Ana Maria Jansen-Franken
Laboratory of Biology of Trypanosomatids, Instituto Oswaldo Cruz/Fiocruz
The habitat of any parasite is constituted by its hosts and in these, the different tissues, represent the exploitable niches. Thus, the greater the diversity of niches (tissues) that a parasite is able to colonize, the greater its adaptability to the host and the greater its ecological valence. The Trypanosoma cruzi taxon is an example of extreme parasitic adaptability, given its ability to infect and multiply in almost all tissues of its various hosts, thus being an example of an eclectic parasite.
Such eclecticism, which results, at least in part, from the significant intraspecific heterogeneity of this taxon, demonstrates that this ancient eukaryote is well adapted to parasitism. The biological plasticity of this kinetoplastid is so expressive that, even today, 100 years after its description by Carlos Chagas, T. cruzi presents aspects of its biology and epidemiology that are still unknown.
What is a reservoir? Most studies still address this issue from an anthropocentric and/or finalist look at the phenomenon of parasitism. Thus, with minor modifications, it is conceptualized as a reservoir: it is “the animal species that in nature is a source of infection of a particular parasite for man.” From the increased recognition of the importance of agricultural production, domestic and/or synanthropic animals were also included in the definition of reservoir. Additionally, it is often thought that a reservoir interacts with the parasite without suffering damage based on the assumption that a long co-evolutionary process results in the elimination of very virulent and/or pathogenic parasites and very sensitive hosts.
This proposition has been increasingly proving to be insufficient since, as has been recognized, virulence can indeed confer, and in some cases confer, greater transmissibility to the parasite even at the expense of some kind of host. An example is T. evansi, a euryxene parasite that is quite virulent and often pathogenic to its wild hosts.
In the same way that living beings have been multiplying and evolving over the course of 3–4 billion years, although more ephemeral and less concrete, parasite-host interactions also evolve. No evolutionary process, and this is for the parasite-host associations, is finalist or follows a kind of pre-defined script as evidenced by the record of the extinction of hundreds of species of hosts (and consequently of their parasites – mainly the stenoxenes). The co-evolutionary process of a parasite-host system includes selection and fixation in the population of both parasites and hosts, changes in morphophysiological, biological, behavioral and molecular patterns, along which some components of the system are eliminated. To consider a parasite-host interaction as being an “arms competition between one component and another” is not to take into account its complexity.
Each parasite-host interaction presents particularities determined by factors related to the host (gender, age, behavioral pattern, genetic characteristics), to the parasite (generation time, reproductive potential, environmental eclecticism, strategies) and to the environment. The latter could be divided into micro and macrolandscapes since the parasite’s landscape is the host’s organism, which in turn is inserted in the environment of its occurrence. Natural landscapes change and one of these changes, ecological succession, is the forger of the dynamic balance of this system. The ecological succession is a consequence of the environmental changes caused by the so-called “pioneer species” (the species that initially settle there), changes that may or may not favor, inhibit or even prevent the colonization of the place by other successor species. This concept also applies to parasitism: throughout its life, a mammal will be exposed to infection by several species of parasites that alter the environment (host) favoring or not the colonization of the “successor” parasitic species. Additionally, it is worth remembering that competition and/or cooperation between species are phenomena already observed among parasites. The result of all these factors is that it will define the role that each host plays in the trophic chain or parasite network in which both are involved.
Food chains/parasitic networks intertwine living beings in a dynamic and complex process subjected to various selective pressures, among which the strategies of reproduction and transmission of the parasite and the habits and life cycle of the host stand out. Therefore, the future of a given parasite-host system is unpredictable since it will depend on the multiple biological variables inherent to the parasite and the host within the different environmental and temporal scenarios of its occurrence. This implies the need to recognize that species do not play the same role as a parasite reservoir in all temporal and spatial aspects of its occurrence.
The dynamic character of these interactions is exemplified by changes in the epidemiological profile of several parasites, the emergence of new diseases or the re-emergence of others considered controlled. All these aspects contributed to the formulation of broader and more comprehensive concepts of what it is and how to study reservoirs. The tendency to increasingly define reservoir from a multidisciplinary focus resulted in the following definition: “reservoir is the species or complex of species that guarantee, in a given landscape, the long-term maintenance of a parasite.” Considering the condition of the reservoir as temporal and spatial is absolutely fundamental in the studies of transmission cycles, of epidemiology and, consequently, in the definition of epidemiological surveillance measures for a given parasitosis. Determining in a target area what role each of the elements of a particular parasitic network plays is only possible through a systemic approach.
This is especially important when dealing with a multi-host and eclectic parasite such as T. cruzi. The amplitude of hosts (mammals and insects) of this parasite results in its dispersion in all biomes and in all forest strata by different strategies since, in addition to the vector transmission classically described – the oral route (probably the most important among wild animals) and the direct route (by scent glands of some marsupial species) ensure their maintenance and significant distribution in nature. The discovery of this alternative transmission strategy and other particularities of the marsupial – T. cruzi interaction clarified new aspects of the biology of this parasite, reinforced the uniqueness of its interaction with each host and alerted to the care that must be taken when proposing generalizations and conclusions based on only in experimental studies.
Although these concepts have been formulated a long time ago, they have not yet been incorporated in fact, since most studies that talk about reservoirs are still only vertical, not taking into account the ecological scenario. A study on reservoirs without this view is likely to result in erroneous conclusions, even because the mere presence of a particular species infected by a parasite is still far from sufficient to define it as a reservoir. As stated above, the role that each species plays in the parasitic network in which it is involved depends on the physiognomy of the landscape and the time frame. Thus, the same species can act as an amplifying reservoir (responsible for expanding the population(s) of a particular species of parasite) or maintainer (which guarantees the parasite’s permanence in nature in times of low population density of vectors, for example).
