Sensitive poliovirus surveillance remains indispensable to the eradication programme because most poliovirus infections are asymptomatic and the few paralytic cases represent only a small fraction of community transmission. Acute flaccid paralysis surveillance is the principal clinical system for identifying suspected poliomyelitis, while environmental surveillance can detect polioviruses in wastewater before or in the absence of a recognised paralytic case. Global guidance therefore treats the two systems as complementary rather than interchangeable components of a single detection architecture [1,2].

The performance of AFP surveillance is conventionally assessed with the non-polio acute flaccid paralysis rate and stool-specimen adequacy. These headline indicators are supplemented by the timeliness of notification and investigation, specimen transportation, laboratory reporting, completeness and timeliness of routine reports, ward-level silence, vaccination-history documentation, non-polio enterovirus isolation and completion of 60-day follow-up. Environmental surveillance is similarly dependent on an uninterrupted chain that begins with appropriate site selection and scheduled sample collection and extends through specimen integrity, transportation, laboratory processing, sequencing and communication of results [1,2].

Recent global assessments have shown that many priority countries attain the principal surveillance indicators at national level, yet important subnational gaps persist in districts affected by insecurity, population movement, difficult terrain, weak reporting or uneven access to health services [3,4]. Earlier international surveillance reviews reached the same conclusion: aggregate national performance can appear satisfactory even when particular geographical areas remain insufficiently represented [5]. Consequently, state and local-government analyses are necessary for identifying blind spots that are concealed by national averages.

African surveillance evaluations have reported considerable heterogeneity. Manyanga and colleagues found that several East and Southern African countries sustained non-polio AFP and stool-adequacy performance above recommended thresholds, although performance varied over time and between countries [6]. In South Africa, Howard and colleagues identified lower stool adequacy and weaknesses in the sensitivity of the AFP system in selected settings [7]. A Zambian evaluation similarly reported acceptable non-polio AFP rates alongside variable specimen quality and very low documentation of 60-day follow-up in some years [8]. These findings demonstrate that a surveillance system may meet one component of the standard while remaining weak in another.

Nigeria has maintained extensive AFP surveillance infrastructure through health facilities, disease-surveillance officers, laboratories, community informants and partner-supported field networks. Nevertheless, continuing vaccine-derived poliovirus detections have shown that strong state or national indicators do not necessarily exclude local surveillance gaps. Nigerian studies have described persistent compatible cases, incomplete follow-up, delayed reporting, silent wards and weaknesses in reverse-cold-chain and documentation processes despite overall achievement of key certification indicators [9–13]. Environmental surveillance has strengthened the detection of silent transmission, but its sensitivity depends on sewage characteristics, catchment populations, sample frequency and laboratory turnaround time [14,15].

Kano and Katsina States are epidemiologically important because they contain large urban, rural, mobile and hard-to-reach populations and have experienced continuing poliovirus programme activity. However, evidence comparing the performance of AFP and environmental surveillance across all 78 local government areas during the recent 2020–2024 period has been limited. This study therefore assessed the level, geographical variation and temporal change in AFP and environmental poliovirus surveillance performance in Kano and Katsina States, Nigeria.

Materials and Methods

Study design and setting

A retrospective longitudinal ecological design was used. The study covered Kano and Katsina States in Northwest Nigeria from 1 January 2020 to 31 December 2024. Kano has 44 local government areas, while Katsina has 34, giving 78 local government areas in the analytical area. The two states include densely populated urban settlements, rural communities, mobile and pastoral populations, interstate and international movement corridors, and areas in which insecurity can affect access to health services. AFP surveillance operates throughout both states, whereas environmental-surveillance sites serve selected sewage catchments rather than every local government area.

Study population and sampling

The documentary population comprised eligible AFP case records, AFP case-investigation forms, poliovirus laboratory records, 60-day follow-up records, environmental-surveillance sampling records and environmental laboratory results generated during the study period. A total-population approach was adopted: every record meeting the eligibility criteria was included, and no sample-size calculation was required. Records were eligible when the state, local government area and reporting period could be established and the variables required for the relevant surveillance indicator were available. Duplicate records and records outside Kano or Katsina States or outside the study period were excluded. Where duplicate entries referred to the same event, the earliest complete record was retained and later records were used only to update final classifications or follow-up information.

