Introduction

Human filariases are vector-borne infections caused by nematodes of the family Onchocercidae. Classified by World Health Organization (WHO) as neglected tropical diseases (NTDs), they disproportionately affect rural and socio-economically marginalized communities where access to diagnostics, treatment, and surveillance is limited [1,2]. Although effective and inexpensive drugs exist, these diseases continue to cause chronic morbidity, restricted mobility, social stigma, and significant economic losses in affected households [2].

Four filarial infections are of major relevance in Africa: lymphatic filariasis (LF), onchocerciasis, loiasis, and mansonellosis [2,3]. LF, primarily caused by Wuchereria bancrofti, affects an estimated 70 million people globally, nearly 40% of whom live in Sub-Saharan Africa [3]. Onchocerciasis (Onchocerca volvulus) remains one of the leading infectious causes of preventable blindness; globally, approximately 25 million people are infected, of whom an estimated 300,000 have lost their vision and nearly one million suffer from visual impairment [1,3]. Loiasis (Loa loa), endemic in Central and West Africa, affects more than 10 million people on the continent and poses unique challenges for elimination programs High microfilaremia is associated with severe and sometimes fatal adverse events following ivermectin administration, creating “treatment exclusion zones” that limit mass drug administration (MDA) [3,4]. In contrast mansonellosis caused mainly by Mansonella perstans and M. streptocerca, rarely M. ozzardi — is estimated to affect over 114 million people in Africa, making it the most widespread filarial infection on the continent [3]. Although previously assumed to be clinically mild, emerging data suggest associations with chronic inflammatory symptoms and interference with serological tests, further complicating differential diagnosis in endemic areas [5,6].

In several regions, these pathogens occur simultaneously, sharing vectors, ecological niches, and risk factors [7]. Co-endemicity — especially involving loiasis creates operational and ethical barriers for elimination campaigns [8]. At the same time, mansonellosis often remain invisible to national surveillance systems, reinforcing epidemiological blind spots [6]. As a result, official prevalence estimates may significantly underestimate the true burden of disease.

Historically, diagnosis in endemic countries has relied on microscopy, due to low cost and minimal infrastructure requirements. However, microscopy has limited sensitivity in low-intensity infections and cannot reliably differentiate between species such as M. perstans and L. loa. Over the past decade, serological methods (antigen detection tests for LF and onchocerciasis) and molecular techniques (PCR, LAMP) have increasingly been used in research settings, improving detection accuracy and revealing infections previously missed by routine surveillance [9,10]. These diagnostic shifts fundamentally influence prevalence estimates — declines in microscopy-based studies may not necessarily reflect real reductions in transmission [11].

Despite nearly 25 years of control programs — including the Global Programme to Eliminate Lymphatic Filariasis (GPELF) and the African Programme for Onchocerciasis Control (APOC) — major gaps persist. Surveillance remains fragmented, particularly in areas without laboratory infrastructure [12–14]. Many countries lack capacity for molecular diagnostics, and research output is unevenly distributed, heavily concentrated in a small number of academic institutions [11,15].

Although numerous studies have examined specific pathogens or regions, no synthesis has evaluated how diagnostic practices and research intensity have evolved across Africa and how these changes influence reported prevalence. This creates uncertainty about whether observed epidemiological patterns reflect true reductions in disease burden or methodological artefacts. This systematic review addresses that gap by examining temporal, geographical, and methodological trends in studies reporting prevalence of lymphatic filariasis, onchocerciasis, loiasis and mansonellosis in Africa between 2000 and 2025. By linking prevalence estimates with diagnostic methods and research activity over time, we provide a continent-wide perspective on progress, persistent inequities, and blind spots that continue to hinder filariasis elimination.. Therefore, the objective of this systematic review was to evaluate how diagnostic practices and research intensity have evolved across Africa and how these changes influence reported prevalence of lymphatic filariasis, onchocerciasis, loiasis and mansonellosis.

Methods

Results

Study selection

A total of 10,122 records were retrieved from four databases. After removing 4,368 duplicates, 5,754 records were screened by title and abstract. Of these, 296 full-text articles were reviewed for eligibility, and 180 met the inclusion criteria and were included in the final synthesis (Fig 1).

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Fig 1. PRISMA flow diagram illustrating the process of identification, screening, eligibility assessment, and inclusion of studies on human filarial infections (2000–2025).

https://doi.org/10.1371/journal.pntd.0014200.g001

Characteristics of included studies

The 180 included studies were published between 2000 and 2025 and covered data from 31 African countries. Most of them focused on local populations (both children and adults; 64.3%), while 15.5% investigated only children and another 14.6% only adults. The remaining studies did not provide detailed information on the age of participants. Across all studies, the total number of participants was 745,568. From the 180 studies, a total of 328 distinct data points were extracted.

