Leishmaniasis epidemiology: all down to the DNA

Molecular Diagnosis of Visceral Leishmaniasis

Visceral leishmaniasis (VL), a deadly parasitic disease, is a major public health concern globally. Countries affected by VL have signed the London Declaration on Neglected Tropical Diseases and committed to eliminate VL as a public health problem by 2020. To achieve and sustain VL elimination, it will become progressively important not to miss any remaining cases in the community who can maintain transmission. This requires accurate identification of symptomatic and asymptomatic carriers using highly sensitive diagnostic tools at the primary health service setting. The rK39 rapid diagnostic test (RDT) is the most widely used tool and with its good sensitivity and specificity is the first choice for decentralized diagnosis of VL in endemic areas. However, this test cannot discriminate between current, subclinical, or past infections and is useless for diagnosis of relapses and as a prognostic (cure) test. Importantly, as the goal of elimination of VL as a public health problem is approaching, the number of people susceptible to infection will increase. Therefore, correct diagnosis using a highly sensitive diagnostic test is crucial for applying appropriate treatment and management of cases. Recent advances in molecular techniques have improved Leishmania detection and quantification, and therefore this technology has  become increasingly relevant due to its possible application in a variety of clinical sample types. Most importantly, given current problems in identifying asymptomatic individuals because of poor correlation between the main methods of detection, molecular tests are valuable for VL elimination programs, especially to monitor changes in burden of infection in specific communities. This review provides a comprehensive overview of the available VL diagnostics and discusses the usefulness of molecular methods in the diagnosis, quantification, and species differentiation as well as their clinical applications.

Evaluation of amastigote reactive cells in human cutaneous leishmaniasis caused by Leishmania aethiopica

Lymphoproliferative responses to three affinity chromatography purified amastigote antigens of Leishmania pifanoi, P-2, P-4 and P-8, were evaluated in peripheral blood mononuclear cells (PBMC) from patients with Ethiopian cutaneous leishmaniasis. Antigen-stimulated cells were analysed for the percentage of CD4+, CD8+ and CD16/56+ cells and the expressed levels of gamma interferon (IFNgamma) and interleukin (IL)-10 were determined in culture supernatants. The amastigote antigens induced cellular responses in leishmaniasis patients with heterologous Leishmania parasite infection. These responses were compared to those of freeze-thawed L. aethiopica promastigote antigen stimulation. The membrane protein (P-8), and to a lesser extent the megasomal/cytoplasmic cysteine proteinase(P-2), induced proliferation with high levels of IFNgamma and IL-10 production in cells from patients with active L. aethiopica lesions. CD16/56+ NK cells were the main cell types induced to proliferate in response to P-8 and P-2 stimulation, followed by CD8+ cell populations. P-4 had no such effect. This contrasts from previous studies of New World human leishmaniasis where P-4 and P-8 were stimulatory. The success of a particular molecule in the induction of a response with a protective phenotype may be dependent on the infecting Leishmania spp. To our knowledge, there are no studies that directly compare the New versus Old World cutaneous leishmaniasis in respect of NK cell and IL-10 responses. Our studies indicate that some leishmanial molecules are recognized across the species, while others are apparently more species specific.

The epidemiology of visceral leishmaniasis in east Africa: hints and molecular revelations

Visceral leishmaniasis appears in the form of notoriously devastating epidemics and as an endemic disease of sporadic nature. In an attempt to understand the nature of this difference and its underlying causes, and to identify possible mechanisms for the instigation of outbreaks, the current status of the characterization of the parasite, its taxonomy, host immunity and genetics, are reviewed with the main focus on eastern Africa, one of the places where the dichotomous epidemiology of the disease is most pronounced. The new molecular tools offer various insights into the understanding of the complex epidemiology of visceral leishmaniasis and the interplay between parasite and host factors. Further insights are also provided on the parts played by demography, genetic history and geography in shaping the overall global portrait of the disease.

Comparison of the specificity of PCR and the histopathological detection of leishmania for the diagnosis of American cutaneous leishmaniasis

More precise and rapid diagnostic methods for American cutaneous leishmaniasis (ACL) are necessary because of the growing number of cases observed in Brazil, including the northeastern region of the State of São Paulo. We applied PCR to 54 skin or mucosal biopsies from patients with a clinical and/or laboratory diagnosis of ACL using primers 13A and 13B, with positive results being obtained for 82% of the samples. When the PCR results were compared to those of histopathological leishmania detection, PCR showed superior results with 81.5% sensitivity and 95% CI of 68.0-95.1%. The Montenegro skin test (MST) was positive in 88.7% of patients. Since MST cannot be used as a diagnostic tool in endemic areas, the present results strongly suggest the use of PCR for the etiological confirmation of ACL, with emphasis on the mucosal form.

