The trypanosomiasis program at ILRAD
Modelling needs of the tick-borne diseases program
The socioeconomics program and perceived modelling needs in the areas of epidemiology, socioeconomics and environmental impact assessment
Modelling needs of FAO
International Laboratory for Research on Animal Diseases
P.O. Box 30709
The International Laboratory for Research on Animal Diseases (ILRAD) has developed a research strategy for devising improved methods of trypanosomiasis control on the basis of four premises. First, control of both tsetse- and non-tsetse-transmitted trypanosomiasis will rely for the next decade on the use of the existing chemoprophylactic and chemotherapeutic compounds in livestock species and, where appropriate, on control of the tsetse fly vector. Second, trypanotolerance is an under-exploited trait which can potentially provide a particularly sustainable means of improving livestock productivity in both tsetse-and non-tsetse-infested areas. Third, development of new means of immunological or chemical control of trypanosomiasis will require continued research effort on parasite biology, host-parasite interactions and host immunology and pathology. Fourth, the effectiveness and sustainability of existing and new control measures will be enhanced by a better understanding of the epidemiology of trypanosomiasis. Among the objectives, those to be achieved in the shorter term are to:
· identify genetic markers of epidemiologically important parasite traits and to determine the limits and extent of genetic exchange occurring between trypanosomes in the field;
· determine the nature of immune cell activation and dysfunction during infection and to identify the parasite molecules responsible;
· determine to what extent defective bone marrow function contributes to the anaemia of trypanosomiasis and to identify parasite and host factors responsible for anaemia development;
· determine the critical factors in host-parasite interactions controlling in vivo parasite growth rates and host-parasite accommodation as it occurs in the carrier state;
· identify genetic markers of trypanotolerance in N'Dama cattle; and
· determine the epidemiological factors in representative field circumstances which are critical for productivity impacts and disease maintenance.
The longer-term objectives are to:
· determine the impacts of control measures on disease, productivity and profitability in representative and defined circumstances in selected study sites;
· develop decision aids for disease controllers to guide design of control strategies based on epidemiological, social, environmental and economic considerations;
· assess the vaccination potential of selected parasite antigens; and
· identify, isolate and characterize trypanotolerance genes in large and small ruminants.
It is envisaged that modelling may assist research progress in several areas. These are development of a more refined understanding of the epidemiology of trypanosomiasis, of the host immune response and host-parasite interactions, and of parasite and host genetics. Further, it is likely that models per se will be essential components of any decision aids which the Program may develop in order to rationalize the disease control process.
International Laboratory for Research on Animal Diseases
P.O. Box 30709
The goals of the Tick-Borne Diseases Program are the investigation of aspects of the biology of Theileria species as they relate to disease control; the development of parasite characterization reagents for epidemiological and immunological studies; the production of defined Theileria populations; the identification of antigens of Theileria and other tick-borne pathogens that may be used for vaccination or diagnosis; the development of vaccines against Theileria and other tick-borne pathogens of cattle based upon characterized antigens and delivery systems; and the evaluation of novel vaccines and the provision of support for tick-borne disease control by national and international organizations. The primary research focus is to develop a subunit vaccine for Theileria parva, the cause of East Coast fever of cattle, to replace an effective but difficult to apply infection-and-treatment method. The infection-and-treatment method is being adopted gradually throughout the eastern, central and southern African region in a complicated environment of tick-borne diseases where the traditional control method has been short-interval acaricide application to cattle. Future research will involve the development of novel vaccines for anaplasmosis, babesiosis and cowdriosis in collaboration with other laboratories working directly on the protective antigens of the causal organisms.
Models exist for the transmission dynamics of T. parva to and from cattle but we know little of transmission from reservoir hosts. ECFXPERT is a site-specific simulation model that can be used to predict changes in tick populations and the incidence of East Coast fever in response to environmental and management factors. This model requires further development to make it more useful under a wider range of circumstance as new data are generated. A large database has been assembled on tick stabilate-induced infections in cattle and its transmission to ticks which may contribute to model construction. The development of new reagents for more specific detection of parasites and antibodies in mammalian hosts and for assessing infections in ticks, together with new information on the biology of the parasites, will greatly improve the quality of epidemiological data for inclusion in such models.
