What evidence guides mass-treatment versus targeted therapy for intestinal parasites in low-resource communities?
Executive summary
Mass drug administration (MDA) is the World Health Organization–recommended strategy to reduce morbidity from soil-transmitted helminths (STH) by treating high-risk groups (particularly school-aged children) without individual diagnosis, while targeted therapy or selective treatment is reserved for contexts where testing, surveillance, or clinical care resources make individual diagnosis feasible; models and empirical reviews show MDA is cost‑effective at high prevalence and for interrupting transmission only under specific coverage and compliance conditions [1] [2] [3]. The choice between mass and targeted approaches hinges on baseline prevalence, parasite lifespan and transmission dynamics, program coverage and systematic non‑compliance, safety/acceptability of presumptive regimens, and operational costs and infrastructure [4] [5] [6].
1. Why the debate exists: transmission biology versus program pragmatism
Natural history and transmission models emphasize that worms with long adult lifespans or aggregated mating structures (for example schistosomes) create transmission breakpoints that require high population‑level drug pressure to push R below 1, favouring MDA in endemic communities; mathematical work shows periodic chemotherapy interacts with age‑dependent exposure and density dependence to determine whether community‑wide or age‑targeted campaigns will reduce transmission effectively [3] [4]. By contrast, when morbidity is concentrated in defined subgroups (school‑age children for many STHs) and diagnostic or treatment resources exist, targeted therapy can focus scarce resources on those who benefit most while avoiding unnecessary drug exposure [1] [7].
2. Evidence on effectiveness and cost: mass treatment wins on scale, not always on nuance
Large cost‑effectiveness analyses and program data find community‑level MDA—especially school‑based deworming—offers good value in reducing morbidity and can be cheaper per treatment than individual diagnosis-and-treat strategies in many low‑resource settings; a Stanford synthesis and WHO programmatic reviews document broad benefits in anemia, growth and cognitive proxies in high‑burden settings and highlight economies of scale for wide coverage campaigns [2] [8]. Systematic reviews and meta‑analyses, however, show variable effects on nutrition and development outcomes and caution that mass deworming’s population impacts depend on baseline prevalence thresholds and statistical power of trials, fueling calls for more nuanced targeting where prevalence is low or heterogeneous [8].
3. Compliance and coverage: the Achilles’ heel of elimination goals
Models and empirical research repeatedly identify systematic non‑compliance as the principal obstacle to elimination by repeated MDA; simulations indicate that for some parasites (e.g., Schistosoma mansoni) reaching untreated pockets is more important than targeting habitual non‑compliers for individual therapy, while for others (e.g., Ascaris) improving overall coverage gives larger returns than selective outreach—so program design must prioritize achieving and sustaining high uptake [5].
4. Safety, co‑administration and operational constraints that favour presumptive therapy
Presumptive co‑administration of albendazole, ivermectin and praziquantel is recommended and tolerated in populations previously exposed to MDA and is included in refugee pre‑departure protocols where screening capacity is limited, illustrating that safety profiles and logistical simplicity can make presumptive mass or presumptive pre‑departure regimens preferable in low‑resource, high‑mobility contexts [9] [6]. WHO guidance allows certain anthelmintics for broad age groups during preventive chemotherapy, reinforcing MDA’s operational appeal where diagnosis is impractical [6].
5. When targeted therapy is better: low prevalence, focal transmission, or high diagnostic capacity
Geographically focal infections (for example pockets of Taenia solium or localized schistosomiasis) and settings with good surveillance or laboratory access support geographically targeted interventions or test‑and‑treat strategies, which can be more efficient and reduce drug pressure that might select for resistance; CDC and journal reviews of targeted interventions for specific parasites argue that focused strategies can outperform blanket MDA when prevalence is low or highly heterogeneous [10] [1].
6. Policy trade‑offs and implicit agendas in the evidence
Programmatic recommendations reflect not only biology and economics but donor priorities, drug donation programs, and feasibility imperatives; advocacy for MDA often aligns with scalable donation-based platforms and school health programs, while calls for targeted approaches frequently stress stewardship, individualized care, and resource constraints—readers should note these institutional incentives when interpreting guidance from WHO, NGOs, and academic cost‑effectiveness studies [1] [2].