Suckling pigs infected with M. hyopneumoniae are considered as initiators for the spread of M. hyopneumoniae infections during the nursery and fattening period [4, 5, 14]. It is hypothesised that identification of risk factors can help to create intervention strategies against a frequent transmission of M. hyopneumoniae from sows to their offspring and thereby further transmission to pen mates in the nursery and growing units. However, many studies investigating potential risk factors for M. hyopneumoniae infections in pigs were predominantly focused on weaned pigs, e.g. growing and finishing pigs [10, 18–21] and did not highlight individual risk factors, which increase the probability of detecting M. hyopneumoniae in suckling pigs. In the present study the occurrence of M. hyopneumoniae infections in suckling and nursery pigs was investigated and various factors potentially influencing the infection status of piglets at the end of the suckling period were analysed.
Detection of M. hyopneumoniae in sows by PCR
An increased positivity for M. hyopneumoniae of the sows’ nasal mucosa was expected around the time of birth. It was hypothesised that a stressful situation as giving birth to piglets would impact the immune system of the sows. Interestingly, the findings of the present study are in contrast to this hypothesis and a previous report, where a decrease in the prevalence from farrowing to weaning was observed at least in some groups . Potentially, the present, ‘delayed’ outcome was due to the slow replication / growth of M. hyopneumoniae, and the loss of antibodies shortly before the birth  could have affected the course of this increase of detection rates.
Detection of M. hyopneumoniae in suckling pigs by PCR
The prevalence 3.6% is low compared to other reports, where higher rates of 7.7%  and 11.3%  were found. However, herds examined in these studies had been selected by either frequent detection of M. hyopneumoniae in other age groups combined with occurrence of enzootic pneumonia or by severe clinical symptoms of respiratory disease in the group of question, i.e. among the suckling pigs. In contrast, herds in the present study were enrolled, when only few suckling pigs were tested positive for M. hyopneumoniae. Noteworthy, in one study describing the course of infection from birth to slaughter, a well comparable detection rate of 3.8% was found in suckling pigs at the age of weaning . Moreover, a recent randomized cross-sectional study reported an overall detection rate of 3.9% in this particular age group . Notwithstanding, the detection rate assessed with PCR on nasal swabs can be influenced by the virulence of the strain (i.e. course of infection) and the imperfect sampling site , and it should be considered that testing 20 suckling pigs per herd results in a maximum possible prevalence of approximately 13%, which - by chance - remains undetected (assumed population size: 100; level of confidence: 95%).
The link between sows colonisation status and positivity in suckling pigs at weaning as found in the present study was already assumed by others , and complies to within-herd transmission pathways described for M. hyopneumoniae.
Detection of M. hyopneumoniae in nursery pigs by PCR
The decreased detection rate of 1.2% among nursery pigs is in accordance to the reproduction ratio of M. hyopneumoniae infection during 6 weeks of nursery, which has been estimated being R0 = 0.56 for unvaccinated and R0 = 0.71 for vaccinated nursery pigs . Another study reported an average R0 = 1.16 during a 6 week nursery period , but this was elaborated in an experimental set-up with inoculation of seeder pigs. It was shown that high virulent strains of M. hyopneumoniae lead to higher reproduction ratios (R0 > 1) than low virulent strains (R0 < 1).
Detection of antibodies against M. hyopneumoniae in sows and suckling pigs by ELISA
Sows have been tested for antibodies against M. hyopneumoniae three weeks prior to and shortly after farrowing in order to determine the transfer of maternal antibodies to the progeny. It is known that a decrease of the concentration of serum antibodies identified by lower S/P ratios or equivalent values over time, as well as a decrease in the overall prevalence of ‘positives’ during this pre-farrowing period is due to a transfer of serum antibodies into the colostrum [28, 29]. In the present study, neither the S/P ratios three weeks prior to farrowing, the S/P rations shortly after farrowing nor the differences (data not shown) were associated with the detection of M. hyopneumoniae by PCR in nasal swabs from suckling pigs. A similar observation was already made in a previous study .
The serological status of the suckling pigs at 14 days of age also did not show any impact on the prevalence of M. hyopneumoniae suckling pigs at the time of weaning. Former studies have shown that high levels of maternally derived antibodies facilitate prevention of M. hyopneumoniae-infection of sucking pigs [30, 31], but this effect could neither be confirmed in the present study nor in others [3, 27].
