BACKGROUND
the wild boar  population is expanding in europe  <cit> .
rearing domestic pigs outdoor also shows an increasing trend  <cit> .
although this type of animal husbandry provides a better quality of life for pigs  <cit> , it exposes them to a higher risk of contact with wild boar, which can carry pathogens with various routes of transmission  <cit> .
important air-born pig diseases include porcine reproductive and respiratory syndrome  and enzootic pneumonia .
in contrast to the prrs virus, mycoplasma hyopneumoniae  seems to be widespread in swiss wild boar, which are considered a potential reservoir for this pathogen  <cit> .
classical swine fever  virus and mycobacterium bovis  are transmitted by indirect or direct close contact  <cit> .
these pathogens are currently not an issue in switzerland  <cit>  but they could be introduced by wild boar emigrating from neighbouring countries  <cit> .
sarcoptes scabiei  is transmitted by close contact  <cit> .
while efforts are done to eradicate the mite from swiss piggeries  <cit> , infestations have been recently diagnosed in wild boar .
in wild boar, aujeszky's disease  virus and brucella suis  are transmitted mainly by the venereal route  <cit> .
while ad virus infection is rare in swiss wild boar  <cit> , nearly 30% of them are carrier of b. suis, including adults with genital organ infection  <cit> .
the objectives of this study were  to document the occurrence of contacts between wild boar and outdoor pigs in switzerland,  to identify risk factors for such contacts in order to propose risk-adapted protection measures for piggeries, and  to test exposed pigs for a possible pathogen spill-over, taking b. suis as an example.
this pathogen was selected because infections have been well documented in swiss wild boar  <cit> , while domestic pigs are officially free of brucellosis  <cit> .
a re-emergence of brucellosis in outdoor pigs has been observed in france following intrusions of and/or mating with wild boar  <cit> , indicating a substantial risk of losing the “free from disease” status in switzerland if close contacts between wild boar and pigs occur.
furthermore, since transmission of b. suis mostly requires the closest type of contact , b. suis spill-over would suggest that transmission of all other pathogens is possible.
here we report different types of interactions between wild boar and outdoor pigs and show that risk factors are not only related to wild boar presence, piggery location and fence characteristics but also include pig breed.
we additionally report field investigations performed in the frame of a brucellosis outbreak in domestic pigs with a history of contacts with wild boar.
RESULTS
contacts between wild boar and pigs
questionnaire surveys among game-wardens and pig farmers
thirty-one percent  of the game-wardens who participated to the study observed contacts at least once between  <dig> and  <dig>  and additionally before  <dig> for four of them.
overall, contacts of any category  were reported in  <dig> swiss cantons.
observations included tracks but also intrusions  and domestic sows with cross-bred piglets .
births of cross-bred animals were reported from 7/ <dig> cantons.
of the  <dig> pig farmers who replied the questionnaire,  <dig> had fattening/finishing pigs,  <dig> had breeding pigs,  <dig> kept both types of animals, one held pigs as a hobby, and no information was available for  <dig> piggeries.
two hundred twenty-two piggeries had concrete run-out,  <dig> had pure pasture run-out,  <dig> had mixed run-out , five had another type of ground , and  <dig> did not answer the question.
herd size ranged from  <dig> to more than  <dig> pigs, with an average of  <dig> animals per farm.
the most represented breeds were landrace and large white pigs  followed by duroc  and mangalitza .
concrete run-outs were fenced with concrete walls, metallic bars, wooden boards, wire mesh, or various combinations of those.
in the case of pasture run-outs, fences consisted in electric wires , wire mesh, wooden boards, metallic bars, or combinations.
none of these pasture fences were fixed deep in the ground.
eighty farmers  reported contacts of any category.
they mentioned at least one contact since they acquired the piggery, which could go back to the 1970’s.
the frequency of contacts ranged from one in 39 years to  <dig> per year.
from these  <dig> piggeries, the majority had a pure pasture run-out.
contacts were significantly more reported from piggeries with pasture and mixed run-out than with concrete run-out .
overall, there was an average of  <dig> contacts of any type per year for a total of  <dig> outdoor farms, or  <dig>  contact/farm/year .
contacts were reported in all calendar seasons and in all  <dig> cantons with wild boar presence.
cases of cross-breeding  of wild boar with different pig breeds were reported in 17/ <dig> piggeries .
most of them were cross-bred with l/lw  and mangalitza sows .
cross-breeding additionally occurred twice with a pure duroc, once with a cross-bred duroc x l/lw, once with a pot-bellied pig, and once with a minipig.
in one of these piggeries holding mangalitza, single and multiple births of cross-bred piglets following intrusions of wild boar or excursions of domestic sows have been observed .
in one instance, eight sows gave simultaneously birth to cross-bred piglets .
cross-breeding occurred more often with mangalitza than l/lw .
overall, there was an average of five mating events per year for a total of  <dig> farms with pure pasture or mixed run-out, which corresponded to an estimated rate of  <dig>  mating/farm/year .
cross-bred piglets were reported in  <dig> cantons, and in one case no information was obtained on piggery location.
