Themes in an Outbreak Investigation Series | Part 1

What is a diagnostic investigation?

A diagnostic investigation is the systematic application of epidemiology and clinical testing to determine the cause(s) of a disease or symptom. Diagnostic investigations are performed when there is a problem resulting in morbidity, mortality, or emotional or financial distress.

There are multiple ways to perform a herd diagnostic investigation, but the basis of an investigation can be summarized into 5 steps: determine if there is a problem, diagnose the problem, make a recommendation, communicate the findings of the investigation, and evaluate the recommendation.

Determine if there is a problem.

The first step of a diagnostic inve stigation is to determine if there is a problem and what it is. Producers have a wide range of what they deem is a problem. For some, it is economic strain while for others, it is a single sick cow. If a producer is reaching out with what they deem is a problem, begin by identifying the who, what, when, and where.

Who is being affected and how many?
What is the producer seeing and what distinguishes a “case”?
When did it start, how long it has been happening, and if they think it is worsening, improving, or staying the same?
Where, physically, is it happening?

A producer can be experiencing an outbreak in which a disease is occurring in a population at a rate above what is normally expected, or a disease has appeared that did not previously exist in the population. An example of an outbreak would be the diagnosis of a case of foot-and-mouth disease which has not been seen in the US since 1929. Novel diseases are easier to recognize as outbreaks because their appearance is unique. The term outbreak is similar in concept to the term epidemic, which is associated with a larger geographic region and rapid disease spread.

Determining if disease incidence is above what is normally expected is often facilitated by records, but even producers without records tend to know when a disease is occurring at a greater than “normal” rate. Determining if an outbreak is occurring relies upon an accurate and consistent case definition. Changes in record-keeping or the individual identifying cases of disease may artificially increase the number of cases and trigger concerns for an outbreak. While the sudden change in case-definition could appear as an outbreak, it could also identify endemic disease that had not previously been noted.

A producer may have an endemic disease that has an expected prevalence or incidence within a herd or population. Diseases that have existed in a population are more difficult to identify as outbreaks. This problem may come to the producer’s attention because, over time, the endemic disease has decreased profitability until the operation is at risk.

Diagnose the Problem

When determining what the problem is evaluate the operation and the individuals in the herd. The adage, “one is a chance, two is a coincidence, and three is a pattern,” is applicable in this situation. Counting the sick, healthy, and dead can help gauge the severity and prognosis of the disease. Necropsy any dead individuals can help to form a list of potential diseases.

A physical exam of affected individuals can help guide towards a diagnosis and ensure appropriate medical intervention. Sample collection of blood, urine, feces, and feed may help guide recommendations. Prior to collecting samples and performing diagnostic tests, think about what will be tested for and how the results will be used.

Further Reading