A model to investigate the optimal seeder-to-naïve ratio for successful natural Mycoplasma hyopneumoniae gilt exposure prior to entering the breeding herd

•A 6:4 seeder-to-naïve ratio was successful to expose all naïve gilts in a group.•The estimated M. hyopneumoniae transmission rate was 1.28 per pig/week.•Acclimation can be based on exposure of naïve gilts to knowingly shedding pigs.•This study is a first step towards a standard M. hyopneumoniae acclimation protocol. Due to the significance of Mycoplasma hyopneumoniae as a swine respiratory pathogen, acclimation measures are taken into consideration when obtaining replacement gilts from negative sources to be introduced to endemically infected herds. The aim of this study was to evaluate the optimum seeder-to-naïve gilt ratio in a 4-week period for successful natural exposure to M. hyopneumoniae. Sixty gilts were divided in two groups, 21 2-week old seeder gilts were inoculated with M. hyopneumoniae, and 39 aged-matched naïve gilts were exposed to the seeders during a 4-week period. The exposure was set by dividing the gilts into six groups of 10 with different ratios of seeder-to-naïve, from 1:9 until 6 seeders and 4 naïve gilts. Laryngeal swabs, oral fluids and blood samples were collected from all gilts prior to, during and after inoculation and exposure. Infection in seeders was confirmed by development of clinical signs, seroconversion post-inoculation, and detection of M. hyopneumoniae genetic material. Naïve were considered positive after 4 weeks if M. hyopneumoniae was detected on bronchial swab or fixed lung tissue. As result, 33% (3/9) naïve gilts were positive in the 1:9 ratio, 75% (6/8) in 2:8, 28% (2/7) in 3:7, 33% (2/6) in 4:6, 80% (4/5) in 5:5 and 100% (4/4) in the 6:4 ratio. The estimated transmission rate (β) and expected probability of infection (ψ) were 1.28 per pig/week and 0.6, respectively. In this study, six seeders were required in a group of 10 gilts for successful exposure to M. hyopneumoniae in a 4-week exposure period.