Anticoccidial drugs are widely used for the control of coccidiosis in the fowl which has inevitably led to the development of drug resistance. Resistance has developed to all of the compounds that have been introduced and if chemotherapy is to remain the principal method of control of coccidiosis, it will be important to continue the search for new anticoccidial agents. Knowledge of biochemical pathways present in the Eimeria parasite and how they differ from those of the host might help identify novel targets for inhibition. Studies of the mode of action of drugs are required if the biochemical mechanisms of resistance are to be understood. Information on the genetic origins of resistance, the stability of resistance and the factors involved in the spread of resistance throughout parasite populations is required. Since there are no methods at present to prevent resistance, more attention should be given to developing strategies for preserving the efficacy of anticoccidial drugs.

A landmark decision was taken in 2002 by the Biotechnology and Biological Sciences Research Council (BBSRC) in the UK to fund the genome sequencing of an eimerian (apicomplexan) parasite from the fowl. The project is a joint collaboration between the Institute for Animal Health (IAH) at Compton, UK and the Wellcome Trust Sanger Institute, Cambridge, UK (see http://www.sanger.ac.uk/Projects/E_tenella/ and http://www.iah.bbsrc.ac.uk/eimeria/). The species chosen, Eimeria tenella, is the best known member of the genus and the Houghton (H) strain is one of the most widely studied. The H strain was isolated in 1949 in the UK. It was maintained initially at the Houghton Poultry Research Station (HPRS), Houghton, and, following closure of that laboratory in 1992, thereafter at the IAH in Compton. At various times the parasite has been provided to other institutions and research groups carrying out coccidiosis research in the fowl. The H strain has been utilised in many fundamental studies on the eimerian life cycle and its relationship with the host. It has also been used to investigate the nature of drug resistance, and to derive attenuated lines-one of which, a "precocious line", is a component of a multivalent live attenuated coccidiosis vaccine (Paracox vaccine, Schering-Plough Animal Health). In this article, some immunological, epidemiological, genetic, and chemotherapeutic investigations with the H strain are reviewed, a summary is provided of the biological characteristics of the parasite and some of the core methods used to prepare purified extracellular life cycle stages for experimental studies or passage are given.

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The electrophoretic mobility of glucose phosphate isomerase (GPI) had been used to identify Eimeria acervulina, E. praecox and E. mivati in isolates of coccidia from commercial sites in the United Kingdom. Mixtures of two or three species gave separate bands upon electrophoresis and each species could be identified provided that it formed at least 10-20% of the total number of oocytes present in a sample of 8 x 10(6) oocysts. The principal forms of GPI corresponded to the GPI of E. acervulina (H), E. praecox (H) and E. mivati (W) and were found in 38, 34 and 8 isolates respectively of the 46 examined. Three further variants of GPI are described.

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