Before being designated in 1991 by the OIE administrative Commission as an “ad hoc Group on NTTAT” the Group devoted its activities since 1983 to the following specific aims: (i) economic impact of T. evansi infections; (ii) development of diagnostic tests; (iii) studies in variation in pathogenicity of isolates of T. evansi; (iv) variation in sensitivity of trypanocidal drugs; (v) exchange of strains between laboratories; (vi) setting up of new means for controlling T. evansi infections. After several years (1983-1991) the following achievements were reached: (i) collection of many epidemiological data from Africa, Asia and South America; (ii) free distribution of diagnostic kits for testing in the field and appreciation of results obtained; (iii) greater interest in the study of this infection worldwide mainly in the camel and buffalo breeding; (iv) synthesis of two new trypanocidal drugs followed by the development and marketing of one of them (melarsomine) devoted to the treatment of Surra in camels. In 1991 the scope of the Group was extended to the other NTTAT, mainly to T. vivax and T. equiperdum infections due to the persistence of T. vivax in tsetse depopulated areas and in some zones of South America and to the difficulty to differentiate T. evansi and T. equiperdum infections by diagnostic means currently in use. The 1st International Seminar on NTTAT was held in Annecy France, 14-16 October 1992 with participation of research workers from 22 countries (Africa, Asia, Europe, South America). Conclusions and Recommendations were published in Rev. Sci. Techn. OIE, 1993, 12: 273-281. The ad hoc Group continues to focus its attention to the problem of Surra in camels (in Africa a coordinating project is being developed in West and East Africa) to control the disease and to the immunosuppression of other animal species unapparently or chronically T. evansi infected.
In 1995 and 1996 Trypanosoma evansi infection (surra) was reported from the following 11 countries in Asia: Bhutan, India, Indonesia, Iran, Laos, Mongolia, Myanmar, Nepal, Pakistan, Philippines and Thailand. However, it appears that the disease has a much wider distribution in Asia. To improve a nationwide diagnostic capability, it is desirable to develop and widely apply simple and reliable diagnostic methods and field veterinarians should be well trained in correct diagnostic procedures. To backup such efforts both regional and national reference laboratories should be established. Research work should be promoted to investigate the pathogenicity of T. evansi strains in Asia, epidemic factors of T. evansi, the role of wild animals, economic impact of the disease, and the selection of the most cost-effective trypanocidal drugs to be used in Asia. To coordinate such activities, it is essential to establish a regionally coordinated programme on protozal diseases of animals in Asia.
Trypanosoma evansi occurs in a variety of ecological regions in both the Old and the New World and although it affects many different domesticated livestock species, there is a dearth of information on its distribution, prevalence and/or incidence and economic significance throughout its geographical range. Although reproductive wastage has been identified occasionally as a source of economic loss, the impact of T. evansi on productivity is recognized usually only when epidemic outbreaks of disease cause widespread loss. The possibility that economically significant production losses might occur also when less acute forms of disease exist has not been considered. The absence of accurate information distorts the overall picture: only outbreaks of disease with a high mortality tend to be quoted in discussions of the effects of surra. Deficiencies in our knowledge of the epidemiology of T. evansi contribute to this situation. These shortfalls include our knowledge and understanding of the natural history of the disease, the absence of any strategies for effective monitoring and surveillance, and the inability to design and implement cost-effective, appropriate strategies for control of the disease. Furthermore, the inadequate data on the effects of T. evansi infection on milk and meat production, draught power, or reproductive efficiency tend to diminish its importance as a disease, and has a negative influence on the attitude of national government and international funding organizations to implement measures to control the disease or to undertake research. Until it is possible to put in place reliable methods for identifying infected animals, estimating morbidity and assessing the economic impact of surra, this situation will continue to prevail.
