Epidemiology of Bovine Trypanosomosis in Ethiopia

Definition of the disease

Bovine Trypanosomosis (Nagana) is a disease complex caused by several species of unicellular protozoan parasites of the phylum Sarcomastigophora, order Kinetoplastida, family Trypanosomatidae and genus Trypanosoma. It is mainly transmitted cyclically by the genus Glossina (Tsetse flies), but also transmitted mechanically by several biting flies like Tabanids, Stomoxys, Haematopota and Chrysops. The disease can affect various species of mammals but, from an economic point of view, tsetse-transmitted trypanosomosis, is particularly important in cattle [10].

Etiology

Trypanosomes are flagellated protozoan parasites that live in the blood and other body fluids of vertebrate hosts. They swim in body fluids by flagellum, boring their way between cells. They generally possess a kinetoplast and undergo cyclical development in an arthropod vector. Their biological adaptations, morphology and pathogenicity are fascinating and are being extensively studied [12]. In Ethiopia there are three important species of trypanosomes are recorded in cattle. These are T. congolense, T. vivax and T. brucei. T. vivax and T. congolense are the main pathogens of cattle. Trypanosomosis outside “tsetse belt” is caused by mechanically biting flies; the main etiological agent of mechanically transmitted trypanosomosis is T.vivax [10]. T. congolense resides in the subgenus Nannomonas, a group of small trypanosomes with medium-sized marginal kinetoplast, lack of free flagella and poorly developed undulating membranes. T. vivax is a member of the subgenus Duttonella, a group of trypanosomes with large terminal kinetoplast, distinct free flagella and inconspicuous undulating membrane, although this organism is considered to be less pathogenic for cattle than T. congolense. This trypanosome readily persists in areas free of tsetse, where it is transmitted mechanically by biting flies or contaminated needles, syringes and surgical instruments. T.b. brucei resides in the subgenus Trypanozoon. T.b.brucei is an extremely polymorphic trypanosome occurring as short, stumpy organisms without flagella, long slender organisms with distinct flagella and intermediate forms that are usually flagellated [4].

Life cycle

The life cycle of trypanosome in tsetse involves cyclical development for varying length of time, depending on species and ambient temperatures. Most tsetse-transmission begins when blood from a trypanosome infected animal is ingested by the tsetse fly. The trypanosome loses its surface coat, multiplies in the fly, then reacquires a surface coat and becomes infective. Trypanosoma brucei species migrate from the gut to the proventriculus to the pharynx and eventually to the salivary glands; the cycle for T. congolense stops at the hypo pharynx and the salivary glands are not invaded; the entire cycle for T.vivax occurs in the proboscis. The animal-infective form in the tsetse salivary gland is referred to as the metacyclic form. The life cycle in the tsetse may be as short as one week with T. vivax or extend to a few weeks for T. brucei species (Figure 1) [13, 14].

Pathogenesis

Pathogenesis of trypanosomosis in most species is a progressive, but not always fatal disease and the main features are anemia, tissue damage and immunosuppression. Metacyclic trypanosomes are inoculated intradermally as the fly feeds. They multiply at this site provoking a local skin reaction (Chancre), which is most pronounced in a fully susceptible host and may be slight or absent with some strains or species of trypanosomes. Within the chancre, metacyclic parasites change to trypomastigote form, enter the bloodstream directly or through the lymphatics and initiate characteristic intermittent parasitemias. Their behavior thereafter depends largely on the species of trypanosome transmitted and the host [16]. T. vivax usually multiplies rapidly in the blood of cattle and is evenly dispersed throughout the cardiovascular system, whereas T. congolense tends to be aggregated in small blood vessels and capillaries of the heart, brain and skeletal muscle. Both species exert their effect mainly by causing severe anemia and mild to moderate organ damage. The anemia has a complex pathogenesis involving mainly increased erythrophagocytosis, some hemolysis and dyshemopoiesis [17]. Very acute infection with T. vivax in cattle causes parasitemias and disseminated intravascular coagulation (DIC) with hemorrhages. T. brucei and rarely T. vivax have the capability of escaping from capillaries into interstitial tissues and serous cavities where they continue to multiply. The cerebrospinal fluid is often invaded by T. brucei alone or mixed with other species or as a relapse after an apparently successful treatment. Animals chronically infected with any pathogenic trypanosome may develop concurrent and even fatal bacterial, viral and other protozoan infections as a result of immunosuppressant [18, 19].