Data sources and collection

Data were extracted from national and state AFP databases, AFP case-investigation forms, poliovirus laboratory records, environmental-surveillance databases and environmental-site sampling records. The extraction tool captured geographical and temporal identifiers; AFP classification; dates of notification, investigation, stool collection, shipment and laboratory reporting; specimen condition; vaccination history; 60-day follow-up; environmental site and sample identifiers; planned and completed collections; enterovirus recovery; sequencing; and final laboratory classification. AFP records were linked to laboratory records with approved epidemiological identifiers, while environmental records were linked by sample number and site code.

Data quality procedures included range and consistency checks, review of duplicate identifiers, reconciliation of conflicting dates and complete verification of confirmed poliovirus classifications and positive environmental detections. At least 10% of the remaining extracted records were checked against their source documents. Missing information was distinguished from a true zero, a non-applicable indicator and the absence of an environmental site.

Surveillance indicators

The non-polio AFP rate was calculated as the number of AFP cases among children younger than 15 years finally classified as non-polio divided by the estimated population younger than 15 years, multiplied by 100,000. The applicable annual surveillance standard was at least two cases per 100,000 children younger than 15 years, or at least three per 100,000 in outbreak-affected local government areas and periods. Stool adequacy represented the proportion of reported AFP cases from which two specimens were collected at least 24 hours apart within 14 days of paralysis onset and received in good condition. A value of at least 80% was considered satisfactory [1].

Additional AFP indicators were notification within seven days, investigation within 48 hours, timely specimen shipment, availability of final laboratory results within 35 days, reporting completeness, reporting timeliness, silent wards, community detection, completeness of vaccination history, non-polio enterovirus isolation, completeness of case-investigation forms and completion of 60-day follow-up among eligible cases. Residual paralysis, death, loss to follow-up and polio-compatible classification were reported separately.

Environmental-surveillance indicators included the number of expected and collected samples, collection completeness, enterovirus detection, collection timeliness, sample-condition adequacy, shipment timeliness, laboratory-result timeliness and sequencing timeliness. The performance thresholds were at least 50% for enterovirus detection and at least 80% for collection completeness, timeliness, adequate condition, timely shipment, laboratory reporting and sequencing [2].

Statistical analysis

The analytical datasets were processed in SPSS version 27. Frequencies and percentages were used for categorical variables, while annual and five-year means were used to summarise continuous surveillance indicators. Indicators were calculated separately by state, local government area and year; no composite surveillance index was created. Five-year local-government summaries were derived from annual observations, with case counts summed and rates or percentages averaged across available years.

Mean performance in Kano and Katsina was compared using Welch’s independent-samples t test because equality of variances was not assumed. Temporal and state differences in adequate stool specimens were examined with mixed-effects binomial logistic regression containing a random intercept for local government area. Environmental sample collection was analysed with mixed-effects binomial logistic regression containing a random intercept for environmental-surveillance site. The non-polio AFP count was analysed using negative binomial regression with the natural logarithm of the population younger than 15 years as an offset and standard errors clustered by local government area. State and year were entered as fixed effects. Results are presented as odds ratios or incidence-rate ratios with 95% confidence intervals. All tests were two-sided and statistical significance was set at p<0.05.

Ethical considerations

Ethical approval was obtained from the Research Ethics Committee of Nasarawa State University, Keffi. Administrative permission to access programme records was obtained from the relevant federal and state health authorities. The ethics committee waived individual informed consent because the study used existing programme records without direct contact with patients or caregivers. Direct personal identifiers were excluded, record-linkage files were stored separately, and findings were reported only in aggregate form.

Results

Distribution of AFP records

A total of 4,218 eligible AFP records were analysed, with 2,109 records from each state. The annual number increased from 485 in 2020 to 1,693 in 2024, although 2021 recorded a lower total than 2020. The marked increase in recorded AFP investigations in the later years was observed in both states (Table 1).

Table 1. Distribution of eligible acute flaccid paralysis records by state and year, 2020–2024

The non-polio AFP rate exceeded the applicable minimum standard in both states in every study year. Katsina increased from 5.31 per 100,000 children younger than 15 years in 2020 to 17.28 in 2024, while Kano increased from 3.67 to 10.64 per 100,000 over the same period. The number of local government areas meeting the annual standard improved from 33 of 34 to all 34 in Katsina and from 37 of 44 to all 44 in Kano.

