MICHAEL THRUSFIELD is Professor of Veterinary Epidemiology at the Royal (Dick) School of Veterinary Studies in the University of Edinburgh in Edinburgh, UK.

ROBERT CHRISTLEY is Professor of Epidemiology and One Health at the Institute of Infection and Global Health and the Institute of Veterinary Science in the University of Liverpool in Liverpool, UK.

Contributors xviii From the preface to the first edition xix From the preface to the second edition xx From the preface to the third edition xxi Preface to the fourth edition xxii About the companion website xxiv 1 The development of veterinary medicine 1Michael Thrusfield Historical perspective 1 Domestication of animals and early methods of healing 1 Changing concepts of the cause of disease 2 Impetus for change 5 Quantification in medicine 10 Contemporary veterinary medicine 12 Current perspectives 12 The fifth period 19 Recent trends 20 Further reading 25 2 The scope of epidemiology 28Michael Thrusfield Definition of epidemiology 28 The uses of epidemiology 29 Types of epidemiological investigation 32 Epidemiological subdisciplines 33 Components of epidemiology 35 Qualitative investigations 35 Quantitative investigations 36 Epidemiology's locale 39 The interplay between epidemiology and other sciences 39 The relationship between epidemiology and other diagnostic disciplines 40 Epidemiology within the veterinary profession 40 Further reading 41 3 Causality 42Michael Thrusfield Philosophical background 42 Causal inference 43 Methods of acceptance of hypotheses 44 Koch's postulates 45 Evans' rules 45 Variables 46 Types of association 46 Non-statistical association 46 Statistical association 46 Confounding 49 Causal models 50 Formulating a causal hypothesis 53 Methods of deriving a hypothesis 53 Principles for establishing cause: Hill's criteria 55 Further reading 56 4 Describing disease occurrence 58Michael Thrusfield Some basic terms 58 Basic concepts of disease quantification 61 The structure of animal populations 62 Contiguous populations 62 Separated populations 65 Measures of disease occurrence 67 Prevalence 67 Incidence 67 The relationship between prevalence and incidence rate 70 Application of prevalence and incidence values 72 Mortality 72 Survival 73 Example of calculation of prevalence, incidence, mortality, case fatality and survival 75 Ratios, proportions and rates 76 Mapping 80 Geographic base maps 80 Further reading 84 5 Determinants of disease 86Michael Thrusfield Classification of determinants 86 Host determinants 89 Genotype 89 Age 90 Sex 91 Species and breed 92 Behaviour 93 Other host determinants 93 Agent determinants 94 Virulence and pathogenicity 94 Gradient of infection 97 Outcome of infection 98 Microbial colonization of hosts 100 Environmental determinants 101 Location 101 Climate 101 Husbandry 104 Stress 105 Interaction 106 Biological interaction 108 Statistical interaction 109 The cause of cancer 110 Further reading 112 6 The transmission and maintenance of infection 115Michael Thrusfield Horizontal transmission 115 Types of host and vector 115 Factors associated with the spread of infection 118 Routes of infection 121 Methods of transmission 123 Long-distance transmission of infection 125 Vertical transmission 129 Types and methods of vertical transmission 129 Immunological status and vertical transmission 129 Transovarial and trans-stadial transmission in arthropods 130 Maintenance of infection 131 Hazards to infectious agents 131 Maintenance strategies 132 Transboundary diseases 135 Further reading 136 7 The ecology of disease 138Michael Thrusfield Basic ecological concepts 139 The distribution of populations 139 Regulation of population size 142 The niche 148 Some examples of niches relating to disease 150 The relationships between different types of animals and plants 152 Ecosystems 155 Types of ecosystem 156 Landscape epidemiology 158 Nidality 159 Objectives of landscape epidemiology 161 Landscape characteristics determining disease distribution 164 Further reading 165 8 Patterns of disease 168Michael Thrusfield Epidemic curves 168 Kendall's Threshold Theorem 168 Basic reproductive number (R 0) 169 Dissemination rate 172 Common-source and propagating epidemics 172 The Reed-Frost model 173 Kendall's waves 175 Trends in the temporal distribution of disease 177 Short-term trends 177 Cyclical trends 178 Long-term (secular) trends 179 True and false changes in morbidity and mortality 180 Detecting temporal trends: time series analysis 180 Trends in the spatial and temporal distribution of disease 186 Spatial trends in disease occurrence 186 Space-time clustering 186 Further reading 187 9 Comparative epidemiology 189Michael Thrusfield Types of biological model 189 Cancer 191 Monitoring environmental carcinogens 191 Identifying causes 192 Comparing ages 193 Some other diseases 196 Diseases with a major genetic component 196 Some non-infectious diseases 197 Diseases associated with environmental pollution 198 Reasoning in comparative studies 199 Further reading 199 10 The nature of data 201Michael Thrusfield Classification of data 201 Scales (levels) of measurement 201 Composite measurement scales 204 Data