Practical Clinical Epidemiology for the Veterinarian (eBook)

Aurora Villarroel (Autor)

eBook Download: EPUB

2015
John Wiley & Sons (Verlag)
9781118472040 (ISBN)

Infos

Practical Clinical Epidemiology for the Veterinarian provides thorough coverage of the fundamentals of epidemiological concepts, situated within the context of daily clinical practice.

• Examines epidemiology from the lens of daily clinical practice to offer a truly practical approach
• Demonstrates the relevance of epidemiology to clinical problems faced in the field using practical examples to clarify the concepts
• Includes clinical cases from all species, with an emphasis on small animal and equine medicine, to demonstrate the concepts
• Uses an easy-to-read approach, with graphs, flowcharts, and tables to promote understanding
• Includes access to a companion website with exercises for study and review

Aurora Villarroel, DVM, MVPM, PhD, DACVPM, served as Assistant Professor of Clinical Sciences at the College of Veterinary Medicine and Extension Veterinarian in the Department of Animal Sciences at Oregon State University in Corvallis, Oregon. She taught epidemiology for 12 years at the veterinary schools of UC Davis, Colorado State University, and Oregon State University, and is currently a consultant at Athyr Vet, LLC.

Inhalt

Preface xi

Acknowledgments xiii

About the companion website xiv

1 Describing health and disease 1

Case definition 1

What is the problem? 2

Who is affected? 4

Where is the disease concentrated? 5

When does disease occur? 5

Types of measurements 5

Counts 5

Proportions 7

Ratios 8

Rates 9

Specific measurements of disease 10

Prevalence 10

Incidence 13

Morbidity 16

Mortality 16

Disease?]specific mortality 16

Case?]fatality 17

2 Basic epidemiology concepts 19

Outcome 19

Risk factor 21

Unit of analysis 22

Types of variables 26

Appropriate statistical analyses for continuous/parametric variables 27

Appropriate statistical analyses for categorical/nonparametric variables 29

Appropriate statistical analyses for multiple samples taken from the same animal 30

Control groups 33

Sample size and P?]value 34

Error and bias 35

Confounding 41

Interaction 42

3 Evidence?]based medicine for the veterinarian 44

Evaluation of a research paper 49

Data presentation in the results 53

Interpretation of results 57

Statistical significance 58

Biological significance 60

4 Study designs 62

Retrospective studies 63

Case-control studies 64

Surveys 65

Cross?]sectional studies 68

Prospective studies 70

Cohort studies 70

Clinical trials 72

Sampling strategies 73

5 Causation versus association 77

Hill's criteria to determine causation 78

Temporal association 78

Consistency of association 79

Specificity of association 79

Dose-response (biological gradient) 80

Biologic plausibility 81

Analogy 81

Measures of association 82

Odds ratio 84

Relative risk 89

Attributable risk 92

6 Diagnostic tests 94

Test quality 95

Accuracy 95

Precision 95

Discrimination ability 98

Test performance 99

Sensitivity 100

Specificity 102

Positive predictive value 104

Negative predictive value 105

Screening 107

Parallel testing 107

Serial testing 107

Gold standard 108

7 Outbreak investigations 109

Definitions 109

Steps in an outbreak investigation 110

Case definition/diagnosis verification 110

Determine the magnitude of the problem 111

Describe the spatial and temporal patterns of disease 112

Analyze potential risk factors 116

Follow?]up 119

Glossary 120

Formulas 125

Final word 128

References 129

Index 134

Rezensionen

"Practical Clinical Epidemiology for the Veterinarianis a new epidemiology textbook that is designed to be a "concise and straightforward" resource for practicing veterinarians." (Journal of the American Veterinary Medical Association, 15 December 2015)

Textauszug

1
Describing health and disease

Disease does not occur at random; if it were we would not have a job! There is a pattern for every disease; we just need to find it.