Leishmaniasis as an emerging infection

Leishmaniasis is a protozoan disease whose diverse clinical manifestations are dependent both on the infecting species of Leishmania and the immune response of the host. Transmission of the disease occurs by the bite of a sand fly infected with Leishmania parasites. Infection may be restricted to the skin in cutaneous leishmaniasis, limited to the mucous membranes in mucosal leishmaniasis, or spread internally in visceral leishmaniasis or kala azar. The overall prevalence of leishmaniasis is 12 million cases worldwide, and the global yearly incidence of all clinical forms approaches 2 million new cases (World Health Organization WHO/ LEISH/200.42, Leishmania/HIV Co-Infection in Southwestern Europe 1990-98: Retrospective Analysis of 965 Cases, 2000). In the last two decades, leishmaniasis, especially visceral leishmaniasis, has been recognized as an opportunistic disease in the immunocompromised, particularly in patients infected with human immunodeficiency virus.

Genetic causes involved in Leishmania Chagasi infection innortheastern: Brazil

A sample of 502 individuals from 94 families from Jacobina, State of Bahia, Brazil, was investigated to determine the causal mechanisms involved in Leishmania chagasi (the causal agent of visceral leishmaniasis in the American hemisphere) infection, as measured by the intradermic reaction to antigens derived from this parasite, using complex segregation analyses. The results showed evidence of a major genetic mechanism acting on infection, with a frequency of a recessive (or additive) susceptibility gene (q) of approximately 0.45. A small multifactorial component (H = 0.29) acting in conjunction with a major recessive gene (q = 0.37) is not ruled out as a concomitant causative factor.

Leishmaniasis

Leishmaniasis is a protozoan disease whose diverse clinical manifestations are dependent both on the infecting species of Leishmania and on the immune response of the host. Transmission of the disease occurs through the bite of a sand fly infected with Leishmania parasites. Infection may be restricted to the skin in cutaneous leishmaniasis, limited to the mucous membranes in mucosal leishmaniasis, or spread throughout the reticuloendothelial system in visceral leishmaniasis or kala azar. Three rare clinical variants of cutaneous leishmaniasis include diffuse cutaneous leishmaniasis, leishmaniasis recidivans, and post-kala-azar dermal leishmaniasis.

Finding genetic markers for complex phenotypic traits in parasites

The identification, mapping and eventual cloning of genes which determine or influence important epidemiological traits in parasites can have great benefits for the control of parasitic disease. In this review, strategies are outlined for identifying genetic markers for complex, quantitative traits. A genetic marker is a variable DNA sequence which co-occurs with a variable quantitative trait. Candidate markers are chosen because they are thought to directly influence the trait whereas random markers are expected to be linked to another DNA sequence which influences the trait. Association studies compare the value of a quantitative trait between different marker genotype classes in a population, without regard to family structure. Linkage studies compare the value of a quantitative trait between marker genotype classes within families or within a population (usually derived from a cross between inbred lines) which is segregating for both marker and quantitative trait loci. The most commonly used analytical methods for determining the significance of association or linkage between marker and quantitative trait loci, and for estimating parameters such as recombination rate and quantitative gene action, are least-squares and maximum likelihood. Both methods may be used to test either single markers or the interval between flanking markers, and both suffer from the need to minimize type I and type II error rates with multiple tests.

Human T-cell responses to secreted antigen fractions of Mycobacterium tuberculosis

The T-cell response of human donors to secreted antigen fractions of Mycobacterium tuberculosis was investigated. The donors were divided into five groups: active pulmonary tuberculosis (TB) patients with minimal and with advanced disease, Mycobacterium bovis BCG-vaccinated donors with and without contact with TB patients, and nonvaccinated individuals. We found that patients with active minimal TB responded powerfully to secreted antigens contained in a short-term culture filtrate. The response to secreted antigens was mediated by CD4+ Th-1-like lymphocytes, and the gamma interferon release by these cells was markedly higher in patients with active minimal TB than in healthy BCG-vaccinated donors. Patients with active advanced disease exhibited depressed responses to all preparations tested. The specificity of the response to secreted antigens was investigated by stimulating lymphocytes with narrow-molecular-mass fractions of short-term culture filtrate obtained by the multielution technique. Considerable heterogeneity was found within the donor groups. Patients with active minimal TB recognized multiple secreted targets, but interestingly, six of eight patients demonstrated a predominant recognition of a low-mass (< 10-kDa) protein fraction which induced high levels of gamma interferon release in vitro. Only a few of 12 previously characterized secreted antigens were recognized by T cells isolated from TB patients, suggesting the existence of a number of as yet undefined antigenic targets among secreted antigens.

