Canine leishmaniosis (CanL) is caused by the protozoan parasite Leishmania infantum. It is a zoonosis and is endemic in the Mediterranean basin, Portugal, Latin America and southern Asia. However, the infection is spreading to non-endemic areas, such as north America and northern Europe, probably due to wider distribution of its vector and especially to larger numbers of dogs travelling from/to endemic countries. Leishmania species completes its life cycle in two hosts, a phlebotomine sandfly vector, which transmits the flagellated infective promastigote form, and a mammal, where the amastigote form develops and replicates inside the host’s macrophages. Other modes of transmission are possible. In utero transmission from an infected dam to its offspring and venereal transmission from infected males to healthy bitches have been documented. Transmission by haematophagus arthropods has been suspected, but is not yet proven to have an epidemiological significance. Rhipicephalus sanguineous ticks have been shown to acquire Leishmania species in their gut after feeding on infected dogs. Blood transfusion is another possible route of transmission and canine blood donors living in endemic countries should be routinely tested for CanL. Direct dog-to-dog transmission has also been suspected, especially in those areas where sandflies are not present. Leishmania species tend to localise in all tissues in which monocytic-macrophagic cells exist in high numbers. Not all dogs infected with L infantum will eventually develop clinical leishmaniosis. …