Epidemiology and Socioeconomics
Estimating the costs of theileriosis and the economics of its control
Studies at the farm level: Kenya
Studies at the national level: Zimbabwe
Geographic Models of the Distribution and Abundance of Ticks
Models of the Economic Impacts of Improved Disease Control
Models of East Coast Fever Transmission
Members of ILRAD's epidemiology and socioeconomics program conduct research into factors likely to enhance or impair disease control initiatives in Africa. The program has three main objectives: (1) to determine what governs the successful application of improved livestock disease control measures, particularly the widespread use of immunization for theileriosis and trypanosomiasis, under diverse circumstances; (2) to predict the likely epidemiological, social, economic and environmental outcomes of improving control of these diseases in given areas; and (3) to suggest control strategies that would most benefit particular agroecological areas, groups of cattle keepers and mixed farmers, and local and national governments.
The program has initially focused on theileriosis and other tick-borne diseases. (Theileriosis encompasses infections with several species of Theileria; in this report, unless otherwise stated, theileriosis refers only to infections with T. parva.) Effective procedures for immunizing cattle against theileriosis using broadly immunogenic stocks of Theileria parva parasites in the infection-and-treatment method are now available, and the prospects of developing genetically engineered vaccines for this and other tick-borne diseases are good. These improved and new technologies will put more effective tools for the control of tick-borne disease at the disposal of veterinarians and farmers in the eleven countries of eastern, central and southern Africa where theileriosis occurs (Figure 32).
Figure 32. The eleven countries of Africa where infections with Theileria parva greatly impede cattle production and improvement. The map shows the distribution of cattle, of the tick Rhipicephalus appendiculatus, the vector that transmits T. parva to cattle, and of areas where the presence of cattle and tick overlap. In general, the distribution of R. appendiculatus is synonymous with that of T. parva. However, in a few areas, such as western Zambia and coastal regions of South Africa, the tick is present without the parasite.
The new technologies being developed, however, will not be used to immunize all cattle in the theileriosis-endemic area. For example, where strong ENDEMIC STABILITY exists. such as areas where most Zebu cattle acquire immunity to tickborne diseases when they are calves through natural exposure to T. parva.no tick-borne disease control strategy may be required. More commonly, however, ENDEMIC INSTABILITY exists because the tick challenge is low or intermittent, because tick control is ineffective, or because highly susceptible taurine cattle (Bos taurus) or taurine-Zebu crosses are kept; in these areas, there will usually be strong justification for implementing new disease control measures.
The risk to livestock of T. parva infection varies considerably from area to area, depending on such variables as climate and vegetation, types of livestock kept, farm management and livestock production systems used and the type(s) of tickborne disease control, if any, currently practised. To determine which livestock populations would most benefit from immunization, these factors must be assessed to ensure that new disease control programs will be compatible with the needs of farmers, with the natural resources of an area, and with the priorities of governments. In this way, the viability as well as productivity of farming systems may be improved over the long term. To make accurate assessments of all the variables has required the development of suitably sensitive methodologies and technologies.
Members of the ILRAD program collaborate, particularly in the collection and analyses of data, with scientists and disease control officers in other organizations. These include the Australian National University, the Commonwealth Scientific and Industrial Research Organisation (Australia), Imperial College of the University of London (UK), the International Livestock Centre for Africa (ILCA, Addis Ababa), the Kenya Agricultural Research Institute (KARI), extension services of the Kenya Ministries of Agriculture and Livestock Development, Texas Agriculture and Mechanical University (USA), the United Nations Environment Programme (Nairobi) and the Zimbabwe Department of Veterinary Services. In much of this collaborative work, scientists from national agricultural research institutes are being trained to use the technologies developed at ILRAD so that research and control organizations throughout the continent may make assessments of the environmental and socioeconomic impact of improved livestock disease control in particular areas.
In 1988, the 11 countries where theileriosis occurs had an estimated human population of 175 million, or 29% of the total population of people in Africa. The total land area in the endemic region is estimated at 156 million hectares, or 19% of Africa's total. The percentages of national land areas affected by theileriosis vary widely, from less than 1% in Sudan to 22% in Kenya, 58% in Zimbabwe and almost 100% in Burundi. The region affected by theileriosis was estimated to have a cattle population of 24 million in 1988, or 13% of all cattle in Africa. In 10 countries (excluding Mozambique), livestock in 1986 contributed 21% of the total agricultural gross domestic product (GDP) of US$16 billion and 8% of the total national GDP of US$42 billion. The contribution of livestock to the agricultural GDP varied substantially from country to country, from about 5% in Burundi, Rwanda and Malawi to 31% in Tanzania and Zambia to 35% and 39% in Zimbabwe and Kenya, respectively.
