Problems of rodent control in rural tropical areas

SESSION IV PREVENTION AND CONTROL Problems of rodent control in rural tropical areas ALLAN M. BARNES 1 Rodent control strategies, techniques, and research needs in rural tropical environments- are reviewed and discussed with special reference to Mastomys natalensis, the possible reservoir of Lassa fever in West Africa. Public health rodent problems are far more serious and widespread in rural tropical areas than in developed countries. In the latter, only the commensal rodents constitute a major problem, whereas in rural tropical areas, native semidomestic species also serve as disease reservoirs and sources of infection to man. The success of rodent control programmes in developed countries depends in large part on the willingness and ability of people and governments to spend relatively large sums on research and control, on an acquired intolerance of people to rats and disease, and on a substantial economic base. These prerequisites are not usually to be found in rural tropical areas. Consequently, the rodent control techniques and programme organizations of developed countries are not directly applicable to such areas, even though the principles are the same. For this reason, it is suggested that a well-funded, integrated research and control programme should be undertaken in a known Lassa fever area, stressing public education, personnel training, and environmental management as well as rodenticidal approaches. INTRODUCTION AND BACKGROUND Interest in rodent control in rural tropical areas, where rodents constitute a perennial and severe threat to man's health and wellbeing, has been stimulated anew by the discovery of Lassa fever in 1969 and the subsequent isolation of the causative virus from Mastomys natalensis in Sierra Leone (1). The unusually high case-fatality rates observed dur- ing epidemics, the high risk of infection in case contacts, and the lack of specific therapeutic mea- sures make Lassa fever a formidable disease (2). The incubation period varies from 3 to 16 days; given the rapidity of modem air travel, this makes it possible for explosive secondary outbreaks to occur any- where in the world, so that Lassa fever is an international problem of considerable dimension. The possibility that contact with inapparent cases might produce disease and a secondary outbreak poses additional problems (3). Up to the present, naturally acquired Lassa fever has been reported from 3 West African countries: Nigeria, Liberia, and Sierra Leone. Antibody to the disease has been found in Guinea. Habitats in which epidemics have occurred range from tropical rain I Chief, Plague Branch, Vector-Borne Diseases Division, Center for Disease Control, Fort Collins, CO, USA. forest to savanna. This geographical and ecological distribution indicates that natural foci of the virus may be more widespread in West Africa than at present known. On the basis of current information, the natural source of human infection appears to be Mastomys natalensis (1). Whether or not Mastomys is also an endemic reservoir is unknown. Related pathogenic viruses (lymphocytic choriomeningitis, Machupo, Junin) are known to be transmitted to man by contamination of the environment with infected rodent urine or other excretory products. Lassa fever virus has been shown to persist in the urine of white mice for at least 82 days (3); therefore, it is likely that Lassa fever is passed from rodent to rodent and from rodent to man in the same manner as related viruses. Unlike murine typhus and bubonic plague, two classic rodent-borne diseases, it is probable that Lassa fever is not transmitted by arthropod vectors. In the absence of an effective vaccine and of any therapy other than isolation and symptomatic treat- ment for patients already ill from the disease, rodent control has been suggested as a viable method of attack for the suppression or prevention of Lassa fever epidemics (4). In the present paper, I shall attempt to give a general view of the problems of rodent control in 3398 - 669 -- BULL. WORLD HEALTH ORGAN., Vol. 52, 1975 A. M. BARNES rural tropical environments, touching on research needs, strategies, techniques, human sociology, and politics, all of which are of great relevance to rodent problems wherever they occur. No solutions and certainly no panaceas will be offered or suggested, but it is hoped that the dimensions of the problem of rodent population management in any environment will be made clearer and some indication given of the steps that might be taken to develop a strategy for Lassa fever control. Public health rodent control in developed coun- tries, except for antiplague campaigns against ground squirrels in Russia (5) and California (6), has concentrated almost exclusively on only 3 species: the Norway rat (Rattus norvegicus), the roof or ship rat (R. rattus), and the house mouse (Mus musculus). The successes of programmes