Veterinary Epidemiology and Economics in Africa
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ILCA Manual No. 3 Veterinary epidemiology and economics in Africa A manual for use in the design and appraisal of livestock health policy S.N.H. Putt, A.P.M. Shaw, A J. Woods, L. Tyler and A.D. James International Livestock Centre for Africa •'— •"jOt^ "^f^-TT^W^^A. Veterinary epidemiology and economics in Africa A manual for use in the design and appraisal of livestock health policy S.N.H. Putt, A.P.M. Shaw, AJ. Woods, L. Tyler and A.D. James Veterinary Epidemiology and Economics Research Unit, Department of Agriculture, University of Reading, Reading, Berkshire, England First published in January 1987 Second edition, March 1988 Original English Designed and printed at ILCA Typeset on Linotype CRTronic 200 in Baskerville lOpt and Helvetica ISBN 92-9053-076-6 Acknowledgements ACKNOWLEDGEMENTS This manual was prepared largely from notes on lectures given by the authors. Many of the concepts introduced in the manual are not new and can be found in most standard textbooks on epidemiology, economics and statistics. The authors wish to acknowledge in particular the contribution of Schwabe, Riemann and Franti's "Epidemiology in Veterinary Practice", Leech and Sellers' "Statistical Epidemiology in Veterinary Science" And Gittinger's "Economic Anal ysis of Agricultural Projects", which provided inspiration for certain parts of this manual. "This One 49QX-EUE-2Y35 in Foreword FOREWORD The value of epidemiological investigation as a basis for the treatment and control of animal disease has been recognised for many decades, but the need to apply economic techniques to the formulation and assessment of disease control activities only became apparent about 15 years ago. This arose in part from burgeoning veterinary expenditure demands associated with new, but costly, technology and in part from growing awareness of the significant influ ence of economic and social factors on patterns of ill-health and disease. FAO published a collation of disease losses in 1963, but it was concern in WHO over the zoonoses which led to the first international initiative, at Reading University in 1972, to develop new methods for the economic, as well as epidemiological, evaluation of animal health programmes. Since then many national and international agencies have become involved and re search and training units have sprung up at several universities around the world. An inter national society and various national societies have also been formed to provide forums for discussion of the more profound understanding that is emerging of how to improve the health, welfare and productivity of animals. The team which has prepared this manual has demonstrated how representatives of a wide variety of disciplines can, and should, work to gether not only to control and avoid the major disease hazards which can still decimate ani mal populations, but also to define how genetics, management, nutrition and environmental adjustment can complement specific veterinary measures. Each member has contributed to a wide variety of research projects and field investigations over the past decade and in so doing, has crystalised a contribution to the training of disease control planners and animal health advisers in Britain and overseas. Recognising the need to provide such material for reference purposes and a wider range of training activities in Africa, ILCA and VEERU decided to join forces in publishing this manual. While Africa is the main focus, we feel sure that this manual will prove useful in other continents of the world and will further the long-term wellbeing of animals, in their many roles, as well as of people. P. R. ELLIS P.J. BRUMBY Director of VEERU, Director General, Department of International Agriculture and Livestock Centre Horticulture, for Africa, University of Reading, Addis Ababa, Great Britain Ethiopia Contents CONTENTS ACKNOWLEDGEMENTS iii FOREWORD v 1 . AN INTRODUCTION TO THE PLANNING AND EVALUATION OF DISEASE CONTROL POLICY 1 1.1 Introduction 1 1.2 The planning process 1 1.2.1 The systems approach to livestock development 1 1.2.2 Stages in the planning process 2 1.2.3 The role of various disciplines in the planning process 3 2. EPIDEMIOLOGY: SOME BASIC CONCEPTS AND DEFINITIONS 5 2.1 Introduction 5 2.2 Intrinsic determinants of disease 6 2.2.1 Disease agents as determinants of disease 6 2.2.2 Host determinants 10 2.3 Extrinsic determinants of disease 11 2.3.1 Climate 12 2.3.2 Soils 12 2.3.3 Man 12 2.4 Describing disease events in populations 13 3. THE USE OF DESCRIPTIVE STATISTICS IN THE PRESENTATION OF EPIDEMIOLOGICAL DATA 15 3.1 Introduction 15 vii Cimlenls 3.2 Tables and graphs 15 3.3 Bar and pic charts 17 3.4 Classification by variable 18 3.5 Quantification of disease events in populations 20 3.6 Methods of summarising numerical data 24 4. THE EPIDEMIOLOGICAL APPROACH TO INVESTIGATING DISEASE PROBLEMS 27 4.1 Introduction 27 4.2 Types of epidemiological study 28 4.2. 