Hematological Ratios in Leishmania infantum—Seropositive and Seronegative Dogs and Their Distribution Across Clinical Stages

The neutrophil-to-lymphocyte ratio (NLR), platelet-to-lymphocyte ratio (PLR), monocyte-to-lymphocyte ratio (MLR), and systemic immune-inflammation index (SII) are derived hematological indices reflecting systemic inflammatory balance in dogs with different serological statuses for canine leishmaniosis (CanL). The aim of this study was to describe and compare hematological ratios (HRs) between dogs seropositive and seronegative for <i>Leishmania infantum</i> and to assess their distribution across LeishVet stages. A retrospective multicenter case-control study was conducted, including 305 seropositive dogs and 305 seronegative controls from referral hospitals in Spain. Hematological data were retrospectively reviewed and analyzed. Significant differences were observed in NLR and MLR between seronegative control dogs and dogs with leishmaniosis, whereas no differences were found for PLR or SII. Median NLR and MLR values were higher in dogs with leishmaniosis compared to controls. In addition, comparisons across LeishVet stages revealed statistically significant differences for all evaluated HRs. These findings indicate that NLR and MLR are increased in dogs with CanL and that HR varies across LeishVet stages, as these indices represent the relative proportions of neutrophils and monocytes in relation to lymphocytes; therefore, changes in their values reflect shifts in leukocyte distribution observed across clinical stages.