Australian College of Veterinary Scientists
You are interested in the effects of age on the occurrence of clinical parvovirus disease in dogs. You go through all practice records for the past 12 months and categorise all dogs on record as either having/not having clinical parvovirus disease over that period. You also record the age of each dog. All of the following factors are associated with age: (a) breed, (b) sex, (c) location (a high proportion of older dogs are found in the north of your practice area), (d) vaccination history, and (e) the number of visits to vet prior to diagnosis. Which of these factors should be considered as potential confounders of the relationship between age and parvovirus disease and why/why not?
Relative to younger cows, older cows are at increased risk of milk fever close to and soon after calving. You wish to estimate the effect of production ability on milk fever incidence. In a cohort study in one large dairy herd, high producing cows (more than 6,000 L of milk produced last lactation) were compared to low producers (less than 4000 L). Under which of the following circumstances is the observed relationship likely to be confounded by age? Why/why not?
Study 1: Age was not considered when selecting high and low production groups. Consequently, cows of all ages are included in the study. As it turned out, there were a higher proportion of old cows in the high production group, relative to the low production group.
Study 2: Only old cows were eligible for inclusion in the study.
Study 3: Age was not considered when selecting high and low production groups. Consequently, cows of all ages were included in the study. On checking the results, you note that the average age of cows in the high production group was similar to that in the low production group.
In an observational study to identify risk factors for neonatal septicaemia in calves, a number of putative risk factors were measured on the day of birth. The outcome (occurrence of neonatal septicaemia) was then determined. Risk factors measured included age and breed of dam, calving ease, colostrum quality (gamma globulin concentration in colostrum), calf suckling vigour soon after birth and calf gamma globulin concentration in serum at 24 to 48 hours after birth (the optimal time to measure these). Only 2 factors were associated with neonatal septicaemia occurrence - suckling vigour (vigorous sucklers had a lower incidence) and calf gamma globulin concentration in serum (calves with low concentrations were at increased risk of neonatal septicaemia). Calf gamma globulin concentration was also associated with suckling vigour (vigorous sucklers had higher concentrations). You are interested in the relationship between suckling vigour and neonatal septicaemia. Is calf gamma globulin a potential confounder of this relationship?
Leptospirosis is contracted by dairy farmers in the dairy, primarily from exposure to urine of infected cows. Herd vaccination is thought to reduce the risk of exposure. This is based on experimental studies where the period of urinary shedding after challenge was much shorter in vaccinated animals relative to non-vaccinates. Supporting this, a cross-sectional study has reported a negative association between leptospirosis serological status of dairy farmers and herd vaccination status. The prevalence of seroconversion was lower in people milking vaccinated herds. Which of the following factors would you seek more information about before you agree with the researchers' conclusion that vaccination reduces the risk of seroconversion in people milking dairy herds? Why?
For the scenarios listed below, under what conditions would you suspect selection bias? Why?
Study 1: In the course of practice, dairy practitioners use bacteriology to confirm diagnoses for some cases of suspected salmonellosis in dairy cows. Some dairy practitioners are concerned about the effects of magnesium oxide feeding on susceptibility of cows to salmonellosis and it is possible that suspected cases are sampled for bacteriology more frequently if they are being fed magnesium oxide, relative to suspected cases not being fed magnesium oxide. A case-control study is conducted to investigate risk factors for salmonellosis in dairy cows. Bacteriologically confirmed cases of salmonellosis in dairy cows are compared with controls (the next dairy cow seen by the same vet). Magnesium oxide feeding is again identified as a risk factor for salmonellosis.
Study 2: Many Western Victorian wool growers consider that ovine dermatophilosis (lumpy wool) is a common problem in weaners sired by rams from a prominent SA merino stud. The disease is readily recognised and accurately diagnosed by most producers. In a case-control study assessing effects of sire source on susceptibility to lumpy wool, case and control flocks (flocks with high and low prevalence of lumpy wool in weaners) are selected from respondents to a series of media items.
Study 3: Following release of the new teat spray "Udderwise", there were extensive concerns amongst producers and vets that it had contributed to epidemics of clinical mastitis in Australian dairy herds. In an effort to investigate this putative association, you survey all of the practice's dairy clients asking about use of "Udderwise" and clinical mastitis frequency over the past month. Thirty four percent of clients respond, about half of which are using "Udderwise" and half are using other teat sprays. "Udderwise" respondents reported a substantially higher incidence of clinical mastitis than respondents using other teat sprays.
Study 1: In 11 dairy herds participating in a herd health program, farmers fail to detect and/or record 50% of cows affected by retained foetal membranes (RFM). However, where records show that RFM occurred, this is invariably true. Not-in-calf cows are identified using whole herd pregnancy testing. A case-control study is then conducted with each not-in-calf cow matched with 5 in-calf cows of similar age and calving date. Records are then examined for a series of potential risk factors for non-pregnancy including occurrence of RFM and other disorders at calving, milk production in early lactation, etc. Effects of RFM on not-in-calf rates are estimated and a strong association observed.
Study 2: In a study looking at effects of dam Johne's disease (JD) status on progeny JD status amongst dairy cows, pathologists take particular care when histologically assessing tissue from the ileum and caecum from progeny of known-infected dams.
Write brief notes to demonstrate your understanding of measurement error (misclassification) (2005 written).
Using examples, write brief notes on handling confounding at the design and analysis stages of a study (2005 written).
Using examples, write brief notes on selection bias in epidemiological studies (2002 written).
You have been asked to consider lamb mortalities on a property divided by a roadway into two blocks - one block is hilly and the other is flatter and more prone to flood. The owner believes that lambs raised on the flatter block are more likely to die than lambs raised on the hilly block. Because he believes them to be hardier, the owner tends to put more wethers on the flatter block than ewe lambs.
Part A. The owner has a total of 100 lambs, evenly split between the two blocks. Of these, he has observed 15 dead on the flatter block and 10 dead on the hilly block. Using these data and a contingency table, calculate the relative risk of mortality for lambs born on the flatter (versus hilly) block.
Part B. Explain how stratified analysis can be used to determine whether a third dichotomous variable (for example, sex of lamb) might confound or otherwise modify the effect of one dichotomous variable on another. Include in your answer, how you would use stratified analysis to delineate between confounding and effect modification. If only confounding is occurring, how might the data be re-analysed?
Part C. The owner now tells you that 30 of his 55 wethers weer sent to the flatter block. Of these, 6 subsequently died, whilst only 4 wethers from the hilly block died. He also tells you that 9 of the ewe lambs on the flatter block died. Use stratified analysis and the data above to determine whether the sex of the lambs is likely to be confounding or otherwise modifying the effect of block on lamb mortality (2003 written).
Australian College of Veterinary Scientists