1 f: fattening, b: breeding.
2 l/lw: large white or landrace, m: mangalitza.
3a: alive, s: slaughtered, so: stamped-out.
4including  <dig> newborn piglets.
5pigs slaughtered before the discovery of the outbreak.
6hybrids were bought from another outdoor piggery for breeding purposes.
in this other piggery, however, hybridization had been accidental .
7a breeding domestic sow, which had mated with a wild boar , was bought pregnant of hybrid piglets from another outdoor piggery
8sampling on request of the pig owner.
sampled domestic pigs from swiss outdoor piggeries  in 2008– <dig> in a study on risk factors for contacts between wild boar and outdoor pigs.
categories of contacts:  presence of wild boar around the enclosure ;  presence of wild boar just outside the fence ;  wild boar intrusions without mating; and  mating .
surveillance of piggeries with camera traps
during one year of surveillance in two piggeries at risk , wild boar were detected only in piggery p <dig>  there were four detections in nearly 16 months, always just outside the enclosure  on the pedestrian way between the forest and the fence, at night , from december to march:  single juvenile;  wild sow with offspring;  single adult ;  two animals .
assessment of risk factors for contacts
information on potential risk factors for contacts between wild boar and domestic pigs  were obtained from  <dig> pig farmers.
selected factors included in the multivariable logistic regression models are listed in additional file 1a-d. the final multivariable models, one for each outcome , showed that  pigs in enclosures separated from the piggery building  or/and located away from other houses  or/and close to a forest  were most at risk for indirect contacts with wild boar roaming around the piggery ;  pigs in enclosures separated from the piggery building  or/and protected with an electric fence were most at risk for closer indirect contacts with wild boar ,  pigs in enclosures located far from the piggery building  or/and protected by an electric fence or/and any fence ≤ 60 cm, were most at risk for an intrusion; and  piggeries holding mangalitza or/and protected by an electric fence were most at risk for the occurrence of cross-breeding .
when comparing the most represented breeds with each other, cross-breeding was more frequent among mangalitza than l/lw .
single variables and hypotheses regarding their influence on the occurrence of contacts between outdoor pigs and wild boar.
multivariable logistic regression models of risk factors in a study performed in 2008– <dig>  significant associations with wild boar contacts are expressed by odds ratios  and respective 95% confidence intervals .
the number of records considered in each model is indicated in parentheses .
testing for b. suis infection of exposed pigs
tested animals of 10/ <dig> initially selected piggeries  were negative both by serology and qpcr, including the pigs from p <dig>  which had been in close contact with an infected wild boar .
in parallel to these investigations, a recently acquired breeding boar  from p <dig> died of a peritonitis thought to be due to a foreign body, and bacteriological culture performed post-mortem revealed a concurrent infection with b. suis biovar  <dig> in the epididymidis  <cit> .
all pigs older than six months subsequently tested seropositive to b. suis  in this piggery were stamped out .
six of  <dig> were positive by qpcr and five of them in culture  <cit> .
five qpcr-positive adult sows  presented severe macroscopic lesions: metritis with miliary abscesses , mastitis, and pyometra.
epidemiological investigations on the brucellosis outbreak in mangalitza pigs
animal movements from and to p <dig> and results of bacteriological investigations in the concerned piggeries are presented in figure  <dig>  two potential sources of infection were considered for p5:  spill-over from wild boar: births of cross-bred piglets in  <dig> indicated successful mating between domestic sows and wild boar.
a prevalence of  <dig> % has been documented for b. suis in wild boar from this area, including adults shedding bacteria  <cit> .
animals had regularly been exchanged between p <dig> and another piggery  since  <dig>  and health problems had been observed since  <dig> in both p <dig> and p <dig>  therefore, p <dig> could have been infected by wild boar, and p <dig> by p <dig>  p <dig> also infected p <dig> in selling fattening pigs.
however, comparison of strains from selected wild boar, brown hares  and affected pigs revealed distinct clusters, and wild boar were considered an unlikely source of infection  <cit> .
introduction of infected pigs: while b <dig> was introduced after the decrease of reproduction and therefore considered an unlikely source of infection, p <dig> could have been contaminated by young piglets bought from other farms and have been the origin of the outbreak in p <dig>
discussion
a considerable geographical overlap between the wild boar population and outdoor piggeries in switzerland was previously documented, pointing at a risk of contacts between wild boar and pigs  <cit> .
