As the objective of the programme is to achieve interruption of transmission, a surveillance system for LF has to cover the entire country, not merely LF endemic areas. LF elimination programmes should establish surveillance that will identify foci of transmission. Efforts should be made to integrate filariasis surveillance within an integrated disease surveillance system of the country.
1. Filariasis (microfilaraemia, hydrocoele or elephantiasis) should be a reportable event throughout the country; a monthly report should be sent from health institutions to the district; a quarterly report from district to state or national headquarters would be adequate.
• Include examination for lymphoedema or hydrocoele in population surveys for other diseases or purposes such as leprosy, family planning, school health or active guinea worm, or podoconiosis, case searches.
• Screening for LF should be done during medical examinations of recruits in the uniformed services such as the military, and the police.
• Random testing for filarial antigenaemia by FTS cards should be done among blood donors in non-endemic areas
Any positive report received from a facility in a LF-free area should be investigated to identify whether the report indicates indigenous transmission or an importation from a LF-endemic areas. If the patient has migrated into the area, the health authority of the area of previous residence should be informed so that investigation for endemicity may be carried out in that area.
Sentinel site surveillance
Progress monitoring in endemic areas
Because of the focal nature of LF distribution, cluster sampling techniques are not effective, nor will monitoring the entire population of the programme IUs be feasible. Monitoring of populations in sentinel sites is therefore recommended.
- Select two sentinel surveillance sites for every million population or in each of the programme IUs whichever is operationally feasible. The selected sites should have relatively stable populations.
- The sites may be geographical units which have a population of at least 500, such as a village, ward, block, sub-district in rural areas and municipal wards or localities in urban areas. If the population of these units is normally greater than 500, then a subunit or locality within the village or area may be selected as a sentinel site.
- Sentinel sites can be selected randomly or on the basis of factors that would be expected to increase the difficulty of eliminating transmission, such as areas of high prevalence, with high vector densities, etc. A list of the selected sentinel sites should be recorded and maintained in a monitoring register. Once selected, the two sentinel sites will continue to operate throughout the period of mass drug administration.
Baseline data collection
In the two sentinel sites, before administering the first round of treatment, examine at least 500 individuals for microfilaraemia by night blood smear examination and filarial disease, and calculate the baseline microfilaraemia prevalence, mean microfilaraemia density, lymphoedema and hydrocoele prevalence.
After the completion of each round of drug distribution, in addition to the sentinel sites selected as described above, randomly select two spot-check sites each year (i.e., each year select two different spotcheck sites)
Activities to be undertaken in the sentinel sites and the spotcheck sites after every round of drug distribution include the following and should be undertaken by personnel at least one supervisory level above those who were responsible for drug distribution:
•Monitor drug coverage through interviews carried out in every household of the sentinel site. Calculate a coverage of ingested drugs (See links to detailed WHO documents below).
•Monitor the proportion and nature of adverse reactions by interviewing the entire population in the sentinel site.
•Assess reasons for non-acceptance of the drug by interviewing the entire population.
•Compare coverage in each age group (2-5, 6-14, >14) of the sentinel sites longitudinally over the years and every year with the spot-check site populations.
•Find reasons for inadequate or drop in coverage and take measures to improve coverage through better and focused management.
•Determine whether a pattern can be found in the defaulters, e.g. adult males being missed due to their absence during drug administration.
•Report on findings, with plans for improvement
Before the fourth round of drug distribution and subsequently before every second round (i.e. before the 4th, 6th, 8th..), in addition to the yearly activities listed above:
•Calculate the microfilaraemia prevalence rate and the mean microfilaraemia density. This is done by night blood survey by finger-stick method covering every individual in the sentinel sites and the two spot-check sites selected for that particular round.
•Compare the microfilaraemia prevalence rate and the mean microfilaraemia density in the sentinel sites with the baseline and longitudinally. Compare the rate and density between the sentinel sites and the spot-check sites for format (See links to detailed WHO documents below).
•When the microfilaraemia prevalence in all four sites, i.e., the two sentinel sites and the two randomly chosen spot-check sites, are all less than 1% and a minimum of 5 yearly rounds of mass chemotherapy has been completed, shift to the Lot Quality Assurance Sampling (LQAS) technique as described below.
Regular assessment and evaluation of drug coverage needs to be done immediately after each round of drug administration through:
- Reported coverage: every peripheral reporting unit/drug distributor to report on the drug coverage, number of individuals actually ingested the drug/ total population
- Observed coverage: The reported coverages should be cross-checked by supervisors from higher administrative levels through surveys in sentinel and spot-check sites
Special reviews should be made of sections of the population not being reached by the drug distribution. Which age groups are missed? Which occupational groups are missed? Are women missed more frequently than men? Do urban populations have a lower coverage that rural populations? Are the advocacy messages inadequate?
Transmission Assessment Survey
Effective monitoring and evaluation is important throughout the lifespan of the programme. National elimination programmes must be able to monitor MDA effectively, assess whether infection has been reduced to levels where transmission is assumed to be no longer sustainable and recrudescence is unlikely to occur, and implement adequate surveillance after MDA to reveal whether recrudescence has occurred. The dynamics of transmission will differ by the type of filarial parasites and prevailing mosquito vectors; as will the target threshold below which transmission is assumed no longer to be sustainable even in the absence of MDA. WHO has published a standard methodology called the Transmission Assessment Survey (TAS) to assess whether a series of MDA have successfully reduced the prevalence of infection to levels equal to or below the critical cut-off threshold for the various vector species and complexes, and to decide whether MDA can be stopped.3 Transmission assessment surveys should be a standard component of monitoring and evaluation for elimination programmes. The objectives of a TAS are:
• to provide a simple, robust survey design for documenting that the prevalence of lymphatic filariasis among 6–7 year old children is below a predetermined threshold;
• to provide the evidence base for programme managers that MDA can be stopped; and
• to assure national governments that national programmes have achieved their elimination goal.