Leishmaniasis is caused by several species of flagellated protozoan parasites found in many areas of the world, particularly in Africa, Latin America, South and Central Asia, the Mediterranean basin and the Middle East. In its more severe forms, the disease can cause serious disfigurement as well as death. WHO estimates the worldwide prevalence to be approximately 12 million cases, with annual mortality of about 60 000. The size of the population at risk is about 350 million. Transmission is most often zoonotic: the parasites (Leishmania) are transmitted from a wild-animal reservoir (small rodents, dogs) by the bite of the female phlebotomine sandfly. It also can be anthroponotic, the parasite being transmitted by the sanfly from an infected human host. Several forms of the disease exist: cutaneous (CL), mucocutaneous (MCL) and visceral (VL, also called “kala-azar”), which, after treatment, is often followed by a dermal manifestation known as “post-kala-azar” dermal leishmaniasis (PKDL). CL and MCL in Central and South America are caused by members of the L. mexicana and L. braziliensis species, whereas CL in South and Central Asia and the Middle East is caused by L. tropica and L. major: The majority of MCL cases occur in Bolivia, Brazil and Peru, and 90% of CL cases occur in Afghanistan, Brazil, Iran, Peru, Saudi Arabia and Syria. VL (“kala-azar”), the most lethal form of the disease, is caused by L. donovani in Bangladesh, Chinnda, Iia, Nepal and Sudan by L. infantum in North Africa and southern Europe, and by L. chagasi in Latin America.
For many years, the public health impact of leishmaniasis has been grossly underestimated, as a substantial number of cases were never recorded. About 1.5–2 million new cases are estimated to occur annually, but only 600 000 are officially declared. In addition, deadly epidemics of VL periodically flare up but go mostly unnoticed in spite of case–fatality rates as high as 10% or more. In the 1990s Sudan suffered a crisis with an excess mortality of 100 000 deaths among people at risk. The expansion of leishmaniases and the alarming rise in the number of cases is related to environmental changes such as deforestation, building of dams, new irrigation schemes and migration of non-immune people to endemic areas, and resulted in significant delay in the implementation of development programmes in the Amazons and the tropical regions of the Andean countries, Morocco and Saudi Arabia.
More recently, as a result of epidemiological changes, a sharp increase in the overlapping of HIV infection and visceral leishmaniasis has been observed, especially in intravenous drug users in South-Western Europe. The situation may soon worsen in Africa and Asia where the prevalence and detection of HIV and Leishmania co-infections still are probably largely underestimated. The first line drugs for treatment of leishmaniasis are antimonials, which remain expensive, require repeated injections, and are associated with important side effects. Drug resistance also is becoming common in certain areas (i.e. Bihar, India). Miltefosin, a recently developed drug which is active orally against VL, has not yet been widely used. Vector and reservoir controls may be useful under certain conditions but are not applicable in every epidemiological setting and require infrastructure and vigilance beyond the capability of many endemic countries. Vaccination, therefore, remains the best hope for control of all forms of the disease.
There is as yet no effective vaccine for prevention of any form of leishmaniasis. A first generation vaccine was prepared using whole killed parasites combined or not with BCG. The combination of autoclaved L. major promastigotes with BCG as adjuvant was tested in Iran against CL and in Sudan against VL. A limited efficacy was noted in converters to positive skin reaction to leishmania antigen (leishmanin) and unexpectedly in boys. Similar observations had been made earlier in Brazil using killed promastigotes without BCG. Alum precipitated autoclaved L. major promastigotes plus BCG have demonstrated safety and substantial immunogenicity in Phase I studies in Sudan. Additional trials are under way to test new formulations with IL-12. It is of note that treatments combining administration of antimonials and first generation leishmania vaccines in patients suffering from “post-kala-azar” dermal leishmaniasis (PKDL) have shown benefit to the patients, suggesting that even suboptimal leishmaniasis vaccines could have a role in the therapeutic setting.
Various subunit recombinant candidate vaccines also have been tested in mice and provided some degree of protection against infection. These vaccines were based on:
- recombinant surface antigen gp63, a glycoprotein with protease activity,
- lipophosphoglycan, a surface glycoconjugate;
- a 46 kD promastigote antigen derived from L. amazonensis;
- or the Leishmania-activated C kinase (LACK), among others.
Protection against L. major infection in mice was provided by DNA constructs encoding a number of Leishmania antigens, including gp63 and LACK.
It has been demonstrated in experimental animal models that a dominant Th1 lymphocyte response (IL-2, IFN-γ) is associated with self-limited disease, whereas a dominant Th2 response (IL-4, IL-5) is linked to progressive disease. Addition of Th1-driving adjuvants such as IL-12 or oligodeoxynucelotides (CpG) to leishmanial antigens (TSA, LeIF, LmSTI-1) resulted in complete protection of susceptible mice against progressive disease, whereas no protection was observed in the absence of adjuvant. The Bill and Melinda Gates Foundation has funded the development of a chimeric vaccine made of these three recombinant leishmanial antigens (LeIF, LmSTI-1 and TSA) in the form of a fusion protein combined with monophosphoryl lipid A in squalene oil as adjuvant. Phase I trials of this vaccine in healthy volunteers in the USA and initial efficacy testing as a therapeutic vaccine in patients in Latin America suggest the safety and immunogenicity of the vaccine.
Recent evidence indicates that a 15 kD protein antigen derived from the salivary glands of the sandfly vector also could be protective in mice when given as a vaccine. Generally, recovery from CL leads to protection against future infections. For centuries, in some of the hyper-endemic areas of the Middle East, the pus of an active lesion was used to inoculate young children to protect them against future lesions on the exposed parts of the body, especially the face. L. major promastigotes grown in culture under good manufacturing practice (GMP) guidelines, rather than the exsudates from active lesions, have been used for inoculation as a live vaccine. The practice is known as leishmanization. Genetically manipulated parasites with attenuated virulence or high sensitivity to chemotherapy might represent the ideal form of a live vaccine.