LF represented the largest proportion of studies (44.5%), followed by onchocerciasis (34.8%), loiasis (12.2%), and mansonellosis (8.5%). Multi-species studies investigating more than one filarial pathogen accounted for 22.2% of the dataset.

The most frequently represented countries were Cameroon, Nigeria, Gabon, and Equatorial Guinea. In contrast, seven countries had only one or two published studies, and 23 had none. The geographic distribution of studies was highly uneven, with a marked research concentration in West and Central Africa (Fig 2).

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Fig 2. Geographical distribution of original studies on human filarial infections in Africa (2000–2025) that met the inclusion criteria of this review.

Base map layer: Natural Earth (Admin 0 – Countries, 1:10m). Source: https://www.naturalearthdata.com/downloads/10m-cultural-vectors/10m-admin-0-countries/. Terms of use: https://www.naturalearthdata.com/about/terms-of-use/b(public domain).

https://doi.org/10.1371/journal.pntd.0014200.g002

Temporal trends and methodological evolution

Between 2000 and 2009, only 1 study were published annually on average, increasing to 8.5 per year between 2010 and 2019 and 12 per year between 2020 and 2025. This rise reflects both expanded surveillance efforts and greater use of molecular diagnostic techniques in recent years.

Microscopy was the dominant diagnostic method in early years (2000–2010), accounting for 55% of studies. However, from 2018 onward, the use of serological (e.g., ICT, FTS, Ov16 ELISA) and molecular assays (PCR, qPCR) increased, comprising nearly 60% of studies in the most recent period. Temporal trends were examined across all diagnostic modalities. While microscopy and molecular methods provide evidence of active infection, serological assays reflect exposure. We formally tested the effect of diagnostic method on prevalence estimates using meta-regression limited to studies of active infections (microscopy vs. molecular), and no significant differences were observed across species (Table B in S4 Appendix Table 2). Serological results were not included in this comparison because they reflect exposure rather than active infection. Combining these data points allows a comprehensive, long-term view of research activity and observed prevalence patterns, acknowledging that short-term studies often capture only limited snapshots of infection dynamics. Pie charts overlaid on prevalence points indicate the proportion of studies using each diagnostic method in each period, providing context for interpreting trends and methodological evolution.

The following sections present species-specific analyses of original studies that met the inclusion criteria. Each subsection summarizes temporal trends, diagnostic approaches, and prevalence patterns based on eligible data points.

Although random-effects pooled prevalence estimates were computed, the main Results focus on descriptive synthesis, while full meta-analytic outputs (pooled estimates, 95% CI, I²/τ², forest plots) are reported in the S4 Appendix (Table 2; Figs 1-4) due to high heterogeneity across studies

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Fig 3. Temporal trends and diagnostic methods, lymphatic filariasis in Africa (2000–2025).

https://doi.org/10.1371/journal.pntd.0014200.g003

Lymphatic filariasis (Wuchereria bancrofti)

Original studies on LF were conducted in multiple African countries, with the highest research activity recorded between 2011 and 2015. The number of eligible publications declined markedly thereafter. Diagnostic approaches were diverse, with serological tests detecting circulating filarial antigen being the most common, followed by microscopy of blood smears and, less frequently, molecular techniques (conventional PCR, qPCR, nested PCR). Average prevalence values decreased steadily from 2000 to 2025—from approximately 10% in 2000–2005 to below 1% in 2021–2025. Studies were concentrated mainly in Central and Eastern Africa, with the largest number of reports from Nigeria and Tanzania (Figs 3 and 7a).

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Fig 4. Temporal trends and diagnostic methods onchocerciasis in Africa (2000–2025).

https://doi.org/10.1371/journal.pntd.0014200.g004

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Fig 5. Temporal trends and diagnostic methods loiasis in Africa (2000–2025).

https://doi.org/10.1371/journal.pntd.0014200.g005

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Fig 6. Temporal trends and diagnostic methods mansonellosis in Africa (2000–2025).

https://doi.org/10.1371/journal.pntd.0014200.g006

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Fig 7. Geographic distribution of studies on (a) lymphatic filariasis, (b) onchocerciasis, (c) loiasis, (d) mansonellosis in Africa (2000–2025).

