The invention pertains to a method to purify coccidial oocysts having dimensions between Dmin and Dmax from faeces comprising the steps of collecting the faeces containing the coccidial oocysts from host animals, diluting the faeces in an aqueous medium, separating a coarse fraction comprising macroscopic particulate matter from the diluted faeces and collecting an aqueous fraction containing the oocysts, characterised in that the method further comprises sieving the aqueous fraction over a first sieve deck having mesh openings to let the oocysts pass, to obtain an aqueous filtrate comprising the oocysts and a first residue comprising particles larger than the oocysts, and sieving the aqueous filtrate over a second sieve deck having mesh openings to obstruct passing of the oocysts through this sieve deck, to obtain a second residue comprising the purified oocysts and a waist filtrate comprising particles smaller than the oocysts. The invention also pertains to a system suitable for applying this method and to oocysts obtained therewith.