1 Prospective studies 28 4.2.2 Retrospective studies 28 4.2.3 Cross-sectional studies 29 4.3 Sampling techniques in epidemiological studies 30 4.3. 1 Random sampling 30 4.3.2 Multi-stage sampling 31 4.3.3 Systematic sampling 32 4.3.4 Purposive selection 32 4.3.5 Stratification 33 4.3.6 Paired samples 33 4.3.7 Sampling with and without replacement 33 4.4 Sample sizes 34 4.4.1 Sample sizes for estimating of disease prevalence in large populations 34 4.4.2 Sample sizes needed to detect the presence of a disease in a population 37 4.5 Methods for obtaining data in epidemiological studies 37 4.5.1 Interviews and questionnaires 37 4.5.2 Procedures involving measurements 39 4.5.3 Errors due to observations and measurements 39 4.6 Basic considerations in the design of epidemiological investigations 42 4.6.1 Objectives and hypotheses 42 4.7 The use of existing data 44 4.7.1 Advantages and disadvantages 44 4.7.2 Sources of data 45 Vlll Contents 4.8 Monitoring and surveillance 47 4.8.1 Epidemiological surveillance 47 4.8.2 Epidemiological monitoring 48 5. STATISTICAL METHODS IN THE ANALYSIS OF EPIDEMIOLOGICAL DATA 49 5.1 Introduction 49 5.2 Estimating population parameters 49 5.2.1 Estimating a population mean 49 5.2.2 Sample size needed to estimate a population mean 52 5.2.3 Estimating a population proportion or rate from a simple random sample 53 5.2.4 Estimating a rate or proportion from a cluster sample 54 5.3 Formulating and testing statistical hypotheses in large-sized samples 56 5.3.1 Testing for a difference in two means 56 5.3.2 Testing for a difference in two proportions 57 5.3.3 Sample size for detecting differences between two proportions in prospective and cross-sectional studies 59 5.3.4 Sample size for detecting differences between two proportions in retrospective studies 59 5.3.5 Testing for differences in prevalence between several groups simultaneously 59 5.3.6 Testing for differences in several means simultaneously 62 5.4 Formulating and testing hypotheses in small-sized samples 62 5.5 Matched comparisons 63 5.6 A word of warni ng 64 5.7 Linear correlation and regression 64 5.8 Time series 66 6. AN INTRODUCTION TO THE USE OF ECONOMICS IN THE PLANNING AND EVALUATION OF DISEASE CONTROL PROGRAMMES 69 6.1 Introduction 69 6.1.1 Basic philosophy 69 6.1.2 Application of economics in disease control policy 70 ix Contents 6.2 Prices appropriate for use in economic analyses 70 6.2.1 Theoretical aspects 70 6.2.2 Opportunity cost and the choice of prices in economic analysis 74 6.2.3 Adjusting for inflation - price conversions and price indexes 77 6.3 Compound interest, discounting, annual rates of growth and annual loan repayments 78 6.3.1 Simple versus compound growth (or interest) rates 78 6.3.2 Discounting and compounding tables 80 6.3.3 Estimating present and future values using annuity tables 81 6.3.4 Loan repayments 81 6.3.5 Interest or discount rates and inflation 82 7. ESTI MATING THE COSTS OF DISEASES AND THE BENEFITS OF THEIR CONTROI 83 7. 1 Introduction 83 7.2 Economic aspects of livestock production systems 83 7.2. 1 Inputs and outputs 83 7.2.2 Factors influencing output and offtake 84 7.2.3 The relationship between livestock prices and output 85 7.3 Estimating the cost of disease 86 7.3.1 Quantifying the direct losses due to disease 86 7.3.2 Methods for estimating annual losses 86 7.3.3 Losses due to disease acting as a constraint on production 90 7.3.4 Other losses due to animal diseases 90 7.3.5 Secondary effects, externalities and intangible effects 90 7.4 The costs of controlling disease 91 7.4.1 Introduction 91 7.4.2 The components of disease control costs 92 7.4.3 The importance of fixed and variable costs in planning disease control policy 93 8. ECONOMICS AND DECISION-MAKING IN DISEASE CONTROL POLICY 95 8.1 Introduction 95 8.2 The principles of partial analysis 95 Contents 8.3 The principles and criteria of benefit-cost analysis 96 8.3.1 The role of the discount rate 96 8.3.2 Dealing with inflation 97 8.3.3 Layout of a benefit-cost analysis 98 8.3.4 The decision-making criteria 99 8.3.5 Dealing with risk and uncertainty 101 8.3.6 The scope of a benefit-cost analysis 102 REFERENCES AND RECOMMENDED READING 103 APPENDIX ONE: TABLES 105 APPENDIX TWO: MODELLING IN VETERINARY EPIDEMIOLOGY AND ECONOMICS 1 1 1 1.