults were active earlier in the dry season (January to April) than those of P. orientalis (April to June); they were collected in small sectors of the forest and only in 1963 and 1964, being particularly common during the 1964 dry season. A few scattered Phlebotomus papatasi also fed on man in forests near villages, but these appeared to be chiefly stragglers.

P. papatasi, the primary pest of man in grassland villages and in older villages within and near forests, was common near and in human habitations. Population densities in long-established grassland hamlets were particularly heavy. This species bit man voraciously, causing many persons to flee to open grass areas for respite and sleep. P. papatasi undoubtedly breeds in or near village huts, though precise information is lacking.

No man-biting sandflies were found in open grassland away from villages or around nearby hafirs (artificial water basins).

Epidemiologically important biological observations of P. orientalis made during our first two field seasons were amply confirmed in later field studies. P. orientalis adults first appeared in the middle of the dry season and increased considerably in numbers until the rains began, when their numbers dropped dramatically. Early records of man-biting began about the same week each year but the population decrease, which depended on the vagaries of the early rains, was more irregular. Ecologically, P. orientalis was strictly confined to Acacia-Balanites forests and to villages in or near these forests. This species was not found in grasslands, urban centers, floodplains, or riparian forests (except for the much modified Taufikia Forest). Human beings were bitten in Acacia-Balanites forests and nearby villages from dusk to shortly after dawn, provided the wind velocity did not exceed certain limits (4.0 m per sec or 14.5 km per hr). Population levels and biting cycles of P. orientalis were almost uniform during each season.

Other biological information on P. orientalis is still scanty. Flight range, demonstrated by studies of marking with fluorescent powder, releasing and recapturing, was established to be at least 730 meters.(19) The effective range, undoubtedly greater than this distance, requires additional study in relation to practical control measures of adult sandflies. Efforts to locate larvae were fruitless. Adult P. orientalis probably rest chiefly in soil cracks in the forest floor, but supporting evidence is circumstantial.

Infections with Leishmania donovani

Of all sandfly species collected, only P. orientalis was infected with L. donovani, and infected flies (proved by inoculation of hamsters) were recovered from each forest collecting site we investigated. Infection rates in sandflies were remarkably consistent, ranging from 1.9 to 5.0% throughout the period of our study.

Among the P. heischi collected in 1964, several were infected with metacyclic trypanosomes that did not infect hamsters, and they remain unidentified.

Comparative studies conclusively showed that P. orientalis is biologically suited to serve as the vector of kala azar to man, while P. papatasi is not.(12) Infections in wild P. orientalis were predominantly heavy, with leptomonads concentrated in the anterior midgut. Laboratory infections of corresponding duration in P. papatasi had very few surviving leptomonads, most of which were inactive and concentrated in the hindgut, in striking contrast to the actively motile clusters of elongated leptomonads in the anterior midgut of naturally infected P. orientalis.

We therefore conclude that P. orientalis is the significant vector to man of a kala azar zoonosis in the Acacia-Balanites woodlands of central Sudan.

Reservoir Studies

Infections in Acacia-Balanites forests

Rodents and dogs were initially selected as the most probable reservoir hosts of a zoonotic L. donovani cycle. Rodents were particularly suspect as they are the most abundant and widely dispersed element of the local mammal fauna. Common species are subject to year-to-year fluctuation of population density. Several rodent species abundant in both grassland and forest are also closely associated with man as village and town commensals. In Gabek Forest (near Tir village) where infected sandflies were taken, we collected two infected rodent species and two infected carnivore species.

Infected carnivores were the Senegal genet (Genetta g. senegalensis) in the civet family (Viverridae), and the Sudanese serval (Felis serval phillipsi) of the cat family (Felidae). Domestic dogs were abundant, and the jackal Canis adustus bweha was the most common wild carnivore in the area, but these animals were not found infected. The viverrid and felid infections may have been acquired by their consuming infected rodents or through bites of sandflies. The reservoir role in these carnivores is of considerable interest, especially in view of the failure to demonstrate infection in domestic and wild canids and the lack of records of natural infection with strains of L. donovani infecting man in wild Felidae (an exceptional case of American visceral leishmaniasis in a domestic cat was reported(146)). We do not rule out the domestic dog or various wild canids, such as the jackal, nor have we eliminated from consideration hyenas, mongooses, zorils, or other carnivores. However, despite much effort we were unable to demonstrate their participation in the transmission cycle.

