It must be human nature to seek that silver bullet that explains a problem.
Whether it is us or our clients, we have this desire to name the exact bug that is causing disease in a group of cattle. Often, reaching this answer would involve running laboratory diagnostics, which can lead to more information, but adds expense.
The question that is then posed to us as veterinarians is if this lab workup will be worth the money or not?
Answering that question involves three parts.
First, we need to have an accurate case definition for these cattle.
Second, we need to know what we hope to learn from diagnostics, keeping in mind what is possible to learn.
Third, we need to collect the right samples to increase the likelihood of a useful result from the diagnostic testing.
Develop your case definition
It’s hard to know whether we need to run tests if we don’t have a firm grip on the challenge the cattle are facing. That’s where a solid case definition comes into play. We need to identify what the problems are and which systems are involved to create this case definition, as well as recognize where the cattle currently are in the disease process.
For example, calves that are off feed and coughing would have involvement of the respiratory and digestive systems. We will want to note how long these problems have been occurring and to what severity, so we can identify if the case is acute or chronic, and if the problems are mild, moderate or severe. Taking all this information into consideration, we can prioritize the clinical signs based on the likelihood of which one is causative, and then create a case definition.
Case definition decides utility of diagnostics
Our case definition then helps us answer the question about what laboratory diagnostics will provide.
By listing our problems and prioritizing them we can decide if the primary cause can be recognized through lab diagnostics via culture, PCR, liver mineral levels, and so forth. If so, by determining chronicity we can ascertain if it is likely the causative agent is still present. It does no good to send in samples from an animal if we cannot respond to these questions in the affirmative.
If these issues are satisfied, we still need to ask what we hope to learn from the laboratory. If, in our example, we decide we are dealing primarily with a respiratory disease, then we must ask if knowing the pathogen will change our therapy or management?
If it is a reoccurring problem in a yard or in cowherd, then yes, there is probably a specific agent that we should identify, as long as we have eliminated other husbandry issues that could be allowing the reoccurring issue.
However, if it is a train wreck in a pen of mixed-source, four-weight calves -- for example -- knowing the gambit of pathogens affecting the group won’t change our treatment strategy and therefore likely makes diagnostics a poor use of the owner’s money.
Sampling technique, diagnostic results
If we decide diagnostics are necessary, the final step is to gather them from the correct source. It is questionable if post-mortem samples from chronic animals will to be representative of the pen. Acute deads are a better option for a post-mortem sample.
But to be representative, consider collecting samples from multiple affected, living animals. Each disease has its own techniques, but in our BRD example two that are commonly used are deep pharyngeal swabs and trans-tracheal washes. If your goal is to screen a large group for pathogens, swabs are quicker and easier to do chute-side. On the other hand, the trans-tracheal washes collect the samples at the actual point of disease, making them potentially more valuable. Consider what will give you the best sample and weigh it against the logistics of collecting samples to make your collection decisions.
Laboratory diagnostics can be a powerful tool if they help us answer specific questions to a case for which we have a good definition. If we don’t meet this criteria, diagnostics can lead us to make inappropriate decisions and waste our client’s money. In the end, our clients don’t care so much about cutting-edge lab techniques as they do about solving their problem. Unless we have a plan to use diagnostics appropriately to solve the client’s problem, then we shouldn’t run them.