Identification of intra- and interspecific Leishmania genetic polymorphisms by arbitrary primed polymerase chain reactions and use of polymorphic DNA to identify differentially regulated genes

Arbitrary primed polymerase chain reactions (AP-PCR) were used to amplify different polymorphic genomic DNA fragments from various Old World Leishmania species. Using four 10-mer AP primers, geographic isolates of L. donovani and various Old World species of Leishmania could be readily distinguished from one another by the pattern of amplified DNA products. Our studies confirmed two important characteristics of AP-PCR: its abilities to amplify a consistent pattern of DNA fragments from the genomes of different isolates of a single species and to identify genetic polymorphisms between the species isolates. We selected three polymorphic DNA fragments that differentiate L. donovani geographic isolates for further analysis. Sequence analysis of the clones derived from these three polymorphic fragments revealed eight unique sequences. Six of eight unique clones hybridized to distinct RNAs upon Northern-blot analysis. Three of these six clones hybridized to RNAs expressed differentially in in vitro grown L. donovani pro- and "amastigotes." One of the differentially expressed clones, LdE-6-1, exhibited restriction length polymorphisms that distinguished L. donovani from L. tropica and L. major. Comparative Northern blotting revealed that LdE-6-1 was differentially expressed in some members of the L. donovani species complex but not in L. major or L. tropica. These results demonstrate that AP-PCR can be used to generate products reflecting particular genes in organisms with low-complexity genomes.

Kinetoplast DNA from Trypanosoma rangeli contains two distinct classes of minicircles with different size and molecular organization

Trypanosomatids are characterized by the presence of kinetoplast DNA (kDNA), a peculiar form of mitochondrial DNA that consists of several thousand minicircles and a few dozen maxicircles catenated in a network. Within a species, the minicircles are known to differ in nucleotide sequence, but are homogeneous in size and always cross-hybridize. In all species of trypanosomatids, kDNA minicircles have at least one copy of a conserved 100-200 nucleotide region containing an almost invariant 'universal' 12-mer sequence (5'-GGGGTTGGTGTA-3'). We here report that Trypanosoma rangeli, a non-pathogenic parasite of man, contains two distinct classes of kDNA, minicircles called KP1 and KP2, which differ in size and molecular organization. Both were cloned and sequenced in both directions. KP2 was 1587 bases along and contained two copies of the conserved region as direct repeats 180 degrees apart. In contrast, KP1 had 1764 bases and showed a single conserved region. Moreover, KP1 differed further from KP2 and from most other previously sequenced trypanosomatid minicircles by containing a nucleotide substitution (5'-GGGGTTAGTGTA-3') in the 12-mer universal sequence tag. Polymerase chain reaction and hybridization studies suggest that the sequence of KP1 is very conserved in several other T. rangeli strains from Honduras, Colombia and Venezuela. It thus could provide a good target for the molecular diagnosis of infection with this parasite.

Intra- and interspecific polymorphisms of Leishmania donovani and L. tropica minicircle DNA

A pair of degenerate polymerase chain reaction (PCR) primers (LEI-1, TCG GAT CC[C,T] [G,C]TG GGT AGG GGC GT; LEI-2, ACG GAT CC[G,C] [G,C][A,C]C TAT [A,T]TT ACA CC) defining a 0.15-kb segment of Leishmania minicircle DNA was constructed. These primers amplified not only inter- but also intraspecifically polymorphic sequences. Individual sequences revealed a higher intraspecific than interspecific divergence. It is concluded that individual sequences are of limited relevance for species determination. In contrast, when a data base of 19 different sequences was analyzed in a dendrographic plot, an accurate species differentiation was feasible.

