Geotargeting

Geographical targeting of interventions is required to ensure programmes are implemented appropriately, focusing resources on areas of greatest need.

Control strategies against worm infections, which focus predominantly on mass drug administration (MDA) with anthelmintic drugs, are most cost-effective when targeted to communities with the highest prevalence of infection and presumed greatest morbidity. The need for mass treatment is therefore determined by infection prevalence within mapping and treatment areas, as assessed by community or school surveys. Deworming is warranted when prevalence of infection exceeds WHO-recommended species specific prevalence thresholds.

  • STH

    For STH infection, the WHO recommends the use of combined STH prevalence – the prevalence of infection with at least one STH species – to decide whether or not to implement targeted mass treatment and, if so, to determine the frequency of treatment.

    The WHO recommendations, as assessed by surveys of stool samples in school-aged children:

    • ≥50% - treatment targeted to all school-aged children twice each year
    • 20-49% - treatment targeted to all school-aged children once each year

    The prevalence of infection is determined by examining 50-100 school-aged children in about five locations in each area targetted for MDA, typically defined as districts. In most countries, these typically represent approximately 200,000 people.

  • Schistosomiasis

    Treatment depends on the prevalence of schistosome infection, as assessed by surveys of stool or urine samples in school-aged children:

    • ≥50% - annual treatment of all school-aged children and high-risk populations
    • 10-49% - treatment once every 2 years of school-aged children and high-risk populations
    • <10% - targeted treatment of all school-aged children twice during primary schooling (at entry and on leaving)

    Definition of the mapping and treatment region for schistosomiasis is dependent upon the focality of infection. For this reason, sampling effort is typically much greater than for other worms. Lot quality assurance sampling (LQAS), which uses small sample sizes to classify communities according to prevalence, is one approach to minimising the time and resources needed to conduct stool surveys for intestinal schistosomiasis. For urogenital, geographical targeting of treatment can be effectively and rapidly achieved through questionnaire-based studies about blood-in-urine administered by teachers to school children.

  • LF

    For treatment of lymphatic filariasis the WHO recommends mass drug administration to the entire population in an endemic geographical region where prevalence exceeds 1%. This is determined using the WHO-recommended mapping strategy, which measures the community prevalence of LF in selected implementation units (IU), the defined mapping and treatment regions. In most countries, these typically represent approximately 200,000 people. 

    An antigen-detection test that is simple, sensitive and specific, called an immunochromatographic test (ICT), is now available and routinely used to estimate prevalence and identify areas requiring mass treatment. The presence of LF antigens in the blood, or antigenemia, is determined by examining 50 -100 individuals over the age of 15 in two locations in each IU using a rapid diagnostic test, immunochromatographic card (ICT).  If the prevalence exceeds 1% in at least one of the pre-selected locations, the entire IU is considered endemic to LF and will require a minimum five rounds of MDA.