An increase in the number of life born piglets per litter was linked to a lower incidence rate of M. hyopneumoniae in suckling pigs at weaning. Even though it was not expected that reproductive performance has an impact on M. hyopneumoniae-infections, there is no doubt about the outcome, since an equal finding has been reported recently in the same context . These findings may indicate that high performing herds truly apply extensive hygiene measures and excellent animal care taking, which however were not captured in the studies.
Another observation with regard to management was that grinding piglets’ teeth was leading to a significant lower incidence rate of M. hyopneumoniae infections. Whether this effect was confounded by an increased colostrum uptake or a higher daily weight gain due to better milk supply by the sow in accordingly treated litters could neither be confirmed nor ruled out. Nonetheless, ‘teeth grinding’ was one of the few variables remaining in the final multivariable Poission regression model with significant impact on positivity to M. hyopneumoniae.
The application of a second dose of iron was associated with a decrease of the incidence rate. Because of the high growth rate of piglets and low iron content of the sows’ milk, conventionally raised suckling pigs usually need additional iron during their first week of life in order to prevent anaemia , and it is possible that some piglets, only receiving 200 mg iron shortly after birth, are borderline anaemic when weaned at day 25 or later. Since it has been shown that increasing the iron supply influences several parameters including immunity , this second iron injection might have prevent piglets also from infection with M. hyopneumoniae.
With an increase of the duration of the suckling period piglets were more often positive to M. hyopneumoniae at weaning. Considering that the transmission of M. hyopneumoniae from sows to their offspring is likely depending on duration of exposure, these results are fairly conclusive. Similar findings have been reported recently . It remains unclear, whether the ‘lower average daily weight gain’ or the increased length of the suckling period, which is an inevitable consequence of the first one, is responsible for the observed effect. Farmers often postpone the weaning date, when piglets are not heavy enough. Unfortunately, there is no way to sort this out, so that further research on this topic is highly recommended.
The floor temperature in the piglet nest demonstrated a significant association with the differences in the incidence rates. Noteworthy, the temperatures as presented in Table 6 do neither consider piglets’ behaviour nor their location in the farrowing pen. Obviously, piglets were not lying in the nest area, when temperatures reached more than 45°C, which was due to a failure of the electrical heating device. The significant impact of piglet nests’ temperatures were also confirmed in the final multivariable model. These findings were in accordance to previous reports describing the interactions between environmental conditions and development of the pig’s immune system. It has been shown that low environmental temperatures aid a reduced suckling activity and a delayed development of immune-competence [35–37].
Vaccination of suckling pigs against PCV2 was linked to an IR of 9.7, but the vaccination, the weaning and the sampling were mainly performed in parallel or in very brief sequences, this ruling out a direct influence of the vaccine. Reasons for an association are more likely respiratory disease in older pigs of the same herd, which demand for vaccination against potential initiators like PCV2. In this context, the high frequencies of co-infections with PCV2 and M. hyopneumoniae and the potentiating effect on clinical symptoms have to be considered [38, 39].
In this observational study, sows and suckling pigs have been treated due to the occurrence of diseases and for metaphylactic reasons, respectively, with various antimicrobials at different points in time. The authors of this article were neither responsible for the application of mass treatment nor have been asked for their opinion. Instead, all treatments had been advised by the herd attending veterinarians and were based on particular disease histories in the herds. It should be noted that attempts of preventing M. hyopneumoniae infections in pigs solely by antibiotic treatment is usually not sustainable and is not accounting for the veterinarians’ responsibility for prudent use of antimicrobials, prevention of bacterial resistance and general public health issues!
Treatments of suckling pigs in two farms included an injection of tulathromycin, which is known to be highly effective against M. hyopneumoniae. Due to the fact that a relatively low prevalence of M. hyopneumoniae in suckling pigs at weaning was observed in the one farm compared to the other, an effect of time of applying antimicrobials could be discussed. However, this does not prove causality and the unequal distribution of positive suckling pigs among the three study herds could likely have biased this outcome. Even though pathogen elimination by applying antimicrobials to suckling pigs has been used to develop the procedure of ‘medicated early weaning’ [41, 42], there are serious concerns regarding this mass treatment: An elimination of M. hyopneumoniae in growing pigs by applying antimicrobials once or even twice during the suckling period should not be expected. Moreover, the use of antimicrobials for metaphylaxis is contrary to recent demands on prudent use of drugs in both human and veterinary medicine.