Base map layer: Natural Earth (Admin 0 – Countries, 1:10m). Source: https://www.naturalearthdata.com/downloads/10m-cultural-vectors/10m-admin-0-countries/. Terms of use: https://www.naturalearthdata.com/about/terms-of-use/b(public domain).

https://doi.org/10.1371/journal.pntd.0014200.g007

Disscussion

Our analysis revealed a marked geographic imbalance in where filariasis research has been conducted in Africa over the past 25 years. Although ecological conditions favorable to transmission — high humidity, the presence of competent vectors (mainly Anopheles, Culex, and Simulium) and large rural areas with limited sanitation infrastructure — occur across much of the continent [23–25], most published research originates from West, Central, and, to a lesser extent, East Africa. These regions are characterized by persistent vector presence and intense, ongoing transmission of parasitic diseases. Entomological data and vector habitat mapping published by WHO shows that nearly the entire African continent lies within the potential transmission zone for filarial infections, even if active epidemiological surveillance is not conducted everywhere [26]. Northern Africa and large parts of Southern Africa were nearly absent from research outputs. Several factors overlap here. First, public health priorities in these regions are different — noncommunicable diseases and infections with distinct epidemiological profiles dominate [27], reducing the pressure to invest in filariasis diagnostics and surveillance. Second, climatic conditions in the north (semi-arid and Mediterranean zones) are less favorable for stable vector populations, and thus for sustained transmission [26]. Third, profound disparities in research and laboratory infrastructure persist. For decades, West and Central Africa have hosted large international control and elimination programs (APOC, GPELF, later ESPEN), which not only implemented interventions but also generated research datasets. Funded projects often translate into scientific publications [28]. Where programs were absent, data are absent.

Unequal access to research funding and diagnostic laboratories is a major driver of geographic bias — repeatedly highlighted in bibliometric studies on scientific production in the Global South [29–31]. Countries with stronger research capacity — such as Cameroon, Nigeria or Gabon — produce research data far more frequently than countries where laboratory infrastructure is limited to basic clinical diagnostics [32]. In many settings, field data are never published in indexed journals (PubMed, Scopus) or appear only in local journals, frequently in French or Arabic, rendering them invisible to standard literature searches. This represents a form of publication bias — not against negative results, but against geography [33,34]. Importantly, the absence of data does not mean the absence of disease. It means the absence of surveillance. For the communities living in these areas, this translates into silent, undetected transmission without access to treatment. The NTD literature consistently shows that neglected diseases occur where scientific systems are neglected [35]. This underscores the need for targeted capacity building and investment in local surveillance networks to capture the true epidemiological picture.This geographic inequality is also reflected in diagnostic approaches. Across studies, we observed a clear evolution in the diagnostic methods used. According to global recommendations, microscopy remains the primary diagnostic tool for filariasis (night blood smears for W. bancrofti, skin snips for O. volvulus, and peripheral blood films for L. loa and Mansonella spp.). Over the past decade, antigen tests — including ICT and FTS for lymphatic filariasis and Ov16 IgG4 serology for onchocerciasis — have become part of global diagnostic standards, both for clinical diagnosis and monitoring elimination programs [36]. In contrast to microscopy and PCR, which confirm active infection, serological assays reflect immunological exposure; antibodies may persist after infection has resolved [37]. Molecular methods (qPCR, nested PCR, F-RT-PCR) are confirmatory tools used mainly in research or high-sensitivity screening. In routine clinical practice in many African countries, advanced diagnostics remain inaccessible. Laboratories often lack essential equipment, stable electricity, cold chain for reagents, and systems for sample storage. Under such conditions, microscopy becomes the first-line tool — not because it is optimal, but because it is available [38–40]. Our data reflect this reality: between 2000 and 2010, microscopy dominated in more than half of all studies. Serology and PCR began appearing only after 2011, coinciding with increased availability of rapid tests and elimination programs supported by international partners. Despite this, microscopy remained dominant across nearly all filarial species throughout the 25-year period. It is important to recognize that microscopy has diagnostic limitations — morphological similarities among microfilariae and operator-dependent interpretation may lead to misidentification, which in some regions has direct therapeutic implications (e.g., risk of severe adverse reactions to ivermectin in individuals with high L. loa microfilaremia) [36,41]. Meta-analytic comparisons between microscopy and molecular diagnostics show no significant differences in prevalence estimates, confirming that historical microscopy data remain informative for assessing long-term trends. Therefore, in resource-limited settings, microscopy can determine patient care. The combination of epidemiological history, clinical assessment, and basic microscopy enables detection of active infections and decisions regarding treatment referral. In contexts where molecular and serological methods are unavailable, microscopy is not a “lesser method” — it is the backbone that keeps fragile healthcare systems capable of responding to patient needs.

