Leishmania and the Leishmaniases

A Parasite Treatment Update and Advances

Table of Contents

Abstract

Chapter 1

1.1 Introduction

1.2 Taxonomic Classification

1.3 Epidemiology

       1.3.1 Malnutrition

       1.3.2 Environmental changes

       1.3.3 Population movements

       1.3.4 Climate change

       1.3.5 Vector Transmission

 1.4 Morphology

        1.4.1 Amastigote

        1.4.2 Promastigote

1.5 Types of Leishmaniasis

       1.5.1 Cutaneous Leishmaniasis

       1.5.2 Mucocutaneous Leishmaniasis

       1.5.3 Visceral Leishmaniasis

1.6 Life cycle of Leishmania

       1.6.1 First stage (from 0-2 days)

       1.6.2 Stage two (from 2-5 days)

       1.6.3 Stage three (after 5 days)

1.7 Pathogeneticity Caused by Leishmania

1.8 The Immune Response and Clinical Diversity in Humans

Chapter 2

2.1 Immunology of Leishmania

2.1.1 T Lymphocytes

2.1.2 The function of B cells

2.1.3 The function of cytokines

2.1.4 Immunosuppression

2.2 Leishmania virulence factors

2.2.1 Lipophosphoglycan (LPG)

2.2.2 Acid phosphatases

2.2.3 Glycoprotein 63 (GP60)

2.2.4 Cysteine proteinases (Cps)

Chapter 3

3.1 Current and Conventional therapies

           3.1.2 Visceral Leishmania Therapies

           3.1.3 Cutaneous Leishmania Therapies

3.2 Vaccine development

        3.2.1 Killed vaccines for Leishmaniasis

        3.2.2 Live vaccination

        3.2.3 Live-attenuated vaccines

        3.2.4 Naked DNA vaccine

Chapter 4

4.1 Visceral Leishmania drug discovery: novel treatments on the way

4.2 The target-free screening approach

4.3 Compound screening: rapidly assessing hits

4.4 Hit-to-lead & lead optimization

Chapter 5

5.1 Control strategies

        5.1.2 Reservoir control

        5.1.3 Vector control

        5.1.4 Insecticide-impregnated materials

Chapter 1

1.1 Introduction

Leishmaniases today remain a main public health problem regardless of the huge amount of research carried on Leishmania pathogens. Leishmania species are basically parasite, the obligate intracellular parasites of macrophage-dendritic cell lineage. They belong to the family known as Trypanosomatida. The genus Leishmania is extensively present in the nature except Antarctica while the morphologically similar leishmanisas live in a single series of cells, leading to diversity of diseases (Murray, Berman et al 2005). It consists of a number of species as shown in the table 1 that are closely identical morphologically. However, the differentiation is based on several epidemiological and biochemical criteria, utilisation of monoclonal probes to find out particular antigens, vectors, promastigote growth prototype in vitro and reservoir hosts. Broadly, leishmanias are divided into cutaneous leishmanias and visceral leishmanias (Center for Food Security and Public Health 2009).

Leishmanias spend their life cycle in two hosts: vertebrate and invertebrate hosts. Former are mammals and latter are the sandflies. Parasites reside in the mammals’ body within their phagolysosomal system of mononuclear phagocytic cell, characteristically macrophages. But in the invertebrate hosts these parasites reside extracellularly, onset occurs completely in the gut and spread is through the mouth-components during blood feeding.

Leishmaniasis has been identified for several hundreds of years and their initial clinical description came out by Alexander Russell in the year of 1756 (Hide, Bucheton, et al. 2007). A number of names have been linked to this group of diseases for example Dum-dum fever, espundia, Kala-azar, white leprosy, and so on. Leishmaniasis is spread by the bite of sand flies, which is a phlebotomine female belonging to the genera Psychodopygus and Lutzomyia and Phlebotomus, in the new and old worlds, respectively.  This parasite is still one of the world’s highly ignored diseases, distressing mainly the poorest people, largely in the developing countries (WHO 2010). Sylvatic mammals are their primary reservoir hosts including hyraxes, domesticated animals, wild canids and forest rodents. Dogs are considered to be the key species among domesticated animals in the disease epidemiology.  At present, leishmaniasis has a broader geographical distribution than ever before, and it is regarded as a rising public health concern for quite a lot of countries. The amplification in the size of leishmaniasis worldwide occurrence is principally the result of the increase in numerous risk factors that are evidently synthetic and incorporate huge migration, treatment failure, urbanization, deforestation, malnutrition and immunosuppression. Currently, man has made a lot of changes to his environment and this may causing changes in the density and range of reservoirs and vectors and thus, may enhance human contact to infected sand flies.