The recent outbreaks of oral Chagas’ disease in Brazil emphasize the need to judge these points. This type of occurrence, observed for the first time in southern Brazil by Nery-Guimarães in 1968, has been repeated more frequently in this decade and is still far from being understood. In common, in all areas where these outbreaks occurred, it was possible to observe the simplification of the fauna of small wild mammals and the favoring of species that are less demanding in terms of habitat, with greater potential for synanthropization.
Progressively, health authorities in Brazil, especially the Ministry of Health, through CGLab, have been sensitized to the importance of multidisciplinary studies in the areas of occurrence of Chagas’ disease outbreaks. The importance of studying the local fauna of domestic and/or wild animals, inserted in the complex methodology necessary for these studies, led to the structuring of a Reference Center for the identification and diagnosis of trypanosomatid reservoirs. This center involves the collaboration of two laboratories of the Instituto Oswaldo Cruz–Fiocruz, the Laboratory of Trypanosomatid Biology and the Laboratory of Biology and Ecology of Small Mammal Reservoirs, and is able to respond to calls from the Ministry of Health to study the enzootic disease of the transmission cycle of the T. cruzi in the most diverse regions of the country.
From the methodological point of view, used by the Reference Center in the identification and diagnosis of trypanosomatid reservoirs, the evaluation of the role that a certain species plays in a certain parasite network must include and/or consider:
- the description of the physiognomy of the landscape with emphasis on the faunal composition and relative frequency of the different local species. The parasite transmission cycle has different characteristics depending on the greater or lesser relative frequency of species with greater or lesser competence as a reservoir;
- adequate sampling that allows the examination of representative samples of the local fauna;
- accurate taxonomic diagnostic methods of the host fauna, since epidemiological conclusions and erroneous management rules may result from insufficient or mistaken identification. This point is exemplified by the very distinct pathogenicity of cruzi in two cryptic species, respectively Thrichomys laurentius and Thrichomys pachyurus: morphologically indistinguishable, these caviomorph rodents were, until very recently, considered a single species;
- precise methods of direct and indirect diagnosis of the parasite as a way of assessing its potential transmissibility;
- that secondary data or data from nearby areas probably do not reflect the local reality: the prevalence of a parasite in host populations differs significantly even in areas very close to the same biome;
- that the temporary disappearance and resurgence of the parasite in a given area are expected phenomena and justify a longitudinal study, as a way of clarifying the temporal and spatial pattern of transmission. It is worth mentioning that this phenomenon does not follow a general pattern and is peculiar to each area studied;
- the prevalence and profile of the infection in animals and the prevalence in the different segments of the populations of the host species (sex, age, seasonality, stability of the infection and parasitemia) is what reflects the transmissibility of the parasite: animals with high rates of positive blood culture demonstrate a expressive parasitemia and, therefore, competence as a reservoir greater than those whose infection is demonstrated only by the presence of specific antibodies;
- that the profile of the experimental infection of the species “candidate” for a reservoir by the parasite clarifies important points in the complementation of the epidemiological picture;
- the analysis of data from a geographic information system coupled to a database with the objective of detailing the environmental processes involved in the present and predicting the tendency of dispersion of the parasite within the environmental scenario in question.
Leidi Herrera and Servio Urdaneta-Morales
Laboratory of Trypanosoma Biology of mammals, Sección de Parasitologia, Instituto de Zoologia Tropical, Facultad de Ciências, Universidad Central de Venezuela, Caracas, Venezuela
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American trypanosomiasis, or Chagas’ disease, is a zoonosis of wild mammals, some of them as old as the American continent. Sloths, bats, rodents, marsupials, rabbits, dogs and others are true reservoirs, which constitute the transmission cycle with at least 100 species of exclusive hematophagous insects (Hemiptera, Reduviidae, Triatominae), which act as vectors.
The etiological agent of Chagas’ disease, Trypanosoma cruzi, circulates among these mammals without causing them any harm, however, population selection events in the transmission elements can change this condition.
Among the most widely distributed wild and/or synanthropic reservoirs in America are the marsupial Didelphis and some rodents, especially Rattus. In Venezuela, we have studied the natural infection of these mammals by TcI (T. cruzi I) which manifests itself with a very low virulence and parasitemia detectable only by xenodiagnosis, blood culture or PCR, without apparent symptoms.Tissue tropism in wild reservoirs, studied in their natural condition or well maintained as a laboratory model (D. marsupialis, R. rattus, and the caviomorph Trichomys apereoides) has been low in terms of the number of invaded organs and parasite load, with a trend observed to cardiotropism, tropism to skeletal muscle, intestinal smooth muscle and, in some cases, parasitism to pancreas cells (Figure 1).
Didelphis, as a wild model in experimental infections, has low mortality, due to the fact that these marsupials control the parasitemia, allowing a rapid advance towards pleomorphic forms of the parasite that are more competent for the infection of the next link, the vector. In these experiments, we have corroborated the transit from the systemic environment to the scent glands. Experimentally infected Didelphis has revealed a reversible infection (blood-anal glands-blood) with nests of amastigotes in the mucosal epithelium and striated musculature of the gland, with no evidence of inflammation (Figure 2).
Secondary cardiotropism and absence of symptoms have also been observed. Xenodiagnosis on these infections closes the cycle with the production of highly infectious metacyclics in the murine model, proving the transmissibility of T. cruzi (Figure 2).
Thus, this reservoir behaves as such while the pathology and virulence are scarce but with high transmissibility due to the double cycle that the parasite develops. This host-parasite system in highly urbanized areas and recreational parks of Caracas-Venezuela, in sympatry with human settlements, generally in marginal conditions and low in health education and environmental management, constitutes a potential risk.
