Does Inflammation Of Pig’s Smell Sensing Organs Increase Aggressive Behaviour? By Christine Pelland from South West Ontario Veterinary Services


It is well known that animals make use of chemical communication to exchange information within their environment. This chemical communication involves the production, emission and detection of these chemicals. Pigs are able to produce chemical messages (odours) from digital, preputial, vulvar, anal, salivary, buccal, pre-orbital, and Harderian glands. They also have one of the largest olfactory receptor repertoires with 1113 functional olfactory receptor genes and 188 pseudogenes. (Probably explains why they are so good at finding truffles!) Pigs and other animals use these chemical messages to mark their territories. Defining your territory helps to avoid all sorts of conflict with other animals. Animals will use chemical messages (pheromones) to attract other animals for the purposes of reproduction. Even on its own boar scent can stimulate a small percentage of female pigs that are in estrus to initiate a standing reflex. Boar scent has also been shown to stimulate the onset puberty in developing gilts. Sows and piglets use chemical messages to identify members of their own litter. Pigs are able to identify pen mates as part of their normal stable pen population. When a individuals are added or removed from the group some fighting can occur as the group re-establishes the social hierarchy within the pen. Reference 1

In pigs, the vomeronasal organ (VNO) is one of the main detectors of these messages. Inflammation of the VNO has been linked to behavioral changes because it potentially prevents molecule detection and, consequently, the translation of the chemical signal into action. Previous research has further shown that as the intensity of vomeronasal sensory epithelium (VNSE) inflammation increases there is increased aggression between pigs. This can be measured by comparing the rate of skin lesions ( fighting ) to the level of inflammation. The VNO inflammation may be related to various infectious diseases that lead to rhinitis or it may be associated with chemicals produced within the barn such as ammonia or hydrogen sulphide. These French researchers wanted to evaluate the cellular and molecular changes that occur during vomeronasalitis. They examined in 76 vomeronasal sensorial epithelia from 38 intensive-farmed pigs. Microscopic examination was used to evaluate the condition of each VNO and classify inflammation as healthy, weak, moderate, or strong. These data were compared to the thickness of the sensorial epithelium and the number of type 1 vomeronasal receptor cells using anti-Gαi2 protein immunohistochemistry (IHC) and analysis. The presence of odorant-binding proteins (OBPs) in the areas surrounding the VNO was also analyzed by IHC and compared to inflammation intensity since its role as a molecule transporter to sensory neurons has been well-established.

The researchers found the following with respect to those 76 samples:

  • 13 (17%) were healthy, 31 (41%) presented with weak inflammation, and 32 (42%) presented with moderate inflammation. No severe inflammation was observed.
  • epithelial thickness and the number of Gαi2+ cells were inversely correlated with inflammation intensity (Kruskal-Wallis and ANOVA tests, p < 0.0001)
  • odorant-binding protein expression in areas around the VNO was increased in inflamed VNO (Kruskal-Wallis test, p = 0.0094), regardless of intensity.

Take Home Messages:

  • Inflammation of the vomeronasal organ which is used by the pig to detect chemical messages was associated with a reduction in the thickness of the sensory epithelium and Gαi2+ cell number and the researchers propose a connection between this condition and the induction of various degrees of sensory nervous olfactory loss. The researchers propose that is may explain how vomeronasalitis could prevent the correct functioning of chemical communication which leads to social conflict. Reference 2
  • Air pollutants such as ammonia, hydrogen sulfide or the occurrence of infectious diseases that lead to rhinitis and by extension vomeronasalitis can cause pigs of all ages and stages of production to have a reduced capacity to recognize pen or littermates.  This leads to increased aggressive behaviour.
  • We might want to add this to the list of potential unintended consequences when we try to save on heating costs by reducing ventilation rates.

Reference: 1. Pietro Asproni , Eva Mainau , Alessandro Cozzi , Ricard Carreras , Cécile Bienboire-Frosini , Eva Teruel , Patrick Pageat . Is There a Link between Vomeronasalitis and Aggression in Stable Social Groups of Female Pigs? Animals (Basel)  2022 Jan 26;12(3):303. doi: 10.3390/ani12030303.

Reference: 2. Violaine Mechin , Pietro Asproni , Cécile Bienboire-Frosini , Alessandro Cozzi , Camille Chabaud , Sana Arroub , Eva Mainau , Patricia Nagnan-Le Meillour , Patrick Pageat    Inflammation interferes with chemoreception in pigs by altering the neuronal layout of the vomeronasal sensory epithelium Front Vet Sci . 2022 Sep 12;9:936838. doi: 10.3389/fvets.2022.936838. eCollection 2022.