Life Cycles Of Emeria spp

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T he life cycle of a typical Eimeria sp. comprises a parasitic and a nonparasitic phase. Th e infective stage – the sporulated oocyst – is ingested and the action of mechanical and chemical factors in the gut (bile salts and trypsin) leads to the release of sporocysts and then sporozoites in the duodenal lumen in poultry. Th e sporozoites invade the mucosa of the intestine, sometimes

passing down the whole length of the alimentary tract before doing so in poultry by Emeria spp. Th ere follow phases of intracellular growth and asexual multiplication with periodic release of merozoites back into the gut lumen. After a number of such schizogonous cycles (the number is primarily a genetically determined characteristic of a given species or strain), sexual forms – the gametocytes – develop intracellularly. Th ese diff erentiate into macro- and microgametocytes. Th e microgametocyte releases many microgametes, which are fl agellated and motile and migrate to the macrogametocytes. Th e macrogametocyte develops into a single macrogamete, which, after fertilization, develops into a zygote and thence an oocyst. During this process, large intracytoplasmic granules appear peripherally and eventually coalesce to form the oocyst wall. In histological sections these are very characteristic and clearly identify the macrogametes . An important feature is that this cycle is quite rapid, with a prepatent period of about 4–5 days (which varies slightly with species), and involves colossal multiplication. Th e degree varies with species but optimally may result in hundreds of thousands or even millions of oocysts produced from one ingested oocyst. When oocysts are passed in faeces they contain an undiff erentiated spherical body. Th ese oocysts only become potentially infective after undergoing sporulation. Th is entails 4 subdivision into four sporocysts, each of which contains two sporozoites. (In genera other than Eimeria there may be a diff erent arrangement.) Sporulation requires three conditions: warmth, moisture and oxygen. Under optimal conditions, around 25–30C, this takes 1–2 days. Sporulated oocysts, protected by the thick oocyst wall, are resistant to a fairly wide range of normal environmental conditions and the ability of at least some to survive for months or years is a key factor in the epidemiology of coccidial infections. Temperatures above 56C and below freezing are lethal, as is desiccation, but oocysts are able to tolerate most disinfectants. Only low-molecular-weight compounds, such as ammonia and methyl bromide, eff ectively kill oocysts and these gases are used to decontaminate experimental facilities. Under practical farm conditions it is more common to use proprietary products that release ammonia in a controlled manner – even then special care is required to protect the operator. T h e identifi cation of diff erent species of Eimeria in a given host necessitates consideration of a number of characteristics. Oocyst morphology may be useful, especially if length:width ratios are determined, but will not distinguish many species. Th e site and nature of lesions are valuable practical criteria. In a research context, identifi cation may be defi nitively determined by cross-immunity

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