Emerging Chagas Disease

Full text available.

Beginning as a silvatic enzoosis involving Trypanosoma cruzi and a range of small mammals and marsupials, human Chagas disease probably emerged as a sparse focal disease at different points in the Americas well before the Christian period. Subsequently spread through internal population migrations, the disease would have become more widespread where the insect vectors became associated with rural settlements. It appears to have spread most widely during the post-Colombian period, especially from the mid 19th to mid 20th centuries when the human infection appears to have peaked in incidence and prevalence. In historical terms, there are sparse indications of probable acute cases, chronic cardiopathy, and megacolon, but such conditions are difficult to diagnose accurately. In contrast, megaoesophagus seems a more specific marker of chronic Chagas disease, with a number of reports of its occurrence in various parts of Brazil, especially since the 18th century. The main impact of chronic Chagas disease corresponds primarily to the occurrence of chronic chagasic cardiopathy, and recognition and characterization of this has been the main stimulus for large-scale control interventions in the endemic countries since the 1950s.

Infections by Trypanosoma cruzi are transmitted commonly by triatomines, hematophagous insects adapted to anthropophilic behavior. With its potential enzootic presence for over 90 million years, Chagas disease in humans has been documented in 9 thousand-year-old mummies from the Atacama Desert. Lately, Chagas disease has shown exponential growth because European and African colonizers dwelling in huts infested by triatomines contaminated with T. cruzi were promptly infected. Nowadays, Chagas disease affects 18 million people and is considered the most lethal endemic infectious disease in the Western Hemisphere.

Chagas disease is now a public health problem due to the detection of cases all around the world. The migration of the Latin American populations to North America, Europe, Asia and Oceania has made the parasite cross the seas, so as to secure the Trypanosoma cruzi survival by means of blood transfusion and by congenital transmission from mother to offspring. The increasing number of cases of Chagas disease detected in various countries all over the world urges the health systems to be prepared to its proper diagnosis and treatment.

The lifecycles of the organisms involved in the circulation and transmission of the Trypanosoma cruzi agent of Chagas disease are intimately related. It appears that these cycles begun after an adaptive process in the quaternary, Mezozoic, Cretaceus, over 90 million years ago. Gradually, the triatomines became intermediate hosts and vectors of the protozoan infection to mammals when the enzootic infections contaminated omnivorous mammals, particularly skunks, armadillos and ant-eaters. Hematophagy resulting from a biochemical requirement for the insect growth has contributed to approximate triatomines and mammalians. Triatomines adapted to human dwells generated the endemic Chagas disease. This chapter describes and characterizes the habits of triatomines, which are important to understanding T. cruzi transmission by the insect-vector, its control and disease prevention. The success obtained with the spray of pyrethroids insecticides to control the triatomines intradomicile recommends a lot more actions to prevent repopulation of the human dwells with the repulsive insect-transmitters of the T. cruzi infections.

This review analyzes the fine structure of Trypanosoma cruzi as visualized by various morphological techniques, including scanning electron microscopy, transmission electron microscopy of thin sections and freeze-fracture replicas, and atomic force microscopy. Data obtained using cytochemistry and immunocytochemistry are also discussed. Various structures, such as the glycocalyx, plasma membrane, flagellar pocket, cytoskeleton, flagellum, kinetoplastmitochondrion complex, glycosome, acidocalcisome, lipid bodies, contractile vacuole, secretory pathway, endocytic pathway and nucleus, are covered.

As a parasite with a potentially decades-long intracellular stage, Trypanosoma cruzi could represent a case study for extreme mechanisms for the maintenance of mitochondrial sequence integrity. While an intact mitochondrial repertoire of membrane-associated cytochromes and NADH dehydrogenases are required for passage through the insect forms, within the vertebrate host many of these products may be dispensable, as in vertebrate-exclusive relatives. The degeneration seen in maxicircle genomes from clinical isolates and from culture indicates that the maintenance of intact maxicircles and a complete library of minicircles is an uphill battle that the parasite manages actively. Both the maxicircle and minicircle have been implicated in the clinical manifestation of Chagas disease. The implications of both these scenarios on the biology of the parasite relative to the host are discussed.

T. cruzi undergoes distinct developmental programs during its replication cycle and during its lifecycle where it shuttles between mammal and triatomine hosts. The information required to achieve these complex morphological and biochemical changes is encoded in the genome at several different levels. A preliminary understanding of the biochemical, physiological, and morphological changes that occur in the different lifestages of T. cruzi requires a detailed knowledge of the parasites genome composition and function. Here, we review the role of the nucleus in parasite identity. We begin with an analysis of how gene organization is resolved by mechanisms of mRNA formation, and the inherent signals in mRNAs that enable stage-specific protein expression. We continue with a description of the genome of the CL Brener strain as determined by the T. cruzi genome-sequencing project, including analysis of repetitive elements, structural genes, and some protein-coding genes. We end with a discussion of T. cruzi population structure and the discovery of rare nuclear hybridization events.

