Livestock Health, Management and Production ›High Impact Diseases›Vector-borne Diseases› Trypanosomoses
Trypanosomoses
Author: Vincent Delespaux
Adapted from: R.J. CONNOR and P. VAN DEN BOSSCHE, 2004, African animal trypanosomoses, in Infectious diseases of livestock, edited by J.A.W. Coetzer & R.C. Tustin. Oxford University Press, Cape Town, 12: 251 – 295
Licensed under aCreative Commons Attribution license.
Table of contents
Introduction
Epidemiology
Pathogenesis
Diagnosis and differential diagnosis
Diagnosis and differential diagnosis
Clinical signs and pathology
Laboratory confirmation
Differential diagnosis
Control / Prevention
FAQs
References
Introduction
The trypanosomoses are diseases of humans and domestic animals that result from infection with parasitic protozoa of the genus Trypanosoma. Trypanosomes parasitize all classes of vertebrates: fish, amphibians, reptiles, birds and mammals. The parasites, with the exception of Trypanosomaequiperdum, the cause of dourine, are transmitted from host to host by haematophagous vectors, and usually cause little appreciable harm to either the vector or the vertebrate host. However, several species of trypanosomes which parasitize mammals are less well adapted and commonly cause disease.
Trypanosomosis is generally characterized by the intermittent presence of parasites in the blood and intermittent fever. Anaemia usually develops in affected animals, and this is followed by loss of body condition, reduced productivity and, often, high mortality.
The first report that associated trypanosomes with disease was made from India in 1880. In 1895 a major discovery was made in Zululand, South Africa that trypanosomes were the causal organisms of ‘nagana’, or tsetse fly disease.
Two forms of human trypanosomosis exist: Chagas' disease occurs in Central and South America and is transmitted by bloodsucking reduviid bugs, certain small wild animals and dogs harbouring the infection. The second form is human sleeping sickness. This occurs in Africa and is transmitted by bloodsucking flies of the genus Glossina, commonly known as ‘tsetse flies’ or simply as ‘tsetse’. The majority of animal diseases caused by trypanosomes occur in the tropics. In Africa, several species of tsetse-transmitted trypanosomes cause African trypanosomoses in domestic animals, which in southern Africa are collectively known as ‘nagana’, a word derived from the Zulu word ‘nakane’ meaning tsetse fly disease. ‘Surra’ is transmitted by biting flies other than tsetse flies and, although it occurs in many parts of the tropics, including northern Africa, it is not present in southern Africa.
The large populations of wild animals, which have thrived for millennia in the tsetse-infested tracts of Africa have evolved with these flies and the trypanosomes they transmit. Hosts and parasites have become mutually adapted and co-exist in a balanced relationship. Humans first brought domestic animals into the tsetse belts of Africa relatively recently. Because of this recent introduction, the relationship between tsetse-transmitted trypanosomes and domestic animals has not fully evolved and infection with these parasites frequently produces disease.
The devastation which resulted from the rinderpest pandemic of the 1890s destroyed almost entire populations of wild animals and millions of cattle. Without hosts on which to feed, tsetse disappeared from large areas. However, a few decades later, tsetse were dispersing from residual pockets, and trypanosomosis again became a problem for livestock owners. By 1931, tsetse were spreading at a rate of 2500 square kilometres (1000 square miles) annually, and game elimination to control tsetse began in 1932. Since then strenuous efforts have been made to contain the tsetse fly. In many other parts of southern Africa, livestock owners have also had to live with the tsetse fly and its consequences.
Tsetse infest 10 million square kilometres and affect 37 countries, which makes African animal trypanosomosis a problem of truly continental magnitude. They live in frost-free areas that have an annual rainfall of 650mm or more. In arid, marginal habitats, tsetse only exist in the better wooded and better watered strips where the host species concentrate during critical times, such as in the late, hot, dry season. Most of the settled areas of the tsetse fly belts of southern Africa are used for traditional mixed farming, but the presence of tsetse seriously handicaps development.
