As described in more detail in this month’s accompanying PROHEALTH review article, a recently published paper from economics work in the project highlighted the importance of good hygiene for pig farm profitability. At the farm level, improved hygiene was determined as the most cost-effective intervention against production diseases of all those examined, with the modelled financial impact of adopting this intervention for an average-sized pig fattening farm being equivalent to approximately 50% of the gross margin.
Where do the large benefits of good hygiene come from?
Whilst the effects of clinical disease are often clearly apparent, the effects of subclinical disease challenges can easily pass unnoticed. Poor hygiene conditions which occur from keeping pigs in pens that were not cleaned or disinfected after a previous occupation by other pigs increase microbial pressure, resulting in a continuous low level challenge from pathogenic agents. This results in a chronic stimulation of the pigs’ immune system, which can be shown by increased blood levels of white cells and haptoglobin, an acute phase protein synthesised by the liver in response to immune system activation, even in the absence of clinical signs of disease.
Stimulation of the immune system results in a series of physiological responses which are detrimental for efficient production. Cytokines, which are messenger molecules synthesised by activated immune cells, regulate many metabolic processes in the body. They can reduce feed intake, reduce muscle protein synthesis and increase protein breakdown in muscles, changing nutrient partitioning away from growth towards metabolic responses in support of immune function.
How big are the production consequences?
In 2012, INRA researchers carried out a review of 122 published experiments reporting the effects on feed intake and growth of pigs subjected to different sanitary challenges. They found that for challenges associated with poor housing conditions including poor hygiene conditions, on average the daily feed intake was reduced by 4% and the daily gain by 10% in comparison with unchallenged control pigs. This indicates a significant deterioration in feed conversion efficiency, because energy and amino acids which would otherwise be used for growth are being diverted to the synthesis of cytokines, antibodies, acute-phase proteins and specific immune cells.
More recent experiments, both within the PROHEALTH project and elsewhere, reinforce these conclusions but also show the great variability in the production penalty of housing pigs in poor sanitary conditions, as illustrated in Figure 1.
Figure 1. The effect on production of housing pigs in poor sanitary conditions. Data from recent experiments in France and The Netherlands show the percentage change in performance parameters of pigs housed in pens receiving no cleaning between batches compared to contemporary groups housed in pens which were cleaned and disinfected prior to entry of the pigs.
Why might some pigs be more adversely affected than others?
Many factors might influence the severity of production deficits in response to poor hygiene, including the quality of ventilation in the housing and the nature of pathogenic challenges present on the farm. However, particular attention has focussed on two issues related to the knowledge that hygiene challenge results in partitioning of dietary protein to components of the immune response rather than lean tissue growth. The possibility that pigs with a higher genetic performance potential, and therefore a higher nutrient demand, are more adversely affected by health challenges was investigated in an experiment within the PROHEALTH project3. Two genetic lines of pig, divergently selected for residual feed intake (low RFI = high feed efficiency, high RFI = poorer feed efficiency), were housed in two contrasting hygiene conditions (clean or dirty pens) from 12 to 18 weeks of age. Pigs housed in dirty conditions showed a higher prevalence of pneumonia (51% v. 8%) and large overall reductions in growth and feed efficiency (see Figure 1). However, perhaps surprisingly, the more efficient low RFI pigs had lower lung lesion scores and a lower acute phase protein response to the unhygienic conditions. In line with these health observations, growth rate and feed efficiency were less affected by poor hygiene in low RFI pigs than in high RFI pigs. Thus, while the results highlighted different responses to the hygiene challenge of two pig lines differing in feed efficiency, further research is needed to understand the genetic links between production traits and resilience to challenges of poor hygiene.
A second area of research relates to nutritional influences on the response to hygiene challenges. Because growth is reduced when dietary protein is repartitioned to an immune response, and a different amino acid profile is needed for this function, it is possible that feeding a diet higher in protein, and especially in the amino acids methionine, threonine and tryptophan, might reduce the performance penalty. This was investigated in a Dutch experiment4 in which pigs, for the whole fattening period, were housed in good or poor sanitary conditions and were given diets either normal or low in protein level, and with or without a further supplement of these three essential amino acids. Once again pleuritis scores at slaughter and acute phase response was higher in pigs kept in poor sanitary conditions, and their performance was reduced (see Figure 1). A 20% increase in dietary supplementation of Met, Thr, and Trp relative to Lys increased feed efficiency to a greater extent in unhygienic pens than in pigs kept in better sanitary conditions. This study indicates the importance of matching dietary amino acid profile to the sanitary status, and consequent degree of immune activation, on the farm.
Possible consequences of poor hygiene for adverse behaviours
A very topical additional observation from the Dutch experiment on the effects of sanitary conditions on performance relates to possible consequences for the risk of damaging behaviours such as tail and ear biting. At a time when European legislators are actively seeking to reduce the practice of tail docking in pigs, and thus increasing the importance of other measures which can reduce the risk of tail biting, the role of health and dietary factors is receiving increased attention. A recent review in this area highlighted the evidence that tail biting might be triggered by the cytokine production and amino acid imbalances associated with immune activation. It is therefore of interest to note that, in the Dutch experiment described previously, where all pigs were tail docked, the frequency of ear biting and prevalence of ear damage were increased in poor sanitary conditions when compared with good sanitary conditions. Furthermore, this effect on ear biting was diet dependent, with a 20% supplementation of methionine, threonine and tryptophan reducing ear biting by 18% in the pigs kept in poor sanitary conditions. This supports the suggestion that the changed partitioning of protein associated with a hygiene challenge may contribute significantly to the risk of outbreaks of injurious behaviour.
In conclusion, many experiments now show that poor sanitary conditions have a major adverse impact on production performance. Despite this, a recently published survey of farm practices from the PROHEALTH project showed that mean scores for cleaning and disinfection practices ranged from only 40/100 to 87/100 in different EU countries. Since improved hygiene has been identified as by far the most cost-effective method to reduce the impact of production diseases and increase financial returns, with possible additional benefits for reduction of risk of injurious behaviours, it clearly merits greater attention.
 Niemi, J. et al. 2020. A value chain analysis of interventions to control production diseases in the intensive pig production sector. PLoS ONE, 15: e0231338.
 Pastorelli, H. et al. 2012. Meta-analysis of feed intake and growth responses of growing pigs after a sanitary challenge. Animal, 6: 952-961.
 Chatelet, A. et al. 2018. Impact of hygiene of housing conditions on performance and health of two pig genetic lines divergent for residual feed intake. Animal, 12: 350–358.
 van der Meer, Y. et al. 2016. Performance of pigs kept under different sanitary conditions affected by protein intake and amino acid supplementation. Journal of Animal Science, 94: 4704–4719
 van der Meer, Y. et al. 2017. A link between damaging behaviour in pigs, sanitary conditions, and dietary protein and amino acid supply. PLoS ONE, 12: e0174688.
 Nordgreen, J. et al. 2020. A proposed role for pro-inflammatory cytokines in damaging behavior in pigs. Frontiers in Veterinary Science, 7: 646.
 Chantziaras, I. et al. 2020. Biosecurity levels of pig fattening farms from four EU countries and links
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