Invited review
The current status of zoonotic leishmaniases and approaches to disease control

https://doi.org/10.1016/j.ijpara.2005.07.001Get rights and content

Abstract

Leishmaniases are a complex of world-wide diseases with a range of clinical and epidemiological features caused by Leishmania spp. of protozoan parasites. Among 15 well-recognised Leishmania species known to infect humans, 13 have zoonotic nature, which include agents of visceral, cutaneous and mucocutaneous forms of the disease in both the Old and New Worlds. Currently, leishmaniases show a wider geographic distribution and increased global incidence of human disease than previously known. Environmental, demographic and human behavioural factors contribute to the changing landscape of leishmaniasis, which includes increasing risk factors for zoonotic cutaneous leishmaniases and new scenarios associated with the zoonotic visceral leishmaniases. The latter consist of the northward spread of Leishmania infantum transmission in Europe and America, the identification of unusual mammal hosts, and the decline of HIV-Leishmania co-infections in southern Europe following the introduction of the highly active antiretroviral therapy. Few advances have been made in the surveillance and control of the zoonotic leishmaniasis, however a number of tools have been developed for the control of the canine reservoir of L. infantum. These include: (i) several canine vaccine candidates, in particular an FML Leishmania enriched fraction showing good clinical protection, has been registered in Brazil for veterinary use; (ii) a number of insecticide-based preparations have been specifically registered for dog protection against sand fly bites. Laboratory and field studies have shown improved efficacy of these preparations for both individual and mass protection.

Introduction

Leishmaniases are protozoan diseases caused by members of the genus Leishmania, parasites infecting numerous mammal species, including humans, and transmitted by the bite of phlebotomine sand flies. Human leishmaniases have diverse clinical manifestations. Visceral leishmaniasis (VL), caused by Leishmania donovani in the Old World and Leishmania infantum in both the Old and New Worlds, is the most severe form, which if left untreated, invariably leads to death. A number of different species of Leishmania cause cutaneous (CL) or mucocutaneous (MCL) leishmaniasis which, if not fatal, are responsible for considerable morbidity of a vast number of people in endemic foci. According to available figures, 66 Old World and 22 New World countries are endemic for human leishmaniases, with an estimated yearly incidence of 1–1.5 million cases of CL forms and 500,000 cases of VL forms (Desjeux, 1996). The incidence of leishmaniases is not uniformly distributed in endemic areas: about 90% of CL cases occur in seven countries only (Afghanistan, Algeria, Brazil, Iran, Peru, Saudi Arabia and Syria), whereas some 90% of VL cases occur in rural and suburban areas of five countries (Bangladesh, India, Nepal, Sudan and Brazil). These figures, however, must be regarded as approximate and, most probably, underestimated, as official data are often obtained through passive case detection and do not include information from private practitioners and non-government organisations.

Currently, it appears that the global incidence of human leishmaniases is higher than before, although it is difficult to differentiate between a real and artificial increase, due to better awareness, case detection, improved reporting and accessibility to treatment. For example, in Brazil CL cases were 21,800 in 1998, while there were 40,000 in 2002; VL cases recorded in the same periods were 1840 and 6000, respectively; in Kabul, Afghanistan, CL cases were 14,200 in 1994, 65,000 in 2002 (Desjeux, 2001, Desjeux and Alvar, 2003).

Undoubtedly, human and animal leishmaniases show a wider geographic distribution than previously known. Autochthonous Leishmania transmission has been recently recorded in traditionally non-endemic areas, for example in western Upper Nile, Sudan (Desjeux, 2001), a number of US states and Canadian provinces (Enserink, 2000), Australia's Northern Territory (Rose et al., 2004), and in some parts of Europe (Koehler et al., 2002, Capelli et al., 2004). It is widely accepted that the leishmaniases are dynamic diseases and the circumstances of transmission are continually changing in relation to environmental, demographic and human behavioural factors. Changes in the habitat of the natural host and vector, immunosuppressive conditions (e.g. HIV infection or organ transplantation-associated therapies) and the consequences of conflicts, all contribute to the changing leishmaniasis landscape.

Section snippets

Zoonotic forms of leishmaniasis

There is no consensus about the named Leishmania species causing disease in humans (Ashford, 2000, Dedet and Pratlong, 2002, Bonfante-Garrido et al., 1981, Boari et al., 2005, Rolao et al., 2005, Rossi et al., 2005, Silveira et al., 2005). The New World species Leishmania chagasi is now widely accepted to be a synonym of L. infantum, however in recent work Latin American authors still consider these species to be distinct. Some authors describe Leishmania archibaldi and Leishmania killicki as

Re-emerging issues

Over the past few years, there have been a number of findings or events which illustrate the changing pattern of Leishmania transmission. As mentioned above, several re-emerging issues were associated with the zoonotic entity of visceral leishmaniasis (L. infantum). Factors underlying such changes could not be easily identified and characterised, however climatic modifications associated with global warming and human behavioural factors were thought to play a major role. As for the former, it

New methodologies of surveillance and control

Over the past few years, international health agencies have increased efforts to improve methodologies for the surveillance and control of leishmaniasis entities characterised by a predominant anthroponotic transmission pattern. Better tools have recently been made available to developing countries for: (i) improved VL case detection, e.g. a direct agglutination test kit for laboratory serology, and a recombinant antigen (K39)-dipstick for field serology; (ii) affordable VL treatment, e.g. the

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