A cross-sectional study including 125 herds was performed evaluating the prevalence of M. hyopneumoniae infections in suckling pigs at the age of weaning and the potential of corresponding risk factors on herd level.
The region where this study was conducted has the highest pig density in Germany (> 500 pigs/ 100 hectare agricultural used space; ), characterized by typical configurations of intensive pig production. Herd structures, husbandry systems and management procedures assessed during this investigation are probably highly representative for many other regions in Europe with intensive pig production. In order to minimize potential selection bias, the herds enrolled in this study were not proposed by field veterinarians but were randomly selected from databases of marketing companies. The participation in the study was voluntary, but all producers asked were willing to participate. The unexpectedly high motivation may be the result of a feeling of obligation to and a degree of economic dependence on the marketing company. Furthermore, producers were highly interested in the diagnostics and veterinary consultancy free of charge.
The transmission of M. hyopneumoniae cannot securely be prevented by vaccinating piglets or pigs against this pathogen . This lack of highly effective vaccines demands for additional actions reducing the impact of M. hyopneumoniae infections on the health of pigs. Any measures lowering the proportion of suckling pigs infected by their mother sow are supposed to decrease the later spread of the agent among growers and finishers [9, 17]. Whether these vertical transmissions of M. hyopneumoniae and corresponding infections in suckling pigs occur with high frequencies and, therefore, have to be considered in prevention programmes is intensively discussed. Studies conducted in single endemically infected herds or herds with a recent epidemic infection with M. hyopneumoniae revealed low prevalences of 1.5 to 3.8% in suckling pigs and 4.4 to 7.2% in nursery pigs , 2.6 to 13.2% in piglets 3 weeks of age  and 9.6% in suckling pigs prior to weaning . In a retrospective study including data from more than 300 herds, comparable low prevalences of 2.0% in suckling pigs and 9.3% in nursery pigs were found . In contrast to these studies, rates of over 30% have been reported previously in suckling pigs, but these studies included only a limited [17, 18] or an unknown number of herds .
In the present study, 3.9% (98/2,500) of all suckling pigs and 36.8% (46/125) of all herds were tested positive for M. hyopneumoniae. A real-time PCR has been used to detect the pathogen in nasal swabs from suckling pigs facilitating a high sensitivity. This PCR assay showed 85 to 90% sensitivity on pig level, when lung tissue was examined and both target sequences (ABC and REP) were amplified . Notwithstanding, it has also been used to determine M. hyopneumoniae infections by testing nasal swabs . Taking this into account, there is evidence that both figures, the overall detection rate among suckling pigs and the prevalence of herds showing M. hyopneumoniae infections in this age group, are good estimates for the current situation in the pig population of Northern Germany. Indeed a true prevalence of 3.9% would require 58 samples per herd in order to detect M. hyopneumoniae in at least one piglet out of a group of 110 piglets, but of course this sampling fraction is not affordable and the reasonable sample size used in the present study was already bigger than in other studies. Thus, the sample size was a good compromise, even though the sampling scheme as applied is missing a maximum possible prevalence of 12.7%, when all collected samples are tested negative.
Noteworthy, the probability of detecting M. hyopneumoniae in suckling pigs was lower than reported earlier , but it seems to be possible in approximately one third of all herds, when the sample size is large enough. These low detection rates are not in contrast to the hypothesis that single infected suckling pigs may act as spreader in the subsequent nursery or growing units .
In the multivariable analysis several herd factors have been identified being associated with the detection of M. hyopneumoniae in suckling pigs at the time of weaning. Whereas three factors could be assigned to risk factors, four others are more likely to be a result than a reason of frequent infections in suckling pigs: herds had a higher chance of at least one positive piglet, when ‘one-shot’ vaccines (OR: 5.5) or ‘two-shot’ vaccines (OR: 4.7) against M. hyopneumoniae were applied to the suckling pigs. Moreover, the risk was significantly increased, when piglets were vaccinated against PRRSV (OR: 4.4) or gilts were vaccinated against PCV2 (OR: 3.6). It is hypothesised that enzootic pneumonia in fattening pigs is eventually complicated by PRRS, both reflecting endemic infections in the herd including a higher probability of vertical transmission of the pathogens from sows to their offspring, drives the farmer to the decision of vaccinating suckling pigs against M. hyopneumoniae and/or PRRSV. Similar interactions can be assumed for PCV2 and gilt vaccination. The association between M. hyopneumoniae infections and the application of different vaccines was even expected, since their use is often an economic incitement for owners of endemically infected farms.
The risk of a herd to have a M. hyopneumoniae infection among suckling pigs was increased when the total number of purchased gilts per year was higher than 120 (OR: 5.8). This observation underlines the necessity of an appropriate acclimatisation period for replacements pigs, which should last approximately four to six weeks . At a certain time point, gilts should get in a close nose-to-nose contact to pigs from the stock herd , because transmission of M. hyopneumoniae by other vectors is insufficient for the purpose of provoked exposure . It should be mentioned that sentinels for nose-to-nose contact must not re-enter the stock herd, when they developed any symptoms of infectious disease during the contact phase. Therefore, the use of old sows or nursery pigs with stunted growth, dedicated for slaughter, are highly recommended as sentinels.
The number of sows housed in compartments of the farrowing units was also associated with the outcome variable. The risk of a herd being positive in suckling pigs significantly increased, when the number of farrowing pens in one compartment was higher than 16 (OR: 3.3). Again there is doubt, whether all-in/all-out policy is strictly implemented, when farrowing units are operated with a high number of pens per compartment, although this factor is known having major impact on the prevention of transmission of M. hyopneumoniae within herds . Beside this, a higher number of pens per compartment also increase the probability of housing gilts in their first parity together with older sows. These gilts shed M. hyopneumoniae more often than older sows [3, 6] and by doing this enable a frequent transmission of the pathogen to their own offspring, but also to piglets of other litters via aerosol .
Herds, where no batch farrowing or any kind of batch-wise farrowing rhythm that does not allow simple integration of sows returning to oestrus (i.e. not 1- or 3-week rhythm) was implemented, were more often found to be positive (OR 2.7). This observation was in accordance with another study reporting higher seroprevalences towards M. hyopneumoniae in sows farrowing in a 2- or 4-week rhythm . When comparing only herds with 2-week farrowing-rhythm to those with 3-week rhythm, the effect was even stronger (OR: 3.8). It can be assumed that in such herds, especially those with 2-week rhythm, a strict realisation of the all-in/all-out policy for the farrowing units is not possible and not performed, because sows which have regularly returned to oestrus approximately 21 days after insemination neither fit into the last nor into the next group. Conclusively, such sows will farrow one week earlier or one week later compared to all other sows, and if the herd does not have appropriate compartments for sows farrowing ‘out of the rhythm’ these sows will be allocated into compartments, where other sows have already farrowed or will farrow later. Taking this fairly conclusive assumption into account, it seems to be the non-compliance with the all-in/all-out policy rather than the farrowing interval that increases the likelihood of M. hyopneumoniae positivity in suckling pigs at the age of weaning.