Critical test evaluation (1977-1992) of drug efficacy against endoparasites featuring benzimidazole-resistant small strongyles (population S) in Shetland ponies.

The research article explores the efficacy of several compounds in treating benzimidazole-resistant small strongyles and other internal parasites in Shetland ponies. The compounds’ effectiveness was observed to decline over time, with resistance observed in several small strongyle species.

Study Design and Procedure

• This long-term study conducted between 1977 and 1992, involved critical evaluations of 13 different compounds, either standing alone or in combinations.
• Ponies less than one year old from a specific breeding herd were used in these tests. This herd had been treated consistently with cambendazole (CBZ) from 1974 to 1978, and then with oxibendazole (OBZ) from 1978 to 1992.
• The study relied on both present critical test data as well as published field data between 1974 and 1992 for evaluation.

Findings on Small Strongyle Resistance

• Initially, average efficacies of OBZ—an anthelmintic commonly used to treat parasite infections—were high, reaching 95% or higher. However, over the years, the effect steadily dropped, hitting a minimum of 1% in 1991. This implies that the parasites developed resistance to the compound over time, leading to its diminishing efficacy.
• Resistance of small strongyles was also evident in critical tests for several other benzimidazoles (BZs) and a pro-BZ. Side-resistance is a concept in pharmacology where resistance to one drug can lead to resistance to other drugs, particularly if they have similar mechanisms of action.

Alternative Compounds and Small Strongyle Resistance

• Other active substances such as ivermectin and piperazine were highly effective. However, weak activity was noted with pyrantel pamoate, a drug otherwise commonly used in the de-worming regime.
• Resistance was observed in six species of small strongyles including Cyathostomum catinatum, Cyathostomum coronatum, Cylicocylus nassatus, Cylicostephanus calicatus, Cylicostephanus goldi, and Cylicostephanus longibursatus.

Results on Other Internal Parasites

• The study also explored the impacts on other types of internal parasites typically found in ponies. Compounds’ efficacies against bots, ascarids, large strongyles, and pinworms were as expected, indicating no signs of drug resistance in these parasites.