Mycoplasma hyopneumoniae gilt aclimatation: investigating the optimal seeder-to-naïve ratio for successful natural exposure

Mycoplasma hyopneumoniae is a swine respiratory pathogen that leads to compromised animal welfare and economic losses in finishing pigs. The lung infection can last up to 7 months and infected animals can become asymptomatic carries capable of infecting other pigs. The pathogen is mainly transmitted by direct contact, and vertical transmission is recognized as risk factor for prevalence of M. hyopneumoniae at weaning. It is known that colonized suckling piglets are considered initiators of the spread of M. hyopneumoniae after weaning, infecting susceptible pen mates. Since the situation of replacement gilts negative to M. hyopneumoniae being introduced into commercial farms endemically positive for the pathogen is still forceful, acclimation measures need to be taken into consideration. An alternative method for gilt acclimation is the controlled natural exposure of naïve gilts to shedding animals. Therefore, the aim of this study was to evaluate the optimum seeder-to-naïve gilt ratio in a 4-week period for successful exposure to M. hyopneumoniae.

Sixty gilts were divided in two groups, 21 2-week old seeder gilts were inoculated with M. hyopneumoniae, and 39 age-matched naïve gilts were exposed to seeders during a 4-week period. The exposure was performed 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 gilts were considered positive after 4 weeks of exposure if M. hyopneumoniae was detected on bronchial swab or fixed lung tissue.

Thirty three percent (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. Considering the results on laryngeal swabs, M. hyopneumoniae genetic material was detected on all naïve gilts in the ratio 5 seeders and 5 naïve gilts, implying that this ratio can possibility be used to achieve exposure of all gilts in the group in longer periods of exposure. The estimated transmission rate (β) and expected probability of infection (ψ) were 1.28 per pig/week and 0.6, respectively.

Six seeders were required in a group of 10 gilts for successful exposure to M. hyopneumoniae in a 4-week exposure period. Nevertheless, a ratio of 1:1 could be possibly used in a less conservative approach and longer exposure periods.

Disclosure of Interest: None Declared.