Three herds, comprising 112 camels, were sampled and examined for Trypanosoma evansi infection (surra) in Moroto district, north-eastern Uganda. The Micro-Haematocrit Centrifuge Technique (MHCT) was used for parasitological diagnosis, while Suratex® and Enzyme-Linked immunosorbent assay (ELISA) were used for trypanosome antigen and anti-trypanosomal antibody detection, respectively. The haematocrit level of each camel was determined during examination with MHCT. Parasite prevalence ranged from 0% to 47%, while Suratex® showed positivity ranging from 35% to 65%. A high antibody prevalence ranging from 78-100% was recorded by ELISA. Low haematocrit values were associated with parasite or antigen-positive camels. Preliminary characterization of 8 trypanosome isolates from these camels by microscopic examination of Giemsa-stained bloodstream forms, transformation in SM-77 medium and isoenzyme characterization, shows characteristics similar to Tripanosoma evansi, as evidenced from their monomorphism, failure to transform in procyclic medium and zymodeme profiles. These findings show a high prevalence of T. evansi infection in Ugandan camels which is associated with anaemia and calls for the institution of interventional measures to save these useful animals. This is the first report of the existence of T. evansi in Uganda, which could be of considerable consequence in the epidemiology of animal trypanosomosis in the country.
Surra occurs in all 13 regions of the Philippines particularly Regions 2, 3, and 4 in Luzon and Regions 9, 10 and 11 in Mindanao. Based on government reports from 1989 to 1997, carabao (swamp buffalo) is the most commonly affected (3,819 cases), followed by horse (3,430 cases) and cattle (2,005 cases). Mortality rate is highest in horse (345 deaths) and least in cattle (122 deaths). These figures are far from the actual numbers of cases and deaths which have not been properly reported due to misdiagnosis and poor monitoring system.
Clinical manifestations in cattle, carabao and horse infected with Philippine strain of Trypanosoma evansi, are intermittent fever, severe anemia, progressive loss of condition despite fairly good appetite, leg weakness, incoordination, prostration, circling movement, running aimlessly, sudden collapse and death if not treated. Other clinical signs are scrotal swelling and anestrus in ruminants. Abortion and congenital transmission in cattle have been observed. Infected animals may remain negative on blood examination but positive by mouse inoculation test. Considerable economic losses due to surra is estimated to be P44.8 M ($1,149,230.70) in nine years, excluding losses attributable to decreased meat and milk yield, poor reproductive performance, cost of labor and medication.
Although it is not always possible to observe the free-living wild animals closely inside the dense forest as we can not confine them. An attempt was made to keep a close watch on them and a six years epidemiological study of trypanosoniasis (surra) infection was undertaking in free living wild animals, feral cattle and domestic animals in the core and adjacent areas of Ranthambhore National Park (India). This study suggest a great aspect regarding the conservation strategies and research for the control of surra in the 21st century.
It is possible that Trypanosoma evansi was introduced into Indonesia at the end of the 19th century since records of the occurrence or disease date from this time. Trypanosomiasis caused by T. evansi (“surra”) is considered to be one of the most important parasitic diseases of domesticated livestock in lndonesia and occurs in most of the islands, including Irian Jaya. Trypanosoma evansi infections in buffalo and cattle are usually chronic or asymptomatic, and result in production losses, including reduced draught power. However, acute cases leading to sudden death have been reported, and occasional epidemics with a high death rate do occur. Infections in horses, which are the livestock species most susceptible to infection with T. evansi, are usually fatal, but surra in this species appears to be of importance only in Sulawesi. The abundance of biting flies throughout the Indonesian archipelago facilitates transmission of T. evansi, and the extensive movements of livestock over recent years within and between the islands have no doubt assisted in its dissemination. Trypanosoma evansi infects wild animals as well as domesticated species and it is possible, though not proven. That certain mammals, such as wild deer, act as reservoirs of infection.