Experimental infections by T. cruzi of caviomorphs, rodents with an ancient co-evolutionary association with the parasite, have revealed myotropism with vacuolization, myocytolysis and lymphomacroeosinophilia, with stable infections, regardless of the genotype of the parasite, but without evident symptoms.
Primates naturally infected by T. cruzi, confined in closed environments such as fauna reserves, develop symptoms similar to human infection, with cardiac abnormalities and biochemical alterations.
Ana Maria Jansen-Franken
Laboratory of Biology of Trypanosomatids, Instituto Oswaldo Cruz/Fiocruz
In popular spirit, no other animal has a more mysterio-saturated reproduction than the opossum. All over the country, both blacks and whites believe that the opossum has its genetic contacts through the snout and that the females keep the fruit of conception in their pouch (…) Such legends should not be surprising, because regarding the use of the pouch as an incubator, it certainly excites the imagination.(…).
Familiarity brings vulgarity, the common ceases to be wonderful. Thus, due to its rarity and its ‘different’ character, the opossum, our only marsupial, figures in folklore in a high degree.
Marsupials have aroused curiosity in Western culture since Vicente Pinzón presented a female opossum to the Catholic kings of Spain. The Brazilian word for opossum, “gambá,” derives from Tupi-Guarani and means ‘open womb’; that is, it was the mode of reproduction that caught the attention of pre-colonial peoples. Among the numerous regional denominations, mucura, sariguê, cassaco stand out, respectively in the north (the first two), northeast and center-west of Brazil. The order Marsupialia, which in our continent only includes the family Didelphidae, presents a significant diversity in Australia, due to the relative isolation that allowed these mammals to occupy ecological niches that in other continents are occupied by several orders of placentals. Although the marsupial pouch is the trait referred to as characteristic of the order, it is the urogenital tract that most significantly distinguishes marsupials from other mammals: female marsupials have two lateral vaginas that unite to form a median vagina. At birth, a passageway for the fetus forms in the connective tissue between the midline vagina and the urogenital sinus. In most marsupials, this channel is transitory and will be formed again with each new birth.
Widely distributed in the Americas, the Didelphidae family represents the oldest group of marsupials (originating in the Upper Cretaceous), and is probably indigenous to America. The genus Didelphis occurs from southeastern Canada to southeastern Argentina, being the marsupial genus with the greatest dispersion on the continent. Until recently, marsupials were considered ‘lower mammals’. Currently, it is known that the similarities between metatherians and eutherians are very great, the differential characteristic being their mode of reproduction: the shorter generation time – 12 to 13 days – and the parturition of individuals almost at an embryonic stage. A marsupial offspring weighs 0.01 to 0.05% of its maternal weight, in contrast to a placental offspring that weighs 2 to 3%.
Marsupials can be considered as an immature placental that will depend, for its development, on the conditions of ‘incubation’ of the marsupium. The intimacy of a marsupial fetus with maternal tissues is much less than that of a placental one. This intimacy becomes important during the long period of breastfeeding.
The gestation period in didelphids is approximately 13 days and at birth, opossums do not have immunoglobulins – these, of maternal origin, appear from the first hours postpartum. A newborn marsupial is deaf and blind, does not yet control its body temperature – which occurs in the middle of the period of dependence on the animals’ marsupium. At birth, its hind legs and tail are vestigial. The precociously developed forelegs are equipped with deciduous nails that help the neonate climb to the marsupium, which happens without maternal assistance. During the first 55 days of life, the still undifferentiated mouth of the offspring is sealed to the nipple and only from then on, they begin to become independent, spending increasing periods outside the baby carrier. Between 80-90 days, the immune system is mature and it is in this period that they start an independent life. A female Didelphidae takes about 112 days to raise a litter – from conception to weaning.
The enormous distribution of didelphids in the Americas is mainly due to their impressive adaptability. Resistance to inbreeding is another factor favorable to the dispersion of the species, as only a small number of animals are needed to found a colony. Despite being described as “not very intelligent”, Didelphis has a significant ability to memorize places where it can find food.
The interaction between man and opossums is probably ancient: in fact, these animals resist well and even benefit from human action in the environment, adapting to degraded natural environments, to the linings of houses and other shelters in the home and peridomicile. They are nomadic (mainly males) and solitary. They survive well by feeding on human food waste, which, on the other hand, often uses them as a source of protein. These characteristics made the Didelphidae to be currently considered as synanthropic.
Didelphid marsupials, mainly of the genus Didelphis, have been referenced since Carlos Chagas, and identified as the most important reservoirs of Trypanosoma cruzi due to the following characteristics:
- high prevalence of natural infection by cruzi in these animals, reported by several authors;
- its ability to maintain the parasite’s extracellular multiplication cycle in the scent glands;
- their nomadic behavior; iv) its ability to colonize households and peridomiciles in heavily devastated forest areas. These characteristics show that these animals should be targets of epidemiological surveillance because they can serve as markers for the presence of an expressive cruzi transmission cycle in the area.
Marsupials of the genera Lutreolina and Didelphis are able to maintain both T. cruzi multiplication cycles simultaneously, both under natural and experimental conditions. In fact, in the light of the scent glands, the parasite can multiply in the epimastigote form and differentiate into metacyclic trypomastigote simultaneously with the intracellular multiplicative cycle of the various tissues of the animal. This means that the opossum can, at the same time, be a reservoir and vector of T. cruzi. Although several authors have reported the presence of T. cruzi in the scent glands of naturally infected Didelphis, its vectorial capacity is not known, in short, the importance of this parasitism in the maintenance of T. cruzi in nature or its vectorial capacity. The scent glands of Didelphis (n=2) are found under the skin, wrapped in a layer of fat, close to the terminal segment of the large intestine. A delicate channel drains the contents at the rim of the anus. Macroscopically, the scent glands have a pearly outer surface and have a capacity of approximately 500 µL. A robust layer of striated muscle covers two-thirds of each gland.
