There are different steps that a country has to take to eliminate LF, including mapping of endemic implementation units (IUs), using at least five MDAs, and conducting surveys in the LF-endemic districts to decide if MDA can be stopped.11
After cessation of MDAs, WHO recommends to implement a passive surveillance before elimination of LF transmission can be certified, although details on how to implement such a system are not elaborated. The surveillance system described in this paper not only meets this need but does so in a way that is feasible in resource-poor settings, and it can be integrated into existing health infrastructure or the Integrated Disease Surveillance and Response (IDSR) program.12
There are several reasons for a country to implement a sustainable nationwide surveillance system. First, the monitoring and evaluation of the LF status of a country before stopping MDAs is focused only on implementation units identified as endemic during the rapid mapping. Based on the focal nature of LF distribution and the convenience sampling used for mapping, it is possible that one or more small endemic area(s) might be missed, which could jeopardize the successful LF elimination. Second, because certification of elimination is based on the certitude that LF transmission is interrupted in the whole country and not only in the area identified during initial mapping, these surveillance data will play an important role during certification. A third reason is that reemergence of the transmission through frequent and massive cross-border movement (e.g., by nomads) is likely in settings where neighboring countries have ongoing transmission. The fourth and probably most important reason to implement a surveillance system is to ensure timely detection and response to any resurgence of transmission after MDAs are interrupted and discontinued. Because of the limited experience with LF elimination, we cannot be totally sure at this time that bringing the LF prevalence below 1% will indeed lead to interruption of transmission.
Other disease elimination programs, such as poliomyelitis, guinea worm, and measles, implement an ongoing surveillance system after interrupting the transmission cycle to ensure that the disease does not recrudesce.13–16
Those systems are proven to be very efficient in detecting new cases in area considered as disease-free.17
However, in contrast to those diseases that can rely on the presence of visible clinical symptoms to identify new cases, LF programs must rely on laboratory testing. Relatively few W. bancrofti
infections lead to visible clinical signs, and the development of signs and symptoms may not correlate with levels of mf; therefore, there is potential to spread the disease.6,18,19
Additionally, the lag time between infection and onset of symptoms can be 10 years or more, making symptom-based surveillance a poor option for LF.1
The most convenient method of field testing for LF is the ICT card antigen test; several factors, however, make this an impractical test for ongoing LF surveillance. First, the test is relatively expensive (approximately $3 per test), has a limited shelf live, and requires a cold chain.3
Second, ICT cards can result in false positives because of several reasons, including incorrect test performance and/or interpretation of the test, which could create the false impression that the disease is not yet eliminated. The enzyme-linked immunosorbent assay (ELISA) evaluation of positive ICT tests conducted at CDC during stop of transmission surveys in the endemic districts in Togo (2008) revealed that the vast majority were false positive. Detection of mf by nocturnal thick blood smear, however, seems well-suited for passive surveillance in resource-poor settings, where diagnose of malaria is routinely done by microscopic examination of thick blood smears. The main cost of surveillance by this method is that of training laboratory technicians to look for mf as well as malarial parasites when evaluating the films. The main drawback of this method for diagnosing LF is the decreased sensitivity in cases of low mf and the requirement that the blood slides be prepared at night. This requirement can be conveniently met, however, in a hospital or emergency care setting, as described here.
There are several issues that will determine the ultimate usefulness of this system, not only in Togo but for other countries needing to institute such passive post-MDA surveillance. The system was specifically designed to be sustainable in resource-poor countries; however, there was a need for some external funding, mainly for initial training of laboratory technicians. Another issue encountered during implementation of the system included the high level of vigilance required on the part of NPEFL staff to remind and encourage participating laboratories to collect and submit slides in a timely manner. However, the most important vulnerability of the system is the lack of a mechanism to ensure that the blood films being evaluated are being prepared at night. The importance of nocturnal blood collection was stressed during training and supervision, but we were not able to devise a practical control system. At the end of the day, the integrity of the system depends on the ability and willingness of the technicians to make the slides at night.
Conducting surveillance among emergency room or hospitalized patients has two distinct advantages. First, because the patients are in the hospital at night, it allows testing to be done by nocturnal blood smear. Second, we felt that the hospital system would provide the best geographical coverage of the country, which was confirmed by the geographical location of the samples as shown in . Military recruits, university students, and blood donors, mentioned in the WHO guidelines as surveillance population, would not give a representative sample, because they tend to come disproportionately from certain regions of the country, whereas the hospitals are located throughout the country.4
The passive surveillance system presented here possesses many of the attributes desired in a good surveillance system.20
More than 30% of the 3,777 villages in Togo were selected by the system, and this, together with a good correlation with the population density, seems to indicate that the system is geographically representative. More samples were collected from the towns where the laboratories are located, probably because of the facts that those localities have a higher population and are in closer vicinity to the health structure. Because the intent of the system is to identify positive cases and not to create prevalence figures, this does not include a significant bias. The system provides a large sample size at minimal cost. It should be stable and sustainable where minimal external funding can be secured or where the political will exists to make LF surveillance a priority. It is well-integrated into pre-existing national health infrastructure and existing programs for malaria in terms of supplies, training, and possibly, funding. It is a simple, flexible, and acceptable system. The system is not without its limitations; the coverage of the system depends on the number and location of the laboratories. More rural areas are less likely to have a laboratory, and data quality and sensitivity of the system depend on the integrity, competence, and compliance of laboratory technicians in correctly collecting nocturnal blood smears. The fact that only 2 of 8,050 patients sampled in previous endemic and non-endemic districts were positive for microfilaria was consistent with the findings of seven 30-cluster surveys conducted in 2008 in the districts receiving MDA; only 1 of 4,230 children tested in the cluster surveys tested positive by both ICT and ELISA. Further validation of this system, including reassessment of the non-endemic districts, is planned. The strengths and weaknesses of the system described above are summarized in .
Strengths and weaknesses of the LF surveillance system in Togo
Overall, the system described here could be a cost-effective and practical solution to the need for passive LF surveillance after cessation of MDA. Togo is the first country that tried to implement a surveillance system for LF, and the purpose was not only to monitor the LF transmission in Togo but also to present to the international LF committee a new way of thinking about LF. Based on this evaluation, the system in Togo will be fine tuned.