Different mechanisms of transmission of the microsporidium Octosporea bayeri: a cocktail of solutions for the problem of parasite permanence.

Periods of low host density impose a constraint on parasites with direct transmission, challenging their permanence in the system. The microsporidium Octosporea bayeri faces such constraint in a metapopulation of its host, the cladoceran Daphnia magna, where ponds frequently lose their host population due to ponds drying out in summer and freezing in winter. We conducted experiments aimed to investigate the mechanisms of transmission of O. bayeri, and discuss how these mechanisms could contribute to the parasite's permanence in the system. Spores accumulate in the fat cells and the ovaries of the host, and vary in morphology, possibly corresponding to 3 different spore types. Horizontal transmission occurred through the release of spores from dead hosts, with the proportion of infected hosts depending on the spore dose. Further, spores are able to persist outside the host both in dry and wet conditions. Vertical transmission occurred to both parthenogenetic and sexual offspring. The former were invariably infected, while the sexually produced resting eggs (= ephippia) had a less efficient transmission. The parasite may be carried by the ephippia, and thus disperse to new ponds together with the host. Together, these mechanisms may allow the parasite to endure periods of harsh environmental conditions both outside and inside the host.

Chronic inflammation impairs hematopoiesis and survival after irradiation.

Our studies show that the induction of a chronic inflammatory lesion in the left hind legs of mice by administration of 5000 rad produced distinct abnormalities of the hematopoietic system. A peripheral neutrophilia accompanied reduced numbers of total nucleated cells, stem cells, stromal cells, erythroblasts, and lymphocytes in the unirradiated femoral marrow, and the spleen was enlarged. Mice with these hematopoietic abnormalities promptly succumbed with bone marrow failure to a sublethal dose of total body irradiation (600 rad TB). Acute inflammation associated with a sterile abscess also impaired survival after 600 rad TB. Hematopoietic abnormalities resembling those in mice with inflammation had been reported in mice bearing a solid extramedullary tumor of sarcoma-180. Concomitant studies showed that bone marrow failure and impaired survival after 600 rad TB administered to mice bearing sarcoma-180 occurred at the same time as that in mice with chronic inflammation. We concluded that chronic inflammation or tumor produced similar abnormalities in the bone marrow and spleen that led to markedly impaired survival and death from bone marrow failure after a sublethal dose of total body irradiation. Although the extramedullary hematopoiesis in the enlarged spleen indicated that its microenvironment supported hematopoiesis, whereas that in marrow was reduced, it was insufficient to compensate for a total body deficit of functional stem cells.

Affective learning in medical education.

The purpose of this study was to test a theory of affective learning and to develop a data-gathering instrument for evaluation of affective learning. Two primary areas of affective learning were conceptualized for study: coping with feelings about professional knowledge and coping with feelings about the necessity for development of professional attitudes toward illness and disability and toward death and dying. This two-year study project (1977-1979) involved a total of 70 first-year medical students. Data were obtained by means of observing students in small group discussions. The observation instrument consisted of a checklist of behaviors. Observer reliability was tested. Study results suggested that students progress through phases of affective learning as described in the theory. Implications of the findings are discussed.

Consistency in ratings of clinical performance of the same students throughout medical school and internship.

Ratings of student clinical performance were analyzed to determine consistency of supervisor and coordinator judgements of the same students as they progressed through medical school and on into the internship. The results suggested that students were consistently and progressively rated higher.