Pooled Testing for Bovine Paratuberculosis: Details Matter

Pooling of fecal samples with subsequent testing for Mycobacterium avium subspecies paratuberculosis (MAP) by culture or real-time PCR is used for 3 main purposes: herd or group classification, prevalence estimation and as a low-cost initial screening for identification of animals infected with MAP. Though pooling is touted to reduce costs for the latter purpose when prevalence is low, some important considerations have been overlooked. First, there is no consensus as to which pool size is optimal for a given within-herd prevalence. Second, rarely are negative pools retested. Although the choice to not retest might be reasonable if the objective is to find animals shedding moderate to high numbers of MAP (i.e. the most infectious animals), it may be sub-optimal if the goal is to detect all infected animals. Some infected pools will invariably test negative because culture and PCR are only about 50 to 60 % sensitive based on a single sample. Third, more sophisticated pooling protocols (for example those requiring re-creation of pools of half or quarter the original size) might offer cost-saving advantages Reticence about application of the latter two testing modifications is understandable since pooling is not supposed to increase laboratory work-load.