The protozoan parasite Trypanosoma cruzi, the etiologic agent of Chagas disease, is an intracellular pathogen in its vertebrate host. Here we review the process of cell invasion by T. cruzi from the perspectives of surface-surface interactions, intracellular signaling, contributions of donor membranes and the cytoskeleton, and discuss cytoplasmic entry and differentiation after escape from the parasitophorous vacuole. The chronic host-pathogen relationship depends upon immune evasion by the parasite and prevention of host apoptosis. Finally, we briefly discuss the development of cardiac autoimmunity in the pathogenesis of Chagas disease.

The obligatory intracellular protozoan Trypanosoma cruzi is an extremely successful parasite infecting a vast number of distinct mammalian hosts across the Americas. T. cruzi relies heavily on the innate and adaptive immunological mechanisms to maintain the host alive during the acute phase and to establish a chronic infection, when the transmission to the invertebrate host occurs. This long term persistence is considered a major force behind the chronic symptoms of the disease observed in humans. Mechanisms that keep host alive during the acute phase have been dissected using genetically deficient mice and they include Toll-like receptors, cytokines (Interferon-gamma, TNF-α, MIF and IL-12), lymphocytes (B, CD4, CD8 and NKT) and NK cells. Target antigens for specific lymphocytes are members of the trans-sialidase, mucin-like, cysteine protease families, etc. Vaccination studies are being performed using different delivery systems such as recombinant proteins, plasmid DNA and microorganisms. Non-antibody mediated cellular immune responses (CD4+ Th1 and CD8+ Tc1) to specific parasite antigens/genes can indeed be used for the purpose of vaccination against acute phase mortality and, in some cases, chronic phase pathology. These results obtained in the mouse model indicate a possible path for a veterinary or human vaccine development

What happens to a person infected with the parasite Trypanosoma cruzi? The answer to this question is essential to the formulation of new ideas throughout the reading of this chapter, as over two thirds of individuals who acquired the infection will never present any clinic manifestation related to Chagas disease. Only 5% or less of acute infected individuals will present symptoms such as fever, generalized joint and muscle pain, indisposition, headache and other unspecific symptoms of a common cold. The other 95% patients are asymptomatic or do not report symptoms that allow the healthcare professional to suspect and/or confirm the acute phase diagnosis of Chagas disease. After three to six months, the patients get to the intermediary phase of the chronic infection in the absence of signs or symptoms. However, three or more decades later, circa of one third of the infected individuals may present chronic Chagas disease symptoms. It is known that 94.5% of chronic chagasic patients will present manifestations of heart disease and the remaining 5.5% will present esophagus disease (megaesophagus) or large intestine manifestations (megacolon).

In the acute phase of Chagas disease the nests of Trypanosoma cruzi amastigote forms are found in tissues, especially in cardiac, skeletal and smooth muscle cells from human body. However the most important element of severe injuries formation in the heart and digestive tube tissues is the lymphocyte and macrophage cells infiltration triggering the destruction of normal, non-parasitized heart muscles, and ganglia from the intestine and peripheral nervous system. The non-parasitized target cells destruction caused by lymphocytes and macrophages from the immune system is particularly evident during the chronic phase of the chagasic infection. The severe inflammation weakens the organs, leading to dilatation and to insufficiency. The cellular death implies in replacement by fibrous scars which are only a sequelae from the rejection of the “self” tissue.

The treatment of the Trypanosoma cruzi infection with anti-trypanosome drugs did not interrupt the progression of the cardiac lesion in rabbits and in humans. Then, the question arose: what would sustain the active lesion in the chagasic heart? A hypothesis of horizontal transference of DNA from T. cruzi to the genome of the host was proposed because it could answer this question. The investigation showed that minicircle sequences from the parasite are transferred into specific sites of the rabbit genome and, also, of the primate genome and a main hotspot of integration was the retrotransposon LINE-1. Using a model of in vitro infection it was possible to demonstrate that the kDNA integrated to the LINE-1 could be mobilized to other genome site within the host cell. This demonstration suggested that the mutation-induced modification in the genotype and in the phenotype of the target cell, which may accumulate along the cryptic chronic infection, might explain the variability of clinic manifestations of the human disease as well as the origin and progression of the rejection of the non-parasitized cell over time. This explanation justifies the long period between the initial infection and the triggering of the disease. According to this theory, the parasite acts as a mutation vector and the altered cells are recognized as not-self and are rejected by the immune defense system.

After the horizontal transference of kDNA minicircle sequences into the genome of chagasic mammals it was possible to show the heritage of the kDNA mutation into those rabbits breed. However, mammals are permissive to infection by T. cruzi, which may persist through the animal life. To ensure that the kDNA mutation was not only a noise produced by the cryptic infection it was necessary to dismiss this possibility. This was possible through experiments using birds that are refractory to T. cruzi infection but are permissive to infection only in the first 10 days of embryonic life. When the fertile eggs were inoculated with T. cruzi the breed was born without infection but presenting the kDNA mutation. In this regard, rabbits and birds with kDNA mutation presented typical Chagas disease lesions: minimal rejection unit which is characteristic of the pathology of this disease where the non-parasitized target cell was destroyed by the cells from the vertebrate host´s immune system. These experiments showed that the minimal rejection unit is the common denominator of the pathogenesis of Chagas disease in vertebrate animals. In the chicken model refractory to T. cruzi the cardiomegaly was linked to the minimal rejection unit’s inflammatory infiltration. Therefore the parasite-free chicken heart pathology seen in kDNA-mutated chickens could be linked to the genotype and phenotype alterations. The mechanism whereby these alterations induce the immune rejection of the chicken’s heart requires further investigation.

