Amastigotes in a chorionic villus
|Classification and external resources|
Visceral leishmaniasis (VL), also known as kala-azar, black fever, and Dumdum fever, is the most severe form of leishmaniasis. Leishmaniasis is a disease caused by protozoan parasites of the Leishmania genus. This disease is the second-largest parasitic killer in the world (after malaria), responsible for an estimated 200,000 to 400,000 infections each year worldwide. 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.
Signs and symptoms
Response to infection by Leishmania donovani varies a great deal, not only by the strength but also by the type of the patient's immune reaction. People with a history of infection by strains of leishmania that cause visceral leishmaniasis show a continuum of immune responses from protective to non-protective. Those who acquired protective immunity (skin test positive) without ever having visceral leishmaniasis have a strong type 1 CD4+ response to leishmania antigens. Antigen specific interferon-gamma and proliferation, as well as the ability to kill intracellular leishmania, are hallmarks of protective immunity. Because visceral leishmaniasis patients lack these responses to leishmania and other antigens, they usually die of secondary infections unless treated. In addition, increased interleukin-10 secretion is characteristic of the disease. Addition of interleukin-12, anti-interleukin-10, or anti-interleukin-4 to peripheral blood mononuclear cells from acute patients sometimes increases interferon-gamma secretion and proliferation. Acute patient peripheral blood mononuclear cells include CD8+ T regulatory cells that decrease interferon-gamma secretion and proliferation responses to leishmania and other antigens and increase interleukin-10 secretion when added to autologous peripheral blood mononuclear cells harvested after successful treatment. Thus, the CD8+ T regulatory cells reproduce the immune response characteristic of visceral leishmaniasis. CD8+ T regulatory cells are also associated with post kala azar dermal leishmaniasis. Addition of interleukin-12 or interferon-gamma does not prevent CD8+ T regulatory activity. The dominance of type 1 CD4+ T cells in skin test positive adults maybe explained by their secretion of factors that inhibit and kill CD8+ T regulatory cells. Successfully treated patients rarely develop visceral leishmaniasis a second time. Their peripheral blood mononuclear cells show a mixed T1/T2 CD4+ and CD8+ T suppressor response but do have the ability to kill intracellular leishmania.
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, and the other symptoms are very easy to mistake 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 kala-azar sufferers, however. Pneumonia, tuberculosis, and dysentery are omnipresent in the 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, measle-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 causing 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.)
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 larvae grow in warm, moist organic matter (such as old trees, house walls, or waste) making them hard to eradicate.
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 a regurgitational 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 are able to prevent. After repeated multiplication, they break down their host cell by sheer pressure of mass, but there is some recent speculation that they are able to 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 cyle.
The gold standard for diagnosis is visualization of the amastigotes in splenic aspirate or bone marrow aspirate. This is a technically challenging procedure that is frequently unavailable in areas of the world where visceral leishmaniasis is endemic.
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 a number of 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.
There are no vaccines or preventive drugs for visceral leishmaniasis. 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:
1. Avoid outdoor activities, especially from dusk to dawn, when sand flies generally are the most active.
2. When outdoors (or in unprotected quarters), minimize the amount of exposed (uncovered) skin to the extent that is tolerable in the climate. Wear long-sleeved shirts, long pants, and socks; and tuck your shirt into your pants.
3. Apply insect repellent to exposed skin and under the ends of sleeves and pant legs. Follow the instructions on the label of the repellent. The most effective repellents generally are those that contain the chemical DEET (N,N-diethylmetatoluamide).
1. Stay in well-screened or air-conditioned areas.
2. Keep in mind that sand flies are much smaller than mosquitoes and therefore can get through smaller holes.
3. Spray living/sleeping areas with an insecticide to kill insects.
4. If you are not sleeping in a well-screened or air-conditioned area, use a bed net and tuck it under your mattress. If possible, use a bed net that has been soaked in or sprayed with a pyrethroid-containing insecticide. The same treatment can be applied to screens, curtains, sheets, and clothing (clothing should be retreated after five washings)."
On February 2012, the nonprofit Infectious Disease Research Institute launched a clinical trial of the visceral leishmaniasis vaccine. The vaccine is a recombinant form of two fused Leishmania parasite proteins with an adjuvant. Two phase 1 clinical trials with healthy volunteers are to be conducted. The first one takes place in Washington (state) and is followed by a trial in India.
As with many diseases in developing nations, (including trypanosomiasis and malaria) effective and affordable chemotherapy is sorely lacking and parasites or insect vectors are becoming increasingly resistant to existing anti-parasite drugs. Possibly due to the lack of financial return, new drugs are slow to emerge and much of the basic research into potential drug targets takes place in universities, funded by charitable organizations. This may change as a result of infection of members of the armed forces from the developed nations that currently occupy nations such as Afghanistan and Iraq, where Leishmania is commonplace.
The traditional treatment is with pentavalent antimonials such as sodium stibogluconate and meglumine antimoniate. Resistance is now common in India, and rates of resistance have been shown to be as high as 60% in parts of Bihar, India.
