Terpenic compounds possess anthelmintic and immunomodulatory properties with potential for controlling equine cyathostomin infections.

Terpenic compounds, such as cinnamaldehyde and carvacrol, show promising anti-parasitic and immunomodulatory effects against equine cyathostomin infections in laboratory tests, but their effectiveness in live horses remains uncertain.

Background and Purpose

• Equine cyathostomins (small strongyle parasites) are common and problematic intestinal parasites in horses.
• Resistance to conventional anthelmintic drugs is increasing, making alternative control strategies essential.
• Dietary additives with anti-parasitic and immune system–modulating properties are proposed as sustainable control options.
• The study aimed to determine whether certain terpenes could provide dual benefits — both killing or inhibiting parasites and modulating the horse’s immune response — for potential use as feed additives against cyathostomin infections.

Methods

• In vitro assays were conducted to test effects of selected terpenes (including cinnamaldehyde and carvacrol) on:
  • Cyathostomin larval development (growth and maturation)
  • Larval migration ability (movement through substrates)
  • Immune modulation in equine immune cells exposed to parasite antigens
• Nemabiome analysis: genetic sequencing was used to assess the impact of terpenes on the composition and community structure of cyathostomin populations.
• An in vivo feeding study was performed where live horses were given cinnamaldehyde for 28 days to observe effects on:
  • Parasite egg excretion in feces
  • Host blood cell profiles (as indicators of immune status)

Key Findings

• In vitro anti-parasitic activity: Cinnamaldehyde and carvacrol significantly inhibited cyathostomin larval development and migration.
• Impact on parasite communities: Both compounds altered cyathostomin community structure, indicating selective effects on parasite populations.
• Immunomodulatory effects in vitro:
  • Both terpenes reduced inflammatory responses in equine mononuclear immune cells.
  • They modulated gene expression induced by parasite antigens in monocytes, upregulating antioxidant defense pathways.
• In vivo results: Despite promising in vitro data, cinnamaldehyde did not significantly impact parasite egg counts or blood immune cell profiles over 28 days in live horses.

Conclusions and Implications

• Terpenes show broad-spectrum bioactivities relevant to parasite control, combining direct anti-parasitic effects with beneficial immune modulation.
• There is a disconnect between laboratory (in vitro) efficacy and live animal (in vivo) effectiveness, highlighting challenges in translating these findings to practical therapies.
• Further research is needed to optimize delivery methods, dosing, or combinations that can realize the potential of terpenic compounds as natural feed additives to help control cyathostomin infections and improve horse health sustainably.