1.2 Taxonomic Classification

Kingdom	Protozoa
Sub-kingdom	Protista
Phylum	Sarcomastigophora
Sub-phylum	Mastigophora
Class	Zoomastigophora
Order	Kinetoplastida
Suborder	Trypanosomatina
Genus	Leishmania

Table 2: Leishmania Parasites Taxonomy

Leishmania organisms are broadly classified two main groups (1) the parasites of old world Leishmanias taking place in Asia, Africa and Europe, and (2) the parasites of new world Leishmanias occurring mainly in America.

About, 30 different species of leishmanias have been explained, and no less than 20 of these parasites are pathogenic for both humans as well as animals (Desjeux 2004).  The diverse zoonotic Leishmania species with their geographical distribution, reservoir hosts and diseases caused by them in humans are summarized below in table 3 and figure 1.

Species of Leishmania	Geographical distribution	Reservoir host	Disease in Humans Caused by L. species
Leishmania(Leishmania)  major	Middle East and Central Asia, North Africa, Sahel belt and Sub-Saharan Africa	Gerbillidae rodents	Localised cutaneous leishmaniasis
Leishmania (Viannia) braziliensis	South America, Mexico and Central America	Rain forest mammals	Mucocutaneous leishmaniasis; Localised cutaneous leishmaniasis
Leishmania(L) infantum	China, Mediterranean Basin, Central and South America, Central Asia, Middle East and to Pakistan	Dog	Localised cutaneous leishmaniasis; Visceral leishmaniasis
Leishmania (V) peruviana	Peruvian Andes	Dog	Localised cutaenous leishmaniasis
Leishmania (L) venezuelensis	Venezuela	Not known	Localised cutaenous leishmaniasis
Leishmania (L) aethiopica	Kenya, Ethiopia	Rock hyraxes	Diffuse cutaneous Leishmaniasis; Localised cutaneous leishmaniasis
Leishmania (L) mexicana	Central America	forest rodents	Localised cutaneous leishmaniasis

Figure 1: Leishmania species distribution according to geographical region.

New World Leishmaniasis: New world leishmaniasis are chiefly cutaneous ones and are found in South America, Mexico, Central America, from Northern Argentina to southern Europe and Southern Texas (Handman 1997). These are:

·         Leishmania mexicana complex

·         L. chagasi

·         L. peruviana

·         Leishmania braziliensis complex

L. chagasi cause visceral leishmania almost entirely in young children, infants, and immunosuppressed individuals (Marfurt, Nasereddin, Niederwieser et al 2003).

Old World Leishmaniasis: Old world leishmanias are responsible for both cutaneous and visceral types of diseases in humans and include:

·         Leishmania donovani

·         Leishmania infantum

·         L. major

·         L. aethiopica

·         L. tropica

The cutaneous ones are mostly found in Middle East, Asia, and Africa. The approximate yearly incidence of the old world cutaneous leishmaniasis is 1 to 1.5 million while above 90% cases are reported in Afghanistan, Algeria, Iran, Iraq, Saudi Arabia, and Syria(Handman 1997). In the old world, L. aethiopica and L. major are responsible for zoonotic cutaneous leishmaniasis. The possibility of cutaneous leishmaniasis is often increased when irrigation systems expanded and agricultural projects are started. In major part of the Central Asia zoonotic cutaneous leishmaniasis is the result of L. major North Africa and Middle East, and spread of infection is sustained in gerbil colonies or wild rodent. These manmade environmental alterations are escorted by the interference of big numbers of nonimmune settlers into a present sylvatic cycle of leishmaniasis. Spread to humans is supported by the routine of sleeping outside without the house without a net over the bed during the hot season. On the other hand, increased human fly exposure happens in villages built on river banks or rock hills particularly in foci of L. aethiopica causing cutaneous leishmaniasis.

Leishmania infantum cause visceral leishmania in infants, immunosuppressed and young children. On the contrary, L. donovani transmit a disease to both adults and children. During the preceding couple of decades, appearance of resistance to pentavalent antimonial has resulted in to an enormous influence on the epidemiology of leishmaniasis.

Leishmania species are usually linked to one or other of the two sections. The species belonging to subgenus Viannia were actually obtained in the ‘New World’, and the subgenus Leishmania were obtained from the Old World, excluding species of the L. hertigi, L. Mexicana complex, and L. deanei—which were discovered in the New World while only—and L. chagasi, L. infantum and L. major were discovered in both the Old and New World.

