An objective of this work was to identify the main risk factors for TB behavior. Several risk factors were identified in multiple papers but there was conflicting evidence regarding the relative importance of each of these factors [12,13]. There were also various study methods used to determine risk factors, as some studies used rope and chew tests as tail models [14,15,16] rather than observing pigs. The most prevalent and consistent risk factor outlined in the charted data was a lack of environmental enrichment. Several other risk factors were presented in the literature, although the importance and success of correcting these risk factors were inconsistent between studies [17,18,19]. Studies have demonstrated that TB occurs most commonly during the grower-finisher phase of production [20,21]; however, this review contained studies which included all stages of production, from breeding age pigs to nursing piglets. Investigation into whether risk factors are consistent over time, or if different risks pose greater challenges at specific phases of production may be an area of study which will prove useful in increasing our basic understanding of TB and decreasing the inconsistencies of study results.

Classifying risks and interventions as unique factors was problematic during analysis, as several studies combined or did not explicitly state which measures were risk factors and which were intervention strategies. Intervention studies have revealed inconsistencies as to the success of the various methods commonly employed on commercial farms. The use of environmental enrichment, including rooting material such as straw, typically decreases TB [22,23]; however, the success of a rooting substrate was found to be dependent upon the quantity of material and the general acceptance of the rooting material by the pigs [24,25]. Furthermore, the addition of rooting material relies on the manure management system; unfortunately, the majority of swine farms use liquid manure systems which may be ill-equipped to handle straw [26], or other types of rooting material. This scoping review highlights that further investigation regarding species relevant enrichment which can be utilized with current manure systems is necessary.

The lack of literature from countries outside of the EU from the past decade was an interesting and concerning finding in this review. Genetics, pig-rearing practices and industry standards have evolved to reflect the current potential of the pigs and the market. Although similar rearing practices are implemented in North America and the EU, and similar quantities of pork and hogs [12,13] are produced in the EU and the US, individual variations within the regulations regarding animal production and animal welfare among the countries represented in the review exist [6,27,28,29,30,31,32,33], and therefore, necessitate original research from all geographic locations. The lack of TB research, outside of the EU, is particularly concerning, as three of the world’s top four pig producing areas: China, the world’s leader in pig production, the US and Brazil [34] only had three publications between them in the last decade, compared to 53 publications from the EU. Pig welfare is becoming more common within China, yet the majority of this work is unavailable in English, as reviewed by Sinclair et al. in 2020 [35], which may have resulted in a perceived lack of data. This apparent lack of research because of limiting this scoping review to English publications may also extend to other non-English speaking pig producing nations. Concern over livestock welfare has been present in the EU for many decades resulting in legislation affecting pig production and animal welfare [4,9,10,11,28,36], and therefore research designed to help guide government policy is likely partly responsible for the large number of publications from the EU. Considering the economic [2,37] and welfare implications associated with TB [2,28], this review demonstrated that there is need for studies relating to TB in pig producing countries outside of the EU to inform modern pig production.

A ban on the tail-docking of pigs has been implemented in the EU [38] due to the negative welfare concerns surrounding the practice and the implication that the procedure does not address the underlying cause of TB, only the visible results; yet, several member states continue to practice this management procedure [4,38,39]. Tail-docking as a routine preventative measure is only acceptable in the EU when TB is present on-farm and all other avenues for decreasing TB behavior have been unsuccessful [4,38,39]. Tail-docking is still permissible and regularly performed on pig farms in North America. Several studies have highlighted the success of tail-docking as a control for TB [12,13,17,22,40]; however, it is known that tail-docking is a painful procedure [41]. Canada has recently required the use of analgesics during tail-docking [30] to diminish post-surgical pain and inflammation. Studies suggest that pigs which are not tail-docked are more susceptible to receiving a tail injury, lowering the pigs’ welfare due to tail damage [41]. Producers have expressed concern over the alternatives to tail-docking, as they may be difficult to implement in current housing environments [3,4,38,39,40].

The majority of studies in this scoping review explored external influences and how they relate to TB occurrences, yet few studies explored genetic predispositions [42,43,44] or nutrition [15] as a precursor for TB behavior. Knowledge of specific genes, such as PDK4, which has been shown to be expressed differently in biter and victim pigs compared to neutral pigs (neither receiving nor performing biting behavior) [44], could encourage genetic companies to purposively select pigs which exhibit a more desirable gene expression. Commercially desirable production traits, such as backfat thickness, have been negatively correlated with TB [43]; therefore it is possible that selection for commercially acceptable and preferred pork products has inadvertently selected for pigs more likely to perform abnormal behavior. It is possible that focusing on specific genetic selection would only address the outcome of TB, rather than the causes of the behavior, similar to the arguments of tail-docking mentioned earlier. However, gene expression can be altered, based on environmental stimuli, such as stress, once again demonstrating the multifactorial nature of TB.

