Effect of repeated oxfendazole treatments on small strongyle infections in Shetland ponies.

This study explores the effect of repeated treatments of Oxfendazole on small strongyles (intestinal worms) in Shetland ponies, especially focusing on hibernating cyathostomes in the third development stage (L3). The study showed that the treatment was more effective in May as compared to July, although its effectiveness dwindled against stage three and four larvae but remained efficacious against adult worms.

Objective and Methodology

• The main objective of this study was to determine the effect of repeated treatments of Oxfendazole (OFZ) on strongyle infections (intestinal worms) in Shetland ponies.
• They investigated this by setting up two groups of three Shetland ponies. Both groups were kept together from May to November and then put in houses.
• Treatments of 10 mg of OFZ per kg of the ponies’ weight were applied in late May, twice in July, and one week before the groups were necropsied (examined post-mortem) in December and April.

Findings and Interpretation

• Worm populations in both groups were found to be dominated by cyathostomes in the early third development stage also known as third-stage larvae or L3. These were found to in a state of hibernation or inhibition.
• The study found that the number of fourth-stage larvae (L4) was significantly higher in April than in December. This suggests that the worms were able to develop from L3 to L4 despite the treatments.
• Through faecal egg counts, the researchers found that the May treatment was more effective than the July treatment. This was demonstrated by high numbers of larvae in the pasture from the end of August onwards, which was a result of the ineffective July treatments.
• The Oxfendazole treatment was found to be less effective against L3 and L4 larvae, however it retained its efficacy against adult worms. This indicates that while the medication was effective at killing adult worms, it struggled with the more resistant larvae stages of the worm’s lifecycle.