Photo of a roundworm.
Photo: Sofia Jonsson

New method for examining anthelmintic resistance of the equine roundworm

Page reviewed:  25/09/2025

Drug resistance in roundworms that infect horses is a growing problem in foals in Sweden and the world. Roundworm infection can cause symptoms as nasal discharge, coughing and poor growth. In cases of very high parasite load the worms can fill the entire small intestine resulting in colic and death.

Background

The aim of this project was to develop a method for quickly and easily detecting resistance in roundworms, to make it easier for animal owners to investigate resistance status and adapt treatment accordingly.

Currently, there is only one method available for investigating the prevalence of resistance in roundworms: the fecal egg count reduction test. This method involves counting the number of parasite eggs in the foal's feces before and after deworming, after which the effectiveness of the treatment is calculated. The method is time-consuming, and therefore a method whereby the parasite's resistance status is investigated in a laboratory environment would save horse owners a lot of work.

The new method, which has been tested and developed in the research project, is performed by taking a collective stool sample from all foals on a farm and sending it to a laboratory, where the parasite eggs in the stool are purified and allowed to develop into larvae. The larvae are treated with different drugs, after which their mobility is measured with a laser instrument. If the larvae are sensitive to the drug being tested, they will move less, while resistant larvae will be as mobile as before treatment.

The method was developed as a collaboration between SLU and the French research institute INRA.

Results

In the developed method, the mobility of roundworm larvae is examined by measuring autofluorescence, i.e., light emitted by the larvae, using a laser instrument. To test the method, the larvae were first exposed to a solution containing various deworming agents. If the parasite is sensitive to the substance being tested, its mobility decreases and thus the autofluorescence decreases, while the movement of a resistant parasite is not affected.

Experiments with larvae sensitive to deworming agents showed that the methodology can detect differences in migration between larvae that have been exposed to deworming agents and unexposed larvae. These results are very promising steps towards developing a method for investigating the occurrence of resistance in roundworms. Further studies will investigate how the mobility of different roundworm populations is affected after exposure to different classes of deworming agents, making it possible to determine their resistance phenotype. Knowing the resistance phenotype, i.e., which types of deworming agents the larvae are resistant to, is important because it enables treatment with the right deworming agent.

In addition to the horse roundworm, the genus Ascaridia also includes the human roundworm (Ascaris lumbricoides), which is a widespread infection among children in low-income countries. Reduced treatment efficacy has also been reported for this parasite, which poses a threat to human welfare and health. As the life cycles of horse and human roundworms are similar, we hope that this research on horse roundworms will be useful in monitoring and preventing the development of resistance in human roundworms as well.

Participants in this project

  • Frida Martin, SLU
  • Eva Tydén, SLU
  • Claude Charvet från INRA, Centre Val de Loire.

The one-year project was launched in 2019 and was financed by SLU Future One Health, that supports interdisciplinary research that is part of the One Health concept – optimal health and welfare for both humans and animals in sustainable ecosystems.

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