here, the occurrence of contacts was assessed, risk factors for these interactions were identified, and pigs from farms at risk were tested for b. suis as an example of pathogen transmissible from wild boar to pigs.
occurrence of contacts between wild boar and pigs
to obtain a data set as complete as possible on the occurrence of contacts between wild boar and domestic pigs, we combined several methodical approaches.
first, we carried out two parallel questionnaire surveys among different target groups, namely pig farmers  and game-wardens .
this proved to be efficient as obtained information was largely complementary.
because questionnaire surveys can be easily performed at a large scale, they give access to a large amount of data within a limited time period and increase the chance to record rare events.
in a second step, we completed the questionnaire surveys by telephone interviews and visits of selected piggeries with the aim to validate information received in written form.
past and present occurrence of cross-bred animals was mostly recorded during such visits, revealing that cross-breeding was under-reported in mailed questionnaires filled by farmers.
this is probably due to the fact that farmers are worried about the consequences it may have for their piggery if close contacts with wild boar are known to occur, such as the request of expensive fence transformations by the veterinary authorities or the loss of the official “piggery health status” attributed by the swiss pig health service.
personal contacts allow to gain the trust of the farmers and thus to obtain more truthful data.
the final data set indicated that the observed yearly contact rate between wild boar and pigs per farm is low but that the number of observed contacts varies greatly among farms.
overall, contacts are not rare and sometimes even lead to cross-breeding.
additionally, we surveyed two selected piggeries at risk with camera traps.
because this method is work-intensive and requests expensive material, it is not applicable on a large number of farms.
also, as contacts with wild boar on a single farm may not be frequent, the expected number of detections is disproportionally low compared to the invested effort.
however, results of camera surveillance confirmed our assumption that many contacts are not observed by the farm personal since they occur at night, which is to be expected given that wild boar are mainly nocturnal  <cit> .
consequently, the derived contact rates are conservative since they underestimate reality.
intrusions of wild boar and mating with domestic sows have been documented in france and germany  <cit>  but to our knowledge this study is the first to quantify contacts between pigs and wild boar including cross-breeding in piggeries.
established populations of hybrids are numerous in the united states  <cit> , sardinia  <cit> , and also exist in southern switzerland   <cit> .
however, these cross-bred animals are free-ranging and related to feral pigs and are relevant rather for conservation issues than for pig farming.
risk factors for contacts
this study is also the first one investigating risk factors for contacts between wild boar and domestic pigs.
because of the limited number of participants and the fact that some farmers had omitted to announce observed close contacts in the mail questionnaire, bias cannot be excluded.
however, most identified factors corresponded to our expectations .
the fact that risk factors for distant contacts were not necessarily identified for closer contact categories may have two explanations.
firstly, the backward selection procedure targeted the most significant factors for each category, some factors may have been eliminated even if playing a certain role once a strong factor was already part of the respective model.
secondly, data sets to a certain extent varied among contact categories, thus potentially influencing the weight of each considered factor.
nevertheless, a large distance to buildings of any kind and poor fence protection were repeatedly identified as risk factors in the final models.
overall, piggeries with concrete run-out  are less at risk for any kind of contact with wild boar than piggeries with pasture run-out .
interestingly, our analysis revealed that the pig breed is also important, mangalitza being more at risk for mating with wild boar than other breeds.
there is a higher proportion of mangalitza pigs kept on pasture run-out than l/lw pigs, but these are generally closer to forested areas.
nevertheless, the final model showed that the enclosure location and the breed are independent risk factors and not confounders.
in switzerland, mangalitza is the breed looking most similar and genetically most closely related to wild boar  <cit> , and it could be more attracting to wild boar than other breeds.
infection status of exposed pigs
pig owner compliance for visits and pig testing was moderate.
this was partly due to the fact that positive results have to be announced to the veterinary authorities with severe consequences for the piggery.
only few piggeries and sometimes only a small proportion of the stock could be sampled but an outbreak of brucellosis was documented.
the piggery first found to be infected  gathered all identified risk factors and had experienced a hybrid birth, suggesting an infection through contacts with wild boar, as documented in other countries  <cit> .
however, phylogenetic analysis of b. suis isolated from pigs and wild boar from the surrounding areas revealed distinct clusters and it was considered unlikely that wild boar were the direct origin of the outbreak, although possibly not all wildlife strains were included in the analysis  <cit> .
to our knowledge, spill-over from wild boar to pigs has so far always been concluded from field evidence but not proven by strain analysis.
