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CHAPTER TWO: LITERATURE REVIEW
2.1 Poultry Production
Poultry farming is a vital component of Uganda’s agricultural landscape, significantly enhancing food security and human nutrition by supplying high-quality animal protein through eggs and meat (FAO, 2020). Beyond nutrition, poultry rearing serves as a crucial source of supplemental income for households, helping to reduce reliance on unstable returns from traditional crop and livestock enterprises. Additionally, the poultry industry has demonstrated potential in generating foreign exchange through regional export of poultry products (MAAIF, 2019).
Socially, poultry holds cultural significance in Uganda, frequently featured in traditional ceremonies such as bride price negotiations and community festivities. While modern poultry farming is still developing in the country, increased investments in the livestock sector over the past two decades have led to a surge in exotic breeds like broilers and layers. Nevertheless, to meet the rising demand for poultry products, farmers increasingly rely on antibiotics for growth enhancement and disease prevention—a trend that has raised concerns regarding drug residues in poultry products (Clarke, 2004; Gerber et al., 2007).
2.2 Antibiotics
2.2.1 Definition of Antibiotics
Antibiotics are chemical substances that either eliminate or inhibit the growth of microorganisms with minimal toxicity to the host (Guardabassi, 2008). These agents can be natural (e.g., penicillin from fungi), semi-synthetic (e.g., amoxicillin), or entirely synthetic (e.g., sulfonamides).
Mara (2001) describes antibiotics as a diverse class of compounds—natural, semi-synthetic, or synthetic—that function by either killing bacteria or arresting their proliferation. Since their introduction in the 1940s, antibiotics have been extensively employed in poultry farming to manage infections, boost growth rates, enhance feed utilization, and prevent diseases. However, misuse or overuse can lead to harmful residues in animal products, posing health risks to consumers.
2.2.2 Classification of Antibiotics
According to Wang (2012), antibiotics fall into two major categories:
Broad-spectrum antibiotics: These are effective against a wide range of bacteria, including both gram-positive and gram-negative types (e.g., doxycycline, aminoglycosides, ampicillin, amoxicillin).
Narrow-spectrum antibiotics: These target specific bacterial groups and are often used when the causative agent is known (e.g., azithromycin, erythromycin, clindamycin).
2.2.3 Mechanism of Action
Antibiotics function by disrupting essential bacterial processes. They may exert bactericidal effects (killing bacteria) or bacteriostatic effects (inhibiting growth) by:
Inhibiting the synthesis of cell walls, proteins, or nucleic acids,
Interfering with metabolic pathways,
Damaging bacterial cell membranes (Wang, 2012).
2.2.4 Commonly Used Antibiotics in Poultry Farming
In poultry production, antibiotics are routinely incorporated into feed and water to prevent diseases, stimulate growth, and improve productivity (Gaudin, 2004). This practice has made poultry products like meat and eggs more affordable (Donoghue, 2003). Broiler chickens, in particular, are frequently given antibiotics to promote rapid weight gain (Nonga, 2009).
In Uganda, penicillin-based antibiotics are commonly used (Mitema, 2001). Misuse—especially the neglect of withdrawal periods—leads to detectable drug residues in meat. Frequently used forms include sodium, potassium, procaine, and benzathine salts (Lee et al., 2001), which are known to trigger allergic reactions and contribute to resistance (Mitema et al., 2001).
2.2.4.1 Tetracyclines
Discovered in the 1940s, tetracyclines inhibit protein synthesis in bacteria. This group includes tetracycline, oxytetracycline, chlortetracycline, and doxycycline (Chopra, 2001; Michalova, 2004). They are widely used due to their broad-spectrum activity and affordability. Resistance is typically mediated by efflux pumps, ribosomal protection proteins, and enzymatic inactivation (Michalova, 2004).
2.2.4.2 β-lactams
Comprising penicillins and cephalosporins, β-lactams are widely applied in veterinary practice (Kowalski, 2007). Their residues have been associated with allergic reactions and growing antibiotic resistance (Konieczna, 2007). Due to resistance, particularly in E. coli and Salmonella, the use of cephalosporins is now restricted in several countries (Schmidt, 2012).