To find how disease behaves we need to answer the following questions:

• What is the problem?
• Who gets diseased?
• Where is the disease concentrated?
• When does disease occur?

Answering all these questions (the essence of epidemiology is describing disease in populations) should lead us to the answer of the ultimate question we have about a certain disease (why does it happen?) and enable us to prevent it.

Case definition

The best explanation of the true substance of the word “definition” in matters pertinent to epidemiology comes from combining two of the meanings of the “definition”: (i) an exact statement or description of the nature, scope, or meaning of something, and (ii) the degree of distinctness in outline of an object (Oxford Dictionaries online).1 Therefore, the more carefully we describe things, the more distinctness we achieve. In defining words, it is important to avoid using another word with the same root as the one we are defining. When defining a case, it tends to be more complete and accurate when following the same rule of not using words with the same root.

Example

When asked to define a diarrheic patient, simply stating it is a dog with diarrhea does not give much distinction to the case. However, if we define a diarrheic patient as a dog with feces that are not well-formed and cannot be picked up without leaving a mark on the ground gives a clear-cut characteristic that allows anyone to categorize a patient as having diarrhea or not.

What is the problem?

Before we start looking into who is diseased or where it is, we need to define what we are going to consider a diseased individual looks like; in other words, we need a case definition. This seems silly at first, but it is the most important step in any study or investigation and is not so clear-cut if you look deeper.

Example

Let us suppose we want to investigate if there is a problem of parvovirus in a kennel. How would you define a case of parvovirus? Most people would say a puppy with diarrhea. The problems with this simple definition of a case of parvovirus are as follows:

• There are other causes of diarrhea in puppies, so you may be overestimating how much parvovirus infection there truly is.
• Parvovirus may have asymptomatic infections, so you may be underestimating infection.
• Parvovirus can have other clinical signs without diarrhea, such as lethargy, anorexia, fever, vomiting, and severe weight loss, so you may be underestimating infection by looking only at puppies with diarrhea.
• How old can a dog be while still being considered a puppy? In other words, what is the “case definition” of a puppy?

To get the best estimate of truly infected dogs in a population, we would have to better define a case of parvovirus infection. An example could be “dogs less than 9 months old with a positive fecal ELISA test for parvovirus.” This definition would minimize the number of dogs with diarrhea due to other causes (because they have to have a positive ELISA test), and it would also minimize the number of dogs excluded because they did not have diarrhea.

The importance of case definition becomes paramount when comparing research studies about a certain disease. If two studies do not have the same case definition, the results of both studies cannot be compared directly.

Example

A study on hip dysplasia in dogs (Paster et al. 2005) showed that inclusion of the caudal curvilinear osteophyte in the definition of canine hip dysplasia significantly altered the diagnosis of a large proportion of dogs, usually toward a higher score but sometimes to a lower score (Figure 1.1).

Figure 1.1 Distribution (frequency [no.]) of subjective hip scores for dysplasia using two different definitions.

(Paster, E.R., LaFond, E., Biery, D.N., Iriye, A., Gregor, T.P., Shofer, F.S., and Smith, G.K. (2005). Estimates of prevalence of hip dysplasia in golden retrievers and Rottweilers and the influence of bias on published prevalence figures. Journal of the American Veterinary Medical Association, 226(3):387–392. © AVMA)

Another example is from a study on diagnosis of staphylococcal infections in a veterinary hospital (Geraghty et al. 2013). In this study, phenotypic appearance of cultured bacteria or genotypic analysis was used to determine which staphylococcal species was isolated from each animal. Figure 1.2 shows a summary of the data presented in the published paper, showing large mismatch in the results using one method versus the other.

Figure 1.2 Distribution of isolation of staphylococcal species defined via phenotypic or genotypic methods.