1.3 Epidemiology

Figure 2: Cutaneous Leishmania Epidemic

Figure 3: Visceral Leishmania Epidemic

Animal and human leishmaniases demonstrate a broader geographic distribution than earlier known. They are widely disseminated around the world and vary over inter tropical zones of Africa, America, and expand in to temperate areas of Asia, South America and southern Europe. The approximate yearly incidence of the old world cutaneous leishmaniasis is 1 to 1.5 million, while above 90% cases are reported in Saudi Arabia, Afghanistan, Iraq, Algeria, Syria and Iran (Arfan, 2006). Their limits of extension are latitude 32° south and 45° north. Geographical circulation of the diseases relies on sand fly species playing a role of vectors, the conditions of internal growth of the parasite and their ecology.  Leishmaniasis is chiefly a zoonosis, though in some areas of the world there is mainly human-vector transmission (Ashford 2000). The world health organization estimates above 500,000 cases of visceral leishmanias and above 1.5 million cases of cutaneous leishmanians arise each year in about 82 countries (WHO 2010; Guerin 2002). Further, it has been estimated that there are nearly 350 million individuals at risk for developing leishmanias, with 13 million people at present infected (WHO 2010).  Moreover, the burden of visceral leishmania remains unidentified globally, because a number of cases are yet not diagnosed. Over 90% cases of visceral leishmania take place in 6 main countries, i.e. Bangladesh, India, Sudan, Nepal, Brazil and Ethiopia.

1.3.1 Malnutrition

Iron, poor protein, deficient vitamin A, energy, and zinc in the nutrition raise the danger that an infection will advance to clinically apparent visceral leishmaniasis. Current experiments in energy, protein, iron and zinc deficient mice propose that this result is caused primarily via failure of the lymph node function to provide obstacle and also because of high early parasite’s visceralization. The risk of mucocutaneous leishmaniasis has also been seen to increase in people with protein-energy malnutrition.

1.3.2 Environmental changes

In the majority of endemic regions, this parasite is distinguished by a patchy distribution, with distinct spread foci. This central distribution of leishmaniasis spread areas is because of micro ecological situations that influence the vector, the reservoir host and the parasite. Environmental changes that can have an effect on the occurrence of leishmaniasis comprise the invasion of agricultural farms urbanization, settling of humans into forested areas, and domestication of the spread cycle (Singh 2006).  

1.3.3 Population movements

Epidemics or spread of both cutaneous and visceral leishmaniasis, in both the new and the old world, are often linked to migration and the coming of non-immune individuals into regions with existing enzootic or endemic cycles of transmission. Forecast of such epidemics counts on the accessibility of ecological knowledge, and on assessment of development areas, prior to achievement of population movements or projects. Cyclic labor movements can also transmit the infection, with the come again of immigrants to non-endemic regions, like seems to have happened in the Ethiopia highlands in the 2000s. Practices, for example sleeping outside the home or living in grassy material made houses or under acacia trees boost risk for the infection (Rijal, Koirala, et al 2006).

1.3.4 Climate change

Leishmanias is sensitive to the change in climate, taking up a distinguishing climate space that is powerfully influenced by alterations in rainfall, humidity and atmospheric temperature (Malaria Consortium 2010). Land degradation and global warming collectively are anticipated to impact the epidemiology of leishmaniasis by countless mechanisms. Changes in temperature, humidity and rainfall, primarily, can have potent consequences on the ecology of reservoir hosts and vectors, by changing their distribution, and affecting their population sizes and survival. Secondly, flood, drought, and famine as a result of changes in climate states, may well cause enormous migration and dislocation of people to regions, with spread of leishmaniasis, and deprived diet can affect their immunity.

1.3.5 Vector Transmission

The Leishmania vector as stated above is transmitted by sandflies known as phlebotomine. These sandflies are broadly present in the warm mainland areas as well as other tropics. Sergentomyia, Phlebotomus and Lutzomyia species suck vertebrates’ blood but Lutzomyia and Phlebotomus can only transmit a disease to humans. The genus Phlebotomus is present in about 50 species in the old world while the genus Lutzomyia is present in the new world (Hide, Bucheton et al. 2007).

1.4 Morphology

  Leishmania exists in two morphological forms, namely

·         Amastigote

·         Promastigote

1.4.1        Amastigote

In the form of amastigote the parasite lives in the reticuloendothelial cells such as monocytes, macrophages, endothelial cells or polymorphonuclear leukocytes. This form is basically non-motile, oval or round body with a length of about 2 to 4 mm along the longitudinal axis (Marfurt, Nasereddin, et al 2003). The cell membrane of amastigote is delicate and can be shown in only fresh specimen. It is present along the cell wall sides or in the middle of the cell.

1.4.2        Promastigote

These are present in the digestive tract of the sandfly and are motile, elongated and extracellular phase of the parasite. The promastigote in its mature form measures 1.5 to 3.5 mm in breadth and 15 to 25 mm in length (Marfurt, Nasereddin, et al 2003). It does not curl round the parasite’s body, thus, no undulating membrane is present.

1.5 Types of Leishmaniasis

       1.5.1 Cutaneous Leishmaniasis

       1.5.2 Mucocutaneous Leishmaniasis

       1.5.3 Visceral Leishmaniasis

1.6 Life cycle of Leishmania

       1.6.1 First stage (from 0 to 2 days)

       1.6.2 Stage two (from 2 to 5 days)

       1.6.3 Stage three (after 5 days)

1.7 Pathogeneticity Caused by Leishmania

1.8 The Immune Response and Clinical Diversity in Humans

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