Journey toward Lymphatic Filariasis Elimination in Sierra Leone
December 11th, 2013
Trying to eliminate neglected tropical diseases (NTD) like lymphatic filariasis (LF)–also known as elephantiasis or big foot–in a country is like trying to permanently rid your garden of invasive plants and weeds. It may seem like a never-ending war that takes not just a long-term commitment and plenty of perseverance and effort, but also constant vigilance and sometimes even a bit of backtracking. This has certainly been the case in Sierra Leone.
Sierra Leone’s LF story
When Sierra Leone declared war on LF in 2007, the first step was to plan its attack on the disease. This meant conducting surveys in each of the country’s 14 health districts, to see where it most needed to focus its treatment efforts. Unfortunately, the survey results confirmed the worst fears of the leaders of the country’s Neglected Tropical Diseases Program (NTDP)–all 14 health districts were endemic for LF and had to be treated for the disease.
Dismayed yet not daunted, the staff at the NTDP began the long nation-wide LF treatment process in 2008. Since 12 of the country’s 14 health districts were co-endemic for both LF and onchocerciasis (a.k.a. river blindness), an NTD that also responds to ivermectin (which is used in conjunction with albendazole to treat LF), NTDP officials first began providing annual mass drug administration (MDA) treatments in 2008 while MDA treatments was started in the other two districts in 2010. In a country with limited healthcare resources, delivering integrated treatments and tracking the diseases together was a wise strategy that reduced costs as well as the amount of time district health workers had to spend away from their health centers.
Sierra Leone’s NTDP achieved a major milestone in August 2013, when all 12 of the health districts that had started MDA in 2008, finished delivering the required five annual MDA treatments in all their at-risk communities. Now it was time for the program’s next big test–making the shift from giving people drugs each year, to testing to see whether those drugs had actually worked.
Shifting Gears to Gauge Progress
After many studies and in consultation with international experts, the World Health Organization (WHO) developed guidelines on monitoring the impact of MDA treatments on disease levels, including when and how to stop treatments and obtain certification for elimination. According to these guidelines endemic countries must conduct a special survey called a transmission assessment survey (TAS) after 6 annual rounds of MDA to determine if treatments for LF can be stopped without fear of the disease coming back. After MDA is stopped and before WHO will certify a country as free of LF, the NTDP must continue doing post-MDA surveillance until it has conducted two additional TAS. The first of these post-MDA TAS is done 2-3 years after the initial TAS, and the second, after yet another 2-3 years have passed (i.e. 4-6 years after the initial TAS).
The purpose of these two additional TAS is to be absolutely sure that the disease is actually gone. If countries pass the post-MDA TAS twice during 4-6 years when treatment is not being delivered, then it is safe to say that the drugs have done their job and the disease really is gone for good. Countries that pass both post-MDA TAS have proven they have successfully treated LF, meaning that they’re ready to apply to the WHO for formal certification of nation-wide LF elimination.
The methodology and pass/fail criteria are the same for both the TAS conducted during MDA and the TAS conducted after it is stopped. Nonetheless, conducting TAS is a complex process that requires highly trained staff and a huge time investment, resulting in considerable costs. Therefore, WHO has determined that countries should not go that route until they know they’re truly ready and their chances of passing the TAS are very good. WHO suggests that countries should conduct what’s known as a pre-TAS a year before embarking on the TAS. The pre-TAS is conducted in districts that have had at least 5 annual MDA treatments for LF, and if they pass the pre-TAS then they become eligible for the TAS. This is the stage that the NTDP in Sierra Leone has reached in the 12 districts that have completed 5 annual rounds of MDA treatments.
Based on their populations, the 12 districts that are now eligible for the pre-TAS were put into six groups of two districts each, according to geographical proximity and epidemiological characteristics, so that the total population for each group was about a million. For each of the six groups, one sentinel site was selected in one district, and a spot check site was selected in the other district, in consultation with the health authorities of each district.
According to WHO guidelines, one sentinel site and one spot check site have to be selected for a population of one million people for the pre-TAS. The sentinel site is a village randomly selected for monitoring impact of treatment before MDA treatment is started, and the spot check site is a village selected in areas that have been identified as the most likely to still have cases of LF. Unlike sentinel sites, which are maintained throughout the MDA treatment program without change, spot check sites are selected before impact assessment surveys such as the pre-TAS, and do change as new ones are selected for each pre-TAS.
Rehearsing for the Final Test
Pre-TAS is not your typical blood test. It uses a specific methodology, called “thick blood film,” which must be done around midnight (between 10 pm and 2 am) and which requires special training for the lab technicians that will prepare, dry, stain, and read the slides. Health workers have to wait six months after a district has had its 5th MDA before doing the test; and they must collect blood samples from at least 300 people from each of the selected sentinel and spot check sites.
The pre-TAS blood draws can only be done at night because that’s when the disease-causing microorganisms comes out of their hiding spots, deep in the body’s tissues, and move into the peripheral blood of their victims. This is the blood that the health workers collect via finger pricks during pre-TAS blood draws.
Once at least 300 people have provided blood samples in each of the villages selected for the survey (sentinel and spot check sites), lab technicians prepare, dry and stain the thick blood films on slides. These are taken back to the lab and read, and the test results are reported to the NTDP. If the pre-TAS results from both the sentinel and spot check sites show that LF prevalence is below 1% (i.e. less than 1 percent of those tested have the microorganism in their blood), then the health district becomes eligible for TAS the following year. In the case of Sierra Leone, districts in a group (as described above) are determined to have passed the pre-TAS if both the sentinel and spot check site selected for that group have a pre-TAS result below 1%.
Sierra Leone’s Pre-TAS Experience
Back in Sierra Leone, the NTDP had already started preparing for its own pre-TAS, having used WHO guidelines to select six sentinel sites and six spot check sites for the 12 districts being tested, and having formed three teams of four technicians each. Each team was assigned at least two technicians that had participated in similar surveys in the past and who had experience in using the pre-TAS methodology.
The day before the teams went out to the field, the less experienced technicians were trained in the pre-TAS methodology and given the opportunity to practice under the supervision of the more experienced technicians. The next day–August 13, 2013–the three teams gathered the materials and equipment they needed for the survey and left for the field.
Since the teams need the villagers to stay awake until the wee hours before they could draw the blood samples, they knew that they’d need to do more than just bring medical equipment and materials. They also needed to entertain the villagers. So, teams traveled with televisions and video players in order to motivate community members to stay up late enough to participate in the survey.
After collecting blood samples from at least 300 people, ages five and older, the teams followed WHO guidelines to prepare the thick blood films, which they then dried and stained. After returning to the NTDP lab in Makeni, the same 12 technicians read the slides they’d prepared in the field using compound microscopes and recorded the pre-TAS results, which were later validated by an independent quality control (QC) expert.
The QC expert reviews all the slides that come back positive for LF, plus 10% of the negative slides, which are selected randomly. The conclusion of the QC expert must match that of the team in order for the Ministry of Health to accept the results as valid.
And the Results Show…
Fortunately, the results of QC expert agreed with those of the team, and were thus accepted as official. Both came to the conclusion that eight of the 12 health districts passed the pre-TAS, demonstrating LF levels below the WHO-determined standard of less than1% of the population surveyed testing positive for the disease. These eight health districts were declared eligible for TAS, which the country plans to conduct six months after the MDA that is scheduled in December 2013 – January 2014. As for the four health districts that did not pass, they will be treated with MDA for two more years, after which time the pre-TAS will be repeated.