• Document: Ionophore toxicity in chickens: A review of pathology and diagnosis
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Avian Pathology ISSN: 0307-9457 (Print) 1465-3338 (Online) Journal homepage: http://www.tandfonline.com/loi/cavp20 Ionophore toxicity in chickens: A review of pathology and diagnosis Laurie Dowling To cite this article: Laurie Dowling (1992) Ionophore toxicity in chickens: A review of pathology and diagnosis, Avian Pathology, 21:3, 355-368, DOI: 10.1080/03079459208418854 To link to this article: https://doi.org/10.1080/03079459208418854 Published online: 12 Nov 2007. Submit your article to this journal Article views: 3588 View related articles Citing articles: 29 View citing articles Full Terms & Conditions of access and use can be found at http://www.tandfonline.com/action/journalInformation?journalCode=cavp20 Avian Pathology (1992) 21, 355-368 REVIEW ARTICLE Ionophore toxicity in chickens: a review of pathology and diagnosis LAURIE DOWLING Pathology Branch, Animal Research Institute, 665 Fairfield Road, Yeerongpilly, Queensland 4105, Australia SUMMARY The ionophores (polyether compounds) have been the predominant means of chemical control of coccidiosis in the past 15 years because of the slow development of resistant strains to them relative to other anticoccidial drugs. However, the ionophores have a narrow range of safety, and it is sometimes difficult to ensure an even distribution of the drug throughout the feed. Diagnosis of toxicity is difficult because of the reversibility of clinical signs and the variability of pathological lesions ranging from none to non-specific. This paper reviews the known pathology of ionophore toxicity and the inadequacies of present diagnostic approaches. Analysis for ionophores in feed may be made by silica gel and high performance thin layer chromatography, but tissue analyses for toxic levels, a more specific diagnostic aid, are not commonly carried out. Limited studies suggest that residues in even severely intoxicated birds remain low. Diagnosis relies upon clinical signs of inco-ordination, leg weakness, diarrhoea and depression, non-specific histopathological lesions of myopathy and the presence of high levels of ionophores in the feed. If, however, toxicity is due to uneven distribution, feed samples may return false negative results. Current diagnostic criteria are, therefore, unsatisfactory and there is a clear need to investigate other diagnostic approaches. INTRODUCTION Ionophores are polyether compounds used in the poultry industry for their anticoccidal activity. Coccidiosis is a continuing problem in the intensive chicken industry and in meat chickens, medication is required continuously. Infection with coccidia causes destruction of the intestinal epithelium which may lead to mortality. More frequently, high morbidity and hypophagia occur leading to reduced weight gain or weight loss (Long, 1984). Controlling subacute disease brings immediate benefits in terms of body weight gain and food conversion. Long (1984) considered that there are three possible approaches for the control of coccidiosis. These are sanitation, chemotherapy and immunological methods. With the use of litter in sheds, control by sanitation is impractical and immunological methods of control, while in use in many broiler breeder flocks, are not safe or economic for use in broiler chickens (Jeffers, 1987). Immunity in chickens to Eimeria tenella can be achieved by giving them daily doses of between one and 20 infective oocysts daily (Joyner & Norton, 1973). Recently, work on live attenuated vaccines has achieved some success Received 23 September 1991; Accepted 16 March 1992. 355 356 L. DOWLING (Shirley & Millard, 1986; Shirley & Bellatti, 1988). Advances towards a genetically engineered vaccine have also been made (Ellis & Tomley, 1991). At the present time, however, chemotherapeutic control of coccidiosis remains the major method of control in broiler chickens. Ideally, chemotherapeutic drugs should have no adverse effects on growth and feed intake, have activity against all important species of Eimeria and have no residues in meat. The drug(s) should minimize the effect of coccidiosis but allow some development of the parasite in order to stimulate a high degree of immunity. Long et al. (1979) found that 40 to 50 mg monensin/kg allowed immunity to develop in replacement breeders, whereas 100 to 121 mg monensin/kg was needed to prevent coccidial disease losses in broilers. Immunity is important in flocks because it enhances overall coccidiosis control and will prevent occurrence of coccidiosis during the withdrawal period. A number of drugs have been used in the control of coccidiosis. In the past 15 years, the ionophorous

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