Pesticide residues and Salmonella contamination in lettuce (Lactuca sativa) sold in Bamako, Mali

Technical Summary: Pesticide Residues and Salmonella Contamination in Lettuce Sold in Bamako, Mali

Problem Statement
Rapid population growth in Bamako, Mali, has driven the expansion of urban and peri-urban agriculture to meet food demands. However, these production systems rely heavily on the intensive use of pesticides and the irrigation of crops with untreated wastewater. These practices pose significant risks to food safety, particularly for leafy vegetables like lettuce (Lactuca sativa), which are frequently consumed raw. While previous studies in West Africa have identified risks associated with either chemical residues or microbiological pathogens individually, there was a lack of recent data simultaneously assessing both chemical and biological contamination in lettuce sold in Bamako. This study addresses the gap by evaluating the prevalence of specific pesticide residues and Salmonella spp. in the city's market supply.

Methodology
The study was conducted between January and February 2026 in Commune I of Bamako, a district covering 35.5 km². Researchers collected 80 composite lettuce samples (approximately 600 g each) from four randomly selected markets: Banconi, Korofina, Djélibougou, and Boulkassoumbougou (20 samples per site).

Each sample was divided into two portions for parallel analysis:

1. Pesticide Residue Analysis: A 300 g portion was processed using the QuEChERS (Quick, Easy, Cheap, Effective, Rugged, and Safe) extraction method, followed by dispersive solid-phase extraction (d-SPE) for purification. Quantification was performed using Gas Chromatography equipped with a micro-electron capture detector (GC-µECD). The study targeted four specific compounds: deltamethrin (a pyrethroid), endosulfan A, endosulfan B, and o,p'-DDT (an organochlorine). Detection limits (LOD) ranged from 0.00167 to 0.00197 mg/kg, and quantification limits (LOQ) ranged from 0.00278 to 0.00328 mg/kg.
2. Microbiological Analysis: A 300 g portion was tested for Salmonella spp. following the standard NF EN ISO 6579:2002 protocol. This involved pre-enrichment in buffered peptone water, selective enrichment in sodium selenite broth, isolation on Salmonella-Shigella (SS) agar, and biochemical confirmation using the API 20E panel.

Key Results

• Pesticide Prevalence: Of the 80 samples analyzed, 55 (68.8%) contained detectable pesticide residues above the limit of quantification (LOQ).

  • Deltamethrin was the most frequently detected pesticide, found in 38 samples (47.5%), with concentrations ranging from 0.07 to 0.45 mg/kg.
  • o,p'-DDT was detected in 22 samples (27.5%), with concentrations up to 0.82 mg/kg.
  • Endosulfan A and Endosulfan B were detected in 15 (18.8%) and 10 (12.5%) samples, respectively.
  • Compliance: While most residues were below Maximum Residue Limits (MRLs), 2.5% of samples exceeded the MRL for deltamethrin, and 5.0% exceeded the MRL for o,p'-DDT. Notably, o,p'-DDT and endosulfans are banned in many countries, yet their presence suggests either long-term environmental persistence or illicit use.
  • Co-contamination: 36.25% of samples contained two or more pesticide residues simultaneously.

• Microbiological Prevalence: Salmonella spp. was isolated from 46 of the 80 samples (57.5%).

  • Spatial Variation: Contamination rates varied significantly by market (p < 0.05). Korofina and Boulkassoumbougou showed the highest prevalence at 65% (13/20 samples each), while Banconi and Djélibougou showed 50% (10/20 samples each).

Key Contributions
This study provides the first integrated assessment of both chemical (pesticide) and biological (Salmonella) hazards in lettuce sold in Commune I of Bamako. By utilizing standardized analytical methods (QuEChERS/GC-µECD and ISO 6579), the research quantifies the extent of dual contamination in a specific urban context. The identification of banned organochlorines (o,p'-DDT and endosulfans) alongside currently used pesticides (deltamethrin) highlights a complex contamination profile involving both historical environmental legacy and potential ongoing illegal practices.

Significance and Claims
The authors claim that the study highlights a significant public health risk posed by the consumption of raw lettuce in Bamako. The simultaneous presence of chemical residues and Salmonella creates a "dual hazard" scenario. The detection of banned pesticides, particularly o,p'-DDT, in 27.5% of samples is described as a "particularly concerning result," suggesting either exceptional environmental persistence or active illicit use. The high prevalence of Salmonella (57.5%) is attributed to poor irrigation water quality and hygiene practices.

The paper concludes that these findings necessitate immediate interventions, including:

1. Strengthening health inspections and residue monitoring.
2. Promoting Good Agricultural Practices (GAPs), specifically regarding safe pesticide use and the composting of manure.
3. Improving the quality of irrigation water to prevent fecal contamination.
4. Educating consumers on proper vegetable washing and disinfection.
5. Establishing an integrated "One Health" surveillance system to monitor chemical and biological contaminants in the food chain.

The study emphasizes that while the population relies on urban agriculture for food security, the current production and distribution practices compromise food safety, requiring coordinated efforts from health authorities, agricultural sectors, and consumers.