VEM 5503 Veterinary Epidemiology Cross sectional studies

Transcription

1 VEM 5503 Veterinary Epidemiology Cross sectional studies Jorge A Hernandez UF CVM LACS Deriso Hall Rm 120 Hernandezja@ufl.edu VEM 5503 Veterinary Epidemiology Cross sectional studies Objectives Know key elements to be considered in the design and performance of a cross-sectional study 1

2 Suggested reading Vet Epi Research (Dohoo et al): Chapter 7 Epidemiology (Gordis): Chapter 10 Case control study Exposure Yes Exposure No Exposure Yes Disease No Disease Yes Disease No Time 2

3 Cohort study Exposure Yes Exposure No Disease Yes Disease No Disease Yes Disease No Time Cross sectional study Defined population Gather data on exposure and disease Exposed Have disease (A) Exposed Don t have disease (B) Not exposed Have disease (C) Not exposed Don t have disease (D) 3

4 Cross sectional study Disease Exposure Yes No No A B Yes C D N Advantages and limitations Disease Exposure Yes No No A B Yes C D N Advantages General population Time period Limitations Cause and effect Disease of long vs short duration 4

5 How many farms do I need? Objective: What s the prevalence of farms with 1 or more horses affected with anhidrosis? Population size: 12,750 farms Expected prevalence: 20% Accepted error: 3% Confidence: 95% Sample size 1 = 649 Expected response = 33% Sample size 2 = 649 x 3 = 1,947 Sample size 3 = 1,207 5

6 Results Response: 500 / 1,247 = 41% North: 45% Central: 39% South: 41% P = 0.60 Prevalence (farm): 56 / 500 = 11% (8, 14%) North: 5% Central: 13% South: 26% P < 0.05 Prevalence (horses): 83 / 4,620 = 2% (1, 4%) North: 0.08% Central: 1.7% South: 4.3% P <

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8 How many horses do I need? Objective: What s the prevalence of selected (abnormal) radiographic findings in TB yearlings sold the Keeneland September sale? Population size: 5,196 Expected prevalence: 10% P1 fragments in MCP joint Accepted error: 3% Confidence: 95% Sample size = = 397 R = 0.94 P <

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10 Is there a need to vaccinate all dogs against CDV and CPV immediately upon admission to shelters regardless of source or physical condition? CDV outbreak in Apr:Jul dogs admitted in dogs admitted in Aug:Sep /637 dogs were tested 431 (68%) had complete records Is there a need to vaccinate all dogs against CDV and CPV immediately upon admission to shelters regardless of source or physical condition? 153/431 (36%): CDV+ CPV+ 33 (8%): CDV+ 136 (31%): CPV (25%): CDV CPV - 10

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13 Exercise What s the seroprevalence of canine distemper virus in dogs entering an animal shelter with a history of infectious disease outbreaks? Population size: 637 (admitted over a 2-month period) First study tested 441 dogs Cost: 441 X $10 = $4,410 USD This time you only have $1,200 USD Exercise 1 continued What s the seroprevalence of canine distemper virus in dogs entering an animal shelter with a history of infectious disease outbreaks? Population size: 637 (admitted over a 2-month period) Expected prevalence: Accepted error: Confidence: Sample size? Cost? Which dogs? 13

14 Exercise 2 What s the prevalence of dogs with PAT for canine rabies virus in a rural community with 2500 people (600 households)? Population size: Expected prevalence: Accepted error: Confidence: Sample size? Cost? Exercise 3 How many dogs do I need to sample:test to determine if 80% have PAT for canine rabies virus in a rural community with 450 dogs? Population size: Confidence level: Sensitivity: Expected prevalence: Sample size? 14

15 Se = 100% (86, 100%) Sp = 100% (87, 100%) 15

16 Seroprevalence: 693 / 761 = 91% (89, 93%) 16

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18 Müller G et al. Vet Pathol 2003;40: Copyright by American College of Veterinary Pathologists 18

19 Question No. 1 In cross-sectional studies, the study population (sample) is selected A) Based on exposure status B) Based on disease status C) From the reference population Question No. 2 In cross-sectional studies, it is possible to establish the temporal relationship between an exposure factor and disease A) True B) False 19

20 Question No. 3 The following results indicate that community of origin is associated with PAT for CDV: True or False? Question No. 4 The following results indicate that gender is associated with exposure to parapoxvirus in California sea lions: True or False? 20

21 VEM 5503 Veterinary Epidemiology Cross sectional studies: Q&A 21