due to the regular pig exchanges between p <dig> and p <dig>  it was not possible to determine which piggery had infected the other, and to date the source of infection remains unidentified.
risk of pathogen spill-over from wild boar to outdoor pigs
wild boar can carry many pathogens and contacts between wild boar and pigs bear a considerable risk of spill-over  <cit> .
although the risk of transmission is expected to increase from category  <dig> to  <dig>  it is largely dependent on the transmission route of the pathogens  <cit> .
thus, the risk of spill-over is highest for infectious agents transmitted by aerosols, to which all outdoor piggeries in areas with wild boar presence may be exposed , whereas a spill-over is least likely for agents transmitted mainly via the venereal route.
here, investigations focussed on b. suis, which is one of the wild boar pathogen of major concern for domestic pigs in switzerland  <cit> .
although b. suis infection is widespread in swiss wild boar and mating of wild boar with pig sows regularly occurs, a spill-over from wild boar to pigs could not be documented, suggesting that the risk of transmission of b. suis from wild boar to pigs is negligible.
however, a spill-over cannot be completely ruled out because the number of sampled piggeries was low, mating had been documented in only few of them, and the origin of the brucellosis outbreak could not be definitely elucidated.
furthermore, while the risk of b. suis spill-over to pigs may have been very low up to now, the situation could change due to the significant increase of both wild boar abundance and prevalence of b. suis in wild boar  <cit> .
prevention strategies
the outbreak was discovered in the frame of a research project.
brucellosis may be unapparent among living animals since infected sows can still give birth, though after few oestrous cycles of sexual rest  <cit> , and infected boars can remain fertile  <cit> .
furthermore, metritis can easily be overlooked at slaughter, and infections are not always associated to pathological changes.
despite decreased reproduction success in two affected piggeries , veterinary investigations were not carried out and pigs were further exchanged.
this indicates a need to increase the awareness of farmers for infectious diseases.
in switzerland, targeted surveillance of porcine brucellosis is carried out only for breeding boars of conventional breeds.
mangalitza pigs are not represented in semen collection centres but boars are exchanged for mating, and they are not tested for brucellosis because switzerland is considered free of this disease.
it is therefore difficult to completely exclude b. suis infection in the pig population.
based on the present results, serological testing should be carried out before exchanging animals.
our data indicate that outdoor piggeries with low electric fence , located up to 500 m to a forest within the wild boar range and distanced from buildings, in particular those holding mangalitza, are exposed to a serious risk of interactions with wild boar and need better protection.
significant seasonal differences regarding the spatial movements of wild boar were not observed  <cit>  and contacts with domestic pigs were reported in all calendar season.
thus, this protection should be present the whole year round.
because wild boar seem to be generally more attracted by sows than by food  <cit> , it seems appropriate to have the most reliable protection where mature sows are held.
most fences are not wild boar-proof, but wire-mesh fencing added to an electrified wire are most effective  <cit> .
to avoid intrusions and escapes, the swiss pig health service recommends building two  <dig>  m wire-mesh fences in parallel, fixed 30 cm deep in the ground to avoid the fence to be sapped, or reinforced by an additional electric fence around the pasture.
however, the implementation of this type of fencing is challenging because it is perceived as not feasible by large piggeries regularly performing pasture rotations, both on a financial and practical point of view.
CONCLUSIONS
this study shows that interactions between wild boar and outdoor pigs are not uncommon, pointing at the risk of pathogen spill-over even if a wild boar origin could not be documented in the case of the outbreak described here.
it also provides data on risk factors that could help the risk-based implementation of protection measures for piggeries.
since a further augmentation of the wild boar population is expected  <cit>  and modern animal welfare requirements are in favour of outdoor facilities for domestic livestock, interactions between wild boar and pigs will likely increase in the future.
our study underlines the importance of improving surveillance strategies for pathogens shared between wildlife and domestic animals and the need to increase disease awareness of farmers and veterinary practitioners.

