Strongyle egg shedding and egg reappearance periods in horses with pituitary pars intermedia dysfunction

Adelaina Horner; Nicholas J Bamford; Michael J Stear; David Piedrafita; Abdul Jabbar; Kristopher J Hughes; Charles M El-Hage; Sarah Preston

doi:10.1016/j.vetpar.2024.110176

Strongyle egg shedding and egg reappearance periods in horses with pituitary pars intermedia dysfunction

Vet Parasitol. 2024 Mar 28:328:110176. doi: 10.1016/j.vetpar.2024.110176. Online ahead of print.

Authors

Adelaina Horner¹, Nicholas J Bamford², Michael J Stear³, David Piedrafita¹, Abdul Jabbar², Kristopher J Hughes⁴, Charles M El-Hage⁵, Sarah Preston⁶

Affiliations

¹ Institute of Innovation, Science and Sustainability, Federation University, Victoria 3350, Australia.
² Melbourne Veterinary School, The University of Melbourne, Parkville, Victoria 3010, Australia.
³ School of Life Sciences, La Trobe University, Melbourne, Victoria 3086, Australia.
⁴ School of Agricultural, Environmental and Veterinary Sciences, Charles Sturt University, Wagga Wagga, NSW 2678, Australia.
⁵ Melbourne Veterinary School, The University of Melbourne, Parkville, Victoria 3010, Australia. Electronic address: cmeh@unimelb.edu.au.
⁶ Institute of Innovation, Science and Sustainability, Federation University, Victoria 3350, Australia. Electronic address: s.preston@federation.edu.au.

PMID: 38603926
DOI: 10.1016/j.vetpar.2024.110176

Abstract

Pituitary pars intermedia dysfunction (PPID) is the most common endocrine disorder of older horses. Immune dysfunction in horses with PPID could increase susceptibility to infectious diseases, including strongyle infections; however, few data are available. The aim of this study was to determine if horses with PPID had increased strongyle faecal egg counts (FEC) compared with control horses, over a fourteen-week period in Victoria, Australia. Clinical signs and plasma adrenocorticotropic hormone (ACTH) concentrations were used to categorise horses into PPID (n=14) or control (n=31) groups. Faecal samples were collected for FEC determination prior to anthelmintic treatment, and fortnightly post-treatment for each horse. Generalised linear mixed modelling, using a gamma distribution, was used to compare differences between groups in the repeated measures study. The confounding variable of age was controlled for as a fixed effect. Following anthelmintic treatment, mean FEC was greater for the PPID group compared to the control group on day 56 (405 ± 756 eggs per gram [EPG] vs 40 ± 85 EPG, p=0.05) and day 70 (753 ±1598 EPG vs 82 ±141 EPG, p=0.04). There were no differences in mean FEC between groups on days 84 and 98. Cumulative FEC (day 14 to day 98) was significantly greater for the PPID horses than control horses (2118 ± 4016 EPG vs 798 ± 768 EPG, p<0.0001). Group egg reappearance period was shorter for PPID horses (day 56 post-anthelmintic treatment) compared to control horses (day 70) and 30% of the PPID horses reached a FEC threshold of >200 EPG on day 42, compared to 0% of control horses (p=0.02). These results suggest that the rate of a re-established patent infection between groups could be different due to a comprised immune response in PPID horses or differences in the host-parasite relationship regarding encysted stage larvae. However, despite differences between groups, some horses with PPID consistently had no detectable or low FEC (<200 EPG) during the study period. These findings highlight the importance of individual FEC monitoring to determine if anthelmintic treatment is required, in line with sustainable parasite management practices.

Keywords: Endocrine; Equine; Faecal egg count; Pituitary pars intermedia dysfunction; Strongyle.