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Trypanosoma evansi T. evansi is a flagellated haemoprotozoa that causes a devastating disease called surra. It is the most prevalent pathogenic trypanosome throughout the tropical and subtropical areas of the world, owing to its ability of mechanical transmission by biting flies (Hoare, 1972). T. evansi have a large diversity of mammalian hosts, including wild life animals. The disease is endemic in all Southeast Asian countries, including Malaysia, and causes a significant constraint to livestock development.
Numerous diagnostic methods for T. evansi are direct methods either to detect parasite antigen and DNA by serological and molecular techniques or active parasitaemia by direct parasitological examination of blood or buffy-coat and/or inoculation of the blood into rodents (Paris, 1982, Nantulya, 1990 Desquesnes and Dávila, 2002). The indirect methods concentrate on detection of anti-T. evansi antibodies using serological techniques (Claes et al., 2005b). However, the sensitivity of the parasitological techniques is often limited due to low parasitaemia in the chronically infected host (Nantulya, 1990). In addition, serological techniques for antibody detection cannot differentiate between past and current infection. Polymerase chain reaction (PCR) introduced recently is considered to be the most sensitive and reliable technique in detection of T. evansi, especially in the chronic and early infection (Desquesnes and Dávila, 2002).
Among horses, T. evansi causes disease characterized by anaemia, body weight loss, abortion and high mortality (Losos, 1980, Silva et al., 1995a). However, the severity of the disease depends on the T. evansi strains existing in the different geographical locations, and there is an assumption of enzootic stability in Latin America in horses (Herrera et al., 2004). According to Luckins (1988), T. evansi found in the Southeast Asian countries is highly virulent and cause economic losses not only in horses but also other animal hosts.
Like many other diseases, surra is a multifactorial disease, and its occurrence is exaggerated by some predisposing risk factors. A major concern is the introduction or importation of naïve hosts into endemic areas as observed in Indonesia and Canary Island (Luckins, 1988, Gutierrez et al., 2000). Within endemic areas, certain factors may influence the prevalence in animals at risk such as breed, sex, environmental factors, stress and the abundance of fly vectors.
Trypanosomes have the capacity for antigenic variation, which is the basis of their ability to escape and evade the host immune response through variant surface glycoprotein (VSG); a mechanism which hinders work in vaccine production (Hutchinson et al., 2007). However, most of the VSG published work described the African trypanosomes such as T. brucei, whereas T. evansi had received little attention in this aspect. VSG repertoire and sequential expression trend are not fully understood in T. evansi populations.
The first case of T. evansi in Peninsular Malaysia was detected in an Australian mare in 1903 (Fraser and Symonds, 1909). Episodes of the infection and outbreak occur sporadically and affected many hosts namely, horses, cattle, buffaloes, pigs and deer (Ng and Vanselow, 1978, Abas Mazni and Zainal-Abidin, 1985, Sani et al., 1995, Arunasalam et al., 1995, Adrian et al., 2010). Malaysia is free from other forms of trypanosomosis. Naturally infected horses and ponies were used to study the dynamics of T. evansi infection (Ng and Vanselow, 1978, Ikede et al., 1983). Since then, there have been no other studies on trypanasomiasis in horses in Malaysia. Anecdotal evidence suggests that local ponies in Malaysia are resistant to T. evansi, however this remains to be proven.
With the increasing interest in equestrian activities, many horses were imported from non-endemic trypanosome countries such as Australia, New Zealand, United Kingdom and America (Bashir, 1993).
Therefore, it is vital that the local situation on surra in horses in Malaysia is described. In addition, the risk factors associated with the disease prevalence have not been elucidated and the dynamics of T. evansi in local ponies which is a potential source for cross breeding with exogenous horses have not been examined. The current study was conducted to generate more information to address these gaps in knowledge for potential use in disease management programs.
Aim of the Research
The aims of this study are; first, to determine the prevalence of T. evansi and its associated risk factors in horses in West Malaysia using different diagnostic techniques, and second to study the dynamics of T. evansi infection in the Malaysian local ponies with emphasis on VSG repertoire and its trend of expression.
1.2 Research Hypothesis
The hypotheses of the current research are:
T. evansi prevalence in horses is low and PCR technique serves as a sensitive tool to detect the disease in endemic areas.
Several risk factors are important in determining the occurrence of the pathogen in horses.
Local ponies are resistant to surra due to the absence of clinical signs and changeless of hematological and biochemical parameters.
The VSG repertoire of T. evansi is small in a biological system such as horses and particular VSGs are express in the early infection.
1.3 Specific Objectives
The specific objectives of the study are:
To determine the prevalence of T. evansi in horses in West Malaysia using parasitological techniques and PCR.
To estimate the seroprevalence of T. evansi infection in horses and determine its association with a few risk factors.
To study the dynamics of T. evansi infection in Malaysian local ponies.
To assess the T. evansi VSGs repertoire and its trend of expression in infected local ponies.