Determination of virulence and pathogenesis of a canine strain of Leishmania leishmania infantum in hamsters and dogs

Abstract

SUMMARY Visceral leishmaniasis was experimentally induced in hamsters by the intracardiac inoculation of 107 amastigotes of Leishmania leishmania infantum of canine origin. At postinoculation (pi) days 7, 21, 42, and 63, hamsters were euthanatized. Body weights and total parasite numbers of the liver and spleen were determined. Gross and histologic evaluations of tissues were done. Dogs also were inoculated IV with 108 amastigotes/kg of body weight. Samples were obtained from dogs prior to infection and at biweekly pi intervals for CBC and serum chemical analysis, for lymphocyte blastogenic assay by use of blood leukocytes, and for ELISA to determine antileishmanial antibody titers. At pi week 12, dogs were necropsied; organ weights, tissue imprints of the liver and spleen, and histologic interpretations of tissues were obtained. Hamsters developed high parasite numbers within 7 days after inoculation, at which time the total parasite numbers in the liver (3.51 × 107 amastigotes) was observed to be approximately 11 times that in the spleen (2.93 × 106). The liver had the highest parasite numbers throughout the infection period. Some infected hamsters became either cachectic and emaciated or ascitic. Two of the 10 infected hamsters died at pi days 54 and 58. Moderate to severe hepatosplenomegaly with granulomatous inflammatory reactions characterized by the presence of varied numbers of parasitized macrophages, giant cells, and hepatic Schaumann bodies were observed in infected hamsters. Infected dogs developed significantly altered hematologic values consisting of mild anemia and moderate leukopenia at pi weeks 8 to 12. Hyperproteinemia characterized by hyperglobulinemia (4.5 g/dl) was noticed at pi week 4. Serum globulin values remained high and increased to > 5.0 g/dl at pi weeks 8, 10, and 12. At pi week 4, ELISA titers were > 16. By pi week 12, all infected dogs had titers > 1,024. The dog with the highest antibody titer had the lowest number of parasites and mild pathologic changes. Evidence of lymphoproliferative response was not noticed up to pi week 12. Similarities between infected hamsters and dogs included the presence of higher parasite numbers in the liver than in other organs. The highest total parasite numbers in the spleen and liver were 1.12 × 109 and 2.86 × 109, respectively. Mild to moderate granulomatous inflammatory reactions were observed in the liver, spleen, thymus, and lymph nodes. Within 12 weeks after inoculation, parasitized macrophages were found in the dermis. The overall results indicate that hamsters are highly susceptible to experimental infection, and that infected hamsters develop findings similar to those in natural human infections. Our findings also indicate that dogs are susceptible and develop high antileishmanial titers that correspond to low parasite numbers. This suggests the possible role of antibody in determining the seriousness of disease.