The pathology of the spleen in canine babesiosis

Abstract

The spleen is the largest secondary lymphoid organ in the body and therefore not only does it perform many hematopoietic functions but it also mounts an immunological response to blood-borne antigens. Malaria and babesiosis, both haemoparasitic diseases, reveal many similarities in clinical disease, pathogenesis and post mortem findings and are often compared to one another. Both are protozoan diseases transmitted by mosquitoes and ticks, respectively. Malaria has been more extensively studied than babesiosis. Information regarding the effect of babesiosis on various hosts is limited, particularly with respect to the spleen. The purpose of this study was to investigate the effect of Babesia rossi on the spleen of dogs. We aimed to provide a detailed histomorphological analysis of infected spleens, with the addition of immunohistochemical labelling of leukocyte subsets. One section of splenic tissue from each of 9 Babesia rossi-infected dogs and 4 healthy control dogs was examined under the light microscope. Immunohistochemical markers were applied to infected and control spleens in order to characterise different immunocyte populations. Markers included CD3 (T lymphocytes), CD20 (mature B lymphocytes and normal dog plasma cells), Mum1 (plasma cells), Pax-5 (immature B lymphocytes), Mac387 (monocytes-macrophages of bone marrow origin) and CD204 (resident tissue macrophages). The application of analytic software enabled us to compare leukocyte subpopulations in infected and control spleens in a semi-quantitative manner. Routine histopathology revealed diffuse intermingling of the white and red pulp in Babesia rossi-infected spleens with a clear loss of distinction between these zones. The merging of zones was accentuated by outspoken white pulp dissolution with no discernible germinal centres, mantle or marginal zones. Immunohistochemical labelling revealed a significant increase in the proportion of tissue resident macrophages as well as macrophages of bone marrow origin in the infected spleens. In addition, apart from a few remnant lymphocytes within the peri-arteriolar lymphatic sheaths and follicles, the majority of immunocytes had redistributed to the red pulp, supporting the observation of white and red pulp intermingling. Our study produced novel insights into the pathology of the spleen in canine babesiosis. The majority of our findings are in agreement with histomorphological descriptions of the spleen in a variety of hosts with malaria. The exact causes, consequences and implications for disease pathogenesis require further investigation.