THE EOSINOPHIL LEUKOCYTE, A PHENOTYPIC MARKER OF RESISTANCE TO NEMATODE PARASITES, IS ASSOCIATED WITH CALM BEHAVIOR IN SHEEP

The hypothesis that behaviour in sheep is influenced by resistance to infections with Trichostrongylus colubriformis and Haemonchus contortu s was explored. Sheep were assessed phenotypically as resistant and su sceptible in four ways and thereafter, the effect of resistance on sev eral behavioural traits was measured in an arena test. The behavioural parameters recorded for each sheep were: approach/avoidance distance; travel; number of moves; and spread. Four phenotypic groups of sheep were set up each with two subgroups: ovine lymphocyte antigen (OLA) ty pe (subgroups SY1a type vs other types); blood eosinophil leucocyte co unts (high vs low); T. colubriformis and H. contortus serum antibodies thigh vs low titres) and faecal egg count (FEC) thigh vs low). Only t he behavioural comparison between sheep with high versus low eosinophi l leucocyte count showed consistent differences that were statisticall y significant, although other comparisons favoured associations betwee n OLA type SY1a, low FEC and reduced approach/avoidance distance and/o r locomotor behaviour. The eosinophil leucocyte association was establ ished on two non-overlapping test populations of sheep, with mean eosi nophil leucocyte counts of 3.19 x 10(5)/mL blood for high and 0.83 x 1 0(5)/mL blood for low subgroups; results were reproduced on five separ ate occasions over 6 weeks to detect differences as approach/avoidance distance (P < 0.01), travel (P < 0.01), number of moves (P < 0.01), a nd spread (P < 0.05). High eosinophilia correlated with low FEC for th e infected sheep, which identified sheep that were resistant to parasi tes. We concluded, therefore, that the resistant sheep were more at ea se with the environment of the test than were the sheep with low eosin ophil leucocyte counts. The strong association between high eosinophil leucocyte counts and behaviour is consistent with other reports which suggest that retention of normal levels of circulating eosinophil leu cocytes is associated with resistance to stress.