The presence of bacteria and mycoplasmas associated with BRD is evidenced by culture and/or qPCR (Real-Time Polymerase Chain Reaction). Viruses are detected through qPCR since it aims to demonstrate their presence. Histological tests such as pathological anatomy, immunohistochemistry, and in situ hybridization are less common but can be complementary to other methods as long as the samples are obtained and preserved under optimal conditions (fresh and fixed in formalin).
Bacterial culture (BC) is a widely used method for BRD diagnosis, as it allows the isolation of bacteria, their identification, and the performance of antibiograms. However, BC takes time (days) to obtain results, and the results depend on the quality of the sample (autolysis, heterolysis). Some BRD bacteria grow after 24 hours of inoculation (M. haemolytica, P. multocida), while others do not grow under standard conditions (they require specific culture media and conditions) or take 3-5 days to grow after successive passes of the original culture (H. somni, Mycoplasma bovis) 4. All these factors delay the acquisition of results and the start of treatment. These disadvantages are shared, although in different contexts, with histopathology, which provides detailed information about tissue changes caused by BRD, such as inflammation, necrosis, fibrosis, or other characteristic histological patterns, but it is strongly affected by sample quality and requires time for execution.
The principle of qPCR is comparable to making a "photocopy" of an original document. If qPCR detects molecules of the pathogen's DNA/RNA in the sample, it uses these molecules as a "template" and copies them billions of times until they are detectable. This makes it a rapid, highly sensitive, and specific method. qPCR does not depend on the microorganism being alive (viable) for detection and is less affected by sample quality than BC or histopathology. Additionally, qPCR is performed immediately upon the sample's arrival at the laboratory. A single sample, such as FTA tissue or secretions, allows for the simultaneous detection of multiple pathogens since it can be done in a multiplex format (multiple PCR in one tube) 5. qPCR is currently the method of choice for the diagnosis and monitoring of BRD.
Author: Jaime Maldonado, DVM, MSc, ECPHM Diplomate. Senior Manager, Scientific Manager Unit (HIPRA)
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