Attempts have been made to model the dynamics of T. parva development in cattle but critical data, such as on the rate of multiplication of the parasitized lymphocytes, are not available. The kinetics of immune responses are being elucidated and will be critical data for model development. However the dose of infecting sporozoites is the major factor influencing the severity and outcome of infection. New vaccines are likely to be based upon sporozoite antigens that will block or reduce the number of lymphocytes infected, and/or schizont antigens expressed on lymphocytes that will limit infected cell proliferation and arrest the development of the pathological effects. Animals immunized with a recombinant sporozoite antigen show variable responses to a standard challenge with an overall protection of 70%. The proportion of sporozoites escaping neutralization and establishing as schizonts determine the outcome of challenge. In field situations further differences may be introduced depending upon virulence of the challenge parasite and age, nutrition status and the relative susceptibility of the animal. Modelling in this area may assist in the design of new vaccines.
The development of new models or improvement of existing models should allow us to provide more accurate definition of tick-borne diseases, their vectors, the interplay between different pathogens, hosts and reservoirs, for epidemiological studies, maintenance of disease surveillance, monitoring disease spread or selecting target populations for different control measures. The development and application of models should also identify areas that require improved data or research effort for greater understanding of any one disease or disease interactions. It may be possible to model the impact of the introduction of new parasite types into defined parasite populations and to determine the influence of sexual recombination, now at the level of genetic markers but later at the immunogenic level.
International Laboratory for Research on Animal Diseases
P.O. Box 30709
The Socioeconomics Program was initiated at ILRAD in 1987, and aims to identify the factors which govern the successful application of improved disease control measures for livestock disease, with emphasis on trypanosomiasis and theileriosis, and to assess the likely impact of improved disease control in economic, social and environmental terms in different locations and under different management conditions. The Program has four objectives, two of which (numbers 2 and 4 below) in particular lend themselves to the use of a modelling approach. The objectives are to:
· identify the factors which are essential for the successful production, delivery and adoption of improved control measures for livestock diseases;
· determine the probable economic, social and environmental consequences of the application of improved control measures for tick-borne diseases and trypanosomiasis in different locations, production systems and agroecological zones of the world;
· support ILRAD's tick-borne diseases and trypanosomiasis programs in assessing the cost-effectiveness of alternative research options for the development of disease control technologies and be of use to international, regional and national livestock disease control programs; and
· quantify and predict the relative economic importance of the infectious and non-infectious diseases of livestock in different regions and livestock production systems in Africa and elsewhere, and the justification for their control.
Assessing the economic importance of diseases and the impact of disease control relies on the use of accurate data on the current distribution and occurrence of diseases, and on their effect under different conditions of livestock production economics. With the scarcity of such data with regard to tick-borne diseases in the African continent, the Program started by using existing models to predict the distribution of the tick Rhipicephalus appendiculatus as a surrogate for Theileria parva distribution data. Two models were used, CLIMEX and BIOCLIM, both run on interpolated climate surfaces for the continent. While reasonably effective in predicting current and potential distribution ranges, neither of these models effectively predicts tick abundance on livestock under different conditions, an important prerequisite to predicting the occurrence of the diseases they transmit. At present, it is unclear whether other available models, such as T3HOST and ECFXPERT could be developed to fulfil this function.
An initial attempt has been made to model the dynamics of Theileria parva infection under very specific circumstances of endemic stability (see Medley, G.F., this volume), and this has shown that it is possible to predict incidence and case-fatality under these circumstances. It is important that this modelling approach be applied to other sets of circumstances to assess its value as a predictor of theileriosis occurrence under varying conditions.
Economic impact models developed by the Program have thus relied so far for their calculations on estimates of disease incidence taken from the literature from diverse studies carried out over the years, but they have made use of the developed distribution models in determining the proportion of cattle populations at risk using simple overlay techniques in a geographical information system (GIS). Two economics models have been developed. The first is a deterministic spreadsheet model that can be applied on a herd, on a national or regional basis, and calculates beef, milk, animal traction and manure losses due to morbidity and mortality associated with theileriosis. It also assesses the economic impact of control measures in terms of benefit-cost ratio (BCR). The second model is a stochastic farm-level simulation model which simulates the annual production and consumption aspects of a farm household over a ten-year period, and measures, in terms of BCR and other indicators, the effect of disease control programs at the farm level.
Both these economic models, and a third developed by Alexandra Shaw (A.P. Consultants, UK), are being tested under different conditions to assess their validity, and then refined and applied to both theileriosis and trypanosomiasis control. Their successful application to these and other diseases will depend on the accuracy of data on distribution, occurrence and effect of diseases. Although attempts have been made to model these indicators, none of the currently available models are developed to a stage that will allow their strategic application over widely differing circumstances.
