Resistance of population-B equine strongyles to thiabendazole, oxfendazole, and phenothiazine (1981 to 1987).

The study tests the resistance of five different species of horse intestinal worms (strongyles) to three drugs (thiabendazole, oxfendazole, and phenothiazine). It found that these worms have developed a resistance to these treatments, but not to oxfendazole, prompting further research into possible reasons for this exception.

Study Design and Methods

• The study was conducted on 15 foals that were naturally infected with BZ-resistant strongyles between 1981 and 1987. The researchers conducted tests using three different drugs: thiabendazole, oxfendazole, and phenothiazine.
• The foals were divided into different groups to test the effects of the various drugs. Different dosages were used for each drug: thiabendazole (44 mg/kg), oxfendazole (10 mg/kg), and phenothiazine (55 mg/kg). The drugs cambendazole and fenbendazole were also tested on one foal each, at doses of 20 mg/kg and 5 mg/kg respectively.

Researchers’ Findings

• Of the three drugs under investigation, thiabendazole, cambendazole, and fenbendazole demonstrated only partial effectiveness against the five species of BZ-resistant small strongyles. They could only remove between 48% and 68% of the parasites, indicating their continued resistance to these drugs.
• Interestingly, the drug oxfendazole was found to be significantly more effective, removing an average of 95% of the BZ-resistant small strongyles in the four foals tested. This outcome contrasted starkly with the others and could possibly be due to a lack of previous reference data on oxfendazole.
• Also, all five species of the BZ-resistant small strongyles were observed to have cross resistance to the drug phenothiazine with only a 38% removal rate. However, the same drug effectively removed 99% of BZ-susceptible species, suggesting a similar mode of action between phenothiazine and the BZ compounds.

Conclusion and Implications

• The study’s findings demonstrate the evolution of resistance in strongyles towards popularly used anti-parasitic drugs in equine health management.
• The exception of oxfendazole suggests that there may be different mechanisms at work in its effectiveness against the strongyles, sparking interest for further investigation.
• The similarities in the treatment outcomes with phenothiazine and BZ compounds hint towards a similar mode of action, making it a less effective treatment option for BZ-resistant species.