THE EFFECT OF AGE ON THE RESPONSE OF ROMNEY SHEEP TO GASTROINTESTINALNEMATODES DURING GRAZING

Three groups of Romney sheep, reared worm-free in pens from birth unti l 4, 16 or 28 months of age, were permitted to graze nematode-infected pasture for 4 weeks after which they were returned to pens and slaugh tered 4 weeks later. Three groups of three Romney sheep reared and mai ntained worm-free for similar periods served as uninfected control gro ups. Faecal egg counts (FECs) 8 weeks after grazing commenced were sig nificantly greater in 4-month-old sheep than the 28-month-old animals. At slaughter, no difference in nematode burdens among these three age groups was found. Histological examination of the abomasal and small intestine mucosa, taken from sheep at slaughter, revealed significantl y greater numbers of globule leukocytes/mucosal mast cells (GL/MMC) bu r fewer connective tissue type mast cells (CTMC) in sheep aged 16 and 28 months than in those aged 4 months. The numbers of eosinophils in t he gastrointestinal mucosal tissues of the three groups of infected sh eep were not significantly different. In uninfected sheep the numbers of eosinophils, GL/MMC and CTMC in the mucosal tissues did not differ between the three age groups and the numbers of GL/MMC and CTMC were s imilar to those in 4-month-old infected sheep. Eosinophil numbers in t he mucosal tissue of the infected groups were significantly higher tha n in the uninfected groups. In the infected groups, FECs but not worm burdens were significantly negatively correlated with GL/MMC numbers. These results indicate that sheep aged 16 or 28 months are no more abl e to resist short-term primary nematode challenge than lambs, but more rapidly express a mucosal GL/MMC response and suppress nematode egg c ounts in their faeces.