General distribution of tsetse flies and cattle in Africa
Concerted efforts to control tsetse over the past 50 years have resulted in significant changes in the distribution of tsetse and tsetse-transmitted trypanosomosis. Unfortunately, few of these achievements have been sustained. In many countries of southern Africa, the current distribution of tsetse and, hence, tsetse-transmitted trypanosomosis is not much different from the ecological limits of the fly distribution.
Early work on trypanosomosis, much of it conducted in southern Africa, concentrated on describing the trypanosomes and studying the natural history of the parasites, their vectors and their hosts. The greatest advances in knowledge of trypanosomosis over the past two decades have been made in the areas of molecular biology and immunology.
Epidemiology
Trypanosomes are protozoan parasites of the genus Trypanosoma, order Kinetoplastida, and have, as characteristic organelles, a kinetoplast and a flagellum. Typically, trypanosomes are digenetic parasites and thus require two hosts to complete their life cycle: they multiply in the blood, tissues or body fluids of a vertebrate host and, with the exception of T.equiperdum which is venereally transmitted, are ingested by a haematophagous invertebrate vector. With a few notable exceptions, a cycle of development and maturation occurs in the vector, after which the parasites are transmitted to another vertebrate host as the vector feeds. Transmission is either by inoculation of trypanosomes with saliva or by contamination of mucosa or broken skin with trypanosomes in the vector's faecal material, voided during the blood meal. The type of development cycle within the vector determines whether or not infective, metacyclic parasites are present in saliva or faeces. On this basis mammalian trypanosomes are classified into the two broad sections of ‘salivaria’ and ‘stercoraria’.
In Africa, the pathogenic trypanosomes that cause sleeping sickness in humans and nagana in domestic animals are salivarian, and cyclical development occurs in tsetse flies. Transmission of any trypanosome species can take place mechanically without cyclical changes occurring in the vector. In nature, this is effected by biting flies, such as Tabanus, Stomoxys and Lyperosia spp., which feed on more than one animal before repletion.
Surra is a disease that affects a wide range of host animals, and it occurs in North Africa, the Near and Far East, Central and South America, Philippines and Mauritius. It is caused by Trypanosomaevansi, a dyskinetoplastic form of which — known as Trypanosomaequinum — also causes disease in equids in Central and South America where it is known as ‘mal de Caderas’ or ‘Murrina’. These parasites have adapted to an entirely mechanical, non-cyclical mode of transmission by blood-sucking flies other than tsetse. Trypanosomatheileri is a stercorarian parasite of cattle which deserves greater mention. It was first reported by Theiler in South Africa in 1903, and has since been found to occur in cattle throughout the world. It is transmitted by tabanid flies and is widely regarded as being non-pathogenic, but in certain circumstances it has been associated with disease.
Human sleeping sickness is caused by T.bruceigambiense and T.b.rhodesiense. Whilst these two subspecies do infect some domestic and wild animals, there are other, more significant pathogens of livestock.
The remarkable alternate adaptations of these extracellular parasites to mammalian and insect hosts are reflected in morphological changes which are readily detectable by light microscopy. Bloodstream forms are trypomastigotes; from the posterior portion of an elongated body, some 8 – 35µm long, arises a flagellum which extends anteriorly, and which is connected to the body by an undulating membrane. Beyond the anterior extremity of some species, the flagellum may extend free of attachment to the undulating membrane. The beating of the flagellum pulls the trypanosome forwards, imparting characteristic motility. Within the cell, in a posterior position and at the base of the flagellum, a kinetoplast is found, and a single nucleus is located almost halfway along the body. In the tsetse fly, trypomastigotes transform to epimastigotes in which the kinetoplast has migrated anteriorly, to a position adjacent to the nucleus. Differences in the morphology of the trypomastigote stages of the various species form the basis for differential diagnosis. The major characteristics are clearly seen in thin blood smears, stained with Giemsa's, Leishman's or other Romanovsky stains.