Interleukin 12 and the regulation of CD4+ T-cell subset responses during murine Leishmaniasis

Interleukin 12 is unique among cytokines in that it is capable of protecting genetically susceptible mice against progressive infection with Leishmania major. Because of the probable causal relationships between CD4(+) T-cell differentiation, cytokine production and disease outcome, this cytokine may prove useful as a component of cytokine-bosed therapies and Thl-selective vaccines, as discussed here by Frederick Heinzel.

An RFLP map for 2q33-q37 from multicase mycobacterial and leishmanial disease families: no evidence for an Lsh/Ity/Bcg gene homologue influencing susceptibility to leprosy

The mycobacterial diseases leprosy and tuberculosis (TB) and the leishmaniases are characterized by a wide spectrum of disease phenotypes, and by the fact that the majority of individuals exposed to the causative organisms Mycobacterium leprae, M. tuberculosis and Leishmania sp. become infected but do not present with clinical disease. In order to determine whether a human homologue to the murine macrophage resistance gene Lsh/Ity/Bcg influences susceptibility to human disease, multicase families for all three diseases have been collected, and linkage analysis performed using a panel of markers in the region of human chromosome 2q33-q37 known to be conserved with the Lsh/Ity/Bcg-containing region of murine chromosome 1. Because of the paucity of available polymorphic markers/linkage information for 2q33-q37, data from 35 multicase leprosy, TB and visceral leishmaniasis families (310 individuals) were first pooled to produce a detailed RFLP map of the region. Peak LOD scores well in excess of 3 were observed for linkage between adjacent pairs of a more proximal (2q33-q35) set of markers CRYGP1, MAP2, FN1, TNP1, VIL1 and DES, and between adjacent pairs of a more distal (2q35-q37) set COL6A3, D2S55 and D2S3. These peak LOD scores and the corresponding values for theta were used in the MAP92 program to generate a multiple two-point map with gene order/map intervals (cM) of: CRYGP1-4.65-MAP2-3.45-FN1-5.95-TNP1-3.41-VIL1-3. 01- DES-20.14-COL6A-10.91-D2S55-3.67-D2S3. Although local support for the placement of loci in this order was weak (LOD < 2, except for DES-COL6A3 where LOD = 6.02), the map is consistent with the gene order for those loci (Cryg, Fn-1, Tp-1, Vil, Des, Col6a3) previously mapped in the mouse. Data from 17 multicase leprosy families (149 individuals) were further analysed for linkage between a putative disease susceptibility locus (DSL) controlling susceptibility to leprosy per se and each of the marker loci. Assuming 100% penetrance for the susceptibility allele, no positive LOD score was obtained for linkage between the DSL and any of the marker genes. Instead, the data provide convincing evidence (LOD scores < -2) that a DSL does not fall within 10-20 cM of CRYGP1, MAP2, TNP1, VIL1, DES or D2S55, or within 5-10 cM of FN1, COL6A3 or D2S3. This effectively excludes a putative DSL controlling susceptibility to leprosy per se from the entire region 2q33-q37.(ABSTRACT TRUNCATED AT 400 WORDS)

Leishmaniases of the New World: current concepts and implications for future research

Recent epidemiologic studies indicate that leishmaniasis in the Americas is far more abundant and of greater public health importance than was previously recognized. The disease in the New World is caused by a number of different parasite species that are capable of producing a wide variety of clinical manifestations. The outcome of leishmanial infection in humans is largely dependent on the immune responsiveness of the host and the virulence of the infecting parasite strain. This article reviews current concepts of the clinical forms, immunology, pathology, laboratory diagnosis, and treatment of the disease as well as aspects of its epidemiology and control. Recommendations for future research on the disease and its control are made.

Leishmaniasis epidemiology: the theory catches up

Until recently, almost all studies of leishmaniasis epidemiology were qualitative and descriptive. But now that the natural history of many Leishmania parasites is quite well known, there is growing interest in quantitative analysis. In this paper I use mathematical models in conjunction with field data to try to answer a wider range of questions than has previously been possible with descriptive techniques, and to sharpen some of the outstanding questions for laboratory workers. This is done with reference to the persistence and resilience of canine leishmaniasis, the maintenance of virulence polymorphisms in Leishmania populations, and the possible existence of cycles of human kala-azar. I conclude by posing a set of problems under three headings: diagnosis of infection (as distinct from disease), natural immunity to Leishmania infection in the vertebrate host, and genetic variation in the parasite population. Some solutions from the laboratory can be found in the companion paper by Blackwell (1992).