The BPHS is used by farmers to monitor their within-farm prevalence of respiratory lesions, to identify potential outbreaks of respiratory disease in their herd and to assess whether different management interventions e.g. vaccination are successful at reducing lung lesions present at slaughter. To our knowledge this is the first study to objectively assess the usefulness of BPHS data for the monitoring of respiratory diseases, other than EP. The study found several statistical associations with respiratory lesions examined by BPHS at slaughter.
There are several limitations in the current study; pigs which were blood sampled and tested for respiratory pathogens were not the same pigs examined for gross lesions at slaughter. Lesions may vary substantially between pigs and over time therefore the results must be interpreted with extreme caution. However, the BPHS data used was as close as possible to the time of blood sampling and respiratory pathogens often persist within farms [23, 24]. In order to evaluate the BPHS further, testing would need to be carried out on pigs undergoing BPHS examination and the results of these tests linked with their BPHS reports. Despite these issues the study does not attempt to draw causal inferences between lesions present at slaughter and circulating pathogens in the herd; but simply identifies potential associations which could be used as hypotheses in further studies.
Another drawback is that not all respiratory pathogens were tested for, therefore associations between the pathogens present and BPHS lesions may be confounded by the presence of other respiratory pathogens. Also, analysis was performed at the farm-level and farms were simply classified as positive/negative for most pathogens. The presence of gross lesions in lungs at slaughter is likely to be influenced by within-herd prevalence and pathogen load, in addition to presence/absence of infection.
Management practices of the farm often play an important role in respiratory disease; farms that finished their own meat pigs on site and kept weaners and finishers together had increased odds of submitting pigs with severe pleurisy and an increased score for BPHS component 1, respectively. If further studies were carried out investigating which management factors are associated with respiratory lesions present and slaughter, this information could be used to provide farmers with useful recommendations for tackling respiratory disease problems. Many studies focus on the relationship between management factors and pathogens present in the herd. However, if it could be shown that certain interventions can improve carcass quality this could help farmers become more responsive to their BPHS reports.
In the univariate analysis, farms which vaccinated against M. hyopneumoniae had higher EP scores. Farmers will use their BPHS reports to aid their decision on whether or not to vaccinate against M. hyopneumoniae, with higher scoring farms more likely to start vaccinating. Hence, in the current study, vaccinating and non-vaccinating farms are unlikely to be comparable in terms of initial level of disease and it is not possible to draw strong conclusions on the impact of vaccination. Previous studies have found reduced lung lesions of EP in vaccinated pigs at slaughter , however, in a study by Villarreal et al (2011) this reduction was non-significant . In this study it is likely that M. hyopneumoniae vaccination has reduced the impact of the pathogen to some extent; however farms still have higher EP scores compared to non-vaccinating farms, which are less likely to have had problems with EP. It is important to remember that M. hyopneumoniae vaccination will not eliminate M. hyopneumoniae but reduce extent of clinical signs and lesions of EP at slaughter. Further the presence/absence of certain risk factors will play a role in disease.
EP-like lung lesion score is frequently used to assess herd health with regards to M. hyopneumoniae, but information regarding other lesions present at slaughter could be made more useful. Although, it is not possible to make an etiological diagnosis on the basis of slaughterhouse examination only and many respiratory problems, pleurisy in particular, are multifactoral . However this study, and others , relating gross lung lesions to the presence of certain respiratory pathogens in farms and farm management practices, could be used to give producers and veterinarians some direction when tackling respiratory disease in response to their BPHS reports. The results of the current study and a project conducted by BPEX indicate that PRRS and H1N2 may be associated with the presence of mild and severe pleurisy in pig lungs at slaughter, respectively . In addition, this study highlights the high proportion of farms seropositive for H1N2 (50%). However, with regards to pleurisy; SI and PRRSV do not currently appear in the BPHS guidelines for producers interpreting their reports.
Although, H1N2 was associated with severe pleurisy in the current study, H1N1 was not associated with any of the lung lesions. This was unexpected as H1N1 is the most common isolate identified from passive surveillance in the UK, therefore it was thought that H1N1 may be more pathogenic . The BPHS is carried out in clinically healthy pigs therefore, perhaps if pigs infected with H1N2 are less likely to exhibit clinical signs they may be more likely to reach slaughter. However, the results of the study suggest these pigs may still have lung lesions present at slaughter, which may result in economic losses.
For a typical 10% prevalence of pleurisy at batch level, losses are estimated to be approximately £2.26 per pig due to reduced carcass weight and/or increased time to slaughter . Within farms in the current study, up to 44% of pigs assessed had severe pleurisy, therefore costs to these producers are likely to be high. The study also found that increased scores for BPHS component 1 were associated with a decrease in the average carcass weight indicating that farms with a high prevalence of EP, severe pleurisy and acute pleuropneumonia (plus viral pneumonia and pericarditis) may have an overall reduction in productivity compared with farms that do not. The BPHS report for 2005 to 2008, suggests there has been a linear trend towards reduction in mean monthly prevalence of pleurisy in participating farms , suggesting participation in the BPHS scheme may have economic benefits for producers, although these benefits are difficult to quantify.
Although the recording of lesions is abattoir based, so only pigs of slaughter age are included in the sample, there is potential for BPHS data to support disease surveillance in pigs at a national level. BPEX already monitor the prevalence of the lesions scored by BPHS in participating farms, producing reports detailing changes in the prevalence of lesions. A sudden increase in the prevalence of lesions could act as an early warning system for an emerging disease or indicate a new outbreak of an old one. Spatial analysis of the BPHS data could identify severely affected areas and local/regional outbreaks of disease, providing numbers of farms assessed by the scheme in each area was controlled for. The scheme could also aid evaluation of new control measures, e.g. vaccination or management changes, not only at farm-level but at a national level. For example within the context of pig farms considered in the current study, BPHS reports could be used more efficiently by producers, veterinarians and BPEX to assess the effectiveness of vaccination against PCV2 at reducing disease in slaughter pigs.
The contribution BPHS data can make to the British pig industry depends on the coverage of farms, the more producers enrolled in the scheme the more representative the data. Only specialised abattoirs offer the scheme, participation is voluntary and producers have to pay membership fees . From July, 2011 all assured English pig producers who register for membership of the National Pig Health Improvement Project (http://www.pighealth.org.uk/health/home.eb) will receive free membership to the BPHS scheme and the number of abattoirs offering the scheme has increased. Although the sample is still biased towards large producers these incentives should increase participation to some extent.