The immunology of canine leishmaniosis: strong evidence for a developing disease spectrum from asymptomatic dogs.

Asymptomatic mixed breed dogs (49) from a region of high incidence of visceral leishmaniosis in Portugal were examined for the presence of Leishmania-specific cellular immunity using a proliferation assay and humoral immunity using an indirect antibody fluorescent test (IFAT) and an ELISA. The results were compared directly with 25 mixed breed dogs permanently residing in a non-endemic region (Scotland). Unlike similar studies in humans from non-endemic areas, there was no evidence of any immunological response against leishmanial antigen whatsoever from the latter group of animals. Of the 49 dogs from Portugal, however, 20 had demonstrable parasite-specific cellular immunity. Depending on the assay and criteria used to measure a positive humoral response, 11, 16 or 24 dogs had a Leishmania-specific humoral response with the ELISA being the most sensitive assay system. While 16, 12 or 8 of the dogs had clearly only a cellular response (depending on which criterion was used to constitute a true antibody response) and 7, 8 or 12 had only humoral response, other 4, 8 or 12 of the dogs had both cellular and humoral responses. This study clearly demonstrates that the infection rate of canine leishmaniosis is not only higher than previously thought using serological tests alone but that the response to infection is already highly polarised in many asymptomatic dogs. It is postulated that an individual dog's position within the immunological spectrum is likely to indicate how the disease will progress.