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Background: Trypanosoma evansi is the etiologic agent of the equine trypanosomosis, a disease related to the detriment of the extensive bovine farming in the Venezuelan grasslands. Even though macroscopic pathologies such as anemia, pale mucosa, icteric tissues, generalized edema, splenomegaly, liver and renal hypertrophy, abortion, anoestrus, emaciation, lymphadenopathies, striated muscle atrophy as well as epicardiac and endocardiac hemorrhages have been described for infections with the agent, no reports of any heart ultrastructural change in experimental or natural infections induced by Venezuelan T. evansi isolates are available. So, a transmission electron microscopic approach to the problem was needed. This work describes cell features of the cardiac myocyte and the cardiac microvasculature ultrastructure in mice experimentally infected with an equine local isolate of T. evansi, also providing an account of the infection with the mice’s survival. Materials, Methods & Results: NMRI Mus musculus were inoculated with a Venezuelan T. evansi isolate derived from a naturally infected Equus caballus. From day three post-infection, and every other day until the mice’s death, one rodent was randomly killed, the heart apex was isosmotically removed and cut in symmetrical blocks, which were fixed, post-fixed, dehydrated, infiltrated, included, sectioned, contrasted and studied by means of transmission electron microscopy, with the subsequent characterization of the cardiac myocyte and the cardiac microvasculature transformations. The evaluation of the micrographs demonstrated ultrastructural time-increasing harmful mitochondrial alterations that included reduction in the number of mitochondria per cell, decrease in mitochondrial dimensions and lessening of the number of cristae per mitochondrion. Myofibrillar destruction, myofilament loss and atrophy were also evident. In addition, damaging augmentation of the vascular endothelium thickness, appearance of abnormal endothelial projections and caveolae loss were incontestable changes. The presence of trypanosomes in the lumen of the heart capillary system was indubitable; however, neither intra-endothelial nor intra-cardiac myocyte parasites were observed; no inter-tissular parasites were found either. Discussion: The ultrastructural modifications in the muscular heart tissue and in the heart capillaries of experimentally infected mice with a Venezuelan isolate of T. evansi, derived from a feral domesticated E. caballus, were incontrovertible being characterized by the deleterious gradual mitochondrial decline. In such a context, the close relationship between the mitochondrion and the ribosome disposition is related to protein synthesis being associated to diverse functions and stress reactions to non-proper substances like T. evansi, such circumstance could lead to cardiac myocyte mitochondrial deterioration. Additionally, changes in the mitochondrial dimensions and/or the number of cristae/mitochondrion are related to the mitochondrial enzyme activity. The myofilament loss and the myofibrillar destruction reported in this work could derive from the capillary damage per se. The overexpression of serum deprivation protein response induces caveolae deformation and endothelial cell membrane tubulation. The heart’s myodamage could be additionally caused by autoimmunity and/or electrolytic unbalance induced by the trypanosome. The endothelial cell detriment could be the result of a distant effect of parasitic toxic catabolites, intense edema, hypoxia and/or ischemia. The atrophy was put in evidence by a growing volume reduction as a result of myofibril loss probably due to collateral ischemic and hypoxic mechanisms caused by the parasite. Furthermore, the effect due to toxins could cause intramuscular microvasculature damage, hypoxia and fibrillar atrophy.The trypanosomes were present in the cardiac capillary circulation, being able, as an inducible result of the liberation of active materials, to provoke mononuclear and polymorphonuclear infiltration, contributing to the inflammatory response.The subcellular damage in the cardiac myocites and in the cardiac microvascularure, along with the presence of trypanosomes in the coronary circulation, and the lack of association between parasites and cardiac myocites or parasites and cardiac endothelial cells, are attributes with a remarkable pathological meaning since it represents a non described phenomenon of gradual ultrastructural change that take part of the events, resulting in the murine host death through a degenerative mechanism.

Trypanosoma evansi: ultrastructural cardiac muscle and cardiac microvasculature changes in experimental murine infections