Failure of vaccination in brook trout Salvelinus fontinalis against Loma salmonae (Microspora)

Loma salmonae OA- and SV-strains were isolated from Oncorhynchus kisutch after passage of infective gill material through rainbow trout Oncorhynchus mykiss and brook trout Salvelinus fontinalis, respectively. In infection Trial I, six groups of thirty-five 10-g fish were isolated into 70-litre fibreglass tanks supplied with fresh water at 15 degrees C. Using oral gavage, they were exposed to the following inocula: Group I, brook trout exposed to 200 000 spores of L. salmonae OA; Group II, brook trout exposed to 200 000 spores of L. salmonae SV; Group III, brook trout sham-exposed to water only; Group IV, rainbow trout exposed to 200 000 spores of L. salmonae OA; Group V, rainbow trout exposed to 200 000 spores of L. salmonae SV; and Group VI, rainbow trout exposed to water only. In infection Trial II, initiated at week 7, brook trout from Groups I-III were orally re-challenged with 200 000 spores of L. salmonae SV, while those in Groups IV-VI were orally re-challenged with 200 000 spores of L. salmonae OA. It was found that, whereas prior exposure to either of the two strains of L. salmonae led to marked resistance of rainbow trout to a second challenge, brook trout failed to mount an effective protective response..