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CHAPTER TWO: LITERATURE REVIEW
2.1 Poultry Production
Poultry farming holds a significant position within Uganda’s agricultural sector due to its contribution to improved nutrition and food security, providing a major source of high-quality protein in the form of eggs and meat (FAO, 2020). It also serves as a vital supplementary income source to crop and other livestock ventures, thereby reducing dependence on volatile traditional commodities. Furthermore, the sector shows promise in earning foreign exchange through the export of poultry products to regional markets (MAAIF, 2019).
Culturally, poultry is valued in various social events such as dowries and festivals. Although poultry farming in Uganda is relatively young, the past two decades have seen a rapid rise in the population of exotic breeds such as broilers and layers, driven by increased investment in the livestock sector. However, the growth in demand for poultry products has led to increased use of antibiotics for disease prevention and growth enhancement, raising concerns about the presence of drug residues in poultry products (Clarke, 2004; Gerber et al., 2007).
2.2 Antibiotics
2.2.1 Definition of Antibiotics
Antibiotics are chemical agents that kill or inhibit the growth of microorganisms with minimal harm to the host (Guardabassi, 2008). They may occur naturally, such as penicillin from fungi and tetracycline from bacteria, or be semi-synthetic (e.g., amoxicillin) or fully synthetic (e.g., sulfonamides) in nature.
Mara (2001) defines antibiotics as a broad class of natural, semi-synthetic, or synthetic compounds that exhibit antibacterial effects by either killing pathogens or impeding their growth. Since the 1940s, antibiotics have been extensively used in poultry farming not only for disease treatment, but also to promote growth, enhance feed efficiency, and prevent infections. However, their improper use can result in residues in animal products, which pose risks to human health (Mara, 2001).
2.2.2 Classification of Antibiotics
Wang (2012) classifies antibiotics into two major types:
Broad-spectrum antibiotics: Effective against a wide array of bacteria, including both gram-positive and gram-negative organisms (e.g., doxycycline, minocycline, aminoglycosides, ampicillin, amoxicillin).
Narrow-spectrum antibiotics: Target specific bacterial groups and are typically used when the causative organism is known (e.g., azithromycin, clarithromycin, erythromycin, clindamycin).
2.2.3 Mode of Action
Antibiotics work by disrupting critical bacterial functions. They may be bactericidal (killing bacteria) or bacteriostatic (inhibiting growth) by:
Inhibiting cell wall, protein, or nucleic acid synthesis;
Disrupting metabolic pathways;
Damaging the cell membrane (Wang, 2012).
2.2.4 Common Antibiotics Used in Poultry
Antibiotics are routinely administered in poultry farming to manage diseases, promote growth, and improve feed efficiency (Gaudin, 2004). Their use has led to more affordable poultry products, such as meat and eggs (Donoghue, 2003). Broilers are frequently treated with antibiotics to accelerate weight gain (Nonga, 2009).
In Uganda, penicillins are among the most widely used antibiotic groups (Mitema, 2001). Inappropriate use—especially failure to observe withdrawal periods—leads to drug residues in edible tissues. Commonly used forms include sodium, potassium, procaine, and benzathine salts (Lee et al., 2001), which have been linked to allergic reactions and antibiotic resistance (Mitema et al., 2001).
2.2.4.1 Tetracyclines
Discovered in the 1940s, tetracyclines inhibit protein synthesis and include compounds such as tetracycline, oxytetracycline, chlortetracycline, and doxycycline (Chopra, 2001; Michalova, 2004). These antibiotics are popular in veterinary medicine due to their broad-spectrum activity and affordability. Resistance mechanisms include efflux pumps, ribosomal protection, and enzymatic inactivation (Michalova, 2004).
2.2.4.2 β-lactams
This group includes penicillins and cephalosporins, which are widely used in veterinary practices (Kowalski, 2007). Residues from these antibiotics may cause allergic reactions and foster antibiotic resistance (Konieczna, 2007). Due to increased resistance, especially in E. coli and Salmonella, cephalosporins are restricted in many countries (Schmidt, 2012).
