So far, resistance to one or more of the three trypanocidal drugs used in cattle has been reported in at least 13 countries in sub-Saharan Africa. In addition to the 11 countries (Burkina Faso, Chad, Côte d'Ivoire, Ethiopia, Kenya, Nigeria, Somalia, the Sudan, the United Republic of Tanzania, Uganda, Zimbabwe) reported by Peregrine (1994), the Central African Republic (Finelle and Yvore, 1962) and Zambia (Mubanga and Sinyangwe, 1997) should be included. This is probably an underestimation of the true situation, because in several countries surveys for resistance have not yet been carried out or cases of resistance have not been published. In eight of the 13 countries, multiple resistance has been reported. Most of the currently available information on drug resistance, however, is derived from limited numbers of case reports and does not give any indication of the prevalence of resistance in a region or a country as systematic surveys have not been conducted. There is also considerable variation in the criteria that have been used to diagnose drug resistance. The table summarizes the published reports in which a number of trypanosome isolates have been examined.
Very few authors provide information on the method of sampling (randomized or not). There is an urgent need for surveys in which representative numbers of trypanosome isolates are examined for drug resistance. Such surveys should be taken at random and use agreed methods of diagnosis. This type of survey should provide more reliable data on the true prevalence of drug resistance in regions and countries. In addition, risk analysis should help to identify the factors that influence sensitivity or resistance to trypanocidal drugs.
It is also important to stress that drug resistance is not an "all or nothing" phenomenon and the degree of drug sensitivity and resistance varies considerably between individual trypanosomes. A further factor that can influence drug effectiveness is identified in the interesting observations of Sones and Holmes (1992), Silayo et al. (1992), Burudi et al. (1994) and Mamman et al. (1995a; 1995b), who reported differences in drug sensitivity according to the timing of treatment after infection and the concentration of trypanosomes in the blood.
Surveys for drug resistance in trypanosomes
Country |
Trypanosome species |
No. of isolates |
% of R isolates |
Resist to |
Reference | |
exam. |
resist | |||||
Burkina |
Tc |
12 |
9 |
75 |
I |
Pinder & Authié, 1984 |
Ethiopia |
Tc |
12 |
12 |
100 |
D |
Codjia et al., 1993 |
11 |
92 |
I |
||||
Ethiopia |
Tc |
10 |
10 |
100 |
D, H, I |
Mulugeta et al., 1997 |
Kenya |
Tc |
7 |
2 |
29 |
I |
Gray et al., 1993 |
Kenya/Somalia |
Tv |
7 |
6 |
86 |
I |
Schönefeld, Röttcher & Moloo, 1987 |
|
|
|
3 |
43 |
H |
Ainanshe, Jennings & Holmes, 1992 | |
|
|
|
5 |
71 |
Q |
||
Nigeria |
Tv |
19 |
12 |
63 |
D, H, I |
Ilemobade, 1979 |
Nigeria |
Tb |
12 |
2 |
17 |
D, I |
Kalu, 1995 |
1 |
8 |
I |
||||
Sudan |
Tc, Tv, Tb |
12 |
5 |
42 |
H |
Abdel Gadir et al., 1981 |
Uganda |
Tb |
36 |
1 |
3 |
D, I |
Matovu et al., 1997 |
Zimbabwe |
Tc |
14 |
6 |
43 |
D |
Joshua et al., 1995 |
D = diminazene; H = homidium bromide (ethidium); I = isometamidium
R = resistant; Tc = T. congolense; Tv = T. vivax; Tb = T. brucei.