in these countries in eliminating or drastically reducing the incidence of rodent-borne diseases such as urban plague and murine typhus have been due in large part to the evolution of high standards of sanitation, willingness and ability on the part of their populations to spend large sums for research and control, and an acquired intolerance on the part of the people to rats and disease. The implementation of such programmes requires a substantial economic base. Even in the developed countries failures occur, particularly when cultural resistance is encountered and basic pro- grammes for reducing rodent food supplies and harbourage are not supported by the people (7, 8), or when rodenticidal materials and techniques come to be regarded as the ultimate solution and are used routinely, leading to the development of pesticide resistance among target rodent populations (9, 10, 11). Public health rodent problems in tropical areas are far more serious, widespread, and difficult to solve than are those in temperate regions. Many of the rodents that act as disease reservoirs are well- adapted native species. In addition to the commensal species characteristic of developed countries in tem- perate climates, semidomestic rodents also may play a significant role as direct reservoirs of diseases affecting man, or alternatively, as a vehicle of infec- tion from the field to household rodents, which in turn may serve as sources of infection for man. This role is exemplified in Indonesia by Rattus exulans, a field rodent that invades households in the moun- tains of Central Java after the harvest of dry land crops or the development of other conditions ad- verse to its populations and, in so doing, transports infective fleas and bubonic plague to the house rat, Rattus rattus diardi (12). Mastomys natalensis, the multimammate mouse, appears to play a similar role in parts of Africa, as does Zygodontomys pixuna in north-eastern Brazil. In North America, the nearest ecological corollary is Peromyscus maniculatus, a highly adaptable, widespread, and geographically variable species that also invades human habitations (13). In the south-western areas of the USA, it has been associated epidemiologically with human plague cases after it has invaded habitations. Such species have been aptly described as " weeds of the animal world ", and all appear to be characterized by their tendency to occupy rapidly any disturbed habitats (as opposed to native climax habitats), and by high reproductivity, wide geographic distribution, a broad gene pool and a consequent wide range of phenotypic, behavioural, and biological variability. All appear adapted to a wide range of climatic regimes. Rattus exulans occurs from near sea level to 2 500 m elevation in Java (14); Mastomys natalensis occupies a wide variety of biotopes, from tropical rain forest and savanna to subtropical areas in Africa (15), and Peromyscus maniculatus occurs in North America from subarctic regions to southern Mexico (16). In addition to the limiting factors of food and harbourage, the presence of more narrowly adapted or more aggressive competitors appears to be a deterrent to these species' occupation of a given niche. For example, Rattus exulans in Java does not occupy human habitations in the presence of the closely commensal Rattus rattus diardi, but in Malaysia where R. exulans is called the " little house rat ", it resides in habitations with impunity (17). In natural habitats, Rattus exulans and Mastomys na- talensis would appear to be relatively minor com- ponents of the rodent fauna. None of the species appears to be a strong burrower, but they often use the abandoned artifacts of other rodents or the structures ofman for shelter. In north-eastern Brazil, overwhelming numbers of Zygodontomys pixuna tend to develop in one breeding season after native chapada vegetation has been cleared in preparation for cultivation. The development of such large popu- lations appears to be facilitated by the " weedy " forms of agriculture practised in north-eastern Brazil and other rural tropical environments. In such envi- ronments, food supply out-of-doors is rarely a limit- ing factor. Worse, when food is reduced by harvest or cultural practices, rodents move to adjacent fields or into habitations. Against such problems, attempts at the direct transfer of techniques and approaches 670 RODENT CONTROL IN RURAL TROPICAL AREAS designed for rodent control in temperate, developed countries is not often likely to be successful. The need for basic information concerning the biology, ecology, and particularly the population dynamics of pest species has been stressed by numer- ous authors (7, 8, 19). Such information is rarely available in rural tropical areas where rodent prob- lems are more severe than in developed countries in temperate regions but where far less basic research has been done (8). Most often, when the need for rodent