Mycoplasma hyopneumoniae (M. hyopneumoniae) is the causative agent of enzootic pneumonia, a chronic respiratory disease that affects mainly growing and finishing pigs. This disease causes significant economic losses on the affected farms by worsening production parameters such as average daily gain and feed conversion rate and by increasing susceptibility to infection by other pathogens as well as medication costs (Thacker and Minion, 2012).
Transmission of M. hyopneumoniae occurs mainly by direct contact (nose to nose) between susceptible and infected animals. This transmission can occur between animals in the same pen (horizontal) and/or between the sow and its offspring (vertical). Vertical transmission (sow to piglet) during the lactation period is considered a key point to control the infection on affected farms (Fano, 2007; Sibila, 2007), as infected piglets during lactation period could transmit the bacteria during their subsequent growing/finishing stages. Indeed, several studies showed a correlation between a high prevalence of M. hyopneumoniae at weaning with a prevalence and severity of lung lesions associated with this pathogen in slaughterhouses (Fano 2007; Sibila, 2007).
One factor that seems to influence the M. hyopneumoniae shedding is the sow’s parity. Gilts and young sows are considered the main shedders of this respiratory pathogen (Boonsoongnern, 2012). Therefore, acclimation strategies of the replacements focused on achieving the correct immunization and reducing M. hyopneumoniae shedding by gilts and first parity sows, could be useful to stabilize the status of this respiratory pathogen on affected farms.
The farm stability depends on the health status of the recipient sow herd, as well as the health status of incoming gilts. Therefore, before implementing an acclimation program against M. hyopneumoniae, we must know: What is the health status of the recipient sow herd against M. hyopneumoniae? What is the health status of the incoming replacements?
In order to determine the M. hyopneumoniae health status of recipient sow herd and incoming replacements , we must take into account: 1) the observation of clinical parameters (clinical signs and lung lesions) and 2) laboratory confirmation of pathogen exposure (ELISA) and/or presence of the pathogen (PCR). Recently, a classification has been proposed considering the presence or absence of those parameters that enables the occurrence of the different scenarios (Fano and Pieters, 2016; Garza-Moreno, 2018).
What strategies are available to achieve a correct acclimation of the replacement gilts against M. hyopneumoniae?
Traditionally, as with other diseases, gilts coming from an external source tend to be exposed to infected animals shedding the specific pathogen. However, in the case of M. hyopneumoniae, achieving sufficient acclimation through natural exposure is complicated due to its low transmission ratio (Meyns, 2004). In fact, a recent study demonstrated the low efficiency of natural exposure to M. hyopneumoniae showing that 6 positive shedding gilts were needed to infect 4 susceptible gilts (6:4 ratio) during a 4-week period (Roos, 2016).
To facilitate the exposure, different artificial exposure methods as M. hyopneumoniae inoculation have been proposed: intranasally, intra-tracheally, or using aerosols (Yeske, 2019). These methods provide higher efficiency to expose replacementsto M. hyopneumoniae. However, these strategies might be a risk for the stability of the farm due to the prolonged shedding of M. hyopneumoniae by infected animals (up to 214 days post infection; Pieters, 2009). Thus, an adequate acclimation period (cool down) is required to ensure that infected gilts will not shed M. hyopneumoniae at entry. This fact implies the entry of younger gilts into the operation, an increased need for facilities, as well as the use of laboratory techniques to verify the status of these inoculated gilts prior to their entry into the farm.
Currently, one of the most efficient tools to achieve a homogeneous immunization of the replacement animals in a relative short period of time could be the vaccination against M. hyopneumoniae. Although vaccination against M. hyopneumoniae does not prevent infection, it decreases clinical signs and infection pressure on the vaccinated population (Maes, 2017). A recent study compared different vaccination protocols for replacement gilts against M. hyopneumoniae during acclimation period (Garza-Moreno, 2019). Under the conditions of this trial, gilt vaccination against M. hyopneumoniae using 4 or 2 doses reduced the infection pressure of this pathogen and increased significantly the maternal immunity transferred to the piglets compared to the unvaccinated group (Garza-Moreno, 2019).
In conclusion, an effective acclimation program for the replacements could minimize M. hyopneumoniae shedding from the sow to its litter during lactation, reducing the prevalence of this pathogen at weaning and its impact on later stages. Therefore, the implementation of strategies that allows increasing and homogenizing the immunity in a feasible period of time, could be very useful to control M. hyopneumoniae within farms.