Previous studies have demonstrated Leishmania infantum infection in colonic samples with histiocytic or lymphoplasmacytic inflammation from seropositive sick dogs. However, there are no studies that have investigated the presence of L. infantum infection in dogs diagnosed with inflammatory bowel disease (IBD) by clinical and histopathological examination. The objectives of this study were to retrospectively investigate the presence of Leishmania infection by immunohistochemistry (IHC) in archived canine colonic biopsies previously diagnosed with IBD in an area endemic for canine leishmaniosis as well as to describe the main histopathological findings. A total of 109 cases of canine colitis were retrospectively retrieved from the archived biopsies of Servei de DiagnÁstic de Patologia of the Universitat AutÁnoma de Barcelona. Information regarding clinicopathological data including signalment, histological results and further diagnostic testing to detect Leishmania infection such as Leishmania IHC staining was compiled from the selected cases. Lymphoplasmacytic (n=101), histiocytic (n=5), and lymphoplasmacytic with mild eosinophilic/neutrophilic component (n=3) colitis were diagnosed between January 1997 and September 2015 performed by endoscopic colonic biopsies. Interestingly, Leishmania IHC was only carried out in 13 of 109 (11.9 %) colonic samples to confirm or exclude Leishmania infection based on the diagnostic database. From those, 5 were diagnosed as granulomatous and 8 as lymphoplasmacytic colitis. Four biopsies were classified as positive, two as unclear results due to unspecific background staining, and the rest were negative for Leishmania IHC. Leishmania IHC was performed in 56 of the remaining 96 colonic samples with a diagnosis of lymphoplasmacytic or histiocytic inflammation that were not previously tested to confirm or exclude this infection. Due to economical restrictions, only 56 colonic samples retrieved could be investigated. Only one out of 56 (1.8%) colonic biopsies examined was classified as positive by IHC. A total of 5 out of 109 (4.6%) dogs were diagnosed with Leishmania infection. Two dogs presented intense histiocytic inflammation while three dogs showed mild to intense lymphoplasmacytic inflammation. The number of amastigotes per 5 microscopic fields viewed at 40x assessed by IHC was variable and ranged between 2 to countless.

In conclusion, Leishmania infection should be included in the list of differentials of inflammatory colonic biopsies. Leishmania amastigotes are commonly not visualized by routine histological staining. Therefore, IHC for Leishmania should be routinely used as a diagnostic tool in endemic areas of leishmaniosis, to exclude or confirm an infection by this parasite in patients with a diagnosis of IBD.

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