Stool adequacy remained above 96% in every state-year observation. Investigation within 48 hours also remained above 98%. Notification within seven days improved in Katsina from 69.79% in 2020 to 83.66% in 2024, whereas Kano recorded 100% notification timeliness in each year of the analysed dataset. Reporting completeness and reporting timeliness improved in both states, and the proportion of silent wards declined substantially. However, specimen shipment, community detection, vaccination-history completeness, non-polio enterovirus isolation and case-investigation-form completeness varied across years (Table 2).

Table 2. Acute flaccid paralysis surveillance performance indicators by state and year

Panel B: reporting, ward coverage, community detection and documentation. NPEV, non-polio enterovirus; CIF, case-investigation form.

Subnational AFP performance

Most local government areas attained the applicable non-polio AFP standard in all five years. In Katsina, Dan Musa, Daura, Mashi, Matazu and Sabuwa attained the standard in four years. In Kano, Bagwai and Fagge attained it in three years, while 11 additional local government areas attained it in four years. Mean stool adequacy exceeded 90% in every local government area. Notification within seven days was consistently 100% across Kano local government areas but ranged from 53.0% in Rimi to 100% in Kankara in Katsina. Reporting completeness exceeded 93% in every local government area, although reporting timeliness, ward silence, community detection, vaccination-history documentation, non-polio enterovirus isolation and case-investigation-form completeness remained heterogeneous (Table 3).

Table 3. Five-year local-government summary of acute flaccid paralysis surveillance performance

Of 52 eligible AFP cases in Katsina, 46 (88.5%) completed 60-day follow-up; Kano completed 136 of 143 eligible follow-ups (95.1%). Most completed follow-ups documented no residual paralysis. Three Katsina cases and three Kano cases had residual paralysis. Three deaths and six polio-compatible classifications were recorded in Kano. Katsina had the lowest annual follow-up completion in 2021, when one of three eligible cases was completed (Table 4).

Across the five years, 659 of 672 expected environmental samples were collected. Collection completeness exceeded 93% in every state-year observation. Enterovirus detection remained at or above the 50% benchmark, and sample-condition adequacy was generally high. In contrast, collection timeliness, shipment timeliness, laboratory-result timeliness and sequencing timeliness were inconsistent. Laboratory-result timeliness was below 80% in most state-year observations and reached 80.21% only in Kano in 2024. Sequencing timeliness in Katsina declined from 100% in 2020 and 2021 to 25% in 2024. No state-year met every environmental-surveillance standard simultaneously (Table 5).

Table 5. Environmental-surveillance performance indicators by state and year

Katsina had a significantly higher mean non-polio AFP rate than Kano (8.68 versus 5.64 per 100,000; p<0.001) and a slightly higher mean stool-adequacy proportion (98.82% versus 97.68%; p=0.008). Kano had significantly higher notification within seven days (100% versus 76.53%; p<0.001). Investigation within 48 hours and AFP reporting completeness did not differ significantly between the states. Differences in environmental collection completeness and enterovirus detection were also not statistically significant (Table 6).

After adjustment for year and repeated observations within local government areas, Kano had lower odds of an adequate stool specimen than Katsina (OR=0.567; 95% CI: 0.433–0.742; p<0.001). The annual change in stool adequacy was not statistically significant. Kano had higher odds of environmental sample collection than Katsina (OR=4.544; 95% CI: 1.744–11.840; p=0.002), while the annual change in sample collection was not significant. The adjusted non-polio AFP rate was lower in Kano (IRR=0.659; 95% CI: 0.631–0.689; p<0.001) and increased by 34.3% for each successive year (IRR=1.343; 95% CI: 1.311–1.375; p<0.001) (Table 6).

Table 6. Comparison and regression analysis of surveillance performance

Panel A: Welch’s independent-samples t-test comparisons.

Panel B: mixed-effects binomial logistic regression for adequate stool specimens and environmental sample collection; negative binomial regression with an under-15 population offset and LGA-clustered standard errors for the non-polio AFP rate. OR, odds ratio; IRR, incidence-rate ratio; CI, confidence interval.