Revue d’Elevage et de Médecine Vétérinaire des Pays Tropicaux (Mar 2000)
Comparison of three serological tests for field diagnosis of camel surra (<em>Trypanosoma evansi</em> infection) in Chad
Abstract
In many countries, one of the major diseases in camels is the Trypanosoma evansi infection, which is responsible for important economic losses. Field diagnosis is necessary for a rational use of trypanocides. Three field serological diagnostic techniques applicable in the field were tested with positive and negative (parasitological test) sera from eastern Chad: CATT, mercuric chloride test and Suratex®. The collapse of PCV though multifactorial was measured and used as an indicator to increase test sensitivity. The highest sensitivity of CATT (0.88) was obtained with non-diluted sera, doubtful reactions being considered as positive; In this case, however, specificity was as low as 55%. The highest specificity (0.93) was measured with pure or diluted sera, positive results being associated with a PCV less than or equal to 25. The highest combined values of sensitivity/specificity (0.85/0.93) and the highest predictive values were obtained with non-diluted sera, the positive criterion being a positive test associated with PCV less than or equal to 25. With the mercuric chloride test, the highest sensitivity (0.94) was obtained when any flocculation, even very light ones, were considered positive. The highest specificity (0.99) was measured by keeping only very positive reactions associated with PCV less than or equal to 25; In this case, however, sensitivity was only 78%. The highest combined values of sensitivity/specificity (0.90/0.95) and the best predictive values were obtained with the positive criterion being a positive or doubtful test associated with PCV less than or equal to 25. The improvements due to simultaneous interpretation of serological tests and PCV value were still dependent on the absence or low prevalence of other major factors of anemia in the studied population. Sensitivity and specificity of Suratex® test in both tested hypotheses (doubtful reactions considered as positive or negative) were low and did not allow recommending its use. In the studied epidemiological context, the mercuric chloride test associated with PCV gave the highest sensitivity/ specificity rates and predictive values at the lowest cost. It is therefore proposed that this technique be used in the field, but it must be stressed that mercuric chloride is toxic and could be dangerous as a diagnosis test.
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