control becomes apparent, the " fire-fighting" approach is taken, and strategies and methods are used that are often inadequate and started too late to help solve the problem. It is unfortunately true that such operations are often attempted for political and social reasons as well as for disease control. It is also true that such programmes may give the illusion of success, but produce little in the way of immediate, real, or lasting benefits. Such attempts may indeed be necessary, but it is apparent that if means of known effectiveness are not available, the major concern should be to develop strategies directed toward both immediate and long-term solutions. STRATEGIES IN RODENT CONTROL Four general strategies are available for the man- agement of pest populations (19): (1) evasion of the harmful effects caused by the pest, (2) elimination of these harmful effects, (3) suppression of those quali- ties that make a pest species harmful, and (4) reduc- tion in numbers of a pest population to a tolerable level. Of these, reduction in numbers has been most often used and is most often successful. Elimination of harmful effects (e.g., through vaccination of sus- ceptible human populations) has achieved some degree of success. Suppression of harmful effects caused by pest species requires more knowledge than is at present available for any pest rodent population and is therefore not practicable. Evasion of harmful effects and reduction in numbers have more often been successful, and these are discussed more thor- oughly below. Evasion of harmful effects The strategy of evading harmful effects has at times been the only means available for preventing human disease. A classic example is the history of onchocerciasis in the Upper Volta Region of West Africa, where valley areas near the river were left virtually uninhabited and undeveloped for centuries to avoid " river blindness ". In Brazil and in Vene- zuela, areas around houses are cleared to the bare ground in an attempt to create a barrier and reduce contact between endemic plague reservoirs and house rats. Protection of food supplies may be a successful means of reducing rodent populations by eliminating their access to food and also of evading a principal means of contact between rodents and people. In rural tropical areas, including West Africa, food supplies are stored extensively in and around private dwellings. Rarely are such supplies protected ade- quately from rodents. Various kinds of metal food storage bins have been devised and are unquestion- ably effective (8). The chief obstacles to their use appear to be the initial cost, distribution, and con- vincing people of the need to use them. There is a distinct need for research on effective alternatives, using indigenous materials. In Bali, storage buildings on stilts provided with rodent barriers are effective in protecting rice stores. By contrast, in the uplands of Central Java where plague is endemic, food is stored in lofts where it is readily accessible to Rattus rattus diardi, which, with its fleas, is the primary source of human plague infection. Adoption of an effective means of rodent-proof food storage in individual houses there would undoubtedly reduce the Rattus rattus population and also reduce the attractiveness of such houses to invading Rattus exulans. In this way, rodents in houses would be reduced as well as evaded. Reduction ofpest numbers Means of reducing pest numbers fall into two categories: (1) those that affect the characteristics of the species (use of chemosterilants or of toxicants); and (2) those that modify environmental conditions in such a way as to be detrimental or lethal to the pest species (19). Chemosterilants. Rodent chemosterilants have received a great deal of attention (20, 21, 22). Although chemosterilants have achieved some degree of success in the field, primarily against Rattus norvegicus, in actual use such materials pos- sess most of the disadvantages of conventional toxi- cants and, in addition, are extremely slow to act and to show effect (21). For example, in a recent success- ful experiment, investigators were able to reduce a Rattus norvegicus population by 75% in 6 months (23). In many cases, unanticipated biological and physiological factors have reduced or prevented suc- 671 A. M. BARNES cess in field trials and in practice. The use of all chemosterilants requires a level of knowledge con- cerning reproductive biology and behaviour not at present available for most tropical rodent species. Timing and scheduling of treatments is critical and requires a high level of organization and logistics. In any event, use of chemosterilants against rodent populations, while potentially offering a useful alter- native or adjunct to other means of rodent control, requires substantial development and investigative effort before the material and techniques may be found useful in the field. For this reason, any successful