It is interesting to mention that the parasite is distributed in the light of the scent glands in a peculiar pattern, the epimastigote forms being more abundant in the vicinity of the glandular epithelium and the metacyclic forms, more found in the center. The material immediately juxtaposed to the glandular epithelium, where epimastigotes predominate, was characterized as having hyaluronic acid as the major component. The rest of the content is basically composed of lipids. The peculiar location of the parasites in the glands probably results in the preferential elimination of the infective forms of the parasite when the host, in response to a negative stimulus, eliminates the contents of the glands that present a strong and unpleasant odor. Additionally, it was observed that the metacyclic forms derived from epimastigotes of the scent glands present, at least, the same infective competence as the metacyclic forms of axenic culture or intestinal tract of triatomines.
In addition to T. cruzi, Trypanosoma (Megatrypanum) freitasi was found parasitizing the lumen of the scent glands of D. aurita.The invertebrate host of T. freitasi is not known, as little is known about its biology. In the scent glands of D. aurita, epimastigote forms in multiplication, multinucleated plasmodial forms and rosette forms were observed.
Although most T. cruzi isolates derived from marsupials have been characterized as belonging to the TCI genotype, the TCII genotype has also been isolated from these animals: thus, a study on natural infection of D. aurita and P. frenata by T. cruzi in a fragment of the atlantic forest showed 50% of positive blood cultures in these two didelphid species that live in sympatry. The biochemical and molecular characterizations of the parasite isolates showed that both species were parasitized by both TCI and TCII and that these two species, even living in sympatry, could participate in independent cycles of parasite transmission.
Although it lacks empirical evidence, the hypothesis that Marsupials are the probable ancestral hosts of T. cruzi is quite plausible insofar as these are very ancient representatives of mammals. The establishment of a possible association of T. cruzigenotypes with the mammalian fauna is still far from being resolved. Didelphid marsupials include important reservoirs of T. cruzi and deserve special attention during risk assessment studies or outbreaks of Chagas’ disease. However, we must always remember that the role that this group of mammals plays in the different transmission chains of T. cruzi in nature varies according to the temporal and local ecological scenario. Therefore, wrong conclusions will emerge from studies in the field that do not address the complexity of the transmission cycle of a parasite as heterogeneous and eclectic as T. cruzi.
Popularly quite disregarded today, didelphids, due to their peculiarities, offer unique possibilities for ecological, evolutionary and biological studies, being, therefore, an interesting animal model not only for studying the epidemiology of Chagas disease and the transmission cycles of T. cruzi in nature, but also for studies of parasite-host interaction in general.
Cristiane Varella Lisboa
Laboratory of Biology of Trypanosomatids, Instituto Oswaldo Cruz/Fiocruz
Due to inherent difficulties, longitudinal studies on zoonotic parasites in wild mammal species are quite scarce; this is particularly the case of Trypanosoma cruzi, a highly heterogeneous taxon from the genetic point of view, multi-host and with wide geographic distribution in the Americas. The recognized genetic diversity of T. cruzi, as well as its biological plasticity and its enormous distribution represent additional obstacles to understanding the variables underlying the transmission cycle. Six discrete typing units (DTU), TcI to TcVI and one recently recognized, TcBat, have already been recognized in the taxon, being described in different species of mammalian hosts, both wild and domestic, as well as in vectors found in several Brazilian biomes. However, the epidemiology of each of these DTUs is far from known. In fact, the seven T. cruzi DTUs occur in complex transmission cycles that may or may not overlap and must be understood as non-linear and stochastic.
Natural infection by T. cruzi is quite expressive in endemic Neotropical primates from different phytogeographic regions of South America. T. cruzi has already been observed in primates included in the families Alouattinae (howler monkeys and brown howler monkeys), Atelinae (spider monkeys, woolly monkeys, and wooly spider monkeys), Callitrichidae (marmosets and lion tamarins), Cebidae (capuchin monkeys, white-fronted capuchin monkeys and squirrel monkeys), Nyctipithecidae (night monkeys) and Pitheciinae (saki monkeys and uakaris).
Neotropical primates occupy different ecological niches in the forest and have a diversity of behavioral and feeding habits that include, from predation (insects and small mammals) to vegetarian habits; these characteristics allow T. cruzi infection to be acquired by different transmission strategies. The consequences of infection by T. cruzi in non-human primates have already been widely studied, mainly aiming to clarify aspects of Chagas’ disease, however the experimental infection in laboratory animals does not fully reflect what happens under natural conditions or with man or wild primates.
The only longitudinal study (11 years) on the interaction of T. cruzi with a free-living primate species was carried out in the wild population of Leontopithecus rosalia (Family Callitrichidae: golden lion tamarin) at the Poço das Antas Biological Reserve – RBPA (Silva Jardim, RJ), in the Coastal Atlantic Forest of Rio de Janeiro.
As it is endangered, the species has been submitted to the Programa de Conservação do Mico-Leão-Dourado(Conservation Program of the Golden Lion Tamarin) in the Atlantic Forest of Rio de Janeiro since the 1970s, which includes (i)reproduction in captivity, (ii) exchange of specimens between national and international zoos, (iii) reintroductions in their areas of occurrence, and (iv) translocations between forest fragments, being considered as a model program for the conservation of wild fauna in Brazil. The program also emphasizes that the adoption of a species as a flag, as with the golden lion tamarin, helps to draw public attention, thus contributing to the conservation of the species and the Atlantic Forest, since many endemic species of the fauna are endangered, mainly due to (i) habitat modification, (ii) fragmentation of the original landscape, and (iii) constant human interference in the environment.
