Hammondia heydorni-like oocysts shed by a naturally infected dog and Neospora caninum NC-1 cannot be distinguished

This study describes transmission experiments using Hammondia heydorni-like oocysts isolated in 1996 from a naturally infected dog. The isolate was de signated as H. heydorni-Berlin-1996. Examination of sera from infected inte rmediate hosts showed immunoblot reactions that resembled patterns observed after Neospora caninum NC-1 infection. Furthermore, N. caninum DNA could b e demonstrated in tissue samples (e.g. heart, brain) of experimentally infe cted intermediate hosts and in oocyst preparations from H. heydorni-Berlin- 1996. The isolated oocysts did not induce any detectable disease in any of the inoculated adult intermediate hosts (goats, sheep, gerbils, guinea pigs , multimammate rats, BALB/c mice, SCID mice), even upon immunosuppression. Furthermore, neither histological lesions nor parasite stages could be iden tified in the tissues of all fetuses recovered from two multimammate rats t hat had been infected prior to pregnancy. An experiment with one dog fed a second time on infected intermediate host tissue indicated that immunity ma y prevent repeated oocyst shedding in N. caninum-infected dogs. In addition , the study clearly demonstrates that N. caninum can be readily transmitted by dogs that have ingested exclusively skeletal muscles of infected interm ediate hosts. Therefore, the study has consequences for the recommendations for farmers to prevent postnatal transmission of N. caninum to cattle. It indicates that feeding of any tissues of potential intermediate hosts (incl uding sheep, goats, rodents) to final hosts may induce the shedding of oocy sts in these hosts and thus pose a risk for post-natal infection of cattle. With respect to oocyst morphology and the infectivity of muscle tissues fo r final hosts, no differences were seen in comparison with observations mad e in the past on Isospora bigemina/I heydorni/H. heydorni. Therefore, earli er studies made on I. bigemina/I heydorni/H. heydorni have to be re-evaluat ed critically to determine whether they may have included N. caninum or oth er protozoan parasites that use dogs as final hosts and have an oocyst morp hology resembling that of I. bigemina/I heydorni/H. heydorni.