Although dogs can readily be infected with the Malakal strain of L. donovani, comparative studies suggest(20) that both the Sudanese and Kenya strains are less well-adapted to these animals than is the Mediterranean strain, of which the dog is the sole reservoir.

Two rodent species, the Nile grass rat, Arvicanthis niloticus luctuosus, and spiny mouse, Acomys albigena, were implicated in the same forest locality. Infections were found in four Arvicanthis (two confirmed, two equivocal) of 117 of these rodents tested by inoculation of hamsters during the 1962 dry season. That year tests in hamsters for infection in 66 Acomys trapped in Gabek forest gave negative results. But the next year, 1963, tests of all Arvicanthis examined were negative while one of 144 Acomys from this forest was infected. The infected felid and viverrid were collected during the same 1963 period. Considerable year-to-year population density differences noted for rodents in this forest require further study relating to the epidemiology of kala azar.

Our reservoir data should not be construed as the actual infection rate in the local wild mammal fauna. Techniques utilized to isolate Leishmania from wild mammals were not sufficiently sensitive to reveal low-grade infections or those restricted to the skin. Skin infections, in particular, are probably abundant and epidemiologically significant.

The important and unique fact that emerges from this study is that two noncanid carnivore species and two rodent species are reservoirs of L. donovani in a single locality where infected man-biting sandfly species and cases in man are found, and that all harbor identical strains of L. donovani.

Infections in Malakal city

In Malakal city we found evidence for a rodent cycle of L. donovani with no direct involvement of man. Two infected rodents were collected, an Acomys albigena in our laboratory compound and a Rattus rattus along the nearby Nile shore. The common black rat, so pugnacious and successful elsewhere, is unable to replace indigenous rodents in central Sudan except in a riverside city such as Malakal. Man-biting sandflies were not found in Malakal despite careful search for them over several years with daily collections for a year in the Malakal laboratory, where infected rodents were discovered. The sandfly vector of the Malakal rodent focus is not known.

Our investigation of cases of kala azar in man reported as originating in Malakal showed that the patients had been brought to the hospital from outlying villages. Malakal is not an endemic locality of the disease in man.

Presence of this zoonotic cycle is a potential danger to man should sandflies capable of transmitting L. donovani appear in Malakal city.

Infections in Man

We had no opportunity to make a full epidemiologic study of an active outbreak of kala azar. Comparative data on numbers, activity, and species composition of sandflies, rodents, and carnivores before and during a peak period of Leishmania transmission to man would be of great epidemiologic significance.

Khor Falus

One local outbreak of kala azar was observed about 25 km south of Malakal at Khor Falus, a back-wash tributary entering the Sobat River 8 km from the Sobat-White Nile junction. Isolation of the area and difficulties of access prevented investigation of the natural history of this outbreak. Incidence, distribution, and clinical aspects of over 200 cases were recorded by Van Peenen and Reid,(14, 15) who considered that the initial infection from wild reservoirs may have been followed in some cases by direct human-to-human contaminative contagion. The basic ecology and environment of scattered Acacia-Balanites forests and open grassland appeared to be much like those studied in the Paloich area. The question of the triggering stimulus of the Khor Falus outbreak is unanswered.

Infections in NAMRU-3 personnel

During the 1963 dry season, three mild cutaneous cases caused by L. donovani occurred among American members of the NAMRU-3 contingent. Two patients had actively collected wild sandflies in the Paloich area, the other was exposed during a brief visit. All three contracted comparable dermal lesions, an ankle lesion in the first case, a single facial lesion in the second, and two facial lesions in the third. The sores developed from papular to infiltrating ulcerating lesions, regressed within a week after starting Pentostam® therapy, and faded to faint scars within 3 months. No visceral involvement was observed in these three cases. No symptoms of infection were found among other American or Egyptian NAMRU-3 members. These colleagues had been subjected to P. orientalis bites in widely varying degrees ranging from brief exposure to much longer periods than those of the three patients. Three Sudanese field employees became ill with visceral kala azar.

