Morphological changes in red blood cells of calves caused by Leptospira interrogans serovar pomona

Haemoglobinaemia is seen in certain hosts infected by certain serovars of Leptospira interrogans, but is absent from other serovar-host associations. Comparisons were made between calves infected with serovar pomona and those receiving a crude "toxin" prepared from the same organism. Red blood cells from "toxin"-injected calves showed discocyte-echinocyte transformation and contained portions of cytoplasm segregated within vacuoles. These animals showed increased sequestration of RBCs within the spleen but no overt haemoglobinaemia. Red blood cells from infected and haemoglobinaemic animals were spherical and pitted. They also showed vacuoles and tracts under the cell membrane in fully haemoglobinized RBCs and dark granular inclusions within the cytoplasm of those which were only partially haemoglobinized. Intracellular leptospires were not seen within the RBCs. Red blood cell sequestration and erythrophagocytosis were very pronounced within the spleen, liver and bone marrow. The changes in the RBCs are not easily explained by the previously proposed theory that RBC destruction is due to a phospholipase-like toxin acting directly upon the RBC membrane. A more appropriate hypothesis is that the RBC lesions are due to the adverse effects of leptospiral "toxin(s)" on the metabolism of the RBC causing the formation of defective portions of cytoplasm. These are then either degraded and expelled, leaving empty vacuoles, or are degraded and left within the cytoplasm as dark granular inclusions.