WHO Report on Global Surveillance of Epidemic-prone Infectious Diseases - African trypanosomiasis
Description of the data
The data are provided by national African trypanosomiasis control programmes, and have been collected at special treatment centres, primarily through systematic screening programmes and referrals. Data provided are sometimes supplemented by published reports. Countries do not have an obligation to report cases of African trypanosomiasis to WHO, and therefore there are gaps in the database both in the number of cases reported and the number of people screened. The case reports are based on cases registered for treatment and include:
Strengths and weaknesses of the data
Only cases registered at special treatment centres are reported, and there are very few cases that come to the treatment centre without having either acute clinical disturbance or having been screened first. Therefore, the number of cases reported must be interpreted in light of the number of cases screened.
There are also problems of comparability with the reported data. There is a good deal of variability in the serological tests in time and space. In addition, the definition of cases, in the absence of parasitological confirmation, is difficult and depends on the number of tests used and the thresholds selected. These may change from one location to another.
Because sleeping sickness is such a focal disease, prevalence should refer only to the areas at risk. Aggregation to the national level is misleading, and obscures the problem. It is almost impossible to measure incidence rates of gambiense sleeping sickness, because the variable and long asymptomatic period of the disease makes it impossible to know when infection began with any accuracy. There is little or no information on mortality outside hospitals, since most of the deaths take place in rural areas with poor or non-existing civil registration systems. In particular, mortality in infants is difficult to measure, even with systematic screening, because of the well known systematic underreporting of infant deaths. In addition, it is very difficult to obtain age/sex breakdowns.
It is important to understand the context in which the data are collected, in order to be able to interpret epidemic curves produced from surveillance data. This can be illustrated by examining the reports from Uganda (Fig. 8.1). Between 1962 and 1975 no cases were reported. Increased reporting during 1977 to 1983 reflected an epidemic of rhodesiense sleeping sickness in Busuga (south-eastern Uganda). However the increases shown between 1986 and 1992 corresponded to both the resumption of systematic population screening for gambiense sleeping sickness in the western part of the country and to a resurgence of rhodesiense sleeping sickness in Busuga.
Disease eliminated from Uganda
Epidemic of T.b. rhodesiense Busuga, south-eastern Uganda
of T.b. rhodesiense continuing, plus resumption of systematic screening for T.b.
gambiense in western Uganda
Fig. 8.1 Reported number of cases of African trypanosomiasis in Uganda, 1939-1998