Although entomological studies were not included in the systematic synthesis, available vector surveillance data support the biological plausibility of the observed human prevalence patterns. Entomological research conducted across West, East and Central Africa consistently demonstrates the persistence of competent vectors for filarial transmission, including Anopheles and Culex mosquitoes for lymphatic filariasis [42–44] Simulium spp. for onchocerciasis [45,46], and Chrysops spp. for loiasis [47]. In several endemic regions, vector presence and infection rates remain detectable even in areas where human prevalence has substantially declined following long-term mass drug administration [45,46]. This apparent discordance has been reported particularly for onchocerciasis and lymphatic filariasis and does not necessarily indicate failure of interventions, but rather reflect the different temporal dynamics captured by human and entomological indicators. While human prevalence responds relatively rapidly to repeated MDA, vector populations and their infection rates may persist longer, especially in ecologically favorable settings [48]. Importantly, entomological indicators are rarely integrated into routine surveillance outside well-funded elimination programs, and their availability is even more uneven than that of human prevalence data. As a result, entomological evidence remains fragmented and difficult to synthesize at a continental scale. For this reason, and to preserve methodological coherence, vector-based studies were not included in the systematic analysis, but they remain essential for local risk assessment and for interpreting post-elimination surveillance data.

The adoption of improved diagnostics enabled more accurate monitoring of elimination interventions [38,40]. In the early 2000s, LF was highly endemic across sub-Saharan Africa — in some areas, prevalence exceeded 20–30%, particularly in West and East Africa [49]. Our review demonstrates a clear decline: mean prevalence decreased from >10% in 2000–2005 to <1% after 2020. This decline coincides with the implementation of the GPELF, launched by WHO in 2000— one of the largest public health interventions in the history of neglected tropical diseases. A key strategy was MDA, consisting of annual distribution of albendazole with ivermectin (or diethylcarbamazine outside onchocerciasis-endemic areas). In 2017, WHO introduced the IDA regimen (ivermectin + DEC + albendazole), further improving elimination effectiveness. Monitoring relied on antigen detection (ICT/FTS), allowing assessment of active or recent infection and guiding decisions to stop MDA [50–52]. The program’s impact is tangible: Togo was the first African country to declare LF elimination (2017) [53]; Malawi stopped MDA and entered surveillance [53]; Tanzania achieved >90% reduction in infections [54]; in Nigeria — the country with the largest population at risk — transmission has been interrupted in multiple regions [55]. Our data mirror WHO findings, showing substantial reductions in LF prevalence in many African countries where MDA has been implemented across successive rounds, despite substantial logistical barriers, lack of laboratory infrastructure, shortages of personnel, unreliable power supply, and the need to reach remote communities cut off during rainy seasons. These patterns highlight the importance of operational logistics and collaboration with local communities alongside the use of available diagnostics.

Progress in onchocerciasis elimination has been less linear. While transmission has been substantially reduced, hotspots persist (e.g., Cameroon) [56]. The history of onchocerciasis control illustrates how success depends on logistics and implementation strategies. The Onchocerciasis Control Programme (OCP, 1974–2002) focused on vector control through larviciding, eliminating transmission across many West African countries. The subsequent APOC (1995–2015) adopted community-directed ivermectin treatment (CDTI), enabling treatment in areas without health infrastructure [14,57]. In our review, mean prevalence declined from ~15% at the start of the century to ~5% in 2005–2020. After 2020, prevalence appears to rise again (>10%), likely reflecting diagnostic shifts — increased use of Ov16 IgG4 serology, which detects exposure, not necessarily active infection. Progress appears greatest in settings applying dual strategies — vector control + MDA — such as Uganda and Tanzania [14,54]. Implementation remains difficult where conflict, weak infrastructure, or co-endemic loiasis (risk of severe adverse reactions to ivermectin) limit interventions [41,58].

Loiasis infections, endemic to rainforest regions of West and Central Africa, illustrate the clinical importance of choosing the correct diagnostic method. Peripheral blood microscopy in the appropriate time window (due to diurnal periodicity) remains the reference standard, as it provides microfilarial density — critical before diethylcarbamazine (DEC) treatment [36,59]. PCR offers superior sensitivity at low microfilaremia but cannot quantify parasite load, limiting standalone use where assessing treatment risk is essential [60]. Serology has extensive cross-reactivity (with Mansonella spp., O. vulvulus), reducing specificity in endemic settings [60,61]. After the introduction of serology in 2021, our review did not identify a clear increase in pooled laiosis prevalence; instead, prevalence estimates remained consistently high over time. Nevertheless, shifts in diagnostic approaches may influence observed prevalence patterns and should be interpreted cautiously. Diagnostic errors have real clinical consequences — during MDA campaigns, severe neurologic complications occurred in individuals with unrecognized high microfilaremia treated with ivermectin [41,59].