use in tropical rural areas appears to be far in the future. Toxicants. The use of toxicants, traps, clubs, biological agents, and other means of killing may often appear to be the only means of dealing with rodent damage and rodent-borne disease problems. Impromptu killing programmes are rarely success- ful; nevertheless, they provide a certain amount of satisfaction to those beset with problems, particu- larly when body counts are used as a measure of success (24). Properly measured, timed, and organ- ized programmes based on adequate knowledge of the reproductive biology, ecology, and behaviour of target species can, however, be successful. A case in point is the initial reduction in the incidence of Bolivian haemorrhagic fever (BHF) by the use of an acute toxicant, zinc phosphide, to effect a drastic reduction in the number of Calomys callosus and, hence, in the number of human cases of BHF. The poisoning programme was followed by a compre- hensive environmental management programme including rodent-proof food storage to maintain control. Too often, poisoning programmes are used as a substitute for sound management directed at solving the basic problem of altering the environ- mental factors responsible for high pest population densities. In order to achieve any degree of success, rodenti- cidal programmes, if used alone, must be not only well-founded and based on considerable knowledge of the species involved and its environment, but also, of necessity, persistent and repetitive. How repetitive such a programme might need to be in practice can be illustrated by available data, much of which has been provided in the reviews presented by Dr Coetzee (32) and Dr Isaacson (33) in this Symposium. A composite picture of the biology of Mastomys natalensis in West Africa, built up from these data, might read something like this: 1. Breeding season-about 10 months of the year. 2. Average age at sexual maturity-approximately 90 days. 3. Gestation period-23 days. 4. Average time between litters-25 days (post- partum estrus). 5. Average number per litter-10. 6. Average number in litter reaching recruitment age-8.5. 7. Average longevity-approximately 1 year. Given these data and assuming that they represent the approximate picture in tropical West Africa, we can estimate fairly well how long it would take for a Mastomys natalensis population to return to its former capacity after successful application of an acute toxicant. If we assume that 90% control was achieved (which is quite successful for any treat- ment), we are left with 10% of the original popula- tion for replenishment. Even if we ignore in-migra- tion and the acceleration in breeding that is usually associated with a reduction in the population below the carrying capacity of its environment (7), our hypothetical population would return to its carrying capacity density in approximately 4 months. Thus, a well-conducted programme depending entirely on acute toxicants would need to be carried out at least twice per year-perhaps three times to achieve more than temporary success. Unless a large enough area was treated or barriers to in-migration were included in the programme, even more frequent repetition might be needed. One might postulate that a thorough and effective poisoning campaign against Mastomys could con- ceivably bring its populations down to a level at which rodent-to-rodent virus transmission would no longer occur and the cycle would be broken. At least two circumstances militate against this postulate, One is the occurrence of Lassa fever in at least three countries, which indicates that the disease is wide- spread in nature. If this is the case, a rodent population recovering after cessation of control activities would soon be reinfected. Another factor is that, at least in laboratory mice, virus can persist in urine up to 82 days; thus, a single infected animal surviving the control programme could reinfect the resurgent population. The use of anticoagulant rodenticides offers cer- tain advantages over acute toxicants in almost any situation requiring rodent poisoning, primarily be- 672 RODENT CONTROL IN RURAL TROPICAL AREAS cause of their relative safety for man and other non- target species, but also because their slow action makes bait shyness far less likely to develop among target rodents than it would if acute poisons were used. The advantages and disadvantages of anti- coagulants for the control of African rodents in tropical environments is discussed at some length in the paper by Gratz & Arata (34). As far as I am aware, neither appropriate laboratory tests nor field trials have been conducted against Mastomys nata- lensis. In general, anticoagulant poisoning pro- grammes alone, regardless of their effectiveness in killing rodents, have the same flaw as those pro- grammes that are based on the use of acute toxicants without an effective sanitation programme-they do not address the basic