High seroprevalence of Toxoplasma gondii and probability of detecting tissue cysts in backyard laying hens compared with hens from large free-range farms.

•An ELISA based on native TgSAG1 (SRS29B) seems to be suitable to detect Toxoplasma gondii-infected chicken farms.•In large organic farms which provide outdoor access for laying hens, prevalence of T. gondii-infected laying hens was low.•A high prevalence of T. gondii infection suggested that contamination with T. gondii oocysts is common in backyard farms.•Use of cats for rodent control and low chicken density on the chicken run seem to be important T. gondii risk factors.•Toxoplasma gondii infections were rare on large organic farms, even when cats were present on the premises.