Faecal egg counts before and after ivermectin treatment in horses with and without pituitary pars intermedia dysfunction

Pituitary pars intermedia dysfunction (PPID) is common in aged horses and increased susceptibility to endoparasitism from immunological dysfunction may occur. However, the association between PPID and strongyle worm burdens remains unclear. Materials and MethodsFifty mature horses/ponies (mean age: 19±8 years) were categorised as PPID (n=23) or non-PPID (n=27), based on plasma ACTH concentration and seasonally-adjusted reference ranges. Faecal egg counts (FEC) were performed pre-treatment (D0) and then every 14 days until D112 post-treatment (with ivermectin), and the egg re-appearance period (ERP: group mean FEC >10% of group mean on D0) was calculated. Independent t-tests, Mann-Whitney U tests or Wilcoxon rank-sum tests and Chi-squared tests were used for continuous and categorical data, respectively. This study was approved by the Research Ethics and Integrity committee, University of Melbourne (#1814395.2). ResultsMedian plasma ACTH was significantly higher in PPID (144 pg/ml, range 54-1013) than non-PPID animals (31pg/ml, range 20-68) (P<0.001) and was also higher in ponies compared to horses (P=0.017). D0 FEC results were not significant different between groups and were not associated with age. The ERP of the PPID group (D56) was shorter than the non-PPID group (D70). The PPID group FEC was significantly higher than the non-PPID group at D42, D56 and D70 (P<0.05).Relevance to Australian clinical equine practiceThe results indicate that horses with PPID have a shortened ERP and higher egg counts after treatment with ivermectin, compared to healthy horses in similar environmental conditions. These findings have implications for parasite control programmes for horses with PPID.