A Proposed Role for Pro-Inflammatory Cytokines in Damaging Behavior in Pigs

Abstract

Sickness can change our mood for the worse, leaving us sad, lethargic, grumpy and less socially inclined. This mood change is part of a set of behavioral symptoms called sickness behavior and has features in common with core symptoms of depression. Therefore, the physiological changes induced by immune activation, for example following infection, are in the spotlight for explaining mechanisms behind mental health challenges such as depression. While humans may take a day off and isolate themselves until they feel better, farm animals housed in groups have only limited possibilities for social withdrawal. We suggest that immune activation could be a major factor influencing social interactions in pigs, with outbreaks of damaging behavior such as tail biting as a possible result. The hypothesis presented here is that the effects of several known risk factors for tail biting are mediated by pro-inflammatory cytokines, proteins produced by the immune system, and their effect on neurotransmitter systems. We describe the background for and implications of this hypothesis.

The effects of boar on susceptibility to swine inflammation and necrosis syndrome in piglets

“…on the basis of LPS, are likely to be not only limited to local reactions, but also lead to massive reactions in the CNS that can cause sickness behaviour, Post Partum Dysgalactia Syndrome (PPDS [ 32 , 33 ]), and that can even trigger tail biting [ 34 , 35 ]. Nordgreen et al [ 35 ] give a comprehensive synthesis of how problems in the areas of microbiota, blood-gut barrier, housing environment and hygiene, immune activation, mycotoxins, psychological stress, nutritional status and feed composition can synergistically lead to inflammation directly or indirectly in association with LPS, and how the CNS is influenced by the inflammation mediators. Here our hypothesis of the development of SINS [ 15 ] as a further local component in addition to the central effects fits perfectly in.…”

“…An important source for the flooding of LPS is the intestine (for discussion see Nordgreen et al [ 35 ], especially under high protein/crude fibre ratios, intestinal diseases and disturbances to the blood-gut barrier [ 36 ]. The blood-gut barrier in pigs is particularly susceptible to heat stress [ 37 – 40 ], reduced intestinal perfusion [ 38 ] and mycotoxins [ 41 , 42 ].…”

“…Furthermore, psychological stress e.g. because of missing environmental enrichment does appear to cause inflammatory reactions, which are similar to those seen as a response to LPS challenge [ 35 ]. And all these factors, thermoregulation, water supply (quantity and quality), high protein and starch content in the feed, low crude fibre content, mycotoxin contamination and stress are commonly highlighted in the context of husbandry problems in pig production, often together with tail biting and tail lesions (e.g.…”

The multivariate approach identifies relationships between pre-slaughter factors, body lesions, ham defects and carcass traits in pigs

“…Moreover, the immune system is known to modulate the muscular metabolism, especially with regards to protein and fat content, and therefore it is reasonable to hypothesize differences in the lean meat percentage observed at slaughter in batches with a higher prevalence of ham abscesses or muscle tears. Besides, recent research [ 42 ] has been proposed that immune activation, especially the involvement of cytokines compounds, could be a major factor in social interaction in pigs, as an example, in the development of tail biting and tail lesion outbreak. For this reason, one limitation of the study is the absence of physiological indicators that could have been used to validate the present hypotheses.…”

Swine Inflammation and Necrosis Syndrome Is Associated with Plasma Metabolites and Liver Transcriptome in Affected Piglets

“…Recently, it was demonstrated in mice that LPS injected into the intrauterine space gains access to the maternal-fetal compartment and interacts with the cells expressing functional TLR4, a receptor sensing MAMPs, in the placenta and fetus [ 33 ]. Regarding positive associations between plasma LPS concentration of sows and severity of SINS symptoms of the piglets [ 10 , 11 , 13 ], it is therefore likely that the hepatic inflammatory process in SINS-affected piglets was caused by MAMPs.…”