Sparse and insufficient data exist on how theileriosis affects livestock production in endemic areas and what effect immunizing cattle by the infection-and-treatment method could have on disease occurrence and livestock productivity. In 1990, ILRAD staff developed methods to assess the costs of theileriosis and the economics of its control. Using a computer spreadsheet model, staff estimated the direct costs of the disease in 1989. Input values in the model were derived from data published in scientific literature and augmented by value assumptions and informed opinion of scientists with extensive knowledge of the disease in the field. Although not definitive, preliminary results of running the model provide the most complete estimate of the economics of theileriosis now available.
The total regional cost of theileriosis was estimated to be US$168 million in 1989, with an estimated mortality of 1.1 million cattle (Figure 33). The spreadsheet model was then run to test the effect of implementing immunization. The assumption was made that annual immunization using the infection-and-treatment method would be given to all calves under one year old as well as to susceptible adult cattle in, or introduced to, an endemic area. Results indicated that immunizing animals at a cost of US$2.50 per animal would yield high economic returns. The regional cost of the disease in this analysis was reduced by 26, 31 and 36% at a 0, 25 and 50% reduction in acaricide application; the benefit: cost ratio of implementing immunization was 9, 12 and 17, respectively. Results of another analysis indicated that immunization would remain economically viable up to a cost of US$4.75, 6.25 and 7.75 per animal with 0, 25 and 50% reduction in acaricide use, respectively. These estimates will be refined as further data become available.
Figure 33. Calculated economic losses in 1989 due to Theileria parva infections in eleven African countries. The total economic loss was estimated to be US$168 million. (Figures are in US$ millions.)
To determine target cattle populations for the application of vaccines to control East Coast fever (ECF), ILRAD continued to collaborate with national research institutes in Kenya in conducting detailed farm level studies in two contrasting areas of Kenya. In the Kaloleni Division of Kilifi District, a subhumid coastal region, ILRAD, KARI and ILCA are pooling expertise and resources to test the efficacy of the infection-and-treatment method under field conditions, to estimate the size and distribution of livestock populations, to assess and quantify the role livestock play in agriculture and society, to determine the prevalence of T. parva infection, and to determine the age at which infection most commonly occurs (by season) and case-fatality rates in susceptible calves. The farm-level data produced in these studies are being input in computer models to assess the economic impacts of using alternative ECF control strategies under various production systems.
Results of a series of studies undertaken by KARI on government and parastatal farms in Kenya's Coast Province showed that the infection-and-treatment immunization method effectively controlled ECF, as measured by the development of serum antibodies and a reduction in disease incidence. (The numbers of clinical reactors in these studies were generally low, about 1. 4%) A livestock census conducted in Kaloleni Division gives a breakdown of cattle numbers by type, age, location, herd size and agroecological zone. The survey indicates that 1.5% of the 57,516 cattle in the area were genetically improved taurine or taurine-cross cattle.
A farm enterprise study was conducted jointly by ILRAD, KARI and ILCA staff on a stratified sample of 77 farms in the division. The data collected confirm the important role livestock play in the African household economy, although wide variations were found among agroecological zones. Cattle enterprise generated a mean annual gross margin of US$210 per animal in taurine-cross herds and US$84 in Zebu herds. On average, livestock contributed 70% of farm investment, 40% of farm-generated income (and 12% of total household income), again with wide variations occurring among agroecological zones, types of cattle herds and grazing management systems. The contribution of livestock to investment and income tended to be higher in low rainfall areas, on farms that kept Zebu cattle and on farms that practised non-intensive management systems.
Similar studies will be made in Uasin Gishu District, a highland area (altitude 1680. 1980 m) in Kenya's Rift Valley Province, to provide a contrast to the Kaloleni surveys. In preparation for field trials of theileriosis immunization by infection and treatment in Uasin Gishu District, a project is under way to characterize the main farming systems in the district and to identify target populations for tick-borne disease control (Figure 34). Results of this study will portray both spatially and across the socioeconomic spectrum the variability in cattle management conditions and strategies likely to impinge on theileriosis risk and its effective control.