Distribution of bovine trypanosomosis in Ethiopia

Estimates made decades ago indicates that a total area of 220, 000 km2 is infested with different species of tsetse flies in which case livestock reared in this area are exposed to various levels of trypanosomes risk (Figure 2). More than twelve million livestock populations which are reared in this area are at risk of contracting tsetse-transmitted trypanosomosis in Ethiopia [25].

T.congolense T.brucei T.vivax Figure 2: Distribution of pathogenic trypanosomes in Ethiopia. Source: [3].

Risk Factors

Host related factors: The effect of infection varies with the host in that most wild animal and some domestic establish a balance with the parasite and remain as clinically normal carriers for long periods. Specifically, some breeds of cattle indigenous to Africa can tolerate light to moderate challenge with tsetse flies by limiting the multiplication of trypanosomes in their blood and by apparently warding off the infection, especially T. vivax [26]. It is both genetic and environmental in origin and the level of tolerance varies. Crossbreeds of indigenous Taurine and Zebu animals are also more tolerant than pure breed zebu. However, due to the uncertain genetic makeup of animals within these so- called breeds and crossbreeds, the level of trypanotolerance may also vary with individual animals within a given category and it can be overcome by heavy tsetse challenge, malnutrition, or other stress factors [17]. The Sheko breed classified as a humpless short horn is the only known breed of Taurine type in eastern Africa which exhibit trypanotolerance [27]. The breed is found in the Bench-Maji zone of southern Region in the south-western parts of Ethiopia [15]. Environmental related factors: The density of tsetse population in the area and the level of their contact with the host, will determine the level of infection. This is further influenced by the vectorial capacity of the fly and the availability of its preferred host, which may not be livestock. Trekking of cattle through tsetse-infested vegetation is a risk nomadic farmer’s face from time to time and the risk is even greater where cattle routes converge, for example, at major bridges or watering holes [11]. Agricultural and industrial developments generally lead to a lowering of tsetse density by destroying its habitat, whereas the establishment of game or forest reserves provides large numbers of preferred hosts or a suitable habitat for tsetse, respectively. Herds located near such reserves are therefore at a higher risk [13]. The vector for trypanosomosis, the tsetse fly, requires a habitat that strongly influenced by ecological and climatic features particularly rainfall, soil type, temperature and vegetation type. Fly larvae can die as a result of drying soils. Extreme temperature, particularly above 36 °C and below 10°C also led to adult fly mortality through starvation and water loss via respiration [28]. Cumulative effects of long rainy season or dry season are important in influencing advances and recession in tsetse population [29]. The effect of altitude on tsetse distribution is through its effect on climate, mainly temperature. Different species of tsetse flies require particular vegetation type that would provide an optimal condition for growth and survival. Vegetation is also important that provides shelter for their hosts. All environmental factors that affect the tsetse fly indirectly affect the occurrence of trypanosomosis [29].

Pathogen related factors: In cattle, T. vivax generally produces a higher level of parasitemia than other species. And since, its life cycle in the tsetse is also shorter; T. vivax is more readily transmitted than the others when animals are newly introduced into a tsetse infested area. Higher parasitemias also facilitate mechanical transmission. On the other hand, T. brucei is rarely detectable by direct examination of cattle blood, even though infection can be confirmed through other diagnostic methods [30].

