Advice updated following evidence of tapeworm resistance in horses

Treatment resistance reported in tapeworms in the United States has resulted in updated guidelines for the use of a saliva-based test for the horse parasite in the United Kingdom.

Praziquantel and pyrantel pamoate are considered the key anthelmintics for controlling tapeworms in horses.

However, a paper published in August in the International Journal For Parasitology: Drugs and Drug Resistance, described the apparent treatment failure of both drugs against tapeworms.

Professor Martin Nielsen, with the M.H. Gluck Equine Research Center at the University of Kentucky, reported fecal egg count data from a Thoroughbred operation in Central Kentucky in 2023.

Fifty-six yearlings were involved in the study.

Praziquantel reduced tapeworm egg counts by 23.5% and pyrantel pamoate by 50.9%, Nielsen reported.

Praziquantel eliminated tapeworm eggs from 3 of 17 horses. However, pyrantel pamoate did not eliminate tapeworm eggs from any of the 14 tapeworm-positive horses.

Nielsen said the findings provide a sharp contrast to the results from historic field efficacy studies and raise concerns about anthelmintic resistance having possibly developed.

“This emphasizes the need for developing and refining antemortem methodologies for evaluating anti-cestodal treatment efficacy and for searching for possible alternative treatment options,” he said.

Britain’s Austin David Biologics Ltd, which developed a equine saliva test kit to aid in tapeworm control, noted that praziquantel and pyrantel pamoate are the only drugs licensed for tapeworm control in horses in the United Kingdom.

In a bid to clarify how this risk affects tapeworm testing and treatment, the company has reissued guidance on the use of its EquiSal saliva test.

The test can be used to diagnose infection to reduce the number of tapeworm treatments needed, resulting in more than two-thirds of horses tested not needing treatment. This approach reduces the risk of wormer resistance as drug use is saved for horses with potentially disease-causing burdens.

The company described the report of emerging resistance in the US as concerning. It stressed the importance of taking the correct action for horses who routinely have borderline or moderate/high EquiSal saliva scores.

Professor Jacqui Matthews, a specialist in veterinary parasitology and director of veterinary science at Austin Davis Biologics, said in horses that return a borderline or moderate/high saliva score, their management should be evaluated to determine what improvements could be made to help prevent tapeworm reinfection from the pasture.

“As part of this evaluation, a follow-up test can be performed three months after treatment to provide information on whether or not reinfection is occurring.”

This advice is based on the fact that tapeworm-specific antibodies reduce relatively quickly over time, with one published study indicating that in over 70% of praziquantel-treated horses, saliva scores reduced to a “low” diagnosis (no treatment recommended) within five weeks of horses being wormed.

Later studies demonstrated that in all horses grazing fields where dung was removed frequently or horses that were stabled (i.e., very low exposure to new infections), EquiSal saliva scores reduced to a “low” diagnosis within 12 weeks.

Matthews further recommends that when horses test positive in the follow-up test, improvements in pasture management should be undertaken, and/or a tapeworm treatment considered where appropriate, to prevent further grazing contamination with tapeworm eggs.

“All co-grazing horses should be tested at the same time in case they act as a continued source of tapeworm eggs. The best approach to prevent wormer resistance is to evaluate and implement improved pasture management to move to a situation where recurring reinfection is considerably reduced.”

The key take-home message is that owners should not repeatedly test and treat without evaluating management, as this practice could lead to resistance.

If EquiSal saliva scores are persistently high despite the above measures, especially when seen in the majority of a herd and where pasture management is deemed appropriate, anthelmintic resistance should now be considered a possibility. Where this is a concern, horse owners should seek further advice from their vet.

Lightbody, K.L., Davis, P.J., Austin, C.J., 2016. Validation of a novel saliva-based ELISA test for diagnosing tapeworm burden in horses. Vet. Clin. Path. 45: 335-46.

Matthews, J.B., Peczak, N., Engeham, S., 2023. An update on the latest developments in testing for equine helminths, In Practice, Under review.

Nielsen, M.K., 2023. Apparent treatment failure of praziquantel and pyrantel pamoate against anoplocephalid tapeworms. Int J Parasitol Drugs Drug Resist. 22;96-101.