 the wild boar population is expanding in europe. rearing domestic pigs outdoor also shows an increasing trend. although this type of animal husbandry provides a better quality of life for pigs, it exposes them to a higher risk of contact with wild boar, which can carry pathogens with various routes of transmission. important air-born pig diseases include porcine reproductive and respiratory syndrome and enzootic pneumonia. in contrast to the prrs virus, mycoplasma hyopneumoniae seems to be widespread in swiss wild boar, which are considered a potential reservoir for this pathogen. classical swine fever virus and mycobacterium bovis are transmitted by indirect or direct close contact. these pathogens are currently not an issue in switzerland but they could be introduced by wild boar emigrating from neighbouring countries. sarcoptes scabiei is transmitted by close contact. while efforts are done to eradicate the mite from swiss piggeries, infestations have been recently diagnosed in wild boar. in wild boar, aujeszky's disease virus and brucella suis are transmitted mainly by the venereal route. while ad virus infection is rare in swiss wild boar, nearly 30% of them are carrier of b. suis, including adults with genital organ infection. the objectives of this study were to document the occurrence of contacts between wild boar and outdoor pigs in switzerland, to identify risk factors for such contacts in order to propose risk-adapted protection measures for piggeries, and to test exposed pigs for a possible pathogen spill-over, taking b. suis as an example. this pathogen was selected because infections have been well documented in swiss wild boar, while domestic pigs are officially free of brucellosis. a re-emergence of brucellosis in outdoor pigs has been observed in france following intrusions of and/or mating with wild boar, indicating a substantial risk of losing the “free from disease” status in switzerland if close contacts between wild boar and pigs occur. furthermore, since transmission of b. suis mostly requires the closest type of contact, b. suis spill-over would suggest that transmission of all other pathogens is possible. here we report different types of interactions between wild boar and outdoor pigs and show that risk factors are not only related to wild boar presence, piggery location and fence characteristics but also include pig breed. we additionally report field investigations performed in the frame of a brucellosis outbreak in domestic pigs with a history of contacts with wild boar.  contacts between wild boar and pigs questionnaire surveys among game-wardens and pig farmers thirty-one percent of the game-wardens who participated to the study observed contacts at least once between and and additionally before for four of them. overall, contacts of any category were reported in swiss cantons. observations included tracks but also intrusions and domestic sows with cross-bred piglets. births of cross-bred animals were reported from 7/ cantons. of the pig farmers who replied the questionnaire, had fattening/finishing pigs, had breeding pigs, kept both types of animals, one held pigs as a hobby, and no information was available for piggeries. two hundred twenty-two piggeries had concrete run-out, had pure pasture run-out, had mixed run-out, five had another type of ground, and did not answer the question. herd size ranged from to more than pigs, with an average of animals per farm. the most represented breeds were landrace and large white pigs followed by duroc and mangalitza. concrete run-outs were fenced with concrete walls, metallic bars, wooden boards, wire mesh, or various combinations of those. in the case of pasture run-outs, fences consisted in electric wires, wire mesh, wooden boards, metallic bars, or combinations. none of these pasture fences were fixed deep in the ground. eighty farmers reported contacts of any category. they mentioned at least one contact since they acquired the piggery, which could go back to the 1970’s. the frequency of contacts ranged from one in 39 years to per year. from these piggeries, the majority had a pure pasture run-out. contacts were significantly more reported from piggeries with pasture and mixed run-out than with concrete run-out. overall, there was an average of contacts of any type per year for a total of outdoor farms, or contact/farm/year. contacts were reported in all calendar seasons and in all cantons with wild boar presence. cases of cross-breeding of wild boar with different pig breeds were reported in 17/ piggeries. most of them were cross-bred with l/lw and mangalitza sows. cross-breeding additionally occurred twice with a pure duroc, once with a cross-bred duroc x l/lw, once with a pot-bellied pig, and once with a minipig. in one of these piggeries holding mangalitza, single and multiple births of cross-bred piglets following intrusions of wild boar or excursions of domestic sows have been observed. in one instance, eight sows gave simultaneously birth to cross-bred piglets. cross-breeding occurred more often with mangalitza than l/lw. overall, there was an average of five mating events per year for a total of farms with pure pasture or mixed run-out, which corresponded to an estimated rate of mating/farm/year. cross-bred piglets were reported in cantons, and in one case no information was obtained on piggery location. 1 f: fattening, b: breeding. 2 l/lw: large white or landrace, m: mangalitza. 3a: alive, s: slaughtered, so: stamped-out. 4including newborn piglets. 5pigs slaughtered before the discovery of the outbreak. 6hybrids were bought from another outdoor piggery for breeding purposes. in this other piggery, however, hybridization had been accidental. 7a breeding domestic sow, which had mated with a wild boar, was bought pregnant of hybrid piglets from another outdoor piggery 8sampling on request of the pig owner. sampled domestic pigs from swiss outdoor piggeries in 2008– in a study on risk factors for contacts between wild boar and outdoor pigs. categories of contacts: presence of wild boar around the enclosure; presence of wild boar just outside the fence; wild boar intrusions without mating; and mating. surveillance of piggeries with camera traps during one year of surveillance in two piggeries at risk, wild boar were detected only in piggery p there were four detections in nearly 16 months, always just outside the enclosure on the pedestrian way between the forest and the fence, at night, from december to march: single juvenile; wild sow with offspring; single adult; two animals. assessment of risk factors for contacts information on potential risk factors for contacts between wild boar and domestic pigs were obtained from pig farmers. selected factors included in the multivariable logistic regression models are listed in additional file 1a-d. the final multivariable models, one for each outcome, showed that pigs in enclosures separated