Golden lion tamarins are social, diurnal, arboreal animals that sleep in tree hollows. They live in remnants of coastal forest in the Atlantic Forest of southeastern Brazil, where they form social groups of two to eight individuals, divided into defined hierarchies. They are cooperative breeders, that is, all adults in the group help with parietal care and most young tamarins disperse from their natal group up to four years of age. Adults weigh between 550 and 600 gr and measure about 60 cm in length from the head to the tip of the tail and feed on a wide variety of fruits, some invertebrates and small vertebrates. They are extremely territorial – each wild group has a defined home range that varies from 50 to 100 hectares, which is occupied by a single social group and is constantly defended from the entry of other groups of neighboring tamarins.
These studies on the interaction of T. cruzi in RBPA in the Atlantic Forest showed that T. cruzi circulates among its wild hosts in distinct and independent transmission cycles that may or may not be linked. Apparently the main DTUs (TcI and TcII) of T. cruzi circulate in all forest strata (arboreal, intermediate and terrestrial) regardless of the ecological niches that each host occupies in the forest.
Thus, it was verified that the TcI DTU circulates in wild rodents, Nectomys squamipedes and Holochilus brasiliensis (terrestrial species), in the maned sloth, Bradypus torquatus (arboreal species), in the marsupial, Didelphis marsupialis (intermediate species), and in the golden lion tamarin (L. rosalia) while DTU TcII infects only the wild population of golden lion tamarins.
Although the natural infection of wild mammals by T. cruzi is quite common in nature, what drew attention in the RBPA was the high seroprevalence and high percentages of positive blood cultures observed in the wild population of golden lion tamarins, suggesting that this host is a significant reservoir of the DTU TcII of T. cruzi, at least in this Atlantic Forest fragment.
The monitoring of T. cruzi infection in golden lion tamarins showed that these hosts maintain stable infections by both TcI DTU and DTU TcII and that there is no correlation between sex and age. Males and females, regardless of age, are equally infected. Vertical transmission was not observed, that is, infected mothers generate uninfected offspring. The infection by T. cruzi probably occurs either in the hollow of the trees, where the tamarins spend the night, or through the oral route (most likely), which is known to be an efficient mechanism of transmission between wild animals and humans. In the case of primates, it is perhaps the most important transmission route, since these animals are closely associated with palm trees, using them as a source of refuge and/or food. Palm trees disperse rapidly in fragmented environments, thus providing a complex micro-habitat for countless species of small mammals and mainly triatomines, as occurs in several forest fragments scattered in Brazilian biomes.
The distribution of T. cruzi infection among golden lion tamarins, although it occurs homogeneously in the population, varies in relation to the infection rates observed in different social groups. This observation shows that the transmission of T. cruzi in a given biome is quite complex and involves numerous ecological factors and that each home range maintained by a given social group of golden lion tamarins must be considered as a unique biological unit with its micro-environmental peculiarities.
The monitoring of T. cruzi infection in populations of golden lion tamarins distributed in forest fragments adjacent to the Poço das Antas Biological Reserve showed that different patterns of infection can occur in the same host (golden lion tamarin) and in the same biome (Atlantic forest). While the population of golden lion tamarins in the RBPA presented high parasitemias and high seroprevalence, the populations of golden lion tamarins, adjacent to the reserve, showed a low prevalence and few positive blood cultures were obtained. The low parasitemias observed in golden lion tamarins from adjacent areas shows that despite these animals being exposed to the parasite, they are better able to control the infection compared to tamarins that inhabit the RBPA.
The occurrence of DTU TcII in golden lion tamarin in the Atlantic Forest is not a consequence of a presumed association of this subpopulation with Neotropical primates. More recent studies with primate species endemic to the Atlantic Forest and the Amazon Forest have shown that primates maintain stable infections with both DTU TcI and DTU TcII and/or stable infections. This shows that these hosts become infected with the subpopulation that occurs in a given area in a given time frame. It is worth mentioning that DTU TcII has already been observed in other species of wild mammals such as marsupials, caviomorph rodents, bats, and carnivores. Thus, both the TcI DTU as well as the DTU TcII infect a diversity of wild mammals, but the TcI DTU has a wide geographic distribution while the DTU TcII occurs in a focal way in nature.
A similar scenario was observed in a distinct fragment of the Atlantic Forest located in the Northeast Region of Brazil, in the Una Biological Reserve, (Bahia), where another endemic species of golden-headed lion tamarin (Leontopithecus chrysomelas), also demonstrated a high rate of T. cruzi infection with high parasitemias, as demonstrated by the high rates of positive blood cultures.
T. cruzi infection (TcI DTU, TcII DTU or mixed infections) is also quite frequent in different species of primates kept and/or born in captivity, such as in Primatology Centers, Screening Centers, Center for the Study and Management of Wild Animals and Private Zoos distributed in different regions of Brazil. Regardless of the transmission route in which the animals become infected, the issue to be considered is the environmental risk, since most of these management centers use reintroduction techniques, exchange specimens and still receive animals from seizures carried out by ICMBio all over the country. This aspect emphasizes the need for parasitological and serological exams in animals that are kept in captivity and/or submitted to any type of management. Every reintroduced or translocated animal must be monitored, as Neotropical primates can act as sentinels for the emergence and/or re-emergence of infectious diseases.
