*3.2. Antimicrobial Residue Studies (ARS)*

Summarized in Table 5, in this category, 16 studies were identified that dealt with antimicrobial residues in animals and the environment between 2000 and 2016. We considered published research involving qualitative and quantitative assessment of antimicrobial residues in tested samples. We observed the geographical spread of the studies in this category was poor and was skewed to the South West of Nigeria, with few studies in the South East, North Central, and North West, and no studies from the North East and South South (Table 5, Figure S3). The test procedures utilized by the researchers in the studies included microbiological assay (MA), immunological assay, and chromatography. Specifically, the Ridascreen chloramphenicol ELISA kits, Premi test kit (version 0505, Gelen contain *Bacillus stearothermophilus*), MA (seeded with *Bacillus subtilis*), MA (seeded with *Bacillus stearothermophilus*), microbial inhibition test (contain *Micrococcus luteus*), liquid chromatography, High Performance Liquid Chromatography (HPLC), four plate agar diffusion test (FPT), antibody-online ELISA kits, and the agar diffusion method was used (Table 5, Figure 5a). The reference drugs used for the measurement of residue in all studies singly or in pairs were penicillin, amoxicillin, oxytetracycline, and chloramphenicol, and some researchers only measured antimicrobial residue without mentioning a specific drug (Table 5, Figure 5a). Using a unified scale developed (Table 2), no study revealed "No residue"; while they all reported different levels of residue (Table 5, Figure 5a,b). Tetracycline demonstrated to be the most researched (Figure 5a), with reports demonstrating about 40% as a "Very high residue" level (Figure 5b). Other antibiotics demonstrated lower "Very high residue" levels, with the exception of amoxicillin as shown in Figure 5a,b (Table 5, Figure 5b).


**Table 5.** Summary evaluation of antimicrobial residue studies.

TET: Tetracycline, CHL: Chloramphenicol, AMX: Amoxicillin, PEN: Penicillin B.S 1: *Bacillus subtilis.* B.S 2: *Bacillus stearothermophilus* AR: Antimicrobial residue.

**Figure 5.** (**a**) Test procedure for each antibiotic tested in antimicrobial residue studies; (**b**) Relative level of antimicrobial residue. Tetracycline: Very high residue (*n* = 3), High (*n* = 1), Low (*n* = 4); Chloramphenicol: Very high (*n* = 1), High (*n* = 1), Low (*n* = 1); Amoxicillin No: (*n* = 1); Penicillin: Very high (*n* = 1), Slight high (*n* = 1), Low (*n* = 1); and Antimicrobial residue generally: Very high (*n* = 1), Slightly high (*n* = 2), Low (*n* = 1).