from the piggery building or/and located away from other houses or/and close to a forest were most at risk for indirect contacts with wild boar roaming around the piggery; pigs in enclosures separated from the piggery building or/and protected with an electric fence were most at risk for closer indirect contacts with wild boar, pigs in enclosures located far from the piggery building or/and protected by an electric fence or/and any fence ≤ 60 cm, were most at risk for an intrusion; and piggeries holding mangalitza or/and protected by an electric fence were most at risk for the occurrence of cross-breeding. when comparing the most represented breeds with each other, cross-breeding was more frequent among mangalitza than l/lw. single variables and hypotheses regarding their influence on the occurrence of contacts between outdoor pigs and wild boar. multivariable logistic regression models of risk factors in a study performed in 2008– significant associations with wild boar contacts are expressed by odds ratios and respective 95% confidence intervals. the number of records considered in each model is indicated in parentheses. testing for b. suis infection of exposed pigs tested animals of 10/ initially selected piggeries were negative both by serology and qpcr, including the pigs from p which had been in close contact with an infected wild boar. in parallel to these investigations, a recently acquired breeding boar from p died of a peritonitis thought to be due to a foreign body, and bacteriological culture performed post-mortem revealed a concurrent infection with b. suis biovar in the epididymidis. all pigs older than six months subsequently tested seropositive to b. suis in this piggery were stamped out. six of were positive by qpcr and five of them in culture. five qpcr-positive adult sows presented severe macroscopic lesions: metritis with miliary abscesses, mastitis, and pyometra. epidemiological investigations on the brucellosis outbreak in mangalitza pigs animal movements from and to p and results of bacteriological investigations in the concerned piggeries are presented in figure  two potential sources of infection were considered for p5: spill-over from wild boar: births of cross-bred piglets in indicated successful mating between domestic sows and wild boar. a prevalence of % has been documented for b. suis in wild boar from this area, including adults shedding bacteria. animals had regularly been exchanged between p and another piggery since and health problems had been observed since in both p and p therefore, p could have been infected by wild boar, and p by p p also infected p in selling fattening pigs. however, comparison of strains from selected wild boar, brown hares and affected pigs revealed distinct clusters, and wild boar were considered an unlikely source of infection. introduction of infected pigs: while b was introduced after the decrease of reproduction and therefore considered an unlikely source of infection, p could have been contaminated by young piglets bought from other farms and have been the origin of the outbreak in p discussion a considerable geographical overlap between the wild boar population and outdoor piggeries in switzerland was previously documented, pointing at a risk of contacts between wild boar and pigs. here, the occurrence of contacts was assessed, risk factors for these interactions were identified, and pigs from farms at risk were tested for b. suis as an example of pathogen transmissible from wild boar to pigs. occurrence of contacts between wild boar and pigs to obtain a data set as complete as possible on the occurrence of contacts between wild boar and domestic pigs, we combined several methodical approaches. first, we carried out two parallel questionnaire surveys among different target groups, namely pig farmers and game-wardens. this proved to be efficient as obtained information was largely complementary. because questionnaire surveys can be easily performed at a large scale, they give access to a large amount of data within a limited time period and increase the chance to record rare events. in a second step, we completed the questionnaire surveys by telephone interviews and visits of selected piggeries with the aim to validate information received in written form. past and present occurrence of cross-bred animals was mostly recorded during such visits, revealing that cross-breeding was under-reported in mailed questionnaires filled by farmers. this is probably due to the fact that farmers are worried about the consequences it may have for their piggery if close contacts with wild boar are known to occur, such as the request of expensive fence transformations by the veterinary authorities or the loss of the official “piggery health status” attributed by the swiss pig health service. personal contacts allow to gain the trust of the farmers and thus to obtain more truthful data. the final data set indicated that the observed yearly contact rate between wild boar and pigs per farm is low but that the number of observed contacts varies greatly among farms. overall, contacts are not rare and sometimes even lead to cross-breeding. additionally, we surveyed two selected piggeries at risk with camera traps. because this method is work-intensive and requests expensive material, it is not applicable on a large number of farms. also, as contacts with wild boar on a single farm may not be frequent, the expected number of detections is disproportionally low compared to the invested effort. however, results of camera surveillance confirmed our assumption that many contacts are not observed by the farm personal since they occur at night, which is to be expected given that wild boar are mainly nocturnal. consequently, the derived contact rates are conservative since they underestimate reality. intrusions of wild boar and mating with domestic sows have been documented in france and germany but to our knowledge this study is the first to quantify contacts between pigs and wild boar including cross-breeding in piggeries. established populations of hybrids are numerous in the united states, sardinia, and also exist in southern switzerland. however, these cross-bred animals are free-ranging and related to feral pigs and are relevant rather for conservation issues than for pig farming. risk factors for contacts this study is also the first one investigating risk factors for contacts between wild boar and domestic pigs. because of the limited number of participants and the fact that some farmers had omitted to announce observed close contacts in the mail questionnaire, bias cannot be excluded. however, most identified factors corresponded to our expectations. the fact that risk factors for distant contacts were not necessarily identified for closer contact categories may have two explanations. firstly, the backward selection procedure targeted the most significant factors for each