(data source Geraghty, L., Booth, M., Rowan, N., and Fogarty, A. (2013). Investigations on the efficacy of routinely used phenotypic methods compared to genotypic approaches for the identification of staphylococcal species isolated from companion animals in Irish veterinary hospitals. Irish Veterinary Journal, 66(1):7–15)

Case definition is of paramount importance in situations where a range of outcomes is possible. This is typical of outcomes that are measured by scores, which are used to establish a relative degree of the outcome when there is no directly measurable factor.

Example

In a study on gastric ulcers in pleasure horses (Niedzwiedz et al. 2013), the authors used a scoring system to determine the severity of the lesion. The scoring system they described is shown in Figure 1.3. Notice that with this description it would be possible to replicate the study using the same scoring system and therefore comparing results across studies. There could be only a potential problem in determining what “small” and what “large” lesions are—that is, a diameter threshold that would qualify a lesion as small or large. Therefore, it is better to always use objective characteristics to define cases or scores.

Figure 1.3 Lesion severity score description for a study on gastric lesions in pleasure horses.

(Niedzwiedz, A., Kubiak, K., & Nicpon, J. (2013). Endoscopic findings of the stomach in pleasure horses in Poland. Acta Veterinaria Scandinavica, 55:45–55)

Who is affected?

Remember we are looking for patterns of disease, so the question is whether the entire population is affected or there are some specific subgroups more affected than others? Any type of subgrouping can be investigated: age, gender, breed, environment, disposition (mainly used for companionship, racing, hunting, or other), diet, etc. To continue with the parvovirus example, we know that most affected animals are puppies and young dogs. Among the young dogs it is mostly males, in theory reflecting their higher tendency to roam lose compared with females.

An example for the environmental differences can be found in feline leukemia, a disease more common in multicat households and in cats that are allowed access to the outdoors.

You can surely find an example for different diets, breeds, etc.

Where is the disease concentrated?

Defining the spatial distribution of disease may help identify risk factors and the behavior of infection. A risk factor is any characteristic that increases the risk of an animal for a certain condition. For example, which horses get infected, those in pasture or those in the barn? Is the disease spreading to adjacent stalls or are apparently “random” stalls involved? Are neighboring farms affected too? Do affected animals live in specific areas such as downtown (smog), or close to wet areas?

When does disease occur?

Is there a pattern in time? How many animals are affected in winter versus summer, spring, and fall? Is there a difference in the number of diseased individuals before and after a given event (change in disinfectant, vaccination event, etc.)? Is there a cyclical nature to the disease that could coincide with mosquito season or freezing?

Evaluate the epidemic curve–temporal distribution of cases. The first case diagnosed in an outbreak is called the “index case.” A representation of the number of cases by days will show the type of epidemic curve of a disease (Figure 1.4). A “point-source” curve shows a high number of affected animals initially, which fades over time. This is typical of situations where many animals are exposed at the same time, like in outbreaks of food-borne diseases. A “propagated” epidemic curve shows a slow increase in the number of cases and a slow decrease too. This curve is typical of epidemics of infectious (contagious) diseases, where animals get exposed at different points in time (i.e., one animal gets infected and spreads the infection to a few others, which in turn infect others).

Figure 1.4 Epidemic curves: point-source (top) and propagated (bottom).

Answering the who, what, where, and when of a disease leads to the why...

Produkt-Details

• Erscheint lt. Verlag: 23.2.2015
• Sprache: englisch
• Themenwelt: Medizin / Pharmazie
• Themenwelt: Veterinärmedizin
• Schlagworte: animal • canine • Cat • Dog • Epidemiologie • Epidemiologie, Gesundheitswesen u. Statistik • epidemiology • Equine • Factors • feline • horse • Incidence • investigation • Morbidity • Mortality • outbreak • Outcome • Patterns • Prevalence • Studies • Trials • Variables • Veterinärmedizin • Veterinärmedizin • Veterinary • Veterinary Epidemiology, Public Health & Statistics • Veterinary Medicine
• ISBN-13: 9781118472040 / 9781118472040