Evaluation of the sensitivity of Woo test for detection of Trypanosoma vivax

Abstract

The haematocrit centrifuge technique (HCT), or Woo test, is the technique most commonly used for diagnosis of animal trypanosomosis, but its sensitivity is not well defined. Depending on the authors, the sensitivity of the Murray technique (dark ground buffy coat method, or DG/BCM) is higher, equal or lower than that of the Woo test for the detection of Trypanosoma vivax. Most authors have compared particular techniques relatively to other techniques, rather than attempting to measure the sensitivity of the tests in relation to a fixed reference. The relative sensitivity of particular tests appears to vary between technicians. The aim of the present study was to measure the sensitivity of the Woo test for French Guiana T. vivax detection, using blood samples with pre-determined levels of parasitaemia, ranging from 1 to 1767 parasites/ml, that were prepared by mixing infected ovine blood with non infected ovine blood. A simple technique is described for the enumeration of parasites in blood. The mean positivity level of the Woo test in sheep was about 200 ± 110 T. vivax/ml. The sensitivity of the test was 100 % above 700 parasites/ml, about 80 % between 300 and 700, 50 % between 60 and 300, and was negligible below 60 parasites/ml. Parameters are provided to estimate parasitaemia based on the number of parasites observed between slide and cover slip (parasitaemia > 2000) or in the capillary tube (parasitaemia < 2000). Sensitivity of the techniques for detection of active infection could be evaluated in comparison with fixed values such as known parasitaemiae, artificially created, as described here.