IL6 and infection: The expression of interleukins IL-6 and IL-1β in spleen samples of the negative control group (G2), positive control group (G1), the group necropsied 6 weeks and 6 days after a single exposure (G5); and the group that received two challenges 12 weeks apart and necropsied and sampled 6 days after the second challenge (G9), were measured by qRT-PCR to characterise the pro-inflammatory responses following infection.