Mucosal inflammatory responses after experimental infection with gastrointestinal nematodes in resistant and susceptible sheep naturally infected with Oestrus ovis

Recent epidemiological observations from field studies indicated that sheep selected for resistance to digestive nematodes were more infected than susceptible ones with the nasal bot fly, Oestrus ovis. The present study was undertaken to verify these informations and to explore the possible associated mechanisms. After a whole grazing season and an anthelmintic drench, two groups of 6 rams belonging respectively to 2 flocks with different resistance status to parasitic nematodes were experimentally infected with a mixture of Haemonchus contortus, Trichostrongylus colubriformis and Teladorsagia circumcincta infective larvae. The worm populations in the gastrointestinal tract, the Oestrus ovis populations in the nasal cavities and the intensity of inflammatory reactions (number of mast cells, eosinophils and globule leucocytes) in the respiratory and digestive mucosae were compared in the 2 groups. No differences were found in the worm numbers and in the cell populations in the digestive tract and this lack of difference was due to acquisition of immunity to nematodes in both lines of rams at the end of the grazing season. In contrast, the mean number of O. ovis larvae was higher in the resistant group compared to the susceptible one (6.3 vs 2.8) and the number of eosinophils was significantly higher in the septum and ethmoid in this resistant group. These results tend to confirm the higher susceptibility to O. ovis in animals selected for resistance to digestive nematodes. They also suggest that the intensity of inflammation in the nasal mucosae might be positively related to the intensity of infection with O. ovis.