ACQUIRED-IMMUNITY AFTER PRIMARY CASEOUS-LYMPHADENITIS IN SHEEP

Caseous lymphadenitis (CLA) caused by Corynebacterium pseudotuberculos is is a worldwide disease of sheep and goats and is characterized by d evelopment of pyogranulomas in lymph nodes and lungs. Control of this disease by vaccination remains controversial, although toxoid vaccines are now commercially available in some countries. To determine the ef ficacy of acquired immunity to control CIA, the effect of primary infe ction on subsequent challenge exposure was investigated. Adult seroneg ative ewes were primarily inoculated with a streptomycin-sensitive str ain of C pseudotuberculosis on the external part of the left ear and t hereafter challenge-exposed by inoculation of the streptomycin-resista nt strain 19R in the right ear. This protocol indicated that primary i nfection with at least 10(7) viable bacteria induced strong protection against subsequent challenge exposure; the ewes with primary infectio n did not develop lesions as a result of challenge exposure, whereas i mmune-naive ewes developed numerous pyogranulomas in the right car, in lymph nodes draining the inoculation site, and in the lungs. However, ewes with primary infection remained carriers of the disease as a res ult of primary inoculation. These results offer experimental support f or development of more effective vaccination to control CIA in sheep a nd goats, and this model indicates that animals with primary infection can be used as positive controls for protection when testing a candid ate vaccine against CLA.