The objective of this study was to estimate the impact of respiratory disease on production performance indicators. It is accepted that both slaughter checks and serology are mostly related to the health status of the pig by the end of the finisher stage. However, some of the lesions found at slaughter were related to group mortality in the weaner stage. The prevalence of scars was related to higher weaner mortality, which is compatible with the nature of these lesions. Scars are healed pneumonia lesions, most probably occurring in weaner or early finisher stages. The tendencies found for cranial pleurisy and pericarditis were also biologically tenable as higher cranial pleurisy and pericarditis reveal on-farm health issues such as bacterial polyserositis, driving general mortality up especially in weaners. The time distance between the appearance of pericarditis and pleurisy and its finding at the abattoir is not well defined in the literature and needs to be investigated in future research to confirm their relationship to weaner mortality.
Vaccination for M. hyopneumoniae was related to higher finisher mortalities. This association is likely to be explained by the higher health status of farms free from M. hyopneumoniae which, therefore, were not vaccinating for that pathogen. In general, vaccination for M. hyopneumoniae and PRRS were related to worse production performance indicators in the univariate analysis, showing that vaccines are in place when there are issues that affect performance. The number of farms affected by these pathogens but not vaccinating was low, which makes it difficult to estimate the effect of vaccination in positive farms. M. hyopneumoniae infections are also relevant due to the aggravation of the lung lesions with secondary infections, which are commonly linked to lung abscesses [27, 28]. Finisher mortality was also related to the size of the herd. Agostini et al. [29] reported similar results and suggested that in bigger farms, less attention may be paid to individual finisher pigs.
Farms slaughtering pigs at higher live weights had increased ADFI. This finding makes sense as it is well known that the ADFI of pigs increases as they grow. Positivity to M. hyopneumoniae and the level of antibodies for PRRS were both related to a decrease in ADFI. Both diseases are known to be among the main ones affecting performance in pig herds [6, 7, 27]. Of the studied lesions, the prevalence of cranial pleurisy and liver milk spots decreased ADFI. Pleurisy is known to cause respiratory distress to the pig and as an inflammatory process should be expected to reduce intake. The liver milk spots are highly suggestive of infection by Ascaris suum [30, 31], which is also related to decreased ADFI and ADG [32–34].
The models for ADG and age at sale were very similar. Positivity for PRRS and the prevalence of cranial pleurisy were both related to lower ADG and higher age at sale. PRRS is the main disease affecting growth of pigs with (post-outbreak) estimated costs of $17.7 USD per pig in farrow-to-finish farms [35]. Our findings confirm the relevance of PRRS as an important factor affecting performance also in Irish conditions. Pleurisy is also known to result in important production losses. In the UK, 10% pleurisy prevalence at batch level was estimated to cost approximately 226p (GBP) per slaughter pig [36]. In all the models described in this study, cranial pleurisy showed better predictive values than average dorsocaudal pleurisy or moderate to severe dorsocaudal pleurisy lesions. However, these variables were highly correlated and could be used interchangeably in the models. Although cranial pleurisy may not necessarily be linked to a particular disease, dorsocaudal pleurisy is in general related to A. pleuropneumoniae [21] which is very prevalent in Irish pig farms as shown in this study. Taking into account the low use of vaccination for A. pleuropneumoniae in Ireland, the situation could be improved with wider use of vaccination [2, 37, 38], which in turn, would result in a reduction in the use of antibiotics. The only difference for the models for ADG and age at sale was that ADG increased as weight at slaughter was higher but age at sale was more affected by herd size. It is well known that ADG increases as the pig increases in size, thus it makes sense that selling bigger pigs improves ADG. On the other hand, a worsening in performance as the size of the herd increases has been reported previously. In a study analysing production parameters and production cost over time (2010–2014) in Spain, Rocadembosch et al. [39] concluded that herd size affected negatively most performance indicators, including ADG in nursery and finishing stages, as also found in this other study [40].
The models fitted explained a significant percentage of the variability for weaner mortality, finisher mortality, ADFI, ADG, and age at sale. It is interesting to notice that the models could be used to explain almost the double of the variability in ADFI, ADG, and age at sale when compared to the variability of weaner and finisher mortality. The understanding of the morbidity and mortality of disease, especially in the absence of secondary infections, could explain the impact on performance without necessarily causing increased mortality. However, the number of pigs sold per sow per year and FCR did not produce good models. The pigs sold per sow per year were included as an indicator of sow productivity to study the effects of respiratory disease in sow performance. However, in this study, no significant effects were found. On the other hand, FCR was only affected negatively by herd size and cranial pleurisy, but these effects only accounted for 14% of the variability. This result suggests that disease clearly affects the growth rate and feed intake of pigs but does not necessarily make production less efficient in terms of feed use.
One limitation of this study is that serology and slaughterhouse assessments correspond to one batch of each farm, instead of including multiple batches to account for a representative sample of the farm. In fact, a minimum of two batches per farm were assessed at slaughter and a maximum of five batches per farm were assessed throughout the study period, but the reasoning to use data from one batch solely was to assure that serology results were a perfect match to the pluck lesions assessed on the same day. Pluck lesions’ averages for the multiple batches assessed per farm were compared to the values of the batch used for these analyses and only minimal differences were found, mainly in the prevalence of liver milk spots (data not shown). Finally, the production performance figures accounted for the whole year of 2017, as opposed to referring to the batches assessed at slaughter.