3.1 Analysis of multiple specimens
Although the sensitivity of a culture method remains constant across specimens, culturing in parallel multiple specimens increases the probability of detecting the agent on the farm. In other words, it increases the farm-level testing regime sensitivity. This is achieved without compromising testing regime specificity, as a culture’s specificity depends mainly on how stringent the bacterial identification method used is. Multiple specimens may be analysed from a single animal (for instance, analysis of visceral organs and faeces) or multiple animals.
The disadvantage of parallel analysis of multiple specimens is the high cost of testing compared with the cost of a single culture. An alternative option, if the pathogen is expected to remain viable in the specimen for 48–72 hours, is for the submitting professional to request an initial culture of one specimen, with further cultures conditional upon a negative result.
The advantage of parallel testing of multiple specimens is exemplified in the following hypothetical case.
Figure 3 shows the variation in the number of Staphylococcus aureus colony-forming units (CFU) per mL of milk over time in 10 goats, after an experimental infection with a S. aureus strain. The horizontal blue arrow represents the hypothetical limit of detection of culture. Note that none of the goats would be culture-positive in the first six hours post infection. All the goats would be positive between 12 and 24h. From then on, the number of positive goats varies over time, and analysis of multiple specimens can increase the probability of detecting at least one infected goat. Multiple specimen analysis does not compromise culture specificity as the organism is identified to the species level using the same method.
3 Specimen collection for farm-level diagnosis of bacterial infections