Economic Impact

According to the Food and Agricultural Organization of the United Nations, trypanosomosis is probably the only disease which has profoundly affected the settlement and economic development of a major part of SSA of the approximately 7-10 million km2 of land that are infested by tsetse fly, only20 million cattle are raised. Under different circumstances this land could support more than 140 million cattle and increase meat production by 1.5 million tons [31]. Trypanosomosis threatens 50 million head of cattle in SSA. Every year, trypanosomosis causes about 3 million deaths in cattle while approximately 35 million doses of trypanocidal drugs are administered to enable livestock to survive in tsetse-infested areas. While the economic losses in cattle production alone are in the range of US$1.0-1.2 billion, the indirect impact engendered by the disease on the total agriculture-livestock production is estimated at US$4.5 billion a year. The overall negative impact extends to the access and availability of cultivable areas, changes in land use and exploitation of natural resources, restriction of opportunities for diversification and intensification of agricultural activity. The magnitude of the problem requires a multidisciplinary approach for effectively promoting sustainable agriculture and rural development strategies [32]. The disease directly affect the milk and meat productivity of animals, reduces birth rates, increases the abortion rates as well as mortality rate; all of these affect the herd size and herd composition [33]. Indirect impact of trypanosomosis mostly lies on crop production through the availability and cost of animals that provide traction power [34]. Animal trypanosomosis reduces work efficiency of oxen cultivation, reducing access to animal traction or discourages the introduction of draught animals in to crop farming [35]. Evaluation on effect of trypanosomosis incidence on the productivity of oxen used for traction showed that relative inefficiency in the high risk area was 38% less efficient than oxen in the low risk area [36].

Additional traction capacity allows farmers expand the area that they cultivate, increase yield of existing crops; grow different mix of crops or allocated labor land and fertilizer more efficiently. In other study discussed the economic benefits from intervening against bovine trypanosomosis. These authors reported significant benefits especially for Ethiopia, because of its very high livestock densities and the importance of animal traction. The estimated maximum benefit per square kilometer of tsetse infested area is US$ 10,000. Consequently, the total maximum benefits from dealing with bovine trypanosomosis in Ethiopia could be as much as US$ 1billion [37].

Zoonotic Impact

The animal pathogens do not infect humans, but animals can serve as reservoirs of T. brucei rhodesiense and T. brucei gambiense, the causes of human sleeping sickness, which are morphologically indistinguishable from T. brucei brucei. Human infections result from tsetse bites, generally in game parks, forest reserves and along streams or other rural setting [38].

Major Clinical Findings in Bovine Hosts

There are no pathgnomonic signs that would help in pinpointing a diagnosis. The general clinical sign are there but variations determined by the level of tsetse challenge, the species and strain of the trypanosome, the breed and management of the host. Acute episodes last for a few days to a few weeks from which the animal dies or lapses in to a sub-acute to chronic stage, or the illness may be chronic from the beginning. Chronic cases may run a steady course, may be interrupted by periodic incidents of severe illness, or undergo spontaneous recovery [19]. The basic clinical syndrome appears after an incubation period of 8-20 days. There is fever, which is likely to be intermittent and to last for a long period. Affected animals are dull, anorexia and apathetic, have a watery ocular discharge and lose condition. Superficial lymph nodes become visibly swollen, mucous membranes are pale, diarrhea occasionally occurs and some animals have edema of the throat and underline. Estrus cycles become irregular, pregnant animals may abort and semen quality progressively deteriorates. The animal becomes very emaciated, cachectic and dies within 2-4 months or longer. Thin, rough-coated, anemic, lethargic cattle with generalized lymph node enlargement are said to have 'fly-struck ‘appearance. Furthermore, intercurrent bacterial, viral, or other parasitic infections may mask or complicate the basic clinical syndrome [39].

Diagnosis

Diagnosis of Trypanosomosis in tsetse, humans or domestic livestock is a basic requirement for epidemiological studies as well as for planning and implementing chemotherapy and for monitoring vector control operations. Accurate diagnosis of trypanosome infection in livestock is required for a proper appreciation of the disease in any geographical locality. Besides clinical diagnosis, parasitological, serological and molecular methods with varying degrees of sensitivity and specificity are available for the diagnosis of trypanosomosis [40].

Clinical Diagnosis

Severity of disease varies with species and age of the animal infected and the species of trypanosome involved. T. congolense and T. vivax are highly pathogenic for cattle and T. brucei infections are generally regarded as being low pathogenicity. The primary clinical signs are intermittent fever, anaemia and weight loss. Cattle usually have a chronic course with high mortality, especially if there is poor nutrition or other stress factors [41]. Clinical diagnosis is found to have a good sensitivity (78%) but a low specificity (27%) when compared to parasitological tests [42]. Under field conditions, in the absence of simple and portable diagnostic tools or access to laboratory facilities, veterinarians could rely on clinical sings and direct parasitological diagnosis to screen and treat cases of bovine trypanosomosis presented by farmers [42].