It is worth noting that one of the environmental problems for the preservation of wild animals is the fragmentation of our forests. Currently, many biodiversity conservation programs aim at the recovery of forests through the creation of forest corridors in order to increase the forest area for translocations and reintroductions of endangered wild species, however these programs do not take into account the environmental status of the landscape. This fact must be taken into account since wild animals, in general, are considered reservoirs of numerous pathogens, including T. cruzi.
André Luiz Rodrigues Roque and Vanderson Corrêa Vaz
Laboratory of Biology of Trypanosomatids, Instituto Oswaldo Cruz/Fiocruz
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Rodents are not indigenous South American mammals. The local fauna was initially composed of species of the orders Didelphimorphia (marsupials), Cingulata (armadillos), and Pilosa (sloths and anteaters). The greatest diversification of fauna, which occurred with the arrival of other mammals on the continent, including some species of rodents, occurs only in the Eocene or early Oligocene (35-40 million years ago). At this time, South America had already separated from Africa and Antarctica/Australia, which occurred approximately 100 and 70-65 million years ago, respectively, assuming an island character ever since.
The entry of rodents into South America can be divided into two major phases: the entry of rodents of the infraorder Hystricognathi (caviomorphs) and, later, the entry of rodents of the suborder Sciurognathi (cricetids). It is known that, regardless of the group in question, the origin of rodents is related to the diversification of a common group that occurred in the supercontinent located to the north of the globe called Laurasia. The first findings of caviomorphs in South America date back to the Eocene or early Oligocene, when this continent was separated from North America by a maritime channel and from Africa by the Atlantic Ocean. These oldest South American rodents must have migrated aboard logs and floating islands, but it is still debated whether they came directly from an Asian ancestor through North America, through a colonization route through Australia and Antarctica, or, in the most currently accepted, coming from African ancestors (phiomorphs), at a time when the two continents were not so far apart and the sea currents favored migration.
Regarding the arrival of the Sciurognathi, the most accepted hypothesis is related to the diversification of murine and cricetid rodents in Africa and the Old World, and their arrival in the Americas at a more recent time than that described for the arrival of caviomorphs, probably coming directly from Africa, or by a migration route that included an initial settlement in North America (mainly cricetidae). After their arrival, the diversification of cricetid rodents in South America must have been the result of rapid evolution with chromosomal reorganization as the main speciation factor.
Currently, rodents represent perhaps the most diverse and widespread taxon among mammals. The morphological diversity in this taxon is exemplified by the contrast between a 5g pygmy rat and a capybara that can reach up to 70 kg. After their arrival, the rodents adapted well, and diversified into a considerable number of species whose distribution is restricted to the South American continent. These colonized the most diverse types of habitat, from tropical forests to deserts, from high-altitude plateaus to flooded plains, from the wild to the urban environment, in addition to the most diverse natural strata, ranging from fossorial species to species with semi-tropical habits. aquatic and arboreal.
This makes possible the existence of a rich community of rodents by region, allowing, many times, the coexistence of congener species. Because of this high diversity, it is difficult to pinpoint the exact distribution of these organisms, especially because, in most cases, little is known about their biology. The reproduction strategies themselves are quite distinct, therefore, reproductive seasonality, gestation time and number of offspring can differ significantly between the genera of this order. Therefore, what is found in the literature are over- or underestimations of the real distribution area of these animals, many of which are determined by information contained in the scientific collections of museums, which serve as a primary source for this type of study.
Today, rodents are very well established in Brazil and form the largest study biomass in any wild ecotope (Figure 1). Its most diverse microhabitats are often shared with triatomines of the genus Triatoma (associated with rocky terrestrial refugia) and Panstrongylus (associated with holes in the ground), some of the vectors of Trypanosoma cruzi. Thus, due to their own ecological niche, rodents become an important group to study when evaluating their role in the transmission cycle of this parasite.
T. cruzi infection of wild rodents has been widely reported, both in Brazil and in other South American countries, especially Argentina and Chile, where there are good research groups working on the subject, and even in the southern United States. However, many works restrict the capture of animals to those that frequent peridomiciliary environments, where the capture effort is concentrated. This is confirmed by the fact that the rodent species most commonly found infected by T. cruzi is Rattus rattus, a synanthropic species highly abundant in the most urbanized cities of South America, but rare within tropical forests. Although abundant in experimental studies, the natural infection by T. cruzi in Mus musculus mice is rarely reported.
Caviomorph rodents of the genus Proechimys, described as important maintainers of the transmission cycle of different species of Leishmania, also have few reports of infection by T. cruzi, with prevalence that does not reach 2 % in Brazil and Colombia. In Chile, long-term studies have already demonstrated T. cruzi infection of different DTUs in the blood of Octogus degus, Phyllotis darwini, and Abrothrix olivaceus, at infection rates ranging from 40 to 45%.
Other rodent species found naturally infected by T. cruzi are: Agouti paca, Akodon lindiberghi, A. montensis, A. toba, Baiomys musculus, Calomys expulsus, C. callosus, Cavia sp, Cerradomys subflavus, C. marinhus,Clyomys laticeps, Coendou prehensilis, Dasyprocta sp., Echymis chrysurus, E. dasytrix, Galea spixii, Graomys chacoensis, Holochilus brasiliensis, Hylaeamys sp. Kerodon rupestris, Necromys lasiurus, Nectomys squamipes, N. rattus, Neotoma floridana, N. mexicana, N. micropus, Octodon degus, Octodontomys sp., Oecomys bicolor, O. mamorae, O. nigripes, O. stramineus, Oryzomys capito, O. scotti, Oxymycterus sp., Peromyscus gossypinus, P. levipes, Proechimys spp., Rhiphidomys macrurus, Scapteromys aquaticus, Sigmodon hispidus, Thrichomys spp., Trinomys paratus, Tylomys mirae, and Wiedomys pyrrhorhinos.