Reactions to skin tests were positive after cure in all three American patients. Four others of 63 Cairo NAMRU-3 staff who were skin-tested also had positive skin-test reactions. These four included three who had worked in the Paloich area and one who maintained Leishmania cultures in the Cairo laboratory.(21)

The infections in NAMRU-3 staff present an interesting clinicoepidemiologic puzzle. Did the three cases represent greater individual susceptibility to L. donovani? Why did they occur only in 1963? Was there any change in virulence of parasites, in the transmission cycle, or in other local factors during this period? In the group exposed to sandfly bites, there definitely was no relation between duration or intensity of exposure to bites and development of lesions. The presence of superficial lesions with no visceral signs is also noteworthy. We know of no other reports of only cutaneous symptoms in a group of persons exposed in nature to L. donovani. The general good health enjoyed by these Americans may have been a protective factor.

Parasites from all cases in man were identified by intraperitoneal and intracardiac inoculation of hamsters. P. papatasi that fed on lesions from all three patients did not contract anterior infections. Posterior midgut and hindgut infections did not persist more than 8 days. This is in keeping with experimental experience with P. papatasi,(12) and served as a xenodiagnostic verification of the L. donovani diagnosis. Cultures from ulcer scrapings were sent to the W.H.O. Leishmaniasis Reference Center, and, along with all other vertebrate host and insect vector isolates, proved to be identical.

Infections in U. S. Embassy personnel

Three other cases, two of which developed into true visceral leishmaniasis, appeared in a group of five Americans from the American Embassy in Khartoum. They had participated in April 1963 in a hunting expedition in the Dinder River forest of Blue Nile Province (not Upper Nile as previously reported(21)). The NAMRU-3 and embassy cases, otherwise unrelated, were examples of zoonotic transmissions and do not constitute a single “outbreak” in an epidemiologic sense.

One embassy patient, with 18 lesions on arms, legs, and trunk, contracted far more serious cutaneous manifestations than did the three NAMRU-3 patients, who had only one or two lesions each. However, this patient had no visceral symptoms, as did his two embassy companions who in turn had no cutaneous lesions.

A leishmanin skin-test survey of 121 persons of the American Embassy in Khartoum showed six positive reactions.(21, 230) These were the three patients, their two companions on the Dinder River hunting expedition, and an agricultural advisor who frequently visited endemic areas in Central Sudan.

All of the patients with dermal manifestations (NAMRU-3 and Embassy) and one of the two with visceral kala azar responded well to a short course of sodium antimony gluconate (Pentostam®), administered as a 10-day, intravenous course (6 ml daily). The second patient with visceral kala azar responded only after a 30-day intravenous regimen of pentamidine isethionate.

Epidemiologic implications

No explanation can be advanced for the occurrence of four dermal and two visceral cases of kala azar among NAMRU-3 and American Embassy personnel in 1963 and the absence of such cases among other research personnel who were much more frequently exposed to P. orientalis bites. Possible human factors may have been general health, nutrition, age, number of infected sandfly bites at the time of initial exposure, and perhaps the blood level of antimalarial drugs. Some members of the NAMRU-3 group were on a prophylactic regimen with chloroquine.

It is of particular interest that in the NAMRU-3 cases, in an area of proved zoonotic kala azar transmission, there were only a few mild dermal lesions, reminiscent of a light case of the dry or urban form of cutaneous leishmaniasis. These mild expressions of infection cannot be attributed to gradual exposure and development of resistance, as immunization against leishmaniasis characteristically requires a full course of infection to produce protection. Judging from the heavy dermal involvement of one member, the hunting party probably encountered a particularly severe zoonotic locality, one well worth future study.