category, some factors may have been eliminated even if playing a certain role once a strong factor was already part of the respective model. secondly, data sets to a certain extent varied among contact categories, thus potentially influencing the weight of each considered factor. nevertheless, a large distance to buildings of any kind and poor fence protection were repeatedly identified as risk factors in the final models. overall, piggeries with concrete run-out are less at risk for any kind of contact with wild boar than piggeries with pasture run-out. interestingly, our analysis revealed that the pig breed is also important, mangalitza being more at risk for mating with wild boar than other breeds. there is a higher proportion of mangalitza pigs kept on pasture run-out than l/lw pigs, but these are generally closer to forested areas. nevertheless, the final model showed that the enclosure location and the breed are independent risk factors and not confounders. in switzerland, mangalitza is the breed looking most similar and genetically most closely related to wild boar, and it could be more attracting to wild boar than other breeds. infection status of exposed pigs pig owner compliance for visits and pig testing was moderate. this was partly due to the fact that positive results have to be announced to the veterinary authorities with severe consequences for the piggery. only few piggeries and sometimes only a small proportion of the stock could be sampled but an outbreak of brucellosis was documented. the piggery first found to be infected gathered all identified risk factors and had experienced a hybrid birth, suggesting an infection through contacts with wild boar, as documented in other countries. however, phylogenetic analysis of b. suis isolated from pigs and wild boar from the surrounding areas revealed distinct clusters and it was considered unlikely that wild boar were the direct origin of the outbreak, although possibly not all wildlife strains were included in the analysis. to our knowledge, spill-over from wild boar to pigs has so far always been concluded from field evidence but not proven by strain analysis. due to the regular pig exchanges between p and p it was not possible to determine which piggery had infected the other, and to date the source of infection remains unidentified. risk of pathogen spill-over from wild boar to outdoor pigs wild boar can carry many pathogens and contacts between wild boar and pigs bear a considerable risk of spill-over. although the risk of transmission is expected to increase from category to it is largely dependent on the transmission route of the pathogens. thus, the risk of spill-over is highest for infectious agents transmitted by aerosols, to which all outdoor piggeries in areas with wild boar presence may be exposed, whereas a spill-over is least likely for agents transmitted mainly via the venereal route. here, investigations focussed on b. suis, which is one of the wild boar pathogen of major concern for domestic pigs in switzerland. although b. suis infection is widespread in swiss wild boar and mating of wild boar with pig sows regularly occurs, a spill-over from wild boar to pigs could not be documented, suggesting that the risk of transmission of b. suis from wild boar to pigs is negligible. however, a spill-over cannot be completely ruled out because the number of sampled piggeries was low, mating had been documented in only few of them, and the origin of the brucellosis outbreak could not be definitely elucidated. furthermore, while the risk of b. suis spill-over to pigs may have been very low up to now, the situation could change due to the significant increase of both wild boar abundance and prevalence of b. suis in wild boar. prevention strategies the outbreak was discovered in the frame of a research project. brucellosis may be unapparent among living animals since infected sows can still give birth, though after few oestrous cycles of sexual rest, and infected boars can remain fertile. furthermore, metritis can easily be overlooked at slaughter, and infections are not always associated to pathological changes. despite decreased reproduction success in two affected piggeries, veterinary investigations were not carried out and pigs were further exchanged. this indicates a need to increase the awareness of farmers for infectious diseases. in switzerland, targeted surveillance of porcine brucellosis is carried out only for breeding boars of conventional breeds. mangalitza pigs are not represented in semen collection centres but boars are exchanged for mating, and they are not tested for brucellosis because switzerland is considered free of this disease. it is therefore difficult to completely exclude b. suis infection in the pig population. based on the present results, serological testing should be carried out before exchanging animals. our data indicate that outdoor piggeries with low electric fence, located up to 500 m to a forest within the wild boar range and distanced from buildings, in particular those holding mangalitza, are exposed to a serious risk of interactions with wild boar and need better protection. significant seasonal differences regarding the spatial movements of wild boar were not observed and contacts with domestic pigs were reported in all calendar season. thus, this protection should be present the whole year round. because wild boar seem to be generally more attracted by sows than by food, it seems appropriate to have the most reliable protection where mature sows are held. most fences are not wild boar-proof, but wire-mesh fencing added to an electrified wire are most effective. to avoid intrusions and escapes, the swiss pig health service recommends building two  m wire-mesh fences in parallel, fixed 30 cm deep in the ground to avoid the fence to be sapped, or reinforced by an additional electric fence around the pasture. however, the implementation of this type of fencing is challenging because it is perceived as not feasible by large piggeries regularly performing pasture rotations, both on a financial and practical point of view.  this study shows that interactions between wild boar and outdoor pigs are not uncommon, pointing at the risk of pathogen spill-over even if a wild boar origin could not be documented in the case of the outbreak described here. it also provides data on risk factors that could help the risk-based implementation of protection measures for piggeries. since a further augmentation of the wild boar population is expected and modern animal welfare requirements are in favour of outdoor facilities for domestic livestock, interactions between wild boar and pigs will likely increase in the future. our study underlines the importance of improving surveillance strategies for pathogens shared between wildlife and domestic animals and the need to increase disease awareness of farmers and veterinary practitioners. 
