Cytological and histopathological bone marrow findings in dogs with natural Babesia rossi infection

Abstract

The bone marrow is the primary site for haematopoiesis and is also the largest primary lymphoid organ. Malaria and babesiosis share many similarities regarding their pathogenesis, clinical disease and postmortem findings. Both are vector-borne protozoal diseases and characterized by the presence of anaemia, an inflammatory leukogram and thrombocytopaenia. The bone marrow response during human falciparum malaria has been studied previously, however, the bone marrow response during canine babesiosis has, to the authors knowledge, not been documented as yet. This study’s purpose was to document the impact of canine babesiosis on the bone marrow by describing the cytological, histopathological and immunohistopathological changes and comparing the findings to that which has been described in human malaria. Bone marrow smears and sections were prepared and processed for cytological and microscopic examination from each of six Babesia rossiinfected dogs and five healthy control dogs. These findings were interpreted in conjunction with the circulating haemogram. A Perls’ Prussian blue special stain for iron aided in assessing iron stores within the bone marrow. The application of CD3 (T-lymphocytes), CD20 (B-lymphocytes), Mum-1 (plasma cells), Mac387 (bone marrow derived monocytes and macrophages) and CD204 (resident tissue macrophages) immunohistochemical markers aided in differentiation of the various cell populations, after which analytic software provided the percentage of the above cell populations in the bone marrow of each infected and control case. The haemogram of Babesiainfected dogs revealed an inflammatory leukogram and a non-regenerative anaemia as was evident by an inadequate increase in the absolute reticulocyte count. This non-regenerative anaemia is also reported in human falciparum malaria where bone marrow dyserythropoiesis plays a major role in its pathogenesis. The bone marrow of the Babesia rossi-infected dogs were hypercellular on the cytological and microscopic examination, mainly because of the proliferation of erythrocyte precursors, notably rubriblasts. Dyserythropoietic changes were evident within the metarubricyte population of Babesia-infected dogs, where the number of metarubricytes was also decreased as compared to the control group. This suggests that there may be a suppression of erythropoiesis ultimately resulting in inappropriate erythrocyte regeneration as is evident by the decreased absolute reticulocyte count values. This suppression of erythropoiesis is likely caused by either direct or indirect influence of the disease on the bone marrow. A significant increase in the myeloid lineage was evident within Babesia-infected dogs. This is an appropriate response of the bone marrow in view of the systemic inflammation, although half of the infected cases showed a degenerative left shift neutrophilia. A marked decrease in the number of segmented neutrophils within the bone marrow were evident within the infected group as compared to the control group. Increased usage or destruction of neutrophils and prior exhaustion of the neutrophilic bone marrow storage pool may be contributing factors. A significant increase in the number of CD204 positive labelling resident macrophages was evident in the bone marrow of the infected group. This increased number of macrophages observed in tissues during canine babesiosis mirrors the findings of previous studies on canine babesiosis as well as human falciparum malaria. This increase is cytokine driven, forming part of the innate immune system and the first line of defense against the invading parasite. A left shift within the bone marrow megakaryocyte population was observed in the infected group. This is an adequate bone marrow response considering the presence of a peripheral thrombocytopaenia and this study therefore demonstrates that the thrombocytopaenia observed in canine babesiosis is likely of peripheral origin. Perls’ Prussian blue stain for iron revealed a significantly increased iron content within the bone marrow of Babesiainfected dogs. In this study, reticulocyte indices suggestive of iron-restricted erythropoiesis were not decreased within the infected group and taking this, together with the presence of sufficient iron within the bone marrow into account, it could not be established that iron-restricted erythropoiesis plays a role in the anaemia of canine babesiosis. This study provides the first detailed description on the bone marrow changes during canine babesiosis. Our findings largely mirror what has been recorded in the bone marrow of humans with falciparum malaria and confirms that dyserythropoiesis is partly responsible for the inappropriate erythroid response that is evident in canine babesiosis.