Treatment needs are defined by mapping which provides an estimate of prevalence in at least two areas considered to be at higher risk than other areas in an implementation unit (IU)  to assess whether the prevalence of infection is at a level to sustain transmission. It is not conducted to measure the prevalence of microfilaraemia (Mf) or antigenaemia (Ag). Mapping can be done either by reviewing existing information on morbidity due to LF, or by conducting a mapping survey. The results are used to classify the IU as endemic (≥ 1% prevalence of Mf or Ag) or non-endemic.  

Following completion of the recommended number of MDA rounds, WHO recommends a transmission assessment survey (TAS) to determine that infection is reduced below the target threshold and MDA can stop. Once MDA has stopped, TAS is used as a surveillance tool to determine that infection levels are sustained below target thresholds.

Of the 73 countries considered endemic at the start of 2015, MDA was no longer required in 18 countries where post-MDA surveillance has been ongoing, and the remaining 55 countries were considered to require MDA. At least one round of MDA has been implemented in every endemic implementation unit (IU, the admin- istration unit at which the programme is implemented) now in 25 countries (13 in 2015), i.e. achieved 100% geographical coverage. An additional 20 countries are implementing MDA but have not yet reached all endemic IUs. Among 10 countries that have not started MDA, 1 country was determined not to require MDA, and 3 countries still need to con rm their requirement for MDA.