Australian College of Veterinary Scientists
Write brief notes on the important features of the epidemiology and the control, management or eradication of THREE (3) of the following conditions/agents:
Briefly describe the essential features and application of THREE (3) of the following:
Write brief notes to demonstrate your understanding of TWO (2) of the following:
Using examples, write brief notes on THREE (3) of the following:
You have just been asked to provide epidemiological assistance for the investigation of an unusual disease outbreak in a mink farm. The farm houses about 10,000 adult mink, and the owner has reported an increased number of mink becoming ill, and an increased death rate. The farm's veterinarian has been investigating the incident, but has been unable to arrive at a diagnosis or recommend action to prevent further losses. He has asked for your assistance in investigating this incident and providing advice to the owner to resolve the problem.
Please describe your approach to this investigation.
Question provided by Paul Bingham:
Step 1: determine whether there genuinely is a problem on this farm and can it be quantified? For example the increased mortality or morbidity could be a result of changes in observation and recording due to a new manager or recording system. To do this data needs to be collected to quantify the current level of disease and if possible historical data to perform a time series analysis. The question: "is the current level of disease genuinely in excess of the long term average for this property" should be answered. Methods of recording the data should also be examined.
Step 2: Case description: Available clinical, laboratory and epidemiological information should be condensed into a case definition. This can be reviewed recursively however it is likely that on a property of this size there will be a 'background' level of morbidity, mortality and possibly a number of disease conditions at work concurrently. A case definition will allow more accurate disease frequency and descriptive data to be collected.
Step 3: The case definition should be discussed with farm staff, animal health professionals so that increased surveillance can be undertaken to accurately detect cases and non-cases.
Step 4: A descriptive investigation of population description, temporal and spatial patterns (the who, when, where) should be undertaken. Population description describes the demographics of both the underlying population (age, sex, breed, physiological state) and the diseased populations. This could turn up key host factors, which are determinants of disease. Temporal patterns over time should be plotted together with an epidemic curve to try and highlight possible risk times for exposure to causal agents and methods of disease spread. Spatial patterns may highlight areas of greater or lesser disease frequency and through a method of difference indicate possible environmental risk factors including management factors.
Step 5: Form hypothesis: The available data from the descriptive study may allow the formulation of one or more likely more causal hypothesis. The descriptive study may also highlight possible areas where disease control or prevention could occur.
Step 6: Environmental studies: It may be possible from the case description and descriptive study to determine a sampling regime for sample collection and further laboratory testing. If this is the cases samples from both affected and unaffected animals should be taken.
Step 7: Conduct analytical studies: If adequate intervention and description of the disease is not possible at this stage it may be necessary to conduct 1 or more analytical studies. These may take the form of cross sectional studies looking at risk factors in both affected and unaffected animals to uncover associations. Cohort studies looking at incidence may also be possible.
Step 8: Implement interventions: As the farmer will be most interested in finding a solution to the problem the final step, if it has not already occurred, is the formulation of an intervention plan to address the causal determinants uncovered.
Although this approach is described sequentially it is likely that many steps would occur in parallel and some steps may need to be repeated particularly the data collection and description steps as the process is multithreaded and recursive.
Aerial spraying of timber plantations (and other crops) with insecticide has been a recent concern amongst landholders in areas of southern Australia. In particular, beef producers have been concerned about the risk that spray drift poses to their cattle because of possible contamination of the meat. Some beef producers are neighbours to timber plantations and others have strips of trees on their properties that may also need spraying. Although not deliberately applied to pasture, contamination of pasture can occur through spray drift or accidental spraying of pasture when trees are being sprayed.
Alpha-cypermethrin, a synthetic pyrethroid insecticide, has been the most commonly used chemical. Synthetic pyrethroids can cause residues in fat tissue, and have an export grazing interval (EGI) of 56 days (the minimum period that must elapse between exposure to the chemical and slaughter) to allow any residues to fall to acceptable levels. However, there is evidence that synthetic pyrethroids may persist on dry pasture and pose a risk of unacceptable levels of contamination in cattle for longer than 56 days after spraying. The beef industry needs some advice on the risk that drift of chemical spray onto pasture poses to their exports.
Describe what you would do to enable you to advise the beef industry on the effect of pasture contamination from aerial spraying on residues in grazing cattle.
Answer provided by Jenny Weston:
Review the literature to assess current knowledge on alpha-cypermethrin and health effects on grazing species and residues in the carcass.
Survey the aerial spraying industry to quantify the amounts and types of chemicals applied to timber and other crops - this may be most easily done by a postal or phone survey. There may be other chemicals that are being used that are more dangerous. Refer to existing literature to get information on any other chemicals that appear to be in common usage.