Mansonellosis (M. perstans, M. streptocerca, rarely M. ozzardi) remain among the most neglected filarial diseases. Although widespread in Africa and parts of South America, they are not classified by WHO as causing significant disability. Without elimination program status, there are no coordinated interventions, no MDA, and epidemiological data remain fragmentary [10]. Diagnosis relies on microscopy, but microfilariae are easily misidentified, especially in mixed infections. Molecular diagnostics clearly differentiate species but are largely restricted to research settings [62]. Clinical manifestations — pruritus, chronic fatigue, musculoskeletal pain, eosinophilia — are nonspecific. For patients, this is an “invisible disease”: it rarely causes dramatic complications but impairs quality of life and results in repeated healthcare visits without diagnostic clarity [10,62]. Treatment options are limited, as ivermectin and albendazole show inconsistent efficacy [10]. Mansonellosis is common but invisible — absent from WHO reports yet affecting daily functioning of affected individuals.

Comparison of our findings with ESPEN data on endemicity status and MDA implementation shows general agreement regarding disease distribution and control levels (Table C in S4 Appendix). However, in some cases, prevalence remained relatively high despite reported full MDA coverage. This may reflect discrepancies between reported geographic coverage and actual drug uptake by the population, as well as the influence of historical infection burden and local transmission. These observations highlight the complexity of elimination processes and the importance of incorporating field survey data when interpreting ESPEN reports [15].Despite elimination progress, the true burden of filarial infections remains poorly documented. Access to diagnostics is limited by distance, transport costs, lack of laboratory facilities, political instability, and stigma. As a result, prevalence is systematically underestimated. The cruel paradox is that the populations most affected by parasitic diseases are the least represented in scientific research. Sustained data collection requires stable funding, infrastructure, and trained personnel — resources often absent in endemic regions [32–40]. Moreover, elimination success without continued surveillance can lead to resurgence. Several countries have documented transmission re-emergence after stopping MDA campaigns [63–65]. Research studies conducted by local institutions are essential — unlike programmatic reports, they are not shaped by administrative targets and may detect early warning signals. Future control efforts must reflect the realities of resource-limited settings: fragile infrastructure, workforce shortages, and chronic underfunding [29,30,32,35]. Implementation of advanced, costly methods across all sites is unrealistic. Instead, central reference laboratories should perform molecular testing on samples collected from peripheral sites. The greatest impact comes from simple, point-of-care antigen and serology tests that require no electricity, no cold chain, and minimal training [38–40]. Integration with existing diagnostics networks for malaria, HIV, and tuberculosis is critical. When blood is already collected — for malaria smears, HIV testing, or on FTA cards during population screening — the same sample can be used for filariasis testing, pending ethical approval. This approach increases the likelihood of detecting silent transmission, especially among asymptomatic carriers who sustain transmission but do not seek care [66,67]. The priority is not technology itself, but ensuring that infections are detected where no one is currently looking — in small clinics, underserved communities, and among children. Strengthening diagnostics requires training, simplified protocols, and improving sample transport — not expensive equipment. Real progress does not require new laboratories; it requires making the most of every resource already available.

An important unresolved question is whether the apparent decline in human prevalence is paralleled by a comparable reduction in vector infection rates. Addressing this would require temporally and geographically matched human and entomological data, which are currently scarce and unevenly distributed across Africa.

Conclusions

The literature review highlights a striking contrast between filarial diseases incorporated into global public health programs and those classified as “non-priority.” Lymphatic filariasis and onchocerciasis benefit from long-term elimination strategies, which have led to gradual and sustained reductions in prevalence. In contrast, Mansonella infections — despite their wide geographic distribution — remain largely invisible to health systems. Epidemiological data are fragmented, diagnostics rely predominantly on microscopy, and effective molecular tools are only rarely implemented. As a consequence, the global picture of disease burden is distorted, and transmission may remain undetected for years.

Across studies, one pattern consistently emerges: the greatest progress occurs in settings where interventions leverage existing health system structures and where long-term engagement of local teams is prioritized. Sustainable progress in filariasis control relies not only on high-tech diagnostics but also on leveraging available resources, training personnel, integrating filarial screening into routine care, and strategically using molecular and serological tools to guide elimination efforts. Flexible use of available resources and integration of diagnostics into routine clinical care appear to be more realistic and sustainable than reliance on costly, technologically demanding solutions.

Supporting information

Acknowledgments

We are sincerely grateful to Professor Małgorzata Paul for her generous guidance, for taking the time to review our manuscript, and for sharing her valuable insights and expertise.