Presently, the treatment of the infection by Trypanosoma cruzi has been considered unsatisfactory. The eradication of the infection and the interruption of the chronic disease evolution have not been reached by treatment in several clinic and experimental trials. To be unanimous, the indication of determined treatment it should be deposed of undesirable side effects and, even if it did not produce the cure (elimination) of the infection, it should at least stop its evolution. However, the treatment with anti-trypanosome nitro derivatives did not show a clear advantage, when cost and effectiveness were analyzed. Although millions of people with the acute T. cruzi infection do not present a clinic disease, the treatment is clearly indicated in several situations in which the patient’s life is in danger. The controversy on the efficacy of the treatment of T. cruzi infection with the available drugs shows that this is one of the aspects of the investigation on Chagas disease that deserve research incentives. The suggestion that the pathogenesis of the disease is associated to the introduction of kDNA mutations from the parasite’s genome to the host’s defines the need of one or more drugs that are truly effective against the infection. The persistence of the infection, throughout the life of the patient may represent a source of kDNA which introduces cumulative mutations. The effect of these mutations on the evolution of the disease could be avoided with the infection elimination. Maybe, this is an aspect of scientific research with possibilities of generating real benefits to the 18 million people infected by T. cruzi, reminding that one third of them will present clinic manifestations of Chagas disease.

Research has advanced towards the identification of potential drug targets in Trypanosoma cruzi as well as the development of new effective drugs for the chemotherapy of Chagas disease. The first crucial step in this protozoan life cycle within the vertebrate host is the process of entry into a mammalian cell. Enzymes actively participating in this process are naturally good drug-target candidates. Right after cell invasion, trypomastigote forms of T. cruzi must differentiate into amastigotes so as to proceed in their life cycle. The inhibition of key enzymes from the sugar metabolism, the lipids synthesis, the digestion of host-internalized proteins and also from the purines salvage pathway could hinder parasite growth, thus placing these enzymes as potential drug targets. When the cell is populated, amastigotes must differentiate into trypomastigotes, which will then leave the cell to infect others and hence restart the cycle. Once well adapted to the intracellular environment, T. cruzi can use several different pathways for infection and differentiation. Therefore, the best strategy to develop drugs for Chagas disease therapy is the plural conception of drugs, that is, to consider as targets several enzymes involved in different physiological processes. So, a drug cocktail would be efficient to treat T.cruzi infections. Another important step in drug development is the determination of the target’s tridimensional structure. The knowledge of its architecture facilitates the rational design of molecules from the enzyme active site. These strategies are believed to provide drugs not only efficient but also with low side effects for the treatment of Chagas’ disease.

An intricate epidemiological chain involving the flagellate protozoan Trypanosoma cruzi is discussed at the environmental level, and in terms of fine molecular interactions in invertebrate and vertebrate hosts dwelling in different ecosystems. This protozoan has a complex, genetically controlled plasticity, which confers adaptation to approximately 40 blood-suckling triatomine species and to over 1,000 mammalian species, fulfilling diverse metabolic requirements in its complex life-cycle. The T. cruzi infections are deeply embedded in countless ecotypes, where they are difficult to defeat using the control methods that are currently available. Many more field and laboratory studies are required to obtain data and information that may be used for the control and prevention of T. cruzi infections and their various disease manifestations. Emphasis should be placed on those sensitive interactions at the cellular and environmental levels that could become selected targets for disease prevention. In the short term, new technologies for social mobilization should be used by people and organizations working for justice and equality through health information and promotion. A mass media directed program could deliver education, information and communication needed to protect the inhabitants at risk of contracting T. cruzi infections.

Chagas disease has been an important factor that imposes difficulties to the colonization of Latin America during the last five centuries. Such assertive requires an insight about an ailment that propagates in the continent, reaching 18 million people, from which one third will succumb to the disease at the ages ranging from 30 to 45 years of age. The families that dwelled in rural areas for several generations know well enough that ailment because they had at least one beloved that succumbed to Chagas disease. Orphanhood and desolation is the prevalent picture in the inland of this unfortunate continent filled with fear of sudden death caused by Chagas disease. Formerly it was said that Chagas disease was a disease of the poor and, therefore, out of fashion. Nowadays, the disease is found in every social class but the conception of out of fashion persists because it is of the interest of those who insist in not taking care of the problem inside their own house. However, many people understand the importance of this disease that has brought suffering to the population. Unfortunately, sectors with alien thinking sustain the idea that the disease has been controlled and issue geopolitical certificates of exemption of triatomines, vectors of Chagas disease. The solution of this problem depends on the mobilization of the society aiming changes on this secular reality.