Table 1.

Application of the disease framework to pen soiling in existing systems: A summary.

Definition	Pigs and/or pig pens get unduly soiled with faeces or urine, usually due to a change in lying behaviour. No agreed standard as to what is undue soiling, and how to measure it.
Aetiology (Main Cause)	Inadequate thermoregulation (overheated/draughty lying area), faulty pen design (disturbance during elimination), flooring issues (dirty/slippery floors).
Symptoms	Pig and pen soiling (mostly seen at the same time) can be a “signal indicator” related to reduced animal welfare caused by climatic conditions. Additional symptoms: resting in the dunging area, panting due to heat stress, restlessness, lying on the slatted floor also at low ambient temperature, impaired animal health (increased transmission of gastrointestinal pathogens and parasites).
Diagnosis and Differential Diagnosis	An operational definition is needed for a clear diagnosis (e.g., different functional areas and system specifications (e.g., floor type) may determine when a pen is classified as soiled). Differential diagnosis: faecal soiling to be distinguished from mud, enrichment substrate (earth, peat, compost), feed, diarrhoea, dark skin colour, skin disorders (e.g., greasy pig disease).
Pathogenesis	Three possible mechanisms: Inadequate (uncomfortable) resting area: Change in resting behaviour (e.g., due to high ambient temperature) followed by elimination in the area previously used for resting. The choice of the lying area seems to be the most important factor. Inadequate (unattractive) elimination area: Change in eliminative behaviour (e.g., due to disturbance or slippery floors) followed by a change in resting behaviour. This may occur more occasionally or by accident. Animal-related factors: e.g., genotype, individual characteristics, sex, age, medical conditions, previous experience, etc.
Treatment and Prevention *	General advice for farmers to deal with pen soiling in existing systems: Be alert for early signs (e.g., changes in resting behaviour), and take action at an early stage (e.g., clean pens and provide bedding in the resting area); Improve thermal comfort, esp. in the designated resting area (check the ventilation system, reduce draughts, and optimise the microclimate); In newly formed pens, favour the correct distinction between elimination and resting areas (floor of the designated elimination area can be wetted, and dry feed or sawdust can be provided on the floor of the expected resting area during the first few days); Remove (or limit) olfactory clues in the expected resting area as much as possible (thorough pen cleaning before introducing a new group of animals), provide smell of faeces in the designated dunging area; Provide proper enrichment materials to facilitate the distinction of functional areas (to stimulate activity in areas at risk of pen soiling) and if possible, add bedding materials in the resting area (to improve comfort).
Importance	Reported prevalence: 4–9% of pens. Pen soiling indicates, by itself, reduced animal welfare (i.e., rearing conditions overriding pigs’ motivation/tendency to keep the lying area clean). Poor hygiene may reduce growth rates, compromise health, reduce welfare and air quality. Legislation requires pigs to be kept in a thermally comfortable, adequately drained and clean area which allows all the animals to lie at the same time. Reducing pen soiling and/or a “pig toilet” could reduce the environmental impact of pig farming, and further improve pig welfare (e.g., by providing straw or roughage on a solid floor) in the future.

* For the purpose of this table, only the most immediately feasible measures for farmers were presented. A more detailed description of measures requiring either modifications in pen design, pen hardware, or even in the barn layout or business model, is given in the text.