Abstract. The diagnosis of Mycobacterium avium subsp. paratuberculosis infection is difficult, especially in the early stages of disease. This is due to the long incubation period, the variable lag phase associated with bacterial proliferation, and the multifocal distribution of slowly developing lesions. There are few previous studies of the early stages of experimental paratuberculosis in goats. In the present study, the ability of conventional diagnostic methods to detect M. a. paratuberculosis infection during the early stages of infection was assessed. Eight goat kids were experimentally infected with M. a. paratuberculosis and subjected to a series of immunological and bacteriological tests before being euthanatized at various times postinfection. At postmortem examination, the ages of the kids ranged from 1½ to 12 months. Of the eight goats infected, three had histopathological evidence of paratuberculosis. Two of these goats were positive with bacteriology, but only one was also positive with all immunological tests. One animal had a positive immunological response, but infection could not be demonstrated by bacteriologic or histopathologic examination. Histopathologic lesions were found in the jejunum, in the ileum, and in one mesenteric lymph node, but only the mesenteric lymph nodes and one retropharyngeal lymph node gave positive results following bacteriologic culture. The disparity between the localization of histopathologic lesions and bacteriologic results emphasizes the need for exhaustive sampling to confirm a diagnosis during the early phase of an infection. It also highlights the need for a better understanding of the biology of M. a. paratuberculosis and its interaction with the immune system of the host.

Rainbow trout (Oncorhynchus mykiss), immunised with horseradish peroxidase, were given horseradish peroxidase intravenously, and the trapping of antigen in the spleen was followed 1, 24, and 48 h after injection. After 1 h, the localisation of horseradish peroxidase indicated that the antigen had been extensively trapped in the walls of the splenic ellipsoids. The colocalization of horseradish peroxidase with rainbow trout immunoglobulin M and complement factor 3 was shown with a double immunofluorescence technique and suggested that horseradish peroxidase was trapped in the form of immune complexes. After 24 and 48 h, very little horseradish peroxidase was detected in the ellipsoids, and horseradish peroxidase was mainly found in association with large cells with prominent cytoplasmic extensions. In non-immunized fish given horseradish peroxidase intravenously, antigen was not detected in ellipsoids. Thus, the observed difference between immunised and non-immunized trout suggests a specific role for the splenic ellipsoids in rapid immune-complex trapping and invites speculation on its significance in a secondary immune response.