Parasitological Diagnosis

These are made by placing a drop of blood on a microscope slide and covering with a cover-slip. The blood is examined microscopically using an x40 objective lens. Approximately 50-100 fields are examined. Trypanosomes can be recognized by their movement among the RBC. The method is simple, inexpensive and gives immediate results. Depending on the trypanosome size and movements, a presumptive diagnosis can be made of the trypanosome species [43]. Final confirmation of the species is made by the examination of the stained preparation. The diagnostic sensitivity of the method is generally low, but depends on the examiner’s experience and the level of parasitaemia. Sensitivity can be improved significantly by lysing the RBCs before examination using a haemolytic agent such as sodium dodecyl sulfate [44]. Thick Blood Smear Technique is simple and relatively inexpensive, but results are delayed because of the staining process. Trypanosomes are easily recognized by their general morphology, but may be damaged during the staining process. This may make it difficult to identify the species [45]. Usually, both a thin and thick smear is made from the same sample. Thick smears contain more blood than thin smears and, hence, have a higher diagnostic sensitivity. Thin smears on the other hand allow trypanosome species identification. Trypanosome species can be identified by the following morphological characteristics (Table 2) [46].

Treatment, Control and Prevention

Trypanosomosis can be treated with trypanocidal drug for therapeutic and prophylactic purpose. A therapeutic drug includes diminazene aceturate, homidium bromide and homidium chloride. Prophylactic drugs for cattle include homidium (bromide, chloride) and isomethamidium [47].

The control of trypanosomosis in enzootic countries involves control of tsetse fly population, prophylactic treatment and good husbandry of animals at risk and use of trypanotolerant animals. Control of tsetse has been successfully attempted, but reinvasion is frequent if the land is not properly utilized. More recent methods involved the use of insecticides applied strategically in the form of ground and aerial spraying over large expanses of land [48]. As tsetse is sensitive to insecticides and no resistance has developed, considerable successes were achieved in some countries. However, spraying insecticides is costly and harmful to the environment. These harmful effects are considerably reduced if the insecticides. Other effective methods involve targets impregnated with insecticides and traps that attract and catch tsetse. These are simple and cheap and can be constructed and maintained by local communities. Furthermore, they do not pollute the environment and are suitable for both small and large- scale fanning [49]. Another method is the sterile insect technique. Since the female tsetse only mates once in a life time, this technique is theoretically able to eradicate a targeted tsetse species in areas where other methods have been used to reduce its density but it is expensive.

Finally, it should be stated that development of the land for agriculture, industries, highways, etc. will effectively destroy the habitat for tsetse flies [19]. Attempts at trypanosomosis control have also been directed to prophylactic dosing with chemicals such as homidium (bromide, chloride) and isomethamidium. Prophylaxis is used along with other methods in areas where there is a heavy tsetse challenge. The prophylactic effect is supplemented by the development of antibodies and the total period of protection may be as long as 5 months. However, it is customary to give four or five treatments per year [50]. The productivity response to this pattern of treatment is good if general husbandry is also adequate. The downside of this approach is that it has reportedly led to drug resistance in many countries. In the absence of a vaccine, control methods must combine reduced exposure to the vectors (large scale tsetse trapping and pour-on applications) with strategic treatment of exposed animals (chemotherapy Chemoprophylaxis) along with use of trypanotolerant animals when feasible [19].

Trypanocidal drug resistance is increasingly reported all over Africa and now present in 21 sub- Saharan countries including Ethiopia [51]. Drug resistance is the heritable loss of sensitivity of a micro- organism to a drug to which it was before sensitive. The exposure of trypanosomes to sub therapeutic concentrations of trypanocidal drugs, the treatment frequency and the degree of drug exposure of the parasite population are important factors influencing the development of drug resistance (Figure 3) [52, 53].