<em>In Vivo</em> Imaging of <em>Leishmania infantum-</em>infected Hamsters by Gingival Inoculation of Axenic Amastigotes Expressing Luciferase

American tegumentary leishmaniasis (ATL) and visceral leishmaniasis (VL) are considered neglected by the World Health Organization. VL can be lethal if not treated; the drugs used in treatment are toxic, and there are cases of resistance. Preclinical tests can represent a bottleneck in discovering new medicines for treatment, depending on the animal model, the strain used, and the inoculum route. The golden hamster stands out for its high susceptibility to subgenera Viannia and Leishmania species, displaying many of the clinical and immunopathological processes observed in human disease. By hamster anatomy, which has a short tail and limbs, the intracardiac route is usually the choice for intravenous injection of Leishmania. However, it is an inoculum that can lead to bleeding and eventually to animal death. Thus, we standardized an alternative intravenous inoculation route for infection at the gingival vein, which is minimally invasive, allows easy venous access, and causes few local and systemic injuries to the animal. Therefore, hamsters infected by the intraperitoneal (IP) or intragingival (IG) route with Leishmania Infantum expressing luciferase (Luc) were followed up for 22 days by the bioluminescence imaging system and 50 days and 8 months post infection by PCR. After gingival inoculation of both axenic amastigotes and promastigotes of L. infantum-Luc, bioluminescence was restricted for at least 2 weeks at the site of injection, which is an indicator of infection in the tissues around the gingival plexus. Hamsters infected intraperitoneally with L. infantum-Luc displayed bioluminescence dispersed throughout the abdomen, as expected. However, by the bioluminescence imaging system infection declined until the 50th dpi and was only detectable by PCR. Axenic amastigotes showed better infection than promastigotes, as evaluated by PCR. Indeed, 8 months after infection, parasites were detected by PCR in the liver of animals inoculated with axenic amastigotes by the intravenous route, which can be a characteristic of the reference strain of L. infantum MHOM/BR/1974/PP75, whose infection progresses slowly and display low parasite burden, below the bioluminescent imaging resolution. Thus, axenic amastigotes can be a better choice for infection and follow-up than promastigotes, and the gingival inoculum is a feasible route for intravenous injection of Leishmania and other pathogens.