Several of the studies included in the review have examined housing and management; however, research describing the use of nutrition, such as tryptophan, or other feed factors which promote calm pigs were far fewer. Tryptophan is an indispensable amino acid for pigs, as it cannot be produced through metabolic pathways, and can only be obtained through the diet [45,46]. Tryptophan is a precursor for serotonin [45], which is known to increase positive feelings, have a positive effect on feed intake [46] and improve intestinal health [47] in mammals. Feeding high levels of tryptophan as a therapeutic agent has been shown to improve recovery time in pigs experiencing stress [47,48,49]. The small number of studies, relating to genetics and nutrition demonstrates a need for research which explores internal influences of TB behavior. Internal influences may provide an amplified, positive contribution to the study of abnormal behavior, rather than only focusing on the external elements of pig rearing.

The results of this scoping review suggest that the quantity of evidence-based data available to guide decision-making processes regarding risk factors and appropriate intervention methods for decreasing or eliminating TB is insufficient. The overarching factor of management appears to be an important risk factor; however, management is a broad category and deciphering the specific risks within this category are difficult. The origins associated with TB suggest that it is a multifactorial condition [50], and thus, risk factors in general may be hypothesized, yet cause and effect remain difficult to define and possibly specific to each individual farm [51]. The assumed triggering factor, which is believed to initiate TB, once removed, does not guarantee that the problem will cease [50], creating a challenging environment to control the abnormal behavior once it begins. When accepted interventions and known risk factors are addressed, TB may persist [2,52,53,54,55,56]. All aspects of commercial swine production, both internal and external influences, are necessary to investigate when considering the causes and remedies for TB behavior. Multidisciplinary research may be required to truly understand this abnormal behavior. In all likelihood, a factor that clearly triggers a TB outbreak in one situation, may not cause TB when circumstances are altered. The multifactorial nature of the problem makes the study of risk factors and the evaluation of preventative strategies very difficult.

A concern that was revealed by the scoping review is that investigators were sometimes inconsistent in defining TB. Some studies [57,58,59,60,61] included all tail-in-mouth events in their studies of risk factors or prevention strategies and other researchers used models such as pigs chewing on ropes [14,15,16,62,63]. It remains unclear whether conclusions from these studies are equally valid to studies that define TB as aggressive chewing of the tail causing injury. Standardizing the definition of TB and designing studies that can control for the many possible contributing factors are challenges that need to be addressed in future studies.

Future Considerations and Next Steps: The current literature yielded by the scoping review indicates that a systematic review is possible at this time. A systematic review and meta-analysis exploring the most effective risk factors and interventions at each stage of production would aid producers and industry in management and housing legislation in the future. Exploration of study designs used in intervention experiments would be useful to evaluate the significance of each treatment within a research or commercial population to guide recommendations for producers. Ongoing evidence-based research with standardized definitions of TB, pertinent risk factors and interventions at separate stages of production, and further exploration regarding the genetic elements that may contribute to TB will allow further insight into the impacts of direct and indirect costs to the producer.

There is a considerable amount of research which focuses on TB; however, categorizing the most important risk factors and interventions to decrease this abnormal behavior is inconclusive. This review demonstrated that the amount of research concerning TB has increased over the past two decades, particularly in the EU. Although this does appear to indicate that there is interest and willingness to understand TB, this abnormal behavior is under-represented in the literature in three of the four largest global pig producing locations. The main risk factors associated with an outbreak of TB, appear to be both internal and external factors involved in commercial pig production, including; genetics, nutrition, and the absence of species relevant enrichment. Tail-docking, and providing environmental enrichment were shown to be often but not always successful intervention strategies for limiting or decreasing TB behavior, although the timing of the interventions was not always explicitly stated, creating challenges for reproducibility of the studies. Available, consistent evidence-based literature is not definitive, and a more rigorous investigation of study design and significant housing and management strategies related to stage of production is necessary. A major inconsistency among TB research is the definition of the behavior being studied, that is whether TB includes all tail-in-mouth activity or whether it is restricted to chewing on tails to create a wound.