Observational studies 1

Readings

• Cameron A (1999) Survey Toolbox. AusVet Animal Health Services, Brisbane. pp 11 - 36, pp 143 - 189.
• Thrusfield M (2005) Veterinary Epidemiology. Blackwell Science, London. pp 266 - 288 (observational studies), pp 228 - 246 (surveys).
• Dohoo I, Martin S, Stryhn H (2003) Veterinary Epidemiologic Research. AVC Inc, Charlottetown, Prince Edward Island, Canada. pp 139 - 146 (observational studies).
• Baldock F (1998) What constitutes freedom from disease in livestock? Australian Veterinary Journal 76: 544 - 545.

Presentations

• Types of observational studies (including their strengths and weaknesses).
• Discuss the circumstances when a cross sectional study is most appropriate, the key design features and the statistics that can be calculated.
• Is Vitamin D intake associated with the risk of multiple sclerosis? One member of the group should briefly research multiple sclerosis - occurrence, hypothesised risk factors etc and bring their findings to the session (5 minutes). At the meeting members of the group allocate themselves to different study types (cross-sectional, case-control, cohort and intervention/clinical trial) and discuss how they would conduct a study examining this question using their nominated design. The design should include exposure and outcome and concentrate on the strengths and weaknesses of their study type.

Exercises

1. Answer two (or more) of the example examination questions below.

2. Select two (or more) of the following papers and discuss the design and conduct of each:

   • Schouten J et al. (2004) Prevalence estimation and risk factors for Escherichia coli O157 on Dutch dairy farms. Preventive Veterinary Medicine 64: 49 - 61.
   • Jackson R et al. (2004) Survey of the seroprevalence of brucellosis in ruminants in Kosovo. Veterinary Record 154: 747 - 751.
   • Cole F et al. (2004) Prevalence and demographic characteristics of exertional rhabdomyolysis in horses in Australia. Veterinary Record 155: 625 - 630.
   • Garner M et al. (1997) A national serological survey to verify Australia's freedom from porcine reproductive and respiratory syndrome. Australian Veterinary Journal 75: 596 - 600.
   • Small L and Pinch DS (2003) Survey for hydatidosis in cattle bred in the northern region of the Northern Territory of Australia. Australian Veterinary Journal 81: 355 - 358.

Example examination questions

1. Describe the circumstances where collection of data on disease occurrence from an abattoir would be appropriate. What are the limitations of this method? (2005 oral)

2. Write brief notes on selection of subjects for a cross sectional study (2005 written).

3. Babesia gibsoni has recently been found in Pit Bull terriers in Victoria. However little is known about its distribution or prevalence in Australia. You have been asked to design a study to identify the prevalence of Babesia gibsoni in the Australian dog population (2003 written).

   • What type of study would you use and what are the strengths and weaknesses of your chosen study design? Describe the study design, including study objectives, hypothesis, unit of interest, reference and study populations.
   • What sampling methods would you use and how would you select your sample size?
   • Discuss possible means of data collection and the advantages and disadvantages of alternative sources of data.
   • Describe any potential biases and how you may control these.
   • Discuss how you would analyse and interpret the results.

4. Canine coronavirus is becoming an agent of concern to small animal practitioners. In particular there is some controversy regarding the prevalence of seropositive dogs and the role that the virus may play in canine gastroenteritis. As a consultant epidemiologist you have been asked to design and carry out a cross-sectional study to investigate these problems. Describe how you would proceed with planning the study, including discussion of important factors affecting study design. (2002 written)

5. Salmonella enteritidis (SE) is a common cause of food poisoning in many countries, with the main source of infection being raw or partly cooked eggs and egg products. For example human illness from SE positive eggs in the United States is now approximately 637 000 cases per year. SE in chickens causes a silent systemic infection that can be detected by both bacteriological and serological techniques. Prevalence of infection in naturally infected commercial layers has been found to be very low.

   The Australian and New Zealand egg industries are thought to be free of SE infection and relatively few human cases occur in either country compared to overseas. No human cases of SE due to consumption of Australian or New Zealand eggs have been reported. You have been engaged by the egg industry in your country (Australia or New Zealand) to plan an epidemiological study to demonstrate freedom from SE. Describe how you would proceed with this project and discuss the key issues affecting study design.

6. A pharmaceutical company has commissioned you as an epidemiologist to identify the major health issues associated with pet dogs in NZ/Australia to help them review the direction of the research and development program. You have one year in which to complete the project and a generous, although not unlimited budget available to you. Describe how you would go about fulfilling this task, giving details of:

   • The different sources of information on dog health that you might use (25%).
   • The way in which you would collect data from these sources (50%).
   • The strengths and weaknesses of each with respect to data quality and ease with which the information can be collected (25%).

Additional reading

• Epidemiological Skills in Animal Health, PGFVS Proceedings 143; pp 39 – 476 (investigation methods), pp 49 - 55 (surveys), pp 57 - 64 (observational studies and measures of association), pp 111 - 119 (monitoring), pp 151 - 158 (survey design), pp 337 - 351 (using existing data sources), pp 353 - 361 (monitoring and surveillance).
• East I et al. (2004) Survey for the presence of White Spot Syndrome Virus in Australian crustaceans. Australian Veterinary Journal 82: 236 - 239.
• Black P et al. (2001) Serological examination for evidence of infection with Hendra and Nipah viruses in Queensland piggeries. Australian Veterinary Journal 79: 424 - 426.