A further challenge will be to extend economic impact assessments to include sociological and factor effects of control measures on the environment, particularly in the case of trypanosomiasis. In a new project just initiated, research will emphasize the potential implications of trypanosomiasis control on regional environments using modelling and GIS analyses, and drawing on results of field studies.
Animal Production and Health Division
Food and Agriculture Organization
Via delle Terme di Caracalla
00100 Rome, Italy
In attempting to define the modelling requirements of FAO in the control of vector-borne diseases and other parasitic infections, it may be appropriate to reflect on how disease control and the efforts of the Animal Health Service are integrated into the overall objectives of the organization.
The containment of livestock diseases is not the sole objective of FAO but merely a component in an effort to enhance livestock production, better land usage and agricultural resource management, which should result in the development of more profitable and sustainable production systems. Thus FAO's efforts are focusing on achieving among other things the following:
· Improvements in the efficiency of the production and distribution of all food and agricultural products.
· Improvement in the levels of nutrition and standard of living.
· Better conditions for the rural populations through employment and income generation.
· Natural resource conservation and environmental protection.
Considering these overall objectives on one side and the complex inter-relationship which exists between the hosts, the parasites, the environment, the production systems and the available economic resources on the other, integrated disease control has become extremely complicated and FAO realizes that only through an increased use of modelling can the amount of data involved be manipulated and made accessible.
Before entering into a brief description of FAO's needs in relation to modelling of the specific diseases which are on the agenda of this meeting, it should be emphasized that FAO is mainly an applier, a user of models and as such is only marginally involved in developing models through projects and research contracts with individual scientists and laboratories. FAO, therefore, is looking to this workshop for guidance in relation to the possible future use of available models, the potential for their immediate applicability in the-field, possible adaptation and future prospects of developing user-friendly models.
Having established FAO's main objectives in livestock disease control and related development aspects, it may be relevant to consider the geographical areas and the main components in the development of sound control programs which should receive priority attention.
Without ignoring Asia and Latin America, ILRAD and to a large extent FAO have a major interest in animal disease control in sub-Saharan Africa and the demographic explosion in this area requires an urgent response to the increased demand for food production. Considering that the region in spite of possessing 20 percent of the world's permanent pastures only produces 3 percent of the global livestock production, it appears that the potential for livestock development is present, justifying the allocation of resources into disease control in the region.
Data collection, collation, analysis and dissemination has always been a major activity of FAO, but it is still a major concern of the Animal Health Service that lack of data on animal and human populations, livestock movements, disease occurrence, production systems and many other components which is necessary for the development of sustainable livestock production systems still seems to be the single most important constraint for the development of disease control programs.
Improved data collection will enhance the potential for using modelling in the determination of the ecology of vectors and parasites and the epidemiology of parasitic diseases. FAO has a major responsibility with regard to data collection through projects and programs and collaboration with modellers and epidemiologists during the project preparation and implementation phases is essential in order to ensure the highest possible quality of data and results.
It is often assumed that following the establishment of the ecology-and epidemiology of vectors and parasitic diseases, control programs can be developed. However, the information available on climatic, environmental and other factors may only cover part of the extensive variations in agro-ecological zones or sub-units of a production system and only through the use of models will it be possible to develop comprehensive disease control programs based on the obtained data and agricultural, environmental and socioeconomic components.
FAO would also welcome an increased use of models for the implementation and appraisal of disease control schemes as this would improve the monitoring of the dynamic changes of environmental and biological factors enabling the organization to rapidly adapt to the changes.
While there are specific modelling needs for all three groups of diseases which have been targeted by this workshop, the need may be particularly important in the area of the economic impact of these diseases and anthelmintic resistance which has reached emergency proportions in several areas of Latin America and Africa.
There is a growing awareness of the fact that FAO will have to rely on a multidisciplinary approach in the preparation of economically and environmentally sustainable production systems including parasitic disease control. Information is, however, often sparse in many areas regarding ongoing changes of the general ecology, patterns of crop production, disease pressures, resource availability, market forces and improved technology. Without this information it is difficult to define effective disease control programs, let alone strategies for sustainable farming, land usage and resource management. It is obvious that the availability of modelling technologies which will enable the user to assess the importance of these factors either in isolation or combined would constitute a unique tool for FAO and other international agencies and governments involved in the development and implementation of production systems, disease control and the study of market forces.