West African human African trypanosomiasis (HAT) is a slowly progressing chronic disease, which is usually fatal if left untreated. In the initial stages of the disease symptoms are usually absent or non-specific.1
By the time patients consult a health professional the disease is often advanced and has spread to the central nervous system. As a result, the patient may require more toxic and expensive treatment. Moreover, the community has been at risk because the patient has been a potential source of infection for the tsetse fly vector for a prolonged period.2
Therefore, the current HAT control strategy in the Democratic Republic of the Congo (DRC) is based on active screening of the population at risk, with the aim to find and treat cases as early as possible. Such screening is carried out by mobile teams that spend several days in each village. On the spot, they screen the population by a card agglutination test for trypanosomiasis (CATT), followed by parasitological confirmation tests for those who are CATT positive.3,4
The total population at risk for HAT in the DRC is estimated at 12.6 million; the reported annual incidence peaked at over 25,000 cases in 1997 and was reduced to just over 8,000 cases by 2006.5
Thirty-five mobile teams are currently active, each able to screen a maximum of 60,000 persons per year. Lutumba and others6
estimated that in each screening round about 40% of existing cases are missed, mainly because of suboptimal participation by the target population and inadequate sensitivity of confirmatory tests.4
Ninety-eight percent of HAT control activities in the DRC is funded by international development aid.7
If HAT control measures are successful, prevalence levels may be reduced to below 0.1%. At such low prevalence rates, continuation of active case finding becomes a problem.4
The population no longer perceives HAT as a threat and participation rates drop; governments and donors start questioning the rationale behind maintaining costly programs that detect so few cases. Integration of HAT control activities into the general health care system is a logical next step.5
However, such integration faces many challenges, mainly because of the lack of a sensitive and specific diagnostic test and the lack of a treatment that is easy to administer and safe. On top of this there is the problem of the unspecific clinical picture in early stages and the diagnosis is therefore often missed by health facilities; as was described for East African HAT by Odiit and others8
in Uganda. If screening activities are abandoned, the disease usually reemerges within a time span that can vary from 3 to 50 years.9–11
Therefore, some form of continuous surveillance is needed in low endemic areas.
A perfect surveillance system of low prevalence areas should be sensitive to detect all outbreaks at an early stage, but at the same time specific, to avoid raising the alarm without necessity. Laveissière and others12
explored the idea of using samples collected on filter paper as a surveillance tool in Côte d'Ivoire. Higher population coverage was achieved by general health workers collecting samples on filter paper over a 2-month period than by a mobile team spending 10 days in the same region; the latter approach being five times more costly.
Whereas Laveissière and others used results of serological tests on filter paper samples to identify individual suspect cases, such approach could, in theory, also be used to identify “suspect villages” in analogy with the system of lot quality assurance sampling (LQAS) used to monitor vaccination coverage and other healthcare programs.13,14
In LQAS, the population is regrouped into lots and a random sample is then taken from each lot. The lot is rejected if the number of defects observed in the sample exceeds a maximum specified limit (or “threshold”). In our case, a village would be a lot and the number of defects in the sample would equate the number of positive results on HAT serological tests. Any LQAS scheme has to identify the sample size to be tested per lot and the “threshold” value within that sample that leads to rejection of the whole lot. For HAT surveillance, the scheme should take into account the probability of false positive serological results, which are more frequent at low prevalence of disease.
Different tests can be performed on blood samples collected on filter paper, such as micro-CATT, LATEX/T.b. gambiense
and enzyme-linked immunosorbent assay (ELISA)/T.b. gambiense
. Micro-CATT is a serological test that uses the same reagent as the CATT test.15
Several studies, although using different test protocols, report on validation of micro-CATT under field conditions.11,14,16–18
Specificity estimates range from 93.7% to 100%; sensitivity was estimated to be 94% by Chappuis and others18
and 94.2% by Noireau and others.17
The LATEX/T.b. gambiense
is a rapid LATEX agglutination test for detection of antibodies in patients infected with Trypanosoma brucei gambiense
. The reagent consists of beads coated with a mixture of three variable surface antigens of bloodstream form trypanosomes.19
Penchenier and others20
evaluated LATEX/T.b. gambiense
as a tool for mass screening in Cameroon and the Central African Republic. Sensitivity was 100%, whereas specificity ranged from 96.1% in the Central African Republic to 97.6% in Cameroon. The ELISA/T.b. gambiense
is an antibody-detection test based on reaction of specific antibodies with purified variable surface glycoproteins from T.b. gambiense
fixed in an ELISA plate.21
During a field evaluation in Sudan, Elrayah and others22
found specificities of 98.5% for ELISA/T.b. gambiense
on plasma and on eluates from filter paper.
The objectives of this study were 1) to compare the diagnostic accuracy of micro-CATT, LATEX/T.b. gambiense, and ELISA/T.b. gambiense on capillary blood samples collected on filter paper under field conditions (by mobile teams); and 2) to assess the potential of a LQAS-based surveillance system for outbreak detection of West African HAT in low prevalence areas.