Control of the disease requires identification of the infected animals, and in the field, parasitological diagnostic techniques involving microscopical examination of wet blood films, together with examination of Giemsa’s stained blood smears (thick or thin) are widely used. A number of serodiagnostic tests have been used for screening animals, including fluorescent antibody tests, card agglutination tests, and enzyme immunoassays to detect trypanosomal antibodies or antigen. These techniques have not been widely applied however, although they have been shown to have a higher sensitivity than parasitological techniques, and have been used in preliminary studies in controlling infection in animals at risk. Serological tests have given a more comprehensive indication of the distribution of surra, the prevalence in different areas, the risk from infection, and the economic impact of the disease. Control of surra has been directed at the treatment of clinical cases or prevention of disease outbreaks by prophylactic treatment of animals at risk. For many years, suramin has been the mainstay of treatment for surra in cattle, buffaloes and horses in Indonesia as it has both a curative and a prophylactic action. Not surprisingly, the long term use of a single drug has elicited the appearance of drug resistant stocks, but there is no indication of the extent of the problem. Diminazene aceturate has been shown effective in control of infections in buffalo in Central Java; isometamidium chloride appears to be less effective, although it has not been widely tested.
Trypanosoma evansi infection in livestock in Thailand has been reported in various kinds of animals for over 80 years. Clinical manifestations observed vary from asymptom to abortion and dead. Development of diagnostic methods parasitologically and serologically reveal more severe economic losses due to trypanosomiasis outbreaks. Whole herd treatment is costly while preventive control measures are not effectively implemented, especially in reservoir herds. Research is being undertaken in the development of efficient method for field diagnosis.
Surra was first detected in mules in Rachaburi province, Thailand in 1949 but it is now known to occur throughout Thailand not only in horses, but also in cattle, buffaloes, pigs, dogs and deer with varying clinical manifestations. The most severe clinical cases usually occurs in infected horses. Abortion at late stages of pregnancy or premature parturition have been reported in infected buffaloes, cattle and pigs. Nervous signs caused by Trypanosoma evansi invading the central nervous system have also been observed in infected cattle and more recently in hog deer.
Prevalence rates for this disease vary between 12.5%, 20% and 4.6% in cattle, buffaloes and pigs, respectively. The economic losses caused by surra are considerable, particularly where they impact upon the small holder farmer. These losses accrue through the effects of disease upon piglet and calf production, loss of draught power in buffalo and the reduction in meat and milk production in infected animals. Although no estimates of these costs has yet been realized, it has been suggested that the cost of treatment with trypanocidal drugs in order to control the disease would be substantial, in the order of US＄7.6 million. More effort is required to determine the extent of the losses in livestock productivity due to trypanosomosis and the benefits of intervention.
The purpose of this paper is to review some the research activities on T. evansi and trypanosomosis in Vietnam from1960-1998.
1.The epidemiology of trypanosomiasis was investigated by parasitological and immunoassay techniques. It was shown that:
-Trypanosomiasis occurred all over North, Central and South Vietnam, with both acute and chronic disease in cattle and buffalo.
-Infection rates in horses were high by mouse inoculation (MI) and haematocrit centrifuge test (HCT). A greater proportion of buffalo were found infected than the cattle by MI and Antigen Detection.
-Enzyme Linked lmmunosorbent Assay (Ag-ELISA). It is possible that buffalo might be the reservoirs of the parasite.
-Abortion was a clinical sign in the pathology of trypanosomiasis in buffalo.
-In North Vietnam, differences in prevalence rates were observed in animals from mountain, intermediate and delta zones, the highest prevalence was seen in animals from intermediate, mountain zone(1990’s) and adverted with the results in 1970’s.
2. One isolate out of six isolates of T. evansi which were collected from six provinces in North and Central Vietnam was resistant to isometamidium chloride (in mice). The results that it is possible that drug resistance occurs in Vietnam and could be more common than is presently known.
3. Immunoassays that are used in Vietnam include the slide agglutination test (SAT), card agglutination test (CATT), indirect fluorescence antibody test (IFAT), enzyme linked immunosorbent assay for detection of antibody (Ab-ELISA) and enzyme linked immunosorbent assay for detection of antigen (Ag-ELISA).