2.2.4.3 Macrolides
Macrolides are primarily used to treat respiratory infections in poultry. They inhibit protein synthesis by binding to the 50S ribosomal subunit (Stolker & Brinkman, 2005). Resistance often arises from plasmid-borne genes or mutations in ribosomal binding sites (Riviere, 2009).
2.2.4.4 Aminoglycosides
These broad-spectrum antibiotics inhibit bacterial protein synthesis and are used to treat infections and support growth. Common examples include gentamicin, neomycin, and streptomycin (Mingeot-Leclercq, 1999).
2.3 Antibiotic Administration in Poultry
In poultry farming, antibiotics are generally administered to entire flocks via feed or drinking water. In some cases, individual birds may be treated through injections (Ramatla et al., 2017).
2.3.1 Antibiotic Resistance
The misuse and overuse of antibiotics have accelerated the emergence of resistant bacterial strains, which poses a serious public health threat. Resistance genes can be transferred across bacterial populations, making common treatments ineffective (Levy & Marshall, 2004; Ahmed, 2012).
2.4 Poultry Diseases Commonly Treated with Antibiotics
Typical poultry diseases treated with antibiotics include respiratory infections, sinusitis, fowl cholera, and synovitis. Drugs such as tetracyclines, penicillins, and enrofloxacin are frequently used. However, their extensive use raises serious concerns about the presence of residues in poultry products (Kaneene et al., 1997; Miller et al., 1997).
2.5 Antibiotic Residues in Poultry Meat
Antibiotic residues refer to traces of drugs and their metabolites remaining in tissues post-treatment (Doyle, 2006). Improper dosage, failure to observe withdrawal periods, mass medication, and sub-therapeutic dosing contribute to these residues (Stobberingh, 2000; Baker & Leyland, 1983).
2.5.1 Health Effects on Humans
Residues in meat and eggs can lead to various health complications, including allergic reactions, antimicrobial resistance, gastrointestinal disturbances, immune system disorders, and even carcinogenesis (Heshmati, 2015). Low-dose, long-term exposure increases the risk of antibiotic-resistant bacteria.
2.5.2 Maximum Residue Limits (MRLs)
MRLs are the highest permissible levels of antibiotic residues in food products. They are established by the Codex Alimentarius Commission and enforced by national food safety agencies (Myllyniemi, 2004; Codex, 2018a).
2.5.3 Withdrawal Period
This is the minimum time required between the final dose of an antibiotic and the point at which the animal product is safe for human consumption. It ensures that drug concentrations in food remain below MRLs (Codex, 2018a).
2.6 Banned Antibiotics
Due to increasing health risks and antibiotic resistance, several antibiotics have been banned for use in food-producing animals. These include chloramphenicol, nitrofurans, and specific fluoroquinolones (Castanon, 2007; Stolker, 2005; Vass, 2008; Davis, 2009).
2.7 Effects of Cooking on Residue Levels
While cooking can reduce the levels of antibiotic residues in meat, it does not entirely eliminate them. The extent of residue reduction depends on the type of antibiotic, cooking technique, and duration. Some residues may migrate into cooking fluids (O’Brien & Conaghan, 1985; Al-Ghamdi, 2000; Abou-Raya, 2013; Javadi & Khatibi, 2011).
2.8 Detection Methods for Antibiotic Residues
Several analytical techniques are used to identify and quantify antibiotic residues in animal products:
Thin Layer Chromatography (TLC): A low-cost technique utilizing capillary action for separation (Coskun, 2016).
Liquid Chromatography-Mass Spectrometry (LC-MS): A highly sensitive method for detecting multiple residues rapidly (Ramatla et al., 2017).
High-Performance Liquid Chromatography (HPLC): Uses high pressure to separate compounds more efficiently (Coskun, 2016).
Enzyme-Linked Immunosorbent Assay (ELISA): A specific and reliable immunological assay, though relatively expensive (Tajik et al., 2010).
