Abstract: A genetically modified coccidian parasites wherein expression of phosphatidylthreonine synthase (PTS) is disrupted, a polynucleotide including a nucleotide sequence encoding a phosphatidylthreonine synthase (PTS) enzyme, which catalyzes the production of a lipid, phosphatidylthreonine (PtdThr). PtdThr is an exclusive, major and physiologically important lipid in selected coccidian parasites, which is required for a normal growth and virulence of coccidian parasites. Coccidian parasites, having the expression of PTS disrupted as described herein, are useful as vaccines. The phosphatidylthreonine synthase enzyme and the nucleotide encoding sequences thereof as well as the phosphatidylthreonine phospholipid can find use in diagnostic methods and diagnostic kits or in vaccine and drug development applications.