Simple SummaryBabesia gibsoniis rarely reported in cats, and its pathogenic potential in this species is unknown.B. gibsoniDNA was detected using two pan-BabesiaPCRs in stored blood from a cat. The cat had died, but retrospective case review identified regenerative anaemia and thrombocytopenia concurrent withB. gibsonidetection. Clinical signs resolved on treatment for a suspected immune-mediated aetiology until the cat suffered fatal haemorrhage 6 months later. Samples stored 4 and 6 months from presentation tested negative forBabesiaspp. This is the first report ofB. gibsonidetection in a cat to provide clinicopathological information.AbstractTick-borne haemoparasiteBabesia gibsonihas been detected rarely in cats, in surveys of apparently healthy animals. In stored blood from a 6-year-old male-neutered domestic shorthair cat in Hong Kong,B. gibsoniDNA was detected retrospectively using PCR forBabesiaspp. 18S rRNA and mitochondrial cytochrome B genes, followed by sequencing and basic local alignment search tool (BLAST) analysis. The cat presented with severe haemolytic anaemia and thrombocytopenia. The cat responded to supportive care and glucocorticoids and was clinically normal despite persistent subclinical thrombocytopenia until six months after presentation, when it succumbed to a fatal haemorrhagic episode. Necropsy revealed severe intestinal and pulmonary haemorrhage and hypocellular bone marrow with megakaryocytosis but no other causes of immune-mediated thrombocytopenia (IMTP) or immune-mediated haemolytic anaemia (IMHA). Blood stored on days 158 and 180 tested PCR negative forBabesiaspp. This report demonstrates that geographic range ofB. gibsonidetection in cats includes Hong Kong. The exclusion of other causes suggests thatB. gibsonimight have potentially played a role in triggering immune-mediated disease in this case.