due to the parallel increase of the number of free-ranging wild boar and domestic pigs reared outdoor, the risk that they interact has become higher. contacts with wild boar can be the origin of disease outbreaks in pigs, as it has been documented for brucellosis in some european countries. this study aimed at quantifying the occurrence of contacts between wild boar and outdoor domestic pigs in switzerland, and identifying risk factors for these contacts. furthermore, exposed pigs were tested for pathogen spill-over, taking brucella suis as an example because b. suis is widespread in swiss wild boar while domestic pigs are officially free of brucellosis. thirty-one percent of the game-wardens and 25% of the pig owners participating to a country-wide questionnaire survey reported contacts, including approaches of wild boar outside the fence, intrusions, and mating. pigs of the mangalitza breed were most at risk for mating with wild boar. one piggery with previous wild boar contacts was found infected with b. suis, however, epidemiological investigations failed to identify the direct source of infection. results show that interactions between wild boar and outdoor pigs are not uncommon, pointing at the existing risk of pathogen spill-over. provided data on risk factors for these interactions could help the risk-based implementation of protection measures for piggeries. the documentation of a brucellosis outbreak in pigs despite the freedom-of-disease status underlines the importance of improving pathogen surveillance strategies and increasing disease awareness of farmers and veterinary practitioners. blood and tissues of outdoor pigs from piggeries were tested for an infection with brucella suis, using rose bengal test, complement fixation test, and an is711-based real-time pcr. risk factors for contacts identified by a uni- and multivariable logistic regression approach were: distance between pigs enclosure and houses, proximity of a forest, electric fences, and fences ≤ 60 cm.
 due to the parallel increase of the number of free-ranging wild boar and domestic pigs reared outdoor, the risk that they interact has become higher. contacts with wild boar can be the origin of disease outbreaks in pigs, as it has been documented for brucellosis in some european countries. this study aimed at quantifying the occurrence of contacts between wild boar and outdoor domestic pigs in switzerland, and identifying risk factors for these contacts. furthermore, exposed pigs were tested for pathogen spill-over, taking brucella suis as an example because b. suis is widespread in swiss wild boar while domestic pigs are officially free of brucellosis.  thirty-one percent of the game-wardens and 25% of the pig owners participating to a country-wide questionnaire survey reported contacts, including approaches of wild boar outside the fence, intrusions, and mating. seventeen piggeries reported the birth of cross-bred animals. risk factors for contacts identified by a uni- and multivariable logistic regression approach were: distance between pigs enclosure and houses, proximity of a forest, electric fences, and fences ≤ 60 cm. pigs of the mangalitza breed were most at risk for mating with wild boar. blood and tissues of outdoor pigs from piggeries were tested for an infection with brucella suis, using rose bengal test, complement fixation test, and an is711-based real-time pcr. one piggery with previous wild boar contacts was found infected with b. suis, however, epidemiological investigations failed to identify the direct source of infection.  results show that interactions between wild boar and outdoor pigs are not uncommon, pointing at the existing risk of pathogen spill-over. provided data on risk factors for these interactions could help the risk-based implementation of protection measures for piggeries. the documentation of a brucellosis outbreak in pigs despite the freedom-of-disease status underlines the importance of improving pathogen surveillance strategies and increasing disease awareness of farmers and veterinary practitioners. brucella suisbrucellosisinteractionsoutdoor pigsrisk factorsspill-overwild boar 