ABSTRACT
The high turn-over rate and quest for white meat have given more impetus to poultry production. The need to increase production and meet the demand for poultry meat has necessitated the use of veterinary drugs, especially antimicrobials, for therapeutic, prophylactic and growth promotion purposes in poultry farming. These drugs tend to accumulate in tissues and organs as residues. The presence of drug residues in tissues above the maximum residue limit (MRL) becomes violative if withdrawal periods are not observed. The consumption of violative levels of antimicrobial residues could result in the development of antibiotic-resistant strains of microorganisms, allergic reaction in sensitised individuals, distortion of activities of the intestinal flora, carcinogenesis and mutagenesis. There is evidence of excessive prescription, overuse and abuse of antimicrobial drugs in veterinary practice in Nigeria, and legislation regarding drug use in veterinary practice is hardly enforced. It has become necessary to monitor the presence and level of antimicrobial residues in poultry in Nigeria using reliable screening tests. This is imperative since there is presently no established surveillance programme for detecting drug residues. Oxytetracycline (OTC) is the most widely used antimicrobial in poultry production. Although thorough cooking is part of our food culture, studies have shown that some drugs are heat stable, hence there is need to investigate the effects of heat and freezing on the stability of this drug in tissues. The study to: (i) assessed the occurrence of antimicrobial residues in broilers retailed in Enugu metropolis, (ii) evaluated the effects of cooking methods and freezing on OTC residue, (iii) compared the sensitivity of detection of OTC residues in tissues using Three Plate Test (TPT) and Premi® test. The study involved both a cross sectional survey of broilers retailed in Enugu metropolis and two experiments. The survey was an assay of antimicrobial residues in broiler meat and organs using qualitative screening methods (TPT and Premi® Test) and a quantification of tetracycline (TC) residues using specific enzyme-linked immunosorbent xviii assay (ELISA) technique. The three major markets (Artisan, Gariki and Ogbete) in which broilers are retailed in Enugu metropolis, were used for the study. A total of 100 broilers were proportionately selected according to the sale capacity of each of the markets as follows: Artisan (40), Gariki (30) and Ogbete (30). The birds and retailers were selected using systematic random sampling technique. Muscle, liver, kidney and gizzard were collected from each of the 100 broilers for the survey. For the experiments, TPT, Premi® Test and ELISA were used to detect OTC residue in tissues of OTC treated birds and to evaluate the effects of various cooking methods (boiling, microwave grilling and roasting) and freezing time on the level of OTC residues. Fifty 5-week old broilers were raised for 3 weeks for the experiment. Four birds were sacrificed and their organs screened for residues. In the absence of residues, the remaining 46 birds were assigned into 2 equal groups (A and B). Group A birds were injected with long acting OTC at the dose of 20mg/kg body weight and group B birds were given OTC in drinking water at the dose of 4g/l for 5 days. Their organs were screened for OTC residue 24 hours after treatment. For each organ, meat juice was extracted by maceration and centrifugation. The API50 and polymerase chain reaction (PCR) were used to identify and characterize the Bacillus subtilis used for the TPT. Premi® test and ELISA were done following the manufacturer’s instructions. Graphpad Prism 5 statistical package was used to analyse the data generated. Chi-square was used to determine associations between occurrence of residues and organ types. One way analysis of variance was used to analyse other data as appropriate. Dunn’s multiple comparism was used for post hoc analysis. Three plate test inhibition zones were correlated to OTC concentration. Significance was accepted at p < 0.05. For the survey, TPT detected antimicrobial residues in 64% of broilers and the organ distribution were as follows: kidney (60%), liver (54%), gizzard (30%) and muscle (11%). Premi® test detected residues in 60% of broilers and distribution in specific organs were: kidney (49%), liver (25%) gizzard (22%) and muscle xix (14%). The ELISA detected tetracycline residues in 90% of broilers and detection in specific organs were: liver (96%), muscle (96%), kidney (88%) and gizzard (82%). The residue level was above MRL in 84% of liver, 100% of gizzard and 86% of muscle, whereas residues in 100% of positive kidney samples were below the MRL. For the experimental study, microwaving, boiling and roasting significantly (p < 0.05) reduced the inhibition zones produced by raw liver in TPT. There was a significant (p < 0.05) decrease in OTC residue concentration in roasted and boiled liver samples tested with ELISA. The three cooking methods had no significant (p > 0.05) effect on residue levels in muscle samples. Freezing had no significant (p > 0.05) effect on residue levels in muscle and liver samples. Three plate test had a higher sensitivity in detecting OTC residue than Premi® test in muscle and liver of birds treated by injection and those administered OTC in drinking water. There was a positive and significant correlation (r = 0.94; p < 0.05) between the TPT inhibition zones and OTC concentration.
