No signs of worm resistance among intestinal strongyles in tested Sardinian horses

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No evidence of drug resistance was found in intestinal strongyles in a study of horses on the Mediterranean island of Sardinia, an autonomous region of Italy.

Intestinal strongyles are the most important parasites of horses, due to their high prevalence worldwide, potential to cause disease, and the spread of drug-resistant populations.

Italian researchers set out to gather data on the effectiveness of dewormers, known as anthelmintics, in controlling strongyle infection in horses on the island.

The study involved 74 horses from seven farms, across which five commercial worming formulations were tested. One contained fenbendazole as its active ingredient, one contained pyrantel, one used moxidectin, and two were ivermectin formulations

Fecal samples collected on the day of treatment showed that cyathostomins were the predominant parasitic species (98.6%).

The study team subsequently performed fecal egg count reduction tests two weeks after the horses were dosed and also investigated the egg reappearance period in the horses at intervals up to 150 days.

The fecal egg count reduction values at day 14 in each treatment group constantly showed a reduction of 95% or more. For one of the ivermectin formulations, the effectiveness was 100% 14 days after treatment. The moxidectin treatment group was 100% effective until 75 days after treatment.

“The results of the present survey indicate that drug-resistant cyathostomin populations are not present in the examined horse population, contrariwise to that observed in other Italian and European regions,” the study team reported in the journal, Parasite Epidemiology and Control.

There was, they said, no evidence of anthelmintic resistance in the examined animals, in contrast to Italy, where evidence of resistance has been found.

“The present results, although the number of investigated horses was not high, differ from those achieved in continental Italy and other European countries, where drug resistant cyathostomins are spread and the egg reappearance period is reduced for fenbendazole and, pyrantel, though with a lesser extent, for ivermectin and moxidectin as well.”

They said the absence of drug resistance in the examined population of Sardinian horses could be related, besides its insularity – that is, natural boundaries prevent the introduction of resistant parasite populations – to the frequency of anthelmintic treatments on the island.

“In fact, more than half of the population of Sardinian horses receive a parasiticide less than two times per year, thus pressure for selection of resistance is minimal if compared to other management practices.

“Also, climatic variables could play an important role.

“In fact, the mild, temperate weather of the island allows a high survival rate of environmental third-stage larvae all year round, thus preserving the amount of those parasitic stages which do not come into contact with parasiticides and are not selected for resistance.”

They noted that, although in Italy anthelmintics were sold as veterinary prescription-only medications, a parasitological diagnosis before treatment was not required as, for instance, in Denmark where this approach was considered a potential strategy to limit resistance in horses.

“Similar control plans would be desirable in other settings,” they said. “In fact, a recent questionnaire survey on intestinal worm control practices in horses in Italy showed that only 9.3% of respondents usually dewormed animals after a fecal examination and 61.3% dewormed all horses together.”

Appropriate use of these drugs, along with high standards of management, were essential to limit or prevent the development and spread of resistance, they said.

A prescription-based only treatment would be a powerful tool in reducing the spread of resistant cyathostomins where they were already present and to prevent their introduction in the free regions, the study team said.

Anthelmintics efficacy against intestinal strongyles in horses of Sardinia, Italy
G. Sanna, , A.P. Pipia, , C. Tamponi, , R. Manca, , A. Varcasia, D. Traversac, A. Scala.
doi:10.1016/j.parepi.2016.01.001

The study, published under a Creative Commons License, can be read here

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