problem. The topic of pesticide resistance must also be mentioned, if only briefly. Such resistance is known to be a natural consequence of persistent exposure of pest populations to a toxic material acting as a selective agent. Resistance to anticoagulant rodenti- cides is now commonplace among Rattus norvegicus and Mus musculus populations in Western Europe (10). Resistance to acute toxicants among rodents is also well documented. The pine mouse (Pitymys pinetorum) has shown resistance to endrin (25) and both Sigmodon hispidus (the cotton rat) and Mus mus- culus (27) have been found capable of developing resistance to DDT. Recently, Howard and co- workers (18) were able to produce experimentally a fourfold resistance to the highly toxic compound sodium monofluoroacetate in a laboratory popula- tion of Rattus norvegicus in 5 generations. It should be evident that persistent exposure of any highly reproductive rodent population, such as Mastomys natalensis, to either an anticoagulant or an acute rodenticide is quite likely to bring about resistance problems. Field resistance to toxicants usually develops gradually. If the control pro- gramme is not extremely well organized and techni- cally supervised, the onset of resistance is not likely to be suspected or detected until outright failure occurs. In rodent-borne disease control, the results of failure can be tragic. In response to increasing resistance to anticoagu- lants, research seems to have been concentrated mainly on the development and testing of new rodenticides, although there has been some renewed interest in environmental management. Among newer products, calciferol (vitamin D2) has shown some promise, either as an adjuvant to anticoagu- lants or alone (28, 29, 30). On the basis of past experience, it can be expected that initially successful new materials will eventually fail if used persistently and continuously; even newer materials will then be sought. Despite the problems associated with rodenticides, their wise and judicious use is indispensable to management programmes. However, they are not substitutes for hard work, insight, application of known principles, or persistence in attacking a rodent problem, and even in developed countries, dependence on rodenticides alone is insufficient to meet rodent control needs. Environmental management The human population in the countries of West Africa is 80-90% rural, with many living in small villages or towns. In the Eastern Province of Sierra Leone the climate is wet tropical. Agricultural lands are developed by slashing and burning existing vege- tation. Fields are interspersed among patches of primary and secondary rain forest. Clear culture is not practised, and weedy vegetation is interspersed through crops. The agricultural base is rice, grown either in swamp rice fields or as a dry land crop. When harvested, much of the rice is stored by families for their own use. Most houses are con- structed of mud and wattle covered with cement. All houses and shops are rodent infested (2). It is interesting that Tongo, the scene of a 1970-1972 epidemic, is a relatively new town developed for diamond mining (1). The practice of clearing new lands for agriculture has often resulted in damaging rodent outbreaks, as noted by Brown (8). The obstacles to creating an environmental man- agement programme in such a milieu are enormous. Among the many problems to be overcome are the need for specific research on Mastomys as it lives in West Africa, the lack of trained personnel, and the necessary organization and logistics to carry out the programme. Another problem, often unanticipated in planning rodent control programmes in under- developed countries, is that of securing the ac- ceptance of programmes by the people, whose tradi- tions and culture may be subject to change as a result of the management procedures (8, 31). Public education would have to be an integral part of any long-term control programme. Such a programme would be perilously slow to take effect, and meanwhile the problem of Lassa fever would continue. A successful programme would require rodenticidal approaches as well as 673 A. M. BARNES other forms of management in order to ensure immediate benefits. Procedures should be judiciously integrated and carefully planned, with enough flexi- bility to accommodate changing conditions and to make use of new information as it is acquired. In view of the potential scope of Lassa fever in West Africa and the shortage of the necessary resources, trained personnel, and organization in the countries involved, it would be advantageous to begin by establishing an on-site research programme to study the biology and control of Mastomys and Lassa fever; the knowledge and techniques devel- oped could be applied elsewhere as Lassa fever surveillance data indicated the need. The resources and commitment required for such a programme would be substantial. If ongoing