In the case of wild rodents in Brazil, perhaps the most cited and empirically proven species complex as reservoirs of T. cruzi in certain localities are the species of the genus Thrichomys. This is because, in addition to studies of its natural infection, experimental works reinforce the importance of species of this genus in the maintenance and amplification of T. cruzi in their respective natural ecotopes. Thrichomys laurentius was found with infection prevalence ranging from 2 to 44% and from 3 to 76% by blood culture and serology, respectively, in different localities that make up the same National Park, Serra da Capivara, in Piauí. In the Caatinga, these rodents showed higher relative abundance and were able to maintain T. cruzi TcI, TcIV and TcV DTU infections, in simple or mixed infections with T. rangeli. In the study of an outbreak of oral Chagas’ disease, which occurred in Redenção/CE, the infection rate of these animals by serological examination reached 100% of the animals examined (n=12). T. fosteri and T. apereoides are other species of the same genus also found infected by T. cruzi. The importance of these animals as potential reservoirs of the parasite is supported by their normally high relative abundance among small mammals in the areas where they are found.
The importance of rodents as wild reservoirs is also described against several other microorganisms, such as viruses and bacteria. This is because: (i) they are frequently identified as hosts of several zoonotic parasites; (ii) comprise the group of mammals with the highest biomass in any wild ecotope; (iii) they are the main predation targets in nature, providing an alternative route for the dispersion of several species of parasites, including T. cruzi; (iv) although wild, many species often approach human habitation, favoring the formation of a continuous gradient of parasite transmission between wild and domestic environments.
In general, the vast majority of descriptions are punctual encounters of T. cruzi infection in these rodent species mentioned above. The determination of a given species as a reservoir depends on several other factors, such as: (i) determination of the geographic distribution of the host coincident with that of the parasite; (ii) association of this distribution with the particularities of their respective biomes; (iii) determination of its prevalence of infection among demographic subpopulations (young people vs. adults, for example); (iv) establishment of temporal and spatial scales so that the phenomenon known in a given area is not transposed to other areas not studied.
Small rodents are mainly granivores or herbivores and tend to stay in more restricted areas which, as stated above, can be shared with triatomine insects. This would make them more likely to be exposed to infection via the contaminating route. On the other hand, rodents are the main targets of predation among mammals and this is an important characteristic, as it allows the dispersion of T. cruzi through the oral route, transferring the parasite in the same way that energy is transmitted in a prey-predator process. It is also worth remembering that, being at a lower level of the trophic pyramid, rodents exhibit much larger populations that undergo rapid population turnover. T. cruzi infection in these mammals, when found, is quite informative as it indicates a recent transmission of the parasite in the area.
Finally, an aspect that deserves to be highlighted when evaluating several studies already carried out with T. cruzi and wild rodents is the generally low rate of infection reported in these studies. Highly susceptible to experimental infection, these animals appear to play a secondary role as reservoirs. In fact, in the experience of our group examining almost four thousand rodents, only 8.5% of the animals were infected (positive serological tests) and the recovery of parasites through positive blood cultures occurs in only 2% of the rodents examined. Two possible scenarios can explain this fact: (i) rodents do not survive infection by T. cruzi in nature and therefore infected rodents die before being captured; or (ii) rodents exhibit a short life cycle and more focal distribution and are therefore less exposed to T. cruzi infection. These two hypotheses are not mutually exclusive and certainly cannot be generalized to all rodent species.
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Heitor Miraglia Herrera, Filipe Martins Santos, Wesley Arruda Gimenes Nantes
Laboratory of Biology of Trypanosomatids, Instituto Oswaldo Cruz/Fiocruz
Trypanosoma cruzi is found between latitudes 41º N and 46º S of the American continent and has enormous plasticity in terms of infectivity with regard to host species. In fact, this flagellate is widely distributed in all phytogeographic regions of the Neotropics, having been reported in natural infections in about a hundred species of wild and domestic mammals, belonging to different orders such as Chiroptera, Primate, Rodentia, Marsupialia, Cingulata, Pilosa, and Carnivora. It is worth mentioning that, in each biome of Latin America, and in each habitat in particular, we can find different species of mammals sustaining different transmission cycles that can be isolated or connected. This character is particular and unique to each particular location. Thus, Chagas’ disease has a complex zoonotic character.
Within this scenario, when discussing the issue of potential reservoirs for T. cruzi, we need to keep in mind that the simple fact that an individual is found naturally infected does not necessarily mean that he or she will pose a risk to the health of its population. other sympatric or human species. Furthermore, the role that each host species plays in the dispersion and/or maintenance of the parasite can be extremely variable due to (i) the complexity of ecological processes and interrelationships and (ii) human pressure to increasingly modify environments.
Here it is intended only to highlight the wild and domestic mammal species that were found naturally infected by T. cruzi and can, under certain environmental and social circumstances, act as potential reservoirs of infection. Marsupials, small rodents and primates, as they act as important reservoirs of infection in humans, constitute separate topics in the study of transmission cycles of T. cruzi.
Some of the wild animals described below, such as South American coatis (Nasua nasua); crab-eating fox (Cerdocyon thous – Figure 1) and armadillos approach the houses, frequenting chicken coops, corrals and warehouses in the countryside. This fact is also observed in the periphery of some cities. In some cases, like bats, they share environments with humans and domestic animals. Thus, these species may be serving as a source of infection for reduviid vector insects that inhabit the same habitats as humans.
Figure 1: Crab-eating fox (Carnivora, Canidae, Cerdocyon thous) in the natural environment. Picture by Filipe Martins Santos.
