The effect of sex on susceptibility to ovine oestrosis in naturally infested lambs.

The aim of this study was to evaluate the influence of sex on natural infestation by Oestrus ovis in lambs. A total of 26 crossbred lambs (14 males and 12 females), aged between 4 and 5 months at the beginning of the 114-day (D0-D113) trial, were maintained under a semi-intensive system in southeastern Brazil. At 8-9 months of age, animals were slaughtered for larval recovery and quantification from the nasal cavity. A total of 483 larvae were recovered, of wich 321 (66.5%) were first-instar larvae (L1) and 162 (33.5%) were second-instar larvae (L2). All males (n = 14) were parasitized, with burdens ranging from 7 to 54 larvae, while two females (16.7%) were uninfested. Males showed significantly higher total larval counts and L1 burden than females (P = 0.004 and P = 0.002, respectively), with no significant difference in L2 counts (P = 0.08). Clinical signs included sneezing and nasal discharge. Nasal discharge varied from serous to mucopurulent, with no significant difference between sexes (P = 0.091), and no significant correlation was found between larval burden and nasal discharge scores (P = 0.571). Eosinophil counts varied throughout the trial, with significantly higher mean counts in females than in males (P = 0.018). Anti-O. ovis IgG levels increased progressively throughout the study, with a significant sex-by-time interaction (P = 0.009). Male lambs exhibited greater susceptibility to O. ovis infestation under natural conditions, suggesting a sex-related influence on the dynamics of ovine oestrosis. Future studies using hormonal, immunological, and chemical ecology analyses will be key to understanding sex-related factors in susceptibility to this parasite.