Infrared thermography of different tear staining scores in pigs

Abstract

Tear staining is increasingly suggested as a potential indicator for pig welfare. However, current research outcomes on tear staining are inconclusive. There is therefore a need for more information on this potential indicator. The aim of this study was to assess the relationship between tear staining scores and infrared thermography (IRT) of the inner canthus of the eye (with eye temperature relating to emotional states). We expected that pigs with a more severe tear staining score would have a higher eye IRT value. Pigs of a high-welfare research herd were assessed for tear staining using a 6-point scoring scale. Twenty-four female pigs with tear staining scores from 0 to 3 were selected, while scores 4 and 5 did not occur. Pigs were subjected to IRT of the right eye, in a test area specially constructed for IRT. Data were analysed in a generalised linear mixed model with IRT value as response variable and tear staining score, age and temperature-humidity index as predictor variables. The results showed no significant relationship between tear staining score and IRT values. The average IRT value was 37.1 ± 0.34, with the highest IRT values in tear staining score 3. The pigs’ age did not influence the outcomes. In this small-scale study we did not find a relationship between tear staining and eye IRT. Nevertheless, the outcomes show that tear staining is prevalent also in high welfare systems and we therefore urge for caution in using tear staining as a welfare indicator.