Review the restrictions under which aerial spraying is undertaken (i.e. weather conditions) to ascertain whether there is a likelihood of spray-drift occurring - this is likely to be occurring to some extent from either drift or accidental spraying of pasture areas.
The question is a combination of spray persisting on pasture for periods of time and residues persisting within the fat (and possibly other tissues) in the carcass of grazing beef cattle.
The final item of interest is the residue persisting in the carcass and a test has already been developed to detect this so it would be best to concentrate on this rather than pesticide persisting on pasture.
Conduct a prospective cohort study to assess residue levels in beef cattle. Null hypothesis: That residues will not be detected above acceptable levels in cattle slaughtered at 57 days after spraying. Alternate hypothesis: That residues above acceptable levels will be detected in cattle slaughtered at 57 days after spraying.
Method: assume a worst case scenario whereby pasture is accidentally sprayed onto pasture. Perform power analysis with type 1 and 2 errors calculated from the literature for sensitivity and specificity of the test for residues to calculate sample size (number of animals in the trial) and what level of statistical significance we wish to achieve in the study. Aerial spray with alpha-cypermethrin (or other chemical as detected by survey) directly onto pasture. Graze cattle that are close to slaughter weight on this pasture for the next 56 days. Slaughter after the EGI (export grazing interval) of 56 days has elapsed and test for residues. Include a control group of same number and type of cattle that graze non-sprayed (and no possibility of contamination) pasture and are similarly tested and slaughtered to assess whether there is some other source of the chemical appearing in the carcass.
Results: compare results (# above the threshold level for residues) with what would be acceptable according to test specificity.
Although this type of study would be more expensive than a survey-based retrospective study (cohort or case-control), the ability to eliminate recall and response bias makes it worthwhile and provides a definitive answer. It may be useful to have a mixture of breed and sex (bull / steer / cow) in the trial to assess any differences due to carcass composition and metabolism. This would allow us to assess whether the current EGI of 56 days is sufficient. If it proves not to be long enough then a similar study with cattle slaughtered at weekly intervals beyond 56 days could be considered to define the appropriate EGI.
A pharmaceutical company has commissioned you as an epidemiologist to identify the major health issues associated with pet dogs in New Zealand/Australia to help them review the direction of their research and development programme. You have a year in which to complete the project and a generous, though not unlimited budget available to you.
Describe how you would go about fulfilling this task, giving details of:
Infectious bovine rhinotracheitis (IBR) is a viral infection of cattle that is endemic in Australian herds. This virus occurs worldwide, but strains overseas appear to be more pathogenic and have been subject to eradication programs in some countries. Although we know that IBR occurs in Australia, and that Australian strains are less pathogenic than overseas strains, we know very little about its prevalence or distribution. You have been asked to design a survey to estimate the prevalence of IBR infected herds in the country.
Describe how you would proceed with designing such a survey and discuss the factors that influence your decisions at each major step of the design.
Answer provided by Alex Grinberg:
Our aim is to produce an unbiased and precise estimate of the herd prevalence, which is different (and in this case, simpler, as there are no intra-class correlation issues) than the cattle prevalence. The unit of interest is the herd. In the specific case of IBR, we would probably carry out a cross-sectional study of herd seroprevalence. The response is binary: herd infected/herd uninfected. A third possible response is the 'inconclusive herd'. The numbers of herds in this cateogry should be minimised by design and by choosing a test of the right characteristics.
I would first check the option of applying a herd-level test (e.g. bulk milk testing). I will assume that a satisfactory herd-level test is not available and individual animal serological testing of individual animals needs to be applied.
Paul Bingham's approach to answering a sampling question:
| Item | Example |
| Type of study | Cross sectional survey. |
| Target population | Australian beef cattle. |
| Study population | All cattle on farms listed on various databases. |
| Sampling frame | Animals in herds listed in databases a, b, and c. |
| Sampling unit | Individual animals. |
| Unit of interest | The herd. |
| Required precision of estimate | ± 5%. |
| Alpha level | alpha = 0.05 (to provide 95% confidence in estimate). |
| Between and within herd clustering | Estimate intra-class correlation coefficient for outcome of interest (literature). |
A new genetically engineered vaccine has recently been developed for infectious bronchitis virus in poultry. This vaccine can be applied to individuals as day-old chicks or as a mass vaccination. It has undergone extensive laboratory evaluation and the developers are now ready to commence field trials under commercial conditions. You have been asked to design a field study to evaluate the efficacy of the vaccine in preventing mortalities and production losses in commercial broiler enterprises. Describe how you would proceed.
Australian College of Veterinary Scientists