Camel trypanosomosis in Morocco. 2. A pilot disease control trial

A first epidemiological study on camel trypanosomosis was conducted in 1997 and 1998 in five Moroccan Saharan provinces. It revealed two major foci at Merzouga (Tafilalet province) and Zagora (Ouarzazate province) where tour camel seroprevalences were 35.4 and 43.3%, respectively, for each region. The first area was chosen in the present study as a pilot region for a control program based on epidemiological data with the aim of evaluating the efficiency of the trial in terms of seroconversion, incidence and health condition. After finding the endemic focus at Merzouga, four other field studies were organised in this area, at 4-month intervals, in order, firstly, to control the camel reservoir by treating all the camels found in the region during the visit using melarsomine (Cymelarsan, Rh?ne-Mérieux, the only trypanocide registered in Morocco), and, secondly, to collect blood samples for studying temporal changes in seroprevalence as well as in anaemia rate. All the 291 tour camels of the area, that were present in at least one of the field visits were examined and included in the trial. Among them, 162 were treated once, 84 twice and 41 three times. A total of 676 serum samples were collected from these camels. At the end of the trial--1 year later--seroprevalence had dropped from 58% (May 1998) to 19% and the mean Ab-ELISA value had decreased from 0.52 (before treatment) to 0.12 (which is below the threshold level), after the third campaign. The haematocrit of the animals was also slightly improved, indicating that trypanosomosis was a major cause, but it was not the only incriminated factor. A seasonal effect was observed. The overall seroprevalence was reduced between spring 1999 and summer 1999, without treatment, whereas a negative effect was recorded on packed cell volume, more camels being found anaemic in summer than in spring. The application of these strategic treatments was associated with a large reduction in the seroprevalence of trypanosomosis, but there was no complete eradication because of the continual exchange of animals from other foci such that of Zagora. The overall seroconversion after 1 year reached 55% and no post-treatment complication was recorded, whereas the incidence due to new infections approached 4.3%. Moreover, several factors like the unusual way of life of the dromedaries, including stress, excessive work, overcrowding and continual contact with other animals coming from other infected regions contribute to the persistence of the disease.