The mainstay for control of Trypanosoma evansi infections (surra) is chemotherapy and chemoprophylaxis. This strategy rests not only on availability of simple, efficacious drugs, but also on diagnosis. The parasitological techniques frequently employed in diagnosis, however, have very low sensitivity since most infections in the field are not associated with patent parasitaemia. An indirect latex agglutination test, Suratex®, for detecting circulating trypanosomal antigens in the blood of infected animals has the potential to circumvent this problem. The test is simple and rapid. It is carried out by mixing equal volumes of serum, plasma or whole blood with the Suratex® reagent on a test card, and the card rocked manually. ln positive reactions, agglutination develops in 2 minutes. Suratex® has been shown to be specific, with a specificity value of 99%, based on studies carried out using sera from horses, camels and cattle from non-endemic areas. The sensitivity of Suratex® is high: 93-97% of blood samples from animals with parasitologically confirmed diagnosis give positive reactions, and most importantly, the test also diagnoses the latent infections which cannot be detected by the parasitological techniques.
A total of 125 imported Sudanese camels were tested for Trypanosoma evansi infection to determine the prevalence of camels with chronic disease that may act as an exotic source of infection. The research employed direct blood smear examination, card agglutination test (CATT/T. evansi) for antibody detection and latex agglutination test (Suratex) for the detection of circulating trypanosomal antigens. Blood smear examination detected patent parasitemia in five camels (4.0%) (two cases were CATT negative). Thirty six camels (28.8%) were CATT positive, while forty five(36.0％) were positive for circulating antigens. Thirty one cases (24.8%) were positive for both tests. Five (4.0%) CATT reactors were negative for trypanosomal antigens. However, 14(11.2％) camels with circulating trypanosomal antigens, including one case with patent parasitemia, tested CATT negative. Samples of these camels, as well as camels with parasitemia were tested for antibodies to common trypanosomal antigens using indirect hemagglutination test (IHA) using T. brucei gambiense antigen. Eleven (78.60%) of the 14 CATT-negative camels had IHA titers ranging from 1/64-1/2048 (including the case with parasitemia) giving concurrence with Suratex results. All five cases with patent parasitemia were positive for IHA. Detection of circulating T. evansi antigens using Suratex was found to be a more sensitive and reliable means of practical diagnosis of camels with chronic latent infection.
The mouse inoculation test (MI) is generally considered the most sensitive method for the detection of T. evansi in blood from infected animals. However, the effect that strain of mouse used may have on the sensitivity of the test is not known. It was investigated in this study.
Forty mice from each of 4 inbred strains (BALB/c, DPJ, CBA/CaH and C57BL/6J）and 2 outbred strains (ARC and Quackenbush) were inoculated in groups of ten, by intraperitoneal injection, with 0.3ml of bovine blood containing 1,000, 125, 25 or 0 T. evansi per ml. Mortality was monitored twice daily and parasitaemia score was recorded every 2-3 days for 21 days. Totals of 96％, 100％ and 95％ of the mice from all strains developed detectable parasitaemia when inoculated with blood containing 1,000, 125 or 25 T. evansi per ml of blood respectively. CBA/CaH mice survived significantly longer (11.6 days) than Quackenbush, BALB/c and C57BL/6J mice (8.8, 8.2 and 8.8 days respectively) inoculated with blood containing 125 T. evansi per ml (p＜0.05). However, there was no measurable difference in duration of survival of mice from different strains when inoculated with blood containing 1,000 or 25 T. evansi per ml.
It was concluded that the choice of mouse strain for use in the MI test is unimportant and that practical considerations such as colour and robustness are more important in deciding on an appropriate strain. Of the six strains evaluated, the BALB/c strain was considered the strain of choice as it was the most robust, is readily available in laboratory colonies and, being white, identifying marks made on the coat with hair dye were easy to visualise.