Differences in the peripheral immune response between lambs and adult ewes experimentally infected with Mycobacterium avium subspecies paratuberculosis.

The peripheral immune response, and its relationship with the outcome of the infection according to the age of the animal, has been investigated in young lambs and adult ewes experimentally infected with two different doses of Mycobacterium avium subspecies paratuberculosis (Map). Sixteen 1.5-month-old lambs out of 24 and 23 adult ewes out of 30 were orally challenged with an ovine Map field isolate. Animals were divided into two groups: HD, infected with a higher dose of Map and LD, with a lower dose. The remaining animals were used as uninfected control groups. Animals were euthanized at 110-120 and 210-220 days post-infection (dpi). Along the experiment, the humoral response and the specific and non-specific IFN-γ production were assessed. An intradermal skin test (IDT), using avian PPD, was also performed at 90 and 195 dpi. Samples of intestine and related lymphoid tissue were taken for histological, bacteriological and PCR studies. The Ab and IFN-γ production as well as the IDT response appeared earlier and with more intensity in the adult ewes compared to the lambs. The basal non-specific IFN-γ levels increased only in the adult ewes from the HD group. Animals from the LD and HD groups were positive to PCR; however, lesions consistent with paratuberculosis were exclusively observed in the HD group, both in lambs and in adult sheep, but they only progressed to more advanced stages in the former. These results suggest that the peripheral immune response induced by Map infection in the adult ewes is more efficient to control the progression of the infection than in lambs. This could likely be due to the existence of previous contacts with Map or other mycobacteria in the adult sheep compared to the young lambs.