Visceral leishmaniasis

Visceral leishmaniasis (VL), also known as Kala-azar and black fever, is the most severe form of leishmaniasis, a disease caused by parasites of the Leishmania genus. It is the second-largest parasitic killer in the world (after malaria), responsible for an estimated half-million deaths worldwide each year. The parasite migrates to the visceral organs such as liver, spleen and bone marrow and if left untreated will almost always result in the death of the mammalian host. Symptoms include fever, weight loss, anaemia 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^[1].

Species that give rise to VL

Several species of Leishmania are known to give rise to the visceral form of the disease. The "Old World" (Africa, Asia, Europe) species are L. donovani and L. infantum and the "New World" (South America) species is L. chagasi.

Life-cycle of the parasite

Like malaria, kala-azar is spread through an insect vector — in this case not a mosquito but the sandflies of the Phlebotomus genus, tiny creatures 2-3 millimeters long and found in tropical or temperate regions throughout the world. Phlebotomus larvae grow in warm, moist organic matter, so old trees, house walls or garbage are their most common breeding centers — making them harder to eradicate than the mosquitoes that transmit malaria, which need standing water to breed.

The adult female Phlebotomus is a bloodsucker, usually feeding at night on sleeping prey. When the fly bites an animal infected with L. donovani, the pathogen is ingested along with the prey’s blood. At this point the protozoan is in the smaller of its two forms, called an amastigote — round, non-motile, and only three to seven micrometers in diameter.

Taken into the stomach of the sandfly, the amastigotes quickly transform into L. donovani’s second form, called the promastigote. This form is spindle-shaped, triple the size of the amastigote, and has a single flagella that allows for motility. The promastigotes live extracellularly in the sandfly’s alimentary canal for a few days, reproducing asexually, then migrate to the salivary glands. This is their means of transmission back into a mammalian host, as the fly injects its saliva into prey when it bites. The promastigotes are introduced into the prey’s bloodstream along with the fly’s saliva.

Once inside the new host, each promastigote attaches itself to a macrophage using its flagella. It is then taken alive into the cell through the macrophage’s own action of phagocytosis. Once inside, it transforms back into the smaller amastigote form. As an amastigote, L. donovani can only reproduce intracellularly — and the amastigotes do their breeding in the most hostile part of the macrophage cell, inside the phagolysosome organelles, whose normal defensive response they are able to prevent. It has been generally believed that after they have reproduced to a certain extent, the L. donovani lyse 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 macrophage’s exocytosis response. The daughter protozoans then migrate through the bloodstream to find new macrophage hosts. In time, L. donovani becomes a systemic infection, spreading to all the host’s organs, particularly the spleen and liver.

Humans are only one of the reservoir hosts for L. donovani. The protozoan can also form its amastigotes in the macrophage cells of animals; rodents, foxes and dogs are the most common. These alternate hosts, particularly the domestic dog, make it far more difficult to control spread of the disease among human populations. Fortunately for humans there is a canine form of leishmaniasis, so infected pets can readily be identified and destroyed.

Disease progression

In human hosts, response to infection by L. donovani varies a great deal, not only by the strength but also by the type of the patient’s immune reaction. Patients whose immune systems produce large numbers of TH1-type T-Cells, which strengthen cell defenses but do not encourage antibody formation, often recover easily from infection on their own, and after recovery are immune to reinfection. Patients whose systems produce more TH2-type cells, which prompt antibody formation but do nothing for cellular health, are likely to quickly succumb to leishmaniasis. Sadly, some of the stronger strains of L. donovani appear to be able to force a switch in the host from a TH1 to a TH2-type immune response as the infection progresses.

When a human patient does develop visceral leishmaniasis, the most typical symptoms are fever and the enlargement of the spleen, or splenomegaly, with enlargement of the liver — hepatomegaly — 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 doesn’t 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 often sets 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).

Treatments

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. Presumably 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 organisations. This may or may not 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 the treatment of choice for visceral leishmaniasis acquired in India is now Amphotericin B in its various preparations (Ambisome®, Abelcet®, Amphocil®)

• AmBisome dose: total dose 21mg/kg (Mediterranean/Brazilian VL); total dose 7.5mg/kg over 6 days (Indian VL)
• Amphocil dose: total dose 7.5mg/kg over 6 days (Indian VL)

A low dose (0.5–1mg/kg) is given on the first day, increasing to 1–2mg/kg on the second day, followed by 1.5–3mg/kg on the third and subsequent days.

Miltefosine is an oral treatment currently available only in India. Results of trials outside of India have been disappointing. There problems with toxicity (gastrointestinal and renal) as well as the rapid development of resistance. Because it is available as an oral formulation, the expense and inconvenience of hospitalisation is avoided, which makes it an attractive alternative.

The Institute for OneWorld Health charity is developing the drug paromomycin, which they claim will be effective and cheap. A treatment with paromomycin will cost about $10. The drug had originally been identified in th 1960's, but had been abandoned because it would not be profitable, as the decease mostly affects poor people.

History and epidemiology

Kala-azar is not a new disease. It first came to the attention of Western doctors in 1824 in Jessore, India, where it was initially thought to be a form of malaria. India gave kala-azar its common name, which is the Hindi for “black fever”, so called 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 two English doctors, William Leishman and 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, and a slightly different strain of the pathogen, L. chagasi, is responsible for leishmaniasis in the new world.

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 the people are as tied to the place of their birth as any peasant of Europe’s dark ages.

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 Medicins Sans Frontieres, “Where else in the world could 50% of a population die without anyone knowing?”

The world’s failure to notice the epidemic was not due solely to its primitive setting, but also to the realities of politics. When, in 1991, a group of Sudanese researchers with the World Health Organization warned of a coming kala-azar epidemic and proposed the construction of a treatment center, the Sudanese government stepped in and denied the existence of any epidemic. At war with its own people, the government in Khartoum did not wish to have foreigners aiding the population, and preferred to use limitations on foreign aid as a means of political control. The WHO, a non-governmental organization, could do nothing in the face of opposition from its host government, and so nothing was done. For much of the nineties, Medecins Sans Frontieres battled the disease essentially alone, working not only without borders but without hospitals or visas.