Efficacy of moxidectin 2 per cent oral gel against cyathostomins, particularly third-stage inhibited larvae, in horses.

The research study examines the effectiveness of moxidectin 2% oral gel in combating cyathostomins, and specifically third-stage larva, in horses. The study concluded that it’s highly effective against adults, luminal, and mucosal cyathostomin larvae, including inhibited third-stage larvae, thus reducing the overall number of these harmful parasites in the horse population.

Introduction

• Cyathostominosis is a common disease in horses worldwide, characterized by the presence of numerous cyathostomin larvae in the caecal and colonic mucosae.
• It is usually seen in young horses, with symptoms including weight loss, chronic diarrhoea, inappetence, and lethargy. The disease could potentially be fatal in severe cases.

The Lifecycle of Cyathostomins

• The worms have a direct lifecycle, progressing from egg deposition to infective third-stage larvae within two to three weeks under optimum conditions.
• After ingestion by the horse, larvae migrate to the large intestine where they become encysted in the mucosa and develop into the fourth stage. Eventually, they move into the intestinal lumen and mature into adults. The prepatent period – the time between ingestion of third-stage larvae and the appearance of eggs in the faeces – is around six to ten weeks.
• In certain seasons, some of the encysted larvae do not moult into the next stage, but remain as early third-stage larvae (EL3). This is referred to as inhibited larval development.

Fenbendazole and Moxidectin

• Fenbendazole was the first anthelmintic to demonstrate high efficacy against inhibited EL3. Alternatively, early trials with moxidectin showed unclear results regarding its effectiveness against all stages of mucosal larvae.
• However, recent studies suggest the potential of moxidectin to treat mucosal larvae, including inhibited EL3.

Materials and Methods

• The study followed a similar process to earlier research (Bairden et al., 2001), using naturally infected ponies obtained from a single grazing area. Two ‘control’ ponies were euthanised pre-treatment to determine their worm burden.
• The remaining ponies, after being kept in housing for eight weeks, were split into two groups to receive either anthelmintic treatment or a placebo. Ponies in the treatment group received a 2% moxidectin oral gel. Eight weeks after treatment, all ponies were euthanised, and their worm burdens were determined.

Results

• No adverse clinical effects were observed in any of the ponies. The study found moxidectin 100% effective against Probstmayria vivipara and Oxyuris equi L4.
• Significant reductions were seen in the treated ponies’ levels of adult and luminal cyathostomin larvae compared to the control ponies.
• The analysis also shows a notable (92.2%) reduction in the treated group compared with the control group for early third-stage (EL3) cyathostomins.

Conclusion

• The trial confirms the high efficacy of moxidectin 2% oral gel, reinforcing previous findings by Bairden and others (2001).
• Given the growing resistance to benzimidazoles, it’s crucial to delay resistance to macrocyclic lactones. Therefore, moxidectin is an important consideration in the treatment of cyathostomins in horses.