Trypanosomes in thin blood smears, x1 000 stained with Diff-Quick. a = Trypanosomacongolense: note absence of free flagellum; b = Trypanosomavivax : note long free flagellum and large kinetoplast; c = Trypanosomabrucei: note polymorphism, prominent undulating membrane and free flagellum; d = Trypanosomabrucei dividing by longitudinal binary fission. (Unpublished photomicrographs by courtesy of Dr L. Logan-Henfrey, International Laboratory for Research on Animal Diseases, PO Box 30709, Nairobi, Kenya)
Electron microphotograph of Trypanosomacongolense: cross-section showing flagellum (F), nucleus (N), mitochondrion (M) and variable surface glycoprotein coat (VSG), x86 000. Bar represents 0,2μm. (Unpublished electron micrograph by courtesy of Dr P. Webster, Yale University School of Medicine, Department of Cell Biology, New Haven, CT)
Electron micrograph of Trypanosomabrucei : section through the flagellar pocket (FP) region of the cell. Microtubules are longitudinally sectioned, x44 000. Bar represents 0,4μm. (Unpublished electron micrograph by courtesy of Dr P. Webster, Yale University School of Medicine, Department of Cell Biology, New Haven, CT)
Scanning electron micrograph of an intermediate (bloodstream) form of Trypanosomabrucei from the blood of a mouse. Note the prominent undulating membrane, pointed posterior end and long, free flagellum. A ‘streamer’ or filopodium can also be seen. (By courtesy of Dr P. Gardiner and reprinted by kind permission of VinandNantulya and Parasitology Today )
Trypanosomes show remarkable adaptation. They survive not only in the turbulent blood stream, where they face vigorous immunological assault, but they also withstand the digestive enzymes of the tsetse fly's alimentary tract.
Trypanosomes reproduce by longitudinal binary fission, both in the bloodstream and in the fly, although a sexual process can apparently occur in the tsetse fly. Multiplication in each host culminates in the presence of mature trypanosomes, which stop dividing and are pre-adapted to the conditions that they will encounter in the next cyclical host. As a tsetse fly takes its blood meal from an infected host it ingests trypanosomes. Pre-adapted parasites survive in the fly, but trypanosomes that are not metabolically adapted to the new physiological conditions die. The transformation of bloodstream trypanosomes into procyclic or midgut forms within the fly’s midgut is a crucial first step in the establishment of a trypanosomal infection. The mechanism of maturation of a midgut infection is complex and, once established does not always progress to maturation. Before the infection is mature, procyclic forms transform into epimastigote and then to metacyclic forms. From the midgut, trypanosomes migrate to the mouthparts or salivary glands.
As the infective tsetse fly feeds, metacyclic trypanosomes and saliva pass through the hypopharynx and are inoculated intradermally; it is here that infection is established. From the skin, the trypanosomes reach the blood via the draining lymphatics within a few days. Trypanosomes multiply in the bloodstream, and although initially their low numbers make detection difficult, the generation time of only a few hours soon leads to high levels of parasitaemia. Trypanosomes may leave the bloodstream to reach various extravascular sites.
The ability of trypanosomes to establish prolonged infections is attributable to the phenomenon of antigenic variation. Each bloodstream trypanosome is completely clad in a dense surface glycoprotein coat. Within a population of trypanosomes originating from a single infection, almost all bear the same glycoprotein coat and are thus of the same antigen type. As parasitaemia rises, a swift antibody response is elicited against the antigen type exposed on the surface of the bloodstream trypanosomes. These specific antibodies attach to the surface glycoprotein and produce complement-mediated lysis of all trypanosomes of that antigen type. However, before antibodies reach trypanolytic levels, some trypanosomes — as few as one in 100000 — switch off the gene that controls the production of the initial surface glycoprotein and activate a gene that codes for a different protein. Trypanosomes which bear the new surface glycoprotein are of a different antigen type and are not destroyed by antibody against the first antigen type; they survive to produce another parasitaemic wave, which in turn is removed by antibody specific for that antigen type. By this time a third variant has arisen, and, escaping the effect of host antibody, it survives to produce the next parasitaemic peak. This antigenic variation is the result of sequential expression of variable surface glycoproteins (VSGs) which constitute a repertoire of variable antigen types (VATs). Infections arising from a single trypanosome may have a repertoire of more than 100 VATs. Thus shielded from total destruction, trypanosome infections usually run prolonged courses, since each VAT is present for several days before being removed. Although within a parasitaemic peak there is a mixture of a small number of VATs, the sequence of expression of VATs tends to be quite stable in clonally-derived trypanosomes. This imparts immunologically distinct characteristics to a strain of trypanosomes, the distinct strain being called a ‘serodeme’. In the course of successive parasitaemic waves, some trypanosomes stop dividing and transform to the pre-adapted form able to survive in the tsetse.
