Histopathological findings and detection of toll-like receptor 2 in cutaneous lesions of canine leishmaniosis

•Papular dermatitis-stage I leishmaniosis (PD) is a disease of immune competent dogs.•PD is depicted by granuloma formation, low parasite load and low TLR2 expression.•Skin lesions from dogs with stage II–III leishmaniosis rarely show granuloma.•In addition, these patients show high parasite load and high TLR2 expression. A broad spectrum of clinical manifestations ranging from a chronic subclinical infection to a non-self-limiting illness has been described for canine leishmaniosis (CanL). This clinical variation is determined by a variable immune response, presumably genetically determined, against the infection. Although different types of adaptive immune response in dogs with CanL have been investigated in several studies, the mechanisms that underlie and determine this variability are still poorly understood. It is currently thought that innate immune response, and particularly the role of specific mediators of the innate immune system, such as toll-like receptors (TLRs), plays a central role in this polarization. However, there is limited data available concerning the role that TLRs play in canine Leishmania infantum infection. The objective of this descriptive study was to characterize and compare the inflammatory pattern, the Leishmania burden and expression of TLR2 in skin lesions derived from dogs with different clinical stages of leishmaniosis and cutaneous lesions. Routine histology, Leishmania and TLR2 immunohistochemistry assays were performed in 11 patients with papular dermatitis (stage I – mild disease) and 10 patients with other cutaneous lesions (stage II–III – moderate to severe disease). A significantly higher frequency of granuloma formation was demonstrated in skin samples of dogs with stage I when compared with dogs of stage II–III. Although not statistically significant, a trend for a lower parasite burden was observed for skin lesions of dogs with stage I when compared with dogs of stage II–III. A lower expression of TLR2 in skin biopsies from dogs with stage I was statistically significant compared with stage II–III. The results obtained in this study indicated an association with TLR2 in the pathogenesis of canine cutaneous leishmaniosis. Further studies are required to fully elucidate these findings.