investigations on the epidemiology and epizootiology of Lassa fever should indicate that Mastomys natalensis is indeed the definitive reservoir of the virus, it is to be hoped that any control programme aimed at reduction of the disease would be well-planned and well-funded and not conducted on a piece-meal or short-term basis or confined to a single approach. Too often, programmes in the less developed countries, imposed from above and implemented by experts, become routine and repetitive in the face of changing conditions; the experts eventually leave and those in charge shift the objectives. Programmes at the working level may continue, but often out of touch with the scientific community. Finally, it should be pointed out that rodent management as we know it in developed countries today was not imposed from above and did not happen overnight; it evolved over a substantial period of time. Its evolution required a strong industrial economy, the active involvement of people at all levels, including the political, and the development of appropriate scientific information. Thus, successful long-term rodent control is not only a matter of scientific endeavour, it is also a cultural, social, economic, and political matter that requires the cooperation of people and governments at every level. I I RESUME LA LUTTE CONTRE LES RONGEURS EN MILIEU RURAL DANS LES TROPIQUES La fievre de Lassa, maladie virale transmise par les rongeurs, a maintenant et6 signalee dans trois pays d'Afrique occidentale, tandis que des anticorps ont ete d6couverts dans un quatrieme pays. C'est une maladie tres dangereuse du fait que le taux de letalite est eleve et que les individus en contact avec un malade risquent for- tement d'etre infectes. Des epidemies ont eclate dans un spectre d'habitats tres large, depuis la foret ombrophile tropicale jusqu'a la savane. Cette grande extension ecolo- gique du virus de la fievre de Lassa indique que le peri- metre du foyer d'infection naturelle est peut-etre beau- coup plus vaste qu'on ne le croyait jusqu'a present. D'apres les renseignements recueillis jusqu'ici, la source naturelle de l'infection humaine semble etre Mastomys natalensis, mais on ignore si ce rongeur est le reservoir endemique de la maladie. En I'absence d'une th6rapeu- tique specifique ou d'un vaccin efficace, la lutte contre les rongeurs a ete proposee comme moyen valable pour sup- primer les epidemies ou eviter les cas humains. En milieu rural dans les tropiques, comme c'est le cas pour les regions ofu s6vit la fievre de Lassa, la lutte contre les rongeurs pose des problemes beaucoup plus complexes et difficiles a resoudre que dans les pays developpes a climat tempere. Dans cet environnement tropical, en effet, les reservoirs de la maladie sont souvent des especes indigenes semi-commensales et bien adaptees, qui sont capables de subsister dans des conditions ecologiques tres diverses. Elles ont pour caracteristiques un taux de reproduction elev6 et une grande variabilite du point de vue du phenotype, du comportement et de la biologie, et elles sont capables de peupler rapidement des habitats perturbes. A quelques rares exceptions pres, les strategies et techniques elaborees par les services de la sante publique dans les pays temperes pour lutter contre les rongeurs visaient a eliminer Rattus norvegicus, Rattus rattus et Mus musculus, ou a reduire leurs populations, et il se peut qu'elles ne soient pas directement applicables aux regions tropicales rurales. Dans la plupart des cas, il faut adapter les strat6gies Anciennes ou en formuler de nouvelles qui tiennent compte expressement du milieu dans lequel se situe le probleme et de l'espece qu'il s'agit de detruire. Parmi les strategies utilisables pour diminuer les contacts entre Mastomys et la population humaine, il semble que la plus prometteuse associerait en les int&- grant l'evitement, la reduction des populations de ron- geurs par 1'emploi de produits toxiques et la mise au point d'un programme d'amenagement du milieu ambiant. Dans l'immediat, il est necessaire d'elaborer des systemes d'emmagasinage des produits alimentaires a l'epreuve des rongeurs en utilisant des materiaux indigenes. La reduc- tion des populations par l'emploi de produits toxiques s'impose dans l'immediat comme dans l'avenir, mais on ne saurait tabler uniquement sur le poison; la capacite de reproduction de Mastomys est telle que les populations ainsi attaquees ne mettraient pas plus de quatre mois 674 RODENT CONTROL IN RURAL TROPICAL AREAS 675 pour recouvrer leur effectif anterieur, ce qui necessiterait des traitements repetes qui pourraient fort bien favoriser le d6veloppement d'une r6sistance au poison. Des recherches fondamentales a long terme sur la biologie et l'6cologie de Mastomys sont necessaires pour mettre au point un programme de lutte integree combinant diverses methodes, y compris 1'emploi appropri6 de produits toxiques. 