Bovine respiratory disease: Diagnosis in the live calf

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When bovine respiratory disease (BRD) strikes feedlot calves, veterinarians and cattlemen usually don’t spend a lot of time determining the specific bacteria or virus (or mix of both) causing the problem. Rightfully, the initial actions are directed toward choosing the appropriate treatment that will get the calf on the road to recovery.

But circumstances arise that make it necessary to work toward a specific diagnosis when outbreaks occur. Treatment failures are a common reason for seeking more detailed diagnostics, but gaining knowledge to help guide future vaccine, metaphylaxis, or treatment programs are also important reasons to obtain an etiologic (agent-specific) diagnosis. 

When cattle have died from BRD, samples of lung and other organs can be submitted to a diagnostic lab for identification of bacteria and viruses associated with the outbreak. But what happens when death losses have not yet occurred?

The same techniques that identify pathogens in post-mortem (dead calf) samples can be used in antemortem (live calf) samples. These include bacterial culture, virus isolation, fluorescent antibody tests, as well as the very sensitive technique of PCR. How can useful samples from live calves be obtained for these diagnostic procedures?

The easiest, but perhaps least useful, samples are obtained from swabs inserted into the nostrils of the affected calf. The swab is then placed in appropriate transport fluid to send to the lab. The problem with nasal swabs lies in interpretation of the results. Healthy calves can normally harbor bacteria in their nasal pathogens that have the potential to cause illness (Mycoplasma bovis and Pasteurella multocida, among others) when stresses or other conditions cause those bacteria to rapidly spread to the lower lungs. Also, in many cases the population of bacteria in the nasal passages is quite different than what is causing pneumonia deeper in the respiratory tract. Positive findings for viruses such as IBR or BRSV may be significant, however.

More technically difficult techniques can be performed by veterinarians to obtain fluid samples from deep in the lungs. These involve passing a long tube into the depths of the lungs, either through the nose (bronchoalveolar lavage) or through a surgical site in the windpipe (transtracheal wash). These have the advantage of getting diagnostic specimens from where the disease is occurring: deep in the lungs. 

An intermediate technique, use of deep pharyngeal swabs, takes samples through use of a guarded swab passed through the nose and into the back of the throat. While this technique does not get samples from deep in the lungs, it bypasses the bacterial population of the nose, and gets to a portion of the respiratory tract where bacteria and virus populations are more related to pathology in the lung.
 
Even when these deeper samples are obtained, there are still pitfalls to work around when it comes to interpreting results. In addition to the presence of normal bacteria in portions of the respiratory tract, results can also be affected by vaccination of calves with certain modified live and intranasal vaccines. Positive results may be due to vaccine virus if sampling takes place closely after vaccination.

Regardless of technique, choosing several cattle to sample, calves that are in the early stages of BRD, and calves that have not been treated with antibiotics will help ensure that your veterinarian has meaningful results to work with.
 
With the advent of sensitive diagnostic techniques such as PCR, antemortem BRD diagnostics have more utility than ever, but they also have more diagnostic pitfalls to be aware of. Working closely with a veterinarian who has a close relationship with a diagnostic laboratory will help ensure these diagnostic efforts are as helpful as possible.

Source: Russ Daly



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