elements 205 Nomenclature and classification of disease 205 Diagnostic criteria 207 Sensitivity and specificity 208 Accuracy, refinement, precision, reliability and validity 209 Bias 210 Representation of data: coding 210 Code structure 211 Numeric codes 212 Alpha codes 213 Alphanumeric codes 214 Symbols 215 Choosing a code 215 Error detection 216 Further reading 217 11 Data collection and management 219Michael Thrusfield Data collection 219 Questionnaires 219 Quality control of data 228 Data storage 229 Database models 229 Non-computerized recording techniques 231 Computerized recording techniques 232 Veterinary recording schemes 232 Scales of recording 232 Veterinary information systems 234 Some examples of veterinary databases and information systems 237 Geographical information systems 244 Further reading 248 12 Presenting numerical data 251Michael Thrusfield and Robert Christley Some basic definitions 251 Some descriptive statistics 252 Measures of position 253 Measures of spread 254 Statistical distributions 254 The Normal distribution 254 The binomial distribution 255 The Poisson distribution 255 Other distributions 256 Transformations 256 Normal approximations to the binomial and Poisson distributions 257 Estimation of confidence intervals 257 The mean 257 The median 258 A proportion 258 The Poisson distribution 259 Some epidemiological parameters 260 Other parameters 261 Bootstrap estimates 261 Displaying numerical data 262 Displaying qualitative data 262 Displaying quantitative data 263 Monitoring performance: control charts 266 Further reading 269 13 Surveys 270Michael Thrusfield and Helen Brown Sampling: some basic concepts 270 Types of sampling 272 Non-probability sampling methods 272 Probability sampling methods 272 What sample size should be selected? 275 Estimation of disease prevalence 275 Detecting the presence of disease 284 The cost of surveys 290 Calculation of confidence intervals 290 Further reading 294 14 Demonstrating association 296Michael Thrusfield Some basic principles 296 The principle of a significance test 296 The null hypothesis 297 Errors of inference 297 Multiple significance testing 298 One- and two-tailed tests 298 Independent and related samples 299 Parametric and non-parametric techniques 299 Hypothesis testing versus estimation 300 Sample-size determination 300 Statistical versus clinical (biological) significance 300 Interval and ratio data: comparing means 302 Hypothesis testing 302 Calculation of confidence intervals 303 What sample size should be selected? 304 Ordinal data: comparing medians 304 Hypothesis testing 304 Calculation of confidence intervals 308 What sample size should be selected? 309 Nominal data: comparing proportions 309 Hypothesis testing 310 Calculation of confidence intervals 313 What sample size should be selected? 314 2 test for trend 314 Correlation 316 Multivariate analysis 317 Statistical packages 318 Further reading 318 15 Observational studies 319Michael Thrusfield Types of observational study 319 Cohort, case-control and cross-sectional studies 319 Measures of association 321 Relative risk 321 Odds ratio 323 Attributable risk 325 Attributable proportion 327 Interaction 328 The additive model 328 Bias 330 Controlling bias 332 What sample size should be selected? 335 Calculating the power of a study 336 Calculating upper confidence limits 337 Further reading 338 16 Design considerations for observational studies 339Robert Christley and Nigel French Descriptive observational studies 339 Analytical observational studies 340 Design of cohort studies 340 Design of case-control studies 346 Design of cross-sectional analytical studies 352 Overview of other study designs 354 Further reading 359 17 Clinical trials 361Michael Thrusfield Definition of a clinical trial 361 Design, conduct and analysis 364 The trial protocol 364 The primary hypothesis 364 The experimental unit 367 The experimental population 368 Admission and exclusion criteria 368 Blinding 369 Randomization 369 Trial designs 370 What sample size should be selected? 372 Losses to follow-up 373 Compliance 373 Terminating a trial 374 Interpretation of results 374 Meta-analysis 375 Goals of meta-analysis 376 Components of meta-analysis 377 Sources of data 377 Data analysis 378 Further reading 380 18 Validity in epidemiological studies 383Robert Christley and Nigel French Types of epidemiological error 383 Accuracy, precision and validity in epidemiological studies 384 Background factors 385 Interpretation bias 385 Selection bias 386 Examples of selection biases 387 Information bias 390 Examples of information biases 390 Statistical interaction and effect-measure modification 392 Confounding 392 Criteria for confounding 393 Confounding and causal diagrams 394 Controlling confounding 394 Errors in analysis 395 Communication bias 395 Further reading 396 19 Systematic reviews 397Annette O'Connor, Jan Sargeant and Hannah Wood Evidence synthesis 397 Overview of systematic reviews 397 Differences between systematic reviews and narrative reviews 398 Questions that are suitable for systematic reviews 398 Types of review questions suitable for systematic reviews 399...