Skin-test surveys

The leishmanin skin test is not a specific response and may reflect transitory nonclinical infection with Leishmania species not normally found in man. These cases would give a falsely positive reaction and must be evaluated with the aid of appropriate control groups. Furthermore, the test does not elicit response during active kala azar, as it does in clinical tropical sore. Skin-test response after kala azar is least in India and South America, greatest in the Sudan and Kenya.

Brief skin-test surveys were made in Khor Falus,(14) Tir,(31) and, as mentioned, among U. S. Embassy personnel in Khartoum and NAMRU-3 personnel in Cairo.(21, 230) The village surveys showed a wide range of positive leishmanin response from a low of about 10% in nonendemic Bentiu to levels over 80% in the Tir area. Both extremes are of interest. The high values suggest previous disease. clinical or subclinical, with an immunizing, or at least a dermal sensitizing after-effect. Reaction levels in endemic areas showed a rise with age, suggesting that previous experience had a cumulative effect.

The dermotropic tendency of Sudanese L. donovani, along with the leishmanin response, which showed widespread dermal sensitization in hyperendemic areas, suggests that under certain conditions (nonepidemic transmission, high immune response, excellent nutritive state and general health), factors in the human host may determine whether visceral disease is expressed or infection is restricted to the skin and mucosa.

The low-level leishmanin findings were made in Bentiu, a village with no infections, far in the southwest corner of Upper Nile Province, about 200 km SW of Malakal. A skin-test response of about 10% was found in 155 adults tested,(14) somewhat less for those below age 16. This response was comparable to that in adults tested in an area of Egypt where kala azar is unknown, but cutaneous leishmaniasis is present. In Bentiu the 10% positive responses may have been in those previously exposed to lizard leishmaniae or to other mammalian forms capable of converting the skin to a leishmanin-positive reaction. Subclinical infections may also have been present, but would have been unlikely as this is not an endemic area, and the response level did not rise sharply with age.

The leishmanin test remains a useful means to determine endemicity of infection. Van Peenen(14) suggests, as a gauge of endemicity, a level of positive leishmanin response 30% or higher among males 30 years of age or older.

Experimental transmission from infection in man

A complete demonstration of vector capacity requires proof of transmission by the normal biting process. In a single experiment, unfortunately with a limited number of flies, P. orientalis was allowed to feed on a patient with kala azar; from the flies were taken active, anterior leptomonads that were infectious when inoculated into hamsters.(17) Of 13 wild-caught P. orientalis placed on the arm of a patient in the Malakal hospital, eight fed, and in each numerous leptomonads developed in the anterior midgut after 6 or 7 days. Four of these had sufficient parasites to form an anterior plug, a solid immobile mass blocking the stomodeal valve. The infected sandfly material was inoculated intracardially into four hamsters. The spleen of one revealed parasites when cultured;(17) the others subsequently showed development of Leishman-Donovan bodies in their spleens.(30)

This study, though limited, forms one of the basic lines of evidence implicating P. orientalis as the actual man-biting vector responsible for transmission of kala azar to man in Upper Nile Province of the central Sudan.

Sandfly-Control Pilot Project

Our limited attempt to control adult P. orientalis in portions of an Acacia-Balanites forest by conventional applications of DDT, BHC, and malathion was unsuccessful.(24) We did not apply insecticides to large enough areas within the forest and did not have sufficient knowledge of the extensive and rapid movement of large swarms of sandflies in the forest.

We nonetheless believe that it may be possible to control this vector by forest-wide insecticide application. House spraying alone is useless. Control is somewhat simplified as it is aimed at a single species of vector with a strongly periodic pattern of adult activity restricted to specific months of the dry season in relatively small, clearly circumscribed forests. The open, largely leafless forest canopy and the few shrubs and herbs present reduce difficulties in distribution of insecticides. Negative factors that influence the control problem are chiefly economic.


