Kala-azar, also known as visceral leishmaniasis (VL), is a disease caused by Leishmania infantum and L. donovani. Patients experience symptoms such as fever, weight loss, paleness, and enlarged liver and spleen. The disease also affects immunosuppressed individuals and has an overall mortality rate of up to 10%. This overview explores the literature on the pathogenesis of preclinical and clinical stages, including studies in vitro and in animal models, as well as complications and death. Asymptomatic infection can result in long-lasting immunity. VL develops in a minority of infected individuals when parasites overcome host defenses and multiply in tissues such as the spleen, liver, and bone marrow. Hepatosplenomegaly occurs due to hyperplasia, resulting from parasite proliferation. A systemic inflammation mediated by cytokines develops, triggering acute phase reactants from the liver. These cytokines can reach the brain, causing fever, cachexia and vomiting. Similar to sepsis, disseminated intravascular coagulation (DIC) occurs due to tissue factor overexpression. Anemia, hypergammaglobulinemia, and edema result from the acute phase response. A regulatory response and lymphocyte depletion increase the risk of bacterial superinfections, which, combined with DIC, are thought to cause death. Our understanding of VL’s pathogenesis is limited, and further research is needed to elucidate the preclinical events and clinical manifestations in humans.

Recent clinical data suggest that severe kala-azar (or visceral leishmaniasis) is an exaggerated innate immune response mediated by inflammatory cytokines, leading to a systemic inflammatory syndrome similar to what is observed in malaria, sepsis and other diseases. We tested this hypothesis by measuring serum cytokines in individuals with kala-azar. We compared patients with severe kala-azar (i.e. hemorrhagic manifestations, n = 38) with patients without evidence of hemorrhage (n = 96). We conducted a detailed clinical and laboratory evaluation, measuring serum IL-1beta, IL-6, IL-8, IL-10, IL-12, interferon-gamma, and TNF-alpha, and markers of disseminated intravascular coagulation (DIC). Infants had higher levels of inflammatory cytokines, while HIV-infected patients had lower concentrations of IL-10 and interferon-gamma. Higher levels of IL-6, interferon-gamma, and IL-8 were found among deceased patients. IL-8 and interferon-gamma were independently associated with bleeding. Several cytokines were associated with different signs of severe clinical and laboratory manifestations, including DIC. IL-6 was highly positively and independently associated with IL-1beta, IL-8, IL-10, and negatively associated with TNF-alpha. IL-1beta and TNF-alpha were also highly independently associated with disease severity. In its severe form, kala-azar, a neglected tropical disease, initiates a systemic inflammatory response that leads to DIC and other manifestations. Children may have higher risk of death due to the more intense cytokine release. The data supports the notion that IL-6 is the central cytokine that is associated with lethal disease, but interferon-gamma, IL1beta, IL-8, and TNF-alpha are also involved with disease severity. Inhibition of IL-6 is a potential target of adjuvant therapy for severe or pediatric forms of this disease.

INTRODUCTION: The objective of the study is to identify the main risk factors for death by New World visceral leishmaniasis and establish a coherent pathogenic substrate of severe disease based on clinical findings. METHODS: Seventy-six deceased inpatients and 320 successfully treated inpatients with VL were studied in a case control study. RESULTS: Bacterial infection and bleeding were mutually exclusive events leading to death. Five risk factors were unique for death by bacterial infection (malnutrition, pulmonary rales, severe anemia, severe absolute neutropenia and higher neutrophil count), while another six were unique for death by bleeding (jaundice, severe relative neutropenia, severe thrombocytopenia, liver injury, kidney failure, higher bone marrow parasite load). Bacterial infection, bleeding, severe anemia, diarrhea, dyspnea, edema, jaundice and bone marrow parasite load were the main syndromes of visceral leishmaniasis among successfully treated patients. CONCLUSIONS: The data support the idea that bacterial infections are due to immune paralysis. Broad organ and system involvement is plausibly due to the high production of proinflammatory cytokines, whose actions fit well with visceral leishmaniasis. The syndromes and causative mediators are typical of a slowly developing systemic inflammatory response syndrome.

Immunosuppression contributes significantly to the caseload of visceral leishmaniasis (VL). HIV coinfection, solid organ transplantation, malnutrition, and helminth infections are the most important immunosuppression-related factors. This review briefly describes the challenges of these associations. East Africa and the Indian subcontinent are the places where HIV imposes the highest burden in VL. In the highlands of Northern Ethiopia, migrant rural workers are at a greater risk of coinfection and malnutrition, while in India, HIV reduces the sustainability of a successful elimination programme. As shown from a longitudinal cohort in Madrid, VL is an additional threat to solid organ transplantation. The association with malnutrition is more complex since it can be both a cause and a consequence of VL. Different regimes for therapy and secondary prevention are discussed as well as the role of nutrients on the prophylaxis of VL in poverty-stricken endemic areas.

The available hypotheses or explanations for the urbanization of American visceral leishmaniasis are insufficient. An alternative hypothesis is that changes in the ecology and biology of the vector, Lutzomyia longipalpis, might explain all the new urban epidemiological features of the disease. To tackle the knowledge gaps in this process, certain key research areas need to be prioritized: the role of dogs in expanding transmission in cities, community trials to evaluate new insecticides, and research on the ecological and molecular determinants of Leishmania chagasi transmission. Investment of public funds should focus on the development of a human vaccine, since such a vaccine now appears to be within reach. Even small effects from a vaccine could substantially reduce the impact of the disease, which in the last quarter century has challenged and defeated both the scientific community and the public health field around the world.