Thinning, or partial depopulation during the growing cycle, poses a major threat to biosecurity with the introduction of many catchers and modular crates. It has been reported that thinned flocks are eight times more likely to be contaminated with Campylobacter than those having a single out-loading. This confirms the successful intervention carried out in some Scandinavian countries where thinning is no longer permitted. For many countries with significantly higher broiler production, the removal of thinning would place huge strains on the ability to meet production requirements in the short term, as well as increase production costs. This approach is currently being tested in one particular processing plant in the UK.
Another factor in regard to thinning is the increased stress levels being applied to all the birds, not only those being caught, due to the short-term feed removal, disruption and noise during the catching process itself. Stress increases the levels of catecholamine hormones and norepinephrine, known to potentiate bacterial growth and regulate the virulence of Campylobacter jejuni in humans. Norepinephrine supplies iron in the presence of transferrin and lactoferrin in iron-limited conditions, thus aiding the bacterial growth; and if the same is happening in chickens, this may help explain the sudden surge in levels of contamination following thinning.