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Experimental viscerotropic leishmaniasis is regularly caused by Leishmania tropica promastigotes. In the current investigation, the viscerotropic pathogenicities of Leishmania major amastigotes and promastigotes were compared and evaluated based on their heterogeneity traits and number of inoculated parasites in experimental mammalian hosts. Serous exudate from 50 patients was infected, 44 with L. major and 6 with L. tropica; only BALB/c mice inoculated with 1-2 x 10(4-6) L. major amastigotes manifested cutaneous lesions at the base of their tails. Five out of the 44 BALB/c mice inoculated with L. major died of sequela of viscerotropic adverse effect, while 2 x 10(6) L. major promastigotes showed viscerotropic signs in four BALB/c mice. The sequencing of the Cyt b gene showed a strain of L. major (GenBank accession number KM393221: haplotype diversity 0.9) containing two codon mutations, 86 and 126 in dead mice, whereas no significant mutant was observed in internal transcribed spacer (ITS)-ribosomal DNA (rDNA) sequences (haplotype diversity 0). Findings show that a lower dose of L. major amastigotes than promastigotes has more potential viscerotropic intensity in susceptible hosts. It illustrates that testing Cyt b as an evolutionary mitognome marker because of having its semi-conserved structure and low copy number is able to be utilized in the discrimination of new mutants.