Figure 34. Staff members of ILRAD and the Kenya Agricultural Research Institute collecting blood samples in a survey of the presence of antibodies to Theileria parasites in cattle in Uasin Gishu District, Kenya. Results of this survey, conducted in September, will help researchers target which cattle populations in the district will most likely benefit from immunization by the infection-and-treatment method.
In 1989, farm data specific to each of Uasin Gishu District's 640 pre-independence Land Registration Units were collected and entered in a computerized database and a geographic information system computer software program. The 640 land units were stratified according to four key variables: agroecological zone, farm size, land tenure and cattle types. From data collected in 1990, ILRAD and KARI scientists made detailed descriptions of the variabilities in farming and livestock production systems in the district. These descriptions will be key considerations in the strategic implementation of improved tick-borne disease control programs. The data obtained have also been used to make a provisional typology of farming/ livestock production systems in Uasin Gishu District. East Coast fever control strategies may then be tailored to meet the characteristics and apparent requirements of these system types.
In preparation for field trials of the infection-and-treatment method of ECF control in Kaloleni Division, ILRAD scientists conducted an ethnoveterinary survey to gauge the depth of local knowledge of livestock diseases. Understanding how much farmers know about diseases and disease-control measures will help veterinary officers and extension agents to implement this and subsequent disease control programs. Interviews were conducted with 158 cattle-keepers, who volunteered a total of 515 identifiable cattle disease syndromes. In general, there was poor understanding of the role ticks play in spreading ECF and there was much overlap in classification of the diseases. This ambiguity suggests that any campaign directed at controlling a given disease should be identified to the public as a campaign aimed at the entire range of locally defined syndromes likely to correspond with the disease in question.
ILRAD staff continued studies in Kaloleni Division to determine the prevalence of antibodies to T. parva in animals of three age strata and of the two main cattle types (taurine-crosses and Zebu) of this area kept under three different management systems (zero, semi-zero and free-range grazing). A widespread prevalence of antibodies to T. parva was found in all age strata, in both cattle types and under all three production systems. This high prevalence rate demonstrates the widespread endemicity of ECF and a high level of population immunity, which, in the face of continued reports of ECF, suggests that infection, and thus mortality, occurs early in life. Studies are under way to determine at which age immunization should be carried out in Kaloleni Division to reduce mortality as well as to determine case-fatality rates following infection. Another study is correlating the presence of circulating antibodies to T. parva schizont antigen with the prevalence of immunity to the Marikebuni stock of T. parva.
Compulsory dipping. he regular application of acaricides to cattle. was introduced in Zimbabwe in 1914. Dipping was initially introduced to control ECF, but following the eradication of ECF in 1954, compulsory dipping was continued to control other forms of theileriosis (January disease and Corridor disease) as well as the tick-borne diseases babesiosis, anaplasmosis, heartwater and the harmful effects of ticks per se (called 'tick worry'). A breakdown in dipping services in the 1970s, during the war of independence, caused heavy losses from tick-borne diseases. Following independence, in 1980, intensive dipping was reintroduced, but since 1984, Zimbabwe's Veterinary Department has been moving away from a policy of total tick control to one of selective control and endemic stability.
In collaboration with workers in the Zimbabwe Department of Veterinary Services, members of ILRAD's Epidemiology and Socioeconomics Program have carried out a cost efficacy study that compares the control of ticks and tick-borne diseases in Zimbabwe using three alternative control strategies: an'intensive dipping' strategy, where cattle are dipped as often as twice a week; a 'reduced dipping' strategy, where cattle are dipped once a fortnight in the summer months and once a month the rest of the year, making a total of 21 acaricide immersions annually; and a 'strategic and minimal dipping' strategy, which combines strategic dipping (weekly dipping during the summer months, for a total of 12 immersions, supplemented by natural or artificially induced herd immunity to tick-borne diseases) and minimal dipping (a total of 4 immersions annually). The choice of 'intensive', 'reduced' or 'strategic and minimal' dipping, and the selection of diseases for immunization, will vary by region.
The study team has identified target cattle populations by zone for each control strategy and has made projections of the likely consequences of implementing the alternative strategies. The team assumed that strategic and minimal dipping would be applied after a two-year transitional period of reduced dipping. Resources that would be expended under each strategy were costed and projected over a 20-year period. Both the 'reduced' and the 'strategic and minimal' dipping strategies were predicted to be more cost effective than an intensive dipping strategy. Results of the study suggest that adoption of strategies that reduce acaricide use would ensure substantial savings for the Zimbabwe Government while causing no increase in the incidence of tick-borne diseases. The Zimbabwe Department of Veterinary Services is following up this study with a national survey of the prevalence of antibodies to tick-borne diseases for the purpose of validating the assumptions made in the cost-efficacy study.