Since the pioneering studies by Carlos Chagas, in 1912, the nine-banded armadillo (Dasypus novemcinctus) has been recognized as one of the natural reservoirs of Chagas’ disease. In its underground shelters, this mammal is found associated with the triatomine Panstrongylus geniculatus, also infected by T. cruzi. Armadillos, essentially terrestrial mammals, are widely distributed throughout Latin America, with an infection rate in Brazil ranging from 10 to 50%. T. cruzi isolated from these animals belong to the TcIII, TcIV and TcIII/TcIV subpopulations (Table 1), indicating the participation of this mammal in different enzootic cycles. In Brazil, parasitism by T. cruzi was also recorded in the southern naked-tailed armadillo (Cabassous unicinctus) and in the six-banded armadillo (Euphractus sexcinctus – Figure 2), the latter found naturally infected by the TcIII and TcIII/TcIV genotypes (Table 1).
Figure 2: Six-banded armadillo (Cingulata, Chlamyphoridae, Euphractus sexcinctus) in the natural environment. Picture by Filipe Martins Santos.
Other species of animals formerly designated as Edentata (Wilson & Reeder) such as the maned sloth (Bradypus torquatus), the collared anteater (Tamandua tetradactyla) and the sikly anteater (Cyclops didactylus) are also reported to be naturally parasitized by T. cruzi (Table 1). It is important to mention that the animals belonging to the orders Cingulata (armadillos) and Pilosa (anteaters and sloths), previously designated as Edentata (Wilson & Reeder), are part of the ancestral placental native fauna of South America, appearing in the Tertiary period about 65 millions of years ago. As the emergence of T. cruzi precedes the mammal fauna (Cretaceous – 150 million years), we can infer that armadillos, sloths and anteaters, in their respective terrestrial and arboreal niches, have been undergoing a long co-evolution with the different strains of T. cruzi. This ancient association may have generated mechanisms of protection and perpetuation for both the parasite and the host. Also, medium-sized rodents, such as the lowland paca (Cuniculus paca), a caviomorph rodent, and the porcupine (Coendou spp.), an arboreal rodent, have already been found naturally infected by T. cruzi.
Undoubtedly, together with sloths and anteaters, paca and porcupine contribute to the maintenance of T. cruzi in the wild ecotope. However, as they have small population densities and are rarely found near human habitations, these species normally do not pose a risk of infection to humans. Nonetheless, in some situations, they may constitute potential reservoir hosts for T. cruzi infection. In peri-urban and rural areas, it is common for the armadillo to invade chicken coops in search of eggs and/or animals, thus approaching the peridomiciliary environment, thus being able to serve as a source of infection for vectors. Still, along with sloths, anteaters, pacas, and porcupines, armadillos are part of the diet of indigenous peoples and people who inhabit areas with low economic status, and that both handling the carcass and eating undercooked meat from animals infected can be sources of infection.
Several bat species are proven hosts for T. cruzi. However, it should be noted that other trypanosomatids (Trypanosoma vespertilionis, Trypanosoma dionisii, Trypanosoma cruzi marinkellei) are frequently found in bats, causing confusion with the parasitological diagnosis for T. cruzi. Bats should be considered in epidemiological studies of Chagas’ disease due to (i) the adaptation of many species to the human home, (ii) their abundance and (iii) the very high rates of parasitism. Especially Artibeus lituratus, a fairly common species found in many habitats, is typically associated with human dwellings and cities rather than natural environments. T. cruzi isolates from this species were characterized as TcI and TcIV. In the southern Pantanal of Mato Grosso, isolates of Artibeus planirostris were characterized as TcII/TcIII while isolates of Phyllostomus hastatus. were characterized as belonging to the TcI, TcII, TcI/TcII and TcII/TcIV genotypes (Table 1). Thus, like armadillos, bats also participate in different transmission cycles, however, unlike armadillos, they occupy above-ground extracts such as tree hollows, banana leaves and linings of human dwellings and rural buildings.
Wild carnivores such as the coati (Figure 3), the tayra (Eira barbara) and the crab-eating fox are found harboring T. cruzi in nature. These animals, as natural predators, may be infected by ingesting parasitized small mammals (rodents and marsupials). Thus, although vector transmission plays a role in nature, the maintenance of T. cruzi in nature is also carried out through oral infection by carnivores.
Figure 3: Coati (Carnivora, Procionidae, Nasua nasua) in the natural environment. Picture by Filipe Martins Santos.
The different genotypes of T. cruzi found in wild animals show that wild transmission cycles are complex and unpredictable systems, sometimes connected through the encounter of different host species in a single habitat. As an example, we can mention the burrows of giant armadillos (Priodontes maximus) that can provide shelter for other species found infected by T. cruzi, such as the crab-eating fox (Cerdocyon thous), coati, ocelot (Leopardus pardalis), and tayra.
The significance of domestic animals (dogs and pigs) as potential reservoirs of T. cruzi for some species of triatomines that inhabit the domicile and peridomicile becomes especially important for sustaining a transmission cycle in environments close to humans. In general, dogs have a prevalence between 15 and 25% in endemic areas, and animals have been described as infected with T. cruzi isolates of TcI – TcII – TcIII – TcII/TcV/TcVI – TcV/TcVI subpopulations (Table 1). It has been shown that, in endemic and enzootic areas, the presence of dogs in a family environment increases the risk of infection for children by four times. In addition to dogs, in an oral outbreak of Chagas’ disease that occurred in Marajó/PA, they showed that pigs residing in the area had a seroprevalence of 64%, showing the importance of these animals in the local cycle of T. cruzi. As well as the wild boad (Sus scrofa feral) in the Pantanal of Mato Grosso do Sul that has already been found infected with T. cruzi TcI type.