Extension of Basic Research

The research described in this report was undertaken as an introductory, though intensive, epidemiologic investigation in a representative kala azar-infected region of central Sudan. Several important extensions of this work were planned to follow the studies reported here; these are still required to develop an effective program for the prevention and control of kala azar. Two investigations within the scope of the present project should also be undertaken as a basis for planning control of sandflies. First of these is the determination of maximum flight range and movement patterns of P. orientalis. Second is a test of the feasibility of forest-wide control of this sandfly by insecticides. Several related transmission and epidemiologic factors partially elucidated in the present study should be further investigated, both for basic and applied information. Of initial importance, however, is that knowledge already acquired be compared with new epidemiologic field data covering periods of both low and high incidence in man of kala azar over a period of several years. It will also be necessary to compare P. orientalis distribution and infection rates with those in other areas of the Sudan. Ecologic localization and seasonal periodicity of the vector species is now known for the Paloich region, and efficient study methods have been worked out. An extended survey, therefore, should be relatively easy and inexpensive. Any forest harboring P. orientalis is properly considered a potential focus of kala azar.

Anterior leptomonad infections in man-biting sandflies should be screened by inoculation into hamsters, particularly when flies are collected from regions of present or planned human habitation. Surveys for infected man-biting sandflies require a mobile team of highly experienced workers who trap, identify, and dissect flies in carefully selected, epidemiologically significant localities. A permanent laboratory base with a hamster colony and facilities for Leishmania cultivation is required for xenodiagnosis. Sandflies or potential reservoirs may have to be transported by air to the central laboratory for processing. Initial surveys in Upper Nile, Blue Nile, Kordofan, and Equatoria Provinces can be made by the mobile team to determine areas of potential infection in man, but final epidemiologic confirmation rests upon animal-infection and serologic-culture studies.

A special investigation of the Kapoeta area is particularly needed. This region apparently represents a pattern of transmission distinct from that in central Sudan, possibly with close affinities to foci in Kenya and northern Uganda. The Kapoeta focus may provide the key to better understanding of the entire East African kala azar complex.

Surveillance, Control, and Treatment Teams

It appears to us that successful prevention and control of kala azar in the Sudan requires a special Ministry of Health Program designed for the particular conditions in the Sudan. A “Kala Azar Prevention and Control Service,” organized as a group of collaborative, integrated teams, seems most appropriate. These should include a team for Surveillance (biomedical research on sandfly biology and reservoir studies for prediction of danger areas); for Prevention (sandfly control); and for Treatment (including clinical studies). These teams would serve as cadres to train their own specialists; they would determine whether and when survey, treatment, and control measures should be undertaken, supervise and evaluate these programs, and initiate epidemiologic research. Together they would have the responsibility for surveillance, prediction, prevention, treatment, and investigation of outbreaks of kala azar.

Initially, the chief mission of the Service should be to determine foci of infection in man and animals, especially those near populated centers, and to undertake sandfly control along welltraveled roads, near schools, and particularly near new agricultural schemes and settlements. Regional movement of troops, survey teams, and construction groups before resettlement of larger populations (especially in previously uninhabited forest areas) should be emphasized. As kala azar is widespread in central Sudan and chiefly a forest zoonosis, control or prevention of the disease throughout its entire distributional range is not yet economically feasible. Specific areas endangering larger human concentrations can, however, be targeted for efforts to control sandflies, to post appropriate warnings against exposure to sandfly bites in dangerous areas, and to undertake educational activities in schools and communities. The need for this is apparent in the face of widespread population shifts, as in the large resettlement program for the Wadi Halfa populace in the Khasm El Girba area. The disease has been reported from Doka, near Gedaref, 160 km south of the scheme.(203) Development of new villages in conjunction with irrigation and cotton-growing schemes is continuously underway. Most of the present and contemplated major agricultural programs in central Sudan involve endemic areas of kala azar. This disease should, therefore, receive substantial attention and commitment of funds. Recognition of the importance of the problem by planners and higher officials other than medical personnel is a prerequisite for full economic exploitation of this essential area. Explosive disease outbreaks could well endanger or destroy the best-planned land-use schemes and development programs.

Author Notes

Participation in this study was supported by ONR contract XYZ-YZO-1, 867,233.234, which is gratefully acknowledged.