To help predict the impact of implementing different tick and tick-borne disease control regimes in different farming systems and agroecological areas, ILRAD scientists are developing computer models, which are supported by spatial databases that have been assembled on factors pertinent to tick and tick-borne disease control. Data from BASELINE STUDIES at the farm, country, regional and continental levels are being obtained and input in various computer models. Future changes in disease control practice will be measured against these data. The databases are incorporated into computer-based geographical information systems, which are designed to handle large amounts of spatial data quickly and efficiently, allowing research workers rapidly to analyse, compare and combine data and present the results as maps (Figure 35).
Figure 35. Use of computer-based geographical information systems, designed to handle large amounts of spatial information, are enabling researchers at ILRAD to study relationships and interactions among many variables that determine the risks to cattle of developing theileriosis.
The graphical elements on these maps are linked to a considerable body of information held in a database manager. The data can be 'interrogated' by researchers and displayed as overlays, which illustrate the interaction of many variables. The maps and databases produced at ILRAD in this way will be used in conjunction with various computer models and techniques of artificial intelligence that are also being developed at the Laboratory to analyse geographically and systematically the factors that impinge on different ecological, sociological and economic environments, on different farming systems and on use of different disease control strategies for vector-borne diseases in Africa. (For more technical details, see the boxed paragraph in this chapter.) The methodologies being developed at ILRAD for assessing the impact of better control of theileriosis and other tick-borne diseases will subsequently be applied to other economically important African livestock diseases.
To further define the factors that determine the distribution and abundance of the tick vectors of theileriosis, four modelling approaches have been developed at ILRAD.
In June, a memorandum of understanding was signed by ILRAD and Texas Agricultural Experiment Station to initiate a collaborative project that will develop a simulation model for ex-ante economic analysis and prediction of the likely impacts of improved control of livestock diseases at the farm level. ILRAD scientists have already developed a spreadsheet model for estimating the productivity of cattle on farms where current disease control strategies are applied; in addition, as discussed above, this model has been run to estimate the economic importance of theileriosis and the economics of its control.
In collaboration with scientists at Imperial College, of the University of London, ILRAD workers have developed two models of endemic theileriosis. The first estimates rates of infection in a herd that are age and parasite-exposure dependent. The second was constructed to investigate the consequences of manipulating infection dynamics in a herd by immunizing cattle and by varying the breeds of animals kept. This model is also being used to study the effects of a carrier state on the dynamics of theileriosis transmission.
Using computer geographic information systems (GIS), members of ILRAD's Epidemiology arid Socioeconomics Program are developing comprehensive spatial databases that are helping to sort out the complexity of the large number of factors involved in the epidemiology, economics and sustainable control of ticks and tick-borne diseases of cattle at the farm, national and regional levels. Most of the GIS databases have been developed on IBM PS/2 386 microcomputers. GIS software now installed on the system includes pcARC/ INFO, a vector-based GIS program, and IDRISI; a grid-cell-based GIS/image processing system. A newer acquisition in the program is a Sun Sparcstation l graphics workstation, which will become the preferred GIS operating environment. The SUN supports GRASS, a grid-cell-based GIS and image processing system. ILRAD continued this year to collaborate with the Global Environmental Monitoring System (GEMS), a service provided by the United Nations Environment Programme (Nairobi) to manage global environmental information. The Global Resource Information Database (GRID), based within GEMS, is a network of centres that use computer technology to process and analyse environmental data. GRID staff in Nairobi continued in 1990 to give support to ILRAD's Epidemiology and Socioeconomics work by digitizing maps needed for the development of the program's databases. A Microvax III minicomputer at GRID also provides ILRAD with the necessary equipment for data- and processing-intensive models, such as the climate-matching program CLIMEX. Continental climatic databases have been used in the CLIMEX model for the tick species Rhipicephalus appendiculatus, R. zambeziensis and A. variegatum. ILRAD's ongoing collaboration with the Zimbabwe Department of Veterinary Services to study alternative tick and tick-borne disease control strategies has involved the development of an extensive GIS database that has been installed on a PS/2 microcomputer within the Veterinary Department. |
