Despite haemolytic anaemia being the main consequence of Babesia rossi infection in dogs, the bone marrow response has been reported to be mild in the face of severe anaemia. A similar finding has been described in falciparum malaria and has been ascribed to either a decreased production of erythroid precursors or an inability of erythroid precursors to respond to hormonal stimulus. Recently, more information has become available on the use of various reticulocyte indices in an attempt to describe and explain the underlying pathogenesis of various anaemias as their release describes the recent functional state of the bone marrow. The objective of this study was to compare the admission absolute reticulocyte count (ARC) and reticulocyte indices in dogs naturally infected with B. rossi with dogs suffering from immune-mediated haemolytic anaemia (IMHA), unrelated to babesiosis, as well as healthy control dogs. The ARC and reticulocyte indices were also evaluated in five experimentally B. rossi infected dogs throughout the disease course. This was a retrospective observational study looking at the records generated on a haematology analyser, the ADVIA 2120 (Siemens, Munich, Germany). The haematocrit (HCT), ARC and other reticulocyte indices for 103 dogs, naturally infected with B. rossi was compared to 16 dogs with IMHA and 14 control dogs. The experimentally infected dogs consisted of five purpose-bred beagles that were infected with high and low dose B. rossi parasite inoculum. Differences between groups were assessed using the Mann-Whitney U test, whereas the Friedman’s ANOVA was used to assess the change over time during the disease course in the experimentally infected dogs. The Wilcoxon Signed Ranks test was then used to determine the differences within the experimentally infected groups. The median (IQR) HCT for the Babesia (0.16 L/L; 0.12 – 0.27; P < 0.001) and IMHA (0.15 L/L; 0.12 – 0.17; P < 0.001) groups were significantly lower than the control group (0.52 L/L; 0.45 – 0.57). For the Babesia and IMHA groups the HCT did not differ significantly. Compared to the control group (42.1 x109/L; 33.8 – 62.6), the median (IQR) ARC was significantly higher in the Babesia (82.1 x109/L; 48.6 – 174.9; P = 0.006) and IMHA (256.7 x109/L; 79.0 – 436.9; P = 0.004) groups. The ARC was significantly lower in the Babesia group compared to the IMHA group (P = 0.011), despite no significant difference for HCT between groups. On day four of the experimentally infected group, approximately 24 – 48 hours after a peripheral parasitaemia was observed, there was a sudden decrease in the ARC to less than its value on day one, which was inappropriate for the degree of anaemia observed. The reticulocytes of B. rossi naturally infected dogs were larger and more hypochromic with a greater difference in cellular haemoglobin (CH) between reticulocytes and mature erythrocytes compared to the control group, whereas the reticulocytes of the experimentally infected dogs were smaller and more hypochromic than day one of infection. All reticulocyte indices demonstrated significant differences on day four compared to day one of the experimental study. The changes noted in the reticulocyte indices are postulated to be a contribution of anaemia of inflammation (AID), shift reticulocytosis and iron-restricted erythropoiesis. This study concludes that the regenerative response in dogs naturally infected with B. rossi is inappropriate, despite the severity of anaemia observed, compared to dogs with IMHA. Furthermore, the findings of the experimentally group may indicate a possible direct suppressive action of the Babesia parasite on the bone marrow during the time of parasitaemia, resulting in insufficient erythropoiesis. Similar findings have been reported in falciparum malaria and other factors will require further investigation. / Dissertation (MSc)--University of Pretoria, 2020. / Companion Animal Clinical Studies / MSc / Unrestricted