Observational studies 2

Readings

• Dohoo I, Martin S, Stryhn H (2003) Veterinary Epidemiologic Research. AVC Inc, Charlottetown, Prince Edward Island, Canada. pp 151 - 162 (cohort studies), pp 163 - 175 (case-control studies), pp 177 - 184 (hybrid study designs).
• Thrusfield M (2005) Veterinary Epidemiology. Blackwell Science, London. pp 266 - 269 (observational studies).
• Stevenson M (2006) An Introduction to Veterinary Epidemiology. Massey University, Palmerston North, New Zealand. pp 28 - 37.
• Schlesselman J (1982) Case-Control Studies Design, Conduct, Analysis. Oxford University Press, New York.

Presentations

• Advantages and disadvantages of case-control studies.
• Advantages and disadvantages of retrospective and prospective cohort studies.
• Hybrid study designs (for example, nested case-control studies).

Exercises

Critically appraise a selection of the following papers. What are the positive and negative features of the study design used in each study?

• Chesterton R et al. (1989) Environmental and behavioural factors affecting the prevalence of foot lameness in New Zealand dairy herds - a case-control study. New Zealand Veterinary Journal 37: 135 - 142.
• Johansen C et al. (2001). Cellular telephones and cancer --- a nationwide cohort study in Denmark. Journal of the National Cancer Institute, 93: 203 - 237.
• Muscat JE at al (2000). Handheld cellular telephone use and risk of brain cancer. Journal of the American Medical Association, 284: 3001 - 3007.
• Parsonnet J et al (1991). Helicobacter pylori infection and the risk of gastric-carcinoma. New England Journal of Medicine, 325: 1127 - 1131.
• Neilen M (1989) Twinning in dairy cattle: a study of risk factors and effects. Theriogenology 32: 845 - 861.
• Steenholdt C and Hernandez J (2004) Risk factors for umbilical hernia in Holstein heifers during the first two months after birth. Journal of the American Veterinary Medical Association 224: 1487 - 1490.
• Wilesmith JW et al. (1997) A cohort study to examine maternally associated risk factors for bovine spongiform encephalopathy. Veterinary Record, 141: 239 - 243.
• Healy A et al. (2004) A paired case-control study of risk factors for scrapie in Irish sheep flocks. Preventive Veterinary Medicine 64: 73 - 83.

Example examination questions

1. Write brief notes to demonstrate your understanding of case-control studies (2001 written).

2. A veterinary practitioner asks you for epidemiological advice about an apparent problem amongst Huntaway dogs (a breed of sheep dog). On the basis of dogs presented to his clinic the practitioner suspects that Huntaways have a higher incidence of hip dysplasia (HD). This concern is of some importance as many sheep dogs in New Zealand are Huntaways. How would you advise this veterinarian to go about determining in Huntaways are, in fact, at greater risk than other breeds of dog of being affected by HD? (2000 written).

3. Crohn's disease is a disease of humans with many similarities to Johne's disease in ruminants. It is a chronic inflammatory condition of the intestines that is histologically similar to Johne's disease. Mycobacterium paratuberculosis has also been isolated from some cases of Crohn's disease, but this is not a consistent finding. There is increasing concern that exposure to M. paratuberculosis is a cause of Crohn's disease, but existing evidence for causality is limited. There is particular concern about risks to people exposed to dairy cattle infected with M. paratuberculosis. You are part of a research team of epidemiologists which has been asked to investigate this issue on an epidemiological, rather than bacteriological basis, focussing on risks to people exposed to infected dairy cattle. You have reasonable resources at your disposal. Describe how you would proceed (1999 written)

Additional reading

• Kelsey J, Thompson W, Evans A (1986) Methods in Observational Epidemiology. Oxford University Press, New York. pp 77 - 147 and pp 148 - 186.