2.2.4.3 Macrolides
Used to treat respiratory diseases, macrolides inhibit protein synthesis by binding to the 50S ribosomal subunit (Stolker & Brinkman, 2005). Resistance may arise through plasmid-mediated mechanisms or mutations altering the ribosomal target (Riviere, 2009).
2.2.4.4 Aminoglycosides
These broad-spectrum antibiotics interfere with bacterial protein synthesis and are used to treat infections and promote growth. Examples include gentamicin, neomycin, and streptomycin (Mingeot-Leclercq, 1999).
2.3 Antibiotic Administration in Poultry
Antibiotics in poultry are commonly administered to entire flocks via feed or drinking water, although individual dosing (e.g., injections) is also possible (Ramatla et al., 2017).
2.3.1 Antibiotic Resistance
Overuse and misuse of antibiotics have led to the emergence of resistant bacterial strains, posing a major public health challenge (Levy & Marshall, 2004). Resistance genes can be transmitted among bacterial populations, rendering standard treatments ineffective (Ahmed, 2012).
2.4 Diseases Treated with Antibiotics in Poultry
Common diseases treated include respiratory infections, fowl cholera, sinusitis, and synovitis. Drugs such as tetracyclines, penicillin, and enrofloxacin are widely used. However, intensive and frequent usage raises concerns about residues in meat and eggs (Kaneene et al., 1997; Miller et al., 1997).
2.5 Antibiotic Residues in Poultry Meat
“Residues” refer to the active ingredients and their metabolites that remain in animal tissues after treatment (Doyle, 2006). Failure to follow dosage and withdrawal guidelines, along with mass treatments and sub-therapeutic dosing, can lead to these residues persisting in food products (Stobberingh, 2000; Baker & Leyland, 1983).
2.5.1 Effects of Residues on Humans
Residues can lead to allergic reactions, antimicrobial resistance, gastrointestinal issues, immune disorders, and even carcinogenic effects (Heshmati, 2015). Sub-therapeutic exposure is especially problematic due to the risk of developing resistant pathogens.
2.5.2 Maximum Residue Limits (MRLs)
MRLs refer to the legally acceptable concentrations of drug residues in food. The Codex Alimentarius Commission sets global standards, many of which are adopted by national authorities (Myllyniemi, 2004; Codex, 2018a).
2.5.3 Withdrawal Period
The withdrawal period is the time between the last antibiotic dose and the safe consumption of animal products. It ensures residue concentrations fall below MRLs (Codex, 2018a).
2.6 Prohibited Antibiotics
Due to rising resistance and health concerns, several antibiotics have been banned for use in livestock. These include chloramphenicol, nitrofurans, and certain fluoroquinolones (Castanon, 2007; Stolker, 2005; Vass, 2008; Davis, 2009).
2.7 Effects of Cooking on Residues
Cooking may reduce but not entirely eliminate antibiotic residues in meat. The extent of reduction depends on the antibiotic type, cooking method, and duration. Some residues may even leach into cooking liquids (O’Brien & Conaghan, 1985; Al-Ghamdi, 2000; Abou-Raya, 2013; Javadi & Khatibi, 2011).
2.8 Methods for Detecting Antibiotic Residues
Several analytical techniques are used to detect antibiotic residues:
Thin Layer Chromatography (TLC): Simple and cost-effective method using capillary action (Coskun, 2016).
Liquid Chromatography-Mass Spectrometry (LC-MS): High-sensitivity method for detecting various compounds in a short time (Ramatla et al., 2017).
High-Performance Liquid Chromatography (HPLC): Similar to LC but uses high pressure for better separation (Coskun, 2016).
Enzyme-Linked Immunosorbent Assay (ELISA): Highly specific immunological test, though relatively costly (Tajik et al., 2010).
