Visceral leishmaniasis (VL), also known as kala-azar (, "black sickness") or "black fever", is the most severe form of leishmaniasis and, without proper diagnosis and treatment, is associated with high fatality. Leishmaniasis is a disease caused by protozoan parasites of the genus Leishmania.
The parasite migrates to the internal organs such as the liver, spleen (hence "visceral"), and bone marrow, and, if left untreated, will almost always result in the death of the host. Signs and symptoms include fever, weight loss, fatigue, anemia, and substantial swelling of the liver and spleen. Of particular concern, according to the World Health Organization (WHO), is the emerging problem of HIV/VL co-infection.
VL is the second-largest parasitic killer in the world (after malaria), responsible for an estimated 20,000 to 30,000 deaths each year worldwide.
Upendranath Brahmachari synthesised urea stibamine (carbostibamide) in 1922 and determined that it was an effective substitute for the other antimony-containing compounds in the treatment of VL caused by Leishmania donovani.
Signs and symptoms When people develop visceral leishmaniasis, the most typical symptoms are fever and the enlargement of the spleen, with enlargement of the liver sometimes being seen as well. The blackening of the skin that gave the disease its common name in India does not appear in most strains of the disease. The other symptoms are easily mistaken for those of malaria. Misdiagnosis is dangerous, as without proper treatment, the mortality rate for kala-azar is close to 100%. L. donovani itself is not usually the direct cause of death in people with kala-azar, however. Pneumonia, tuberculosis, and dysentery are omnipresent in the immuno-depressed regions where leishmaniasis thrives, and, as with AIDS, it is these opportunistic infections that are more likely to kill, flaring up in a host whose immune system has been weakened by the L. donovani infection. Progress of the disease is extremely variable, taking anywhere from one to twenty weeks, but a typical duration for the Sudanese strain of the disease is narrower, between twelve and sixteen weeks.
Even with recovery, kala-azar does not always leave its hosts unmarked. Some time after successful treatment—generally a few months with African kala-azar, or as much as several years with the Indian strain—a secondary form of the disease may set in, called post kala-azar dermal leishmaniasis, or PKDL. This condition manifests first as small, measles-like skin lesions on the face, which gradually increase in size and spread over the body. Eventually, the lesions may coalesce to form disfiguring, swollen structures resembling leprosy, and occasionally cause blindness if they spread to the eyes. (This disease is not the same as cutaneous leishmaniasis, a milder disease caused by another protozoan of the Leishmania genus, which also causes skin lesions.)
Cause Two species of Leishmania are known to give rise to the visceral form of the disease. The species commonly found in East Africa and the Indian subcontinent is L. donovani and the species found in Europe, North Africa, and Latin America is L. infantum, also known as L. chagasi.
The insect vectors are species of sandfly of the genus Phlebotomus in the Old World, and of Lutzomyia in the New World. Sandflies are tiny flies, measuring 3–6 mm long by 1.5–3 mm in diameter, and are found in tropical or temperate regions throughout the world. The sandfly species Lutzomyia longipalpis is the primary vector of this disease.Alexander, Bruce; Lopes de Carvalho, Renata; McCallum, Hamish; Pereira, Marcos Horácio (December 2002). "Role of the Domestic Chicken (Gallus gallus)in the Epidemiology of Urban Visceral Leishmaniasis in Brazil". Emerging Infectious Diseases. 8 (12): 1480–1485. doi:10.3201/eid0812.010485. PMC 2738513. PMID 12498667. The larvae grow in warm, moist organic matter around human habitations (such as old trees, house walls, or waste), making them hard to eradicate.
Visceral Leishmaniasis/kala-azar samples from India revealed the presence of not only the primary causative protozoan parasite, i.e., Leishmania donovani (LD), but also co-infection with another protozoan member called Leptomonas seymouri (LS). The latter parasite (LS) further contained an RNA virus known as Leptomonas seymouri narna-like virus 1 (Lepsey NLV1). So, it appears that a great majority of kala-azar patients in the Indian subcontinent are exposed to an RNA virus in LS, the co-infecting parasite with LD, i.e., the "LD-LS-Lepsey NLV1 triple pathogen" phenomenon.
Life cycle The life cycle of Leishmania is completed in two hosts, humans and sandflies. The adult female sandfly is a bloodsucker, usually feeding at night on sleeping prey. When the fly bites an individual infected with Leishmania, the pathogen is ingested along with the prey's blood. The protozoan is in the smaller of its two forms, called an amastigote, which is round, non-motile, and only 3–7 micrometers in diameter. Inside the stomach of the sandfly, the amastigotes quickly transform into elongated and motile forms called the promastigotes. Promastigote is spindle-shaped, triple the size of the amastigote, and has a single flagellum that allows mobility. The promastigotes live extracellularly in the alimentary canal, reproducing asexually, then migrate to the proximal end of the gut where they become poised for regurgitative transmission. As the fly bites, the promastigotes are released from the proboscis and introduced locally at the bite site.
