Veterinary Epidemiology Basic Concepts
Veterinary Epidemiology Basic Concepts
K. Rajkumar Assistant Professor (Senior Scale) Department of Veterinary Medicine Rajiv Gandhi Institute of Veterinary Education and Research Formerly : Rajiv Gandhi College of Veterinary and Animal Sciences Puducherry 605009
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Contents
1. Introduction to Veterinary Epidemiology .............................
1
1.
Progress in Epidemiology ..................................................
2
2.
Definition of Epidemiology ................................................
4
3.
The Uses of Epidemiology ..................................................
8
4.
The Major Application of Epidemiology (In Veterinary Medicine) ........................................................................... 13
2. Ecological Concepts of Epidemiology .................................... 15 1.
Ecology ............................................................................... 15
2.
Population Ecology ............................................................ 16
3.
Applied Ecology ................................................................. 16
4.
Biotope ................................................................................ 16
5.
Biotic Community or Biocenosis ........................................ 17
6.
Ecosystem ........................................................................... 17
7.
Basic Ecological Concepts .................................................. 17
8.
Niche ................................................................................... 22
9.
The Relationships between different types of Animals and Plants ........................................................................... 25
10. Ecosystem ........................................................................... 26 11. Landscape Epidemiology (Medical Ecology, Horizontal Epidemiology, Medical Geography) .................................. 29 3. Components of Epidemiology ................................................. 33 1.
Qualitative Investigations .................................................. 33
2.
Quantitative Investigations ................................................ 34
3.
Disease Control ................................................................. 38
v
Contents/vi 4. Epidemiology: Types and Methods ......................................
39
5. The Cause of Disease (Disease Process) .............................
45
1. 2. 3. 4. 5. 6.
Koch’ Postulates ............................................................... Evans’ Postulates .............................................................. Variables ........................................................................... Type of Association .......................................................... Causal Factor .................................................................... Confounding .....................................................................
45 46 47 48 49 53
6. Causal Hypothesis ..................................................................
57
1. 2.
Formulating a Causal Hypothesis ................................... Principles should be considered to Establish a Causal Association ........................................................................
7. Agent, Host and Environmental Strategies of Epidemiology ........................................................................... 1. Determinants .................................................................... 2. Determinants Associated with Host, Agent and Environment ..................................................................... 3.
58 59 61 61 62
Interaction .........................................................................
74
8. Pattern of Diseases ................................................................. 1. Endemic (enzootic) ........................................................... 2. Epidemic (epizootic) ......................................................... 3. Pandemic (panzootic) ....................................................... 4. Sporadic ............................................................................. 5. Exotic ................................................................................. 6. Exzootic ............................................................................. 7. Trends in the Temporal Distribution of Diseases ............ 8. Epidemic Curve .................................................................
75 75 76 81 81 82 83 84 86
9. The Transmission of Infection .............................................. A. Horizontal (lateral) ........................................................... B. Vertical .............................................................................. C. Horizontal Transmission .................................................. 1. Types of Host and Vector .................................................
89 89 89 91 92
2.
Vertical Transmission ........................................................ 106
Contents/vii 10. Maintenance of Infection ....................................................... 109 1.
Hazards to Infectious Agents ........................................... 109
2.
Maintenance Strategies ..................................................... 110
11. Epidemiological Studies ......................................................... 113 1.
Descriptive Studies ............................................................ 114
2. Analytical Epidemiology .............................................. 116 3.
Four Basic Steps in Conducting a Case Control Study ... 119
4.
Nested Case –Control Study ............................................ 126
5.
Ecological Studies .............................................................. 127
6.
Experimental Epidemiology .............................................. 127
12. The Control and Eradication of Diseases ............................ 131 1.
Control ............................................................................... 134
2.
Eradication ........................................................................ 134
3.
Elimination ........................................................................ 135
4.
Prevention ........................................................................ 135
5.
Strategies of Control and Eradication .............................. 137
6.
Important Factors in Control and Eradication Programmes ....................................................................... 142
13. Sources of Epidemiological Data .......................................... 145 1.
Data ................................................................................... 145
2.
Sources of Epidemiological Data ...................................... 147
14. Scales or Levels of Measurement .......................................... 151 1.
The Nominal (Classificatory) Scale .................................. 151
2.
The Ordinal (Ranking) Scale ............................................ 151
3.
The Interval Scale .............................................................. 151
4.
The Ratio Scale .................................................................. 152
5.
The Visual Analogue Scale ............................................... 152
15. Evaluation of Diagnostic Tests .............................................. 153 1.
Importance ........................................................................ 153
2.
To Evaluate the Accuracy of Tests ................................... 153
Contents/viii 3.
Relation between Sensitivity and Specificity .................... 154
4.
Selection of Tests Based on Sensitivity and Specificity .... 155
5.
Calculation of Sensitivity Specificity ................................ 155
16. Epidemiological Measurements ............................................ 157 1.
Prevalence .......................................................................... 157
2.
Incidence ............................................................................ 158
3.
Attack Rate (AR) ............................................................... 159
4.
Secondary Attack Rate (SAR) .......................................... 160
5.
Cumulative Incidence ....................................................... 160
6.
Mortality ............................................................................ 160
7.
Cumulative Mortality ........................................................ 160
8.
Mortality Rate .................................................................... 161
9.
Death Rate ......................................................................... 161
10. Case Fatality ...................................................................... 161 11. Survival .............................................................................. 161 17. Measure of Productions .......................................................... 163 1.
Ratio ................................................................................... 164
2.
Vaccine Evaluation ............................................................ 164
18. Measurement of Causal Association .................................... 167 1.
Relative Risk (Risk ratio, R.R.) .......................................... 167
2.
Chi-Square Test for Independence of Two Attributes ..... 169
3.
Chi-Square Test (McNemar’s) Applied to Differences between Two Correlated Proportions .............................. 170
19. Principles of Survey ................................................................ 173 1.
Types of Survey ................................................................. 173
2.
Sampling ............................................................................ 174
20. Mapping ................................................................................... 181 1.
Geographic Base Maps ...................................................... 181
2.
Geographical Information System .................................... 182
Contents/ix 21. Disease Outbreak Investigation ........................................... 185 1.
Investigation of an Epidemic ............................................ 185
2.
Propagated Epidemic ........................................................ 185
3.
Steps in Investigating a Propagated Epidemic ................ 186
4.
Point Epidemics (Steps in Investigating a Point Epidemic) ........................................................................... 187
22. Epidemiological Software ...................................................... 189 1.
EPIZOO ............................................................................. 191
2.
Win Episcope 2 .................................................................. 192
23. Animal Disease Forecasting .................................................. 195 1.
Early Warning Contingency Planning ............................. 198
2.
Disease Surveillance .......................................................... 199
3.
Passive Disease Surveillance ............................................. 199
4.
Active Disease Surveillance .............................................. 199
24. Economics of Animal Diseases ............................................. 203 1.
Livestock Productivity and the Effects of Disease ........... 204
2.
The Cost of Disease and the Principle of Marginal Returns ............................................................................... 205
3.
Level and Type of Analysis ............................................... 205
4.
Data Requirements for Economic Assessment of Disease Control Projects ................................................................. 206
25. OIE (Office International Des Epizooties) .......................... 207 1.
Objectives ........................................................................... 208
2.
Scientific Information ........................................................ 208
3.
International Solidarity ..................................................... 208
4.
Sanitary Safety .................................................................. 209
5.
Promotion of Veterinary Services ..................................... 209
6.
Food Safety and Animal Welfare ..................................... 210
Glossary of Terms ......................................................................... 211 References ....................................................................................... 219
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