After ingestion by the tsetse, pre-adapted trypanosomes shed the glycoprotein coat, transform, multiply and finally mature. Infective tsetse then transmits metacyclic trypanosomes to another host. Irrespective of the VAT of the bloodstream trypanosomes ingested by a fly, the metacyclic VATs of a serodeme are relatively constant. The antigenic diversity within a species leads to the possibility of animals in a tsetse-infested area being exposed to a large number of antigenically distinct trypanosomes, but although trypanosomes within a species may be antigenically dissimilar, they are morphologically indistinguishable.
The characterization of trypanosomes relied for a long time on comparisons of their morphology, motility, host specificity, tsetse transmissibility and their development within the fly, but more recent characterization methods include isoenzyme typing, analysis of kinetoplast DNA by polyacrilamide gel electrophoresis, pulsed field gradient electrophoresis of chromosomal digests and DNA hybridization.
The sequel to infection with salivarian trypanosomes is not always disease. The outcome is determined by many factors, frequently related to the susceptibility of the host and the pathogenicity of the trypanosome. In the case of wild animals, a natural cycle of trypanosome transmission occurs which is not associated with disease. Similarly, in some breeds of domestic animals, infection with salivarian trypanosomes is tolerated, and host and parasite reach an equilibrium. Disturbance of the equilibrium may precipitate disease a long time after establishment of the infection. Thus, although the tsetse-transmitted trypanosomes are aetiological agents of African trypanosomosis, infection is not always synonymous with disease. The occurrence of T.theileri in healthy cattle throughout the world exemplifies a well-developed host-parasite relationship.
Similar events occur in many wild animals which harbour tsetse-transmitted trypanosomes. Infected animals show no clinical signs, but when they are subjected to the stress of capture, for example, their immunity is reduced, parasitaemia flares up and clinical disease may be precipitated.
The epidemiology of African trypanosomosis is almost entirely dependent on tsetse flies. African trypanosomes are well-adapted parasites of many species of wild animals, and sylvatic cycles of trypanosome transmission occur throughout the 10 million square kilometres infested by this unique vector. The natural hosts of salivarian trypanosomes usually show no clinical signs of infection, host and parasites being in equilibrium. The large numbers of naturally infected wild animal hosts constitute a huge reservoir of trypanosomes. Once infected, tsetses remain so for life and thus they too form a reservoir of infection. Consequently, when domestic animals are introduced into areas in which sylvatic cycles of trypanosome transmission occur, trypanosomosis always emerges as a serious disease. Wild animals are the natural hosts of T. bruceirhodesiense, the aetiological agent of human sleeping sickness in central, eastern and southern Africa. Thus, people living and working in tsetse areas are at risk of contracting the disease, but for animal trypanosomosis to occur it is not always necessary for livestock to enter tsetse-infested areas; tsetse also move.
Changes in land use may also alter the extent of tsetse infestation. The abandonment of cultivation, for various reasons, permits the regrowth of vegetation, which may then provide suitable tsetse habitat. Conversely, the intensive settlement and cultivation seen in some areas destroy tsetse habitat.
The trypanosomal infection rate in tsetse is of prime importance. The ease with which infections develop in tsetse depends upon the fly’s vectorial capacity and specific factors related to the blood of host animals. Generally speaking, the duration of the development of trypanosomes in tsetse increases with increasing complexity of the developmental cycle.
The prevalence of trypanosomal infections in tsetse is also affected by host preference. Two aspects are important in this context: first, there is diversity of host preference among Glossina and, second, different species of hosts vary in their susceptibility to infection with the different species of trypanosomes. Of particular importance is the relationship between absolute tsetse density and biting rate. A good understanding of this relationship is essential when predicting the impact of tsetse control interventions. Usually the challenge increases with the tsetse population density but even at low densities tsetse can still cause a substantial disease problem. This is partly attributed to the often observed, increased frequency with which flies that have metacyclic infections in their mouthparts probe.