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Field trials of calciferol against warfarin resistant infestations of the Norway rat (Rattus norvegicus Berk.) J. Hyg. (Camb.), 73: 361- 367 (1974). 31. SHUYLER, H. R. Rodents in the tropics: their effects and control. PANS, 18: 445-451 (1972). 32. COETZEE, C. G. The biology, behaviour, and ecology of Mastomys natalensis in southern Africa. Bull. World Health Organ., 52: 637-644 (1975) 33. ISAACSON, M. The ecology of Praomys (Mastomys) natalensis in southern Africa. Bull. World Health Organ., 52: 629-636 (1975) 34. GRATZ, N. G. & ARATA, A. A. Problems associated with the control of rodents in tropical Africa. Bull. World Health Organ., 52: 697-706 (1975). DISCUSSION GRATZ: I have two comments. In the USA, there has been a lot of interest in the use of chemosterilants and in the sterile male release method of controlling species of diptera; it has worked here and there, but all too often it is seized on as a panacea. At present this method is no substitute for a rodenticide programme, with all the hard work that entails. From an operational viewpoint, it seems unlikely that the chemosterilants will be useful for several years to come. In addition, some of the chemo- sterilants on the market are general mammalian chemo- sterilants, and in tropical countries where it is difficult to protect the bait, there would be a risk of sterilizing children as well as rodents. Secondly, I would certainly support your plea for ecological studies as a basis for any rodent control campaign. Following a recommendation that was made at an informal meeting on Lassa fever in Geneva about a year ago, WHO made a request to the Government of the Federal Republic of Germany for funds to support studies on Mastomys natalensis in West Africa. We have just been informed that that Government has made a grant to WHO for a 3-year study in Nigeria or elsewhere on the ecology of Mastomys natalensis. We will certainly look to many people in this room for guidance and participation in that study. McLEAN: I would also like to support the idea of basic ecological studies. However, I do not feel so pessimistic about chemosterilants. I believe the problems associated with the use of chemosterilants have been in their appli- cation. To be effective, these agents should be used in combination with other approaches. Rodenticides may be applied first to kill a major portion of the population; those that are remaining are then sterilized. This is the only effective way to use chemosterilants. Environmental management can also be used as an adjunct. I would not discourage chemosterilant programme, because I think they can be effective if used at the proper time and in the proper place. BARNES: I would agree with that. Unfortunately, though, in the tropical environments we are a long way from being able to develop that kind of sophisticated pro- gramme. REEVES: I believe we should also consider the possibility that the stress of control measures may bring about population shifts and alterations in the genetic pools. Can such stress, whether due to chemosterilants or to toxicants, alter the susceptibility or the efficacy of the rodent as a host for the arenaviruses? This happens with other viral agents and their arthropod vectors. Con- tinued analysis of the pathogenesis of the arenaviruses and of the competence of the hosts will therefore be required. ISAACSON: I should like to point out that Lassa fever is not only a problem of West Africa but potentially a problem of the whole of Africa, excluding probably the north in the Sahara region. The whole area covers some 15 to 25 million square miles and is one enormous reservoir of Mastomys. During 50 years of experience in South Africa with rodent control, it has become evident that it is quite unrealistic to attempt eradication of a large rodent population. In addition, one factor that has not been mentioned in this Symposium is that in Africa we have very large game reserves, some 10 000 to 20 000 square miles in extent. How do we control rodent popula- tions without also controlling or, in fact, eradicating the game, which forms a very important source of income for many countries in Africa. SHOPE: There are two potential disease control methods involving rodents that you have not mentioned. One would be to immunize the animals or population of animals. The other, proposed several years ago in the Machupo rodent control programmes, would be to increase the acidity of the urine by dietary control and thereby to prevent transmission of the virus, which is less likely to survive in an acid urine. BARNES: Immunization has been considered for a long time in other contexts. A number of years ago we had some strains of Yersinia pseudotuberculosis that cross reascted immunologically with Yersinia pestis. The idea of immunizing certain rodent populations in North America seemed attractive in theory, but nearly impos- sible to apply in practice.