Miltefosine is the first oral treatment for this disease. The cure rate of miltefosine in Phase III clinical trials is 95%; Studies in Ethiopia show that is also effective in Africa. In HIV immunosuppressed people which are coinfected with leishmaniasis it has shown that even in resistant cases 2/3 of the people responded to this new treatment. Miltefosine has received approval by the Indian regulatory authorities in 2002, in Germany in 2004 and in U.S.A. in 2014. It is now registered in many countries.
The drug is generally better tolerated than other drugs. Main side effects are gastrointestinal disturbance in the first or second day of treatment (a course of treatment is 28 days) which does not affect the efficacy. Because it is available as an oral formulation, the expense and inconvenience of hospitalization is avoided, and outpatient distribution of the drug becomes an option, making Miltefosine a drug of choice.
The nonprofit Institute for OneWorld Health has adopted the broad spectrum antibiotic paromomycin for use in treating VL; its antileishmanial properties were first identified in the 1980s. A treatment with paromomycin costs about $15 USD. The drug had originally been identified in the 1960s. The Indian government approved paromomycin for sale and use in August 2006.
But, while the disease's geographical range is broad, it is not continuous. The disease clusters around areas of drought, famine, and high population density. In Africa, this has meant a knot of infection centers mostly in Sudan, Kenya, and Somalia. Living conditions here have changed very little in the past century, and the people are not normally very mobile. Parts of the Sudan, in particular the Upper Nile region, are almost totally cut off from the rest of the country, and most people tend to remain at their place of birth.
See also History of leishmaniasis
Kala-azar first came to the attention of Western doctors in 1824 in Jessore, India (now Bangladesh), where it was initially thought to be a form of malaria. Assam gave kala-azar one of its common names, Assam fever. Another common name, kala-azar (Hindustani: काला आज़ार (Devanagari) کالا آزار (Nastaleeq) kālā āzār), is derived from kala which means black in Sanskrit, as well is the languages descended from it, including Assamese, Hindi and Urdu; the word azar means disease in Persian and Hindustani; as such the disease is named for the darkening of the skin on the extremities and abdomen that is a symptom of the Indian form of the disease. The agent of the disease was also first isolated in India by Scottish doctor William Leishman (who observed the parasite in spleen smears of a soldier who died of the disease in Dumdum, Calcutta, India - hence the name dumdum fever) and Irish physician Charles Donovan, working independently of each other. As they published their discovery almost simultaneously, the species was named for both of them—Leishmania donovani.
Today, the name kala-azar is used interchangeably with the scientific name visceral leishmaniasis for the most acute form of the disease caused by L. donovani. The disease is endemic in West Bengal, where it was first discovered, but is seen at its most deadly in north and east Africa. It can also be found throughout the Arab world and southern Europe (where the causative organism is L. infantum), and a slightly different strain of the pathogen, L. chagasi, is responsible for leishmaniasis in the new world. Several species of canines serve as reservoir hosts of L. infantum (chagasi).
Contemporary life has made itself felt even here, however—not as "progress" but in the form of the many small wars of Africa's post-colonial era. In the Sudan, where civil war has been continuous since 1983, the violence has been concentrated in the more populated south, and kala-azar was concentrated there too. But the wars have driven a steady stream of refugees out of the region, and these traveled either across the southern border or into the remoter western part of the country called the Upper Nile, where both war and the disease that went with it had not yet penetrated.
These refugees, moving at foot-speed, carried the disease with them, and when it arrived it hit the Upper Nile with a force comparable to smallpox hitting the American Indians. The isolated people of the Upper Nile had no access to medicine or education about the new disease among them. Worse, their immune systems were defenseless against this new pathogen, foreign to them though it came only from another part of their own country. One village at the center of the epidemic, Duar, was left with four survivors out of a population of a thousand, and from the late eighties to the mid-nineties a total of 100,000 succumbed to the sickness in that region alone. In the words of Jill Seaman, the doctor who led relief efforts in the Upper Nile for the French organization Médecins Sans Frontières, "Where else in the world could 50% of a population die without anyone knowing?"
The Indian medical practitioner Upendra Nath Brahmachari was nominated for the Nobel Prize in Physiology or Medicine in 1929 for his discovery of ureastibamine (an antimonial compound for the treatment of kala-azar) and a new disease, post kala-azar dermal leishmaniasis. Brahmachari's cure for visceral leishmaniasis was the urea salt of para-amino-phenyl stibnic acid which he called Urea Stibamine.
Combination drug therapies are currently under investigation, particularly by the Drugs for Neglected Diseases initiative (DNDi). Combination therapies allow for the use of existing drugs in combination, each in lower doses, which helps to decrease the incidence of severe side effects and drug toxicity, as well as the risk for development of resistance against the drugs; they have been shown to be cost-effective strategies. Comparative homology modelling of the enzyme Hypoxanthine-guanine phosphoribosyl transferase (HGPRT; EC 22.214.171.124) in L. donovani suggest that among all of the computationally screened compounds, pentamidine, 1,3-dinitroadamantane, acyclovir and analogs of acyclovir had higher binding affinities than the real substrate (guanosine monophosphate).
The drug development pipeline is lacking significantly, and no novel drug targets are expected for approval in the next 5 years. In the meantime, new combination therapies, and well as improvements to existing drugs targets, are under development. Single-dosage administrations of liposomal amphotericin B have been shown to be effective, and oral formulations are currently under development to increase access and facilitate distribution of the efficacious drug in the field.
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