Once inside the human host, promastigotes invade macrophages. Inside the cells, they transform back into the smaller amastigote form. The amastigotes replicate in the most hostile part of the macrophage cell, inside the phagolysosome, whose normal defensive response they can prevent. After repeated multiplication, they break down their host cell by sheer pressure of mass, but there is some recent speculation that they can leave the cell by triggering the exocytosis response of the macrophage. The daughter cells protozoans then migrate to fresh cells or through the bloodstream to find new hosts. In this way, the infection is progressive, spreading to the host's mononuclear phagocyte system, particularly the spleen and liver. The free amastigotes in peripheral tissues are then ingested by sandfly to enter another cycle.
Regulatory T and B cells The cell-mediated immunity (CMI) that kills Leishmania also produces inflammation. If the inflammation is excessive, it can cause tissue damage. The role of regulatory T and regulatory B cells is to suppress CMI enough to prevent tissue damage. However, an excessive regulatory response can prevent clearance of Leishmania and could explain the anergy of VL, poor response to drug treatment, development of PKDL, and relapses. A role for regulatory cells in VL has long been suspected. A variety of regulatory T and B cells have been implicated in VL, including Type 1 T helper cells that secrete IL-10 in addition to IFN-γ, natural T reg, Tr1, CD8+ T reg, and B reg. All of these lymphocytes act, at least in part, by secreting IL-10 and other suppressive cytokines.
CD4+ T regs are present at increased frequency in the bone marrow of VL patients, are one source of IL-10, and proliferate in response to Leishmania antigen. Levels of FoxP3 mRNA were also up-regulated in lesional tissue from PKDL patients. However, T regs are not elevated in spleen cells from VL patients nor does depletion of T regs increase Leishmania antigen specific IFN-γ secretion The highest levels of IL-10 mRNA in spleen cells is in CD8+ and other non-FoxP3+ T cells. White blood cell CD8+ T cells from VL patients have elevated IL-10 levels. There is a 9.6 fold increase in IL-10 expressing CD8+ T cells among PBMC lymphocytes from PKDL patients. In the one study of T cell clones from VL patients, the clones isolated from VL PBMC were 100% CD8+. When mixed with self PBMC one or three years after successful treatment the CD8+ T cells decreased Leishmania antigen specific proliferation and IFN-γ secretion and increased IL-10 secretion. Depletion of CD8+ T cells from VL PBMC stopped endogenous IL-10 secretion but increased Leishmania antigen-specific IL-10 secretion, suggesting that CD8+ regulatory T cells are responsible for endogenous IL-10 secretion. CD4+ clones could only be isolated from VL PBMC after CD8+ T cell depletion. The CD4+ clones had little effect on IL-10 secretion but decreased IFN-γ secretion when mixed with self PBMC collected after successful treatment.
Regulatory B cells are known to favor development of regulatory T cells and suppress development of Type 1 T helper cells by producing IL-10 and other down-regulatory cytokines. IL-10 levels are elevated in B cells from VL PBMC. A study of dogs with naturally acquired VL showed that the percentage of regulatory B cells increased three-fold during VL. Depletion of B cells increased CD4+ T cell proliferation and IFN-γ secretion but decreased IL-10 secretion. Blocking IL-10 or programmed cell death receptors on B cells increased Leishmania antigen-specific T cell proliferation and IFN-γ secretion. Co-culture of T cells with B cells decreased the percentage of CD4+ T cell proliferation and IFN-γ secretion four-fold.
Diagnosis The gold standard for diagnosis is visualization of the amastigotes in splenic aspirate or bone marrow aspirate. However, the technique is associated with discomfort, high risk of tissue damage, while being expensive and difficult. The detection is also unreliable as it gives high false negative results, while the true positives are often having very low infection.
Serological testing is much more frequently used in areas where leishmaniasis is endemic. A 2014 Cochrane review evaluated different rapid diagnostic tests. One of them (the rK39 immunochromatographic test) gave correct, positive results in 92% of the people with visceral leishmaniasis, and it gave correct, negative results in 92% of the people who did not have the disease. A second rapid test (called latex agglutination test) gave correct, positive results in 64% of the people with the disease, and it gave correct, negative results in 93% of the people without the disease. Other types of tests have not been studied thoroughly enough to ascertain their efficacy.
The K39 dipstick test is easy to perform, and village health workers can be easily trained to use it. The kit may be stored at ambient temperature, and no additional equipment needs to be carried to remote areas. The DAT anti-leishmania antigen test, standard within MSF, is much more cumbersome to use and appears not to have any advantages over the K39 test.
There are many problems with serological testing: in highly endemic areas, not everyone who becomes infected will actually develop clinical disease or require treatment. Indeed, up to 32% of the healthy population may test positive, but not require treatment. Conversely, because serological tests look for an immune response and not for the organism itself, the test does not become negative after the patient is cured, it cannot be used as a check for cure, or to check for re-infection or relapse. Likewise, patients with abnormal immune systems (e.g., HIV infection) will have false-negative tests.
Other tests being developed include detects erythrosalicylic acid.
Prevention As of 2018, there are no vaccines or preventive drugs for visceral leishmaniasis. However, vaccines are in development. The most effective method to prevent infection is to protect from sand fly bites. To decrease the risk of being bitten, these precautionary measures are suggested: Outdoors: 1. Avoid outdoor activities, especially from dusk to dawn, when sand flies are generally the most active.