Sweden’s horse owners could manage parasite burdens better, study finds

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The use of specific diagnostic tools by Sweden’s horse owners to identify parasite burdens in their animals remains low, 10 years after the nation reclassified deworming agents as prescription-only drugs.

Researchers have delved into parasite occurrence and management among Swedish horses with gastrointestinal disease in a just-published study, comparing them to horses with no gut-related issues.

Ylva Hedberg-Alm and her colleagues, writing in the open-access journal Animals, note that colic is a common clinical sign in horses, which is sometimes life-threatening.

One potential cause is parasites, which can be controlled by deworming drugs. However, their frequent use has led to drug resistance, prompting Sweden to introduce a prescription-only policy for their sale about 10 years ago in a bid to move to evidence-based deworming.

There are concerns that parasite-related colics in horses have increased since, with particular concern around large strongyles (Strongylus vulgaris), given their potential to cause life-threatening disease.

The study team set out to learn whether horses with colic differed in parasitological status compared to horses without colic.

They used molecular-based testing to investigate the prevalence of large strongyles and the most common tapeworm in horses, Anoplocephala perfoliata.

Fecal egg counts were conducted to provide insights into the burden of small strongyles carried by the horses.

The study team also used a questionnaire to delve into the parasite control measures used by owners and their pasture management strategies.

The researchers used horses admitted to the University Animal Hospital at the Swedish University of Agricultural Sciences over one year, taking samples from 137 horses with gastrointestinal disease and 137 controls, with no issues related to their gut.

Molecular testing revealed the presence of S. vulgaris in 5.5% of the horses. However, 62% of horses were seropositive for the parasite, indicating a high frequency of exposure; and horses with peritonitis (inflammation of the peritoneum) showed higher antibody levels for S. vulgaris when compared to the control animals.

A total of 15.7% (43 of the 274 horses) were positive for tapeworms, with no significant difference between the two groups.

The overall prevalence of strongyle eggs was 44.9% (123 of the 274 horses) and the prevalence was similar between cases and controls, 47.4% and 42.3%, respectively.

About two-thirds of the horses were classified as low egg shedders, with an eggs-per-gram count of 200 or fewer. In all, 16.4% were classed as high shedders.

Discussing their findings, the authors said that small strongyles appeared to be well tolerated without causing disease in most cases, apart from the well-known syndrome of larval cyathostominosis in young animals.

The study team found no differences in parasite control strategies between cases and controls.

Macrocyclic lactones — the family of dewormers that includes ivermectin — was the most commonly used agent.

“However, within the study population as a whole, differences in anthelmintic routines were present, with 28.8% of the owners deworming their horses on a routine basis without prior parasitic diagnostics.”

The authors described the exposure among Swedish horses to large strongyles as high. In addition, those with peritonitis showed significantly higher seroprevalence compared to controls and other gastrointestinal diagnoses, suggesting that S. vulgaris had a role in causing disease.

They suggest future studies should explore the association between S. vulgaris and peritonitis in Swedish horses to learn more about the clinical implications of the increasing prevalence of S. vulgaris in the country.

Turning to management strategies, the study revealed that 36% of the horse owners used only fecal egg counts, 32% used egg counts combined with specific diagnostics for S. vulgaris or A. perfoliata, and 29% dewormed routinely without prior parasite diagnostics.

“Although the study showed a clear increase in the use of fecal analyses compared with before the prescription-only law was implemented, such legislation does not, per se, result in all owners using fecal diagnostics prior to treatment.

“The use of specific diagnostics is still low, indicating a need for education, of both owners and the veterinary profession, on how to best apply the diagnostic tools available.”

Treating horses annually or bi-annually does not ensure a negative test result for S. vulgaris, they say.

“Moreover, the study revealed that stocking intensity is often high and practices such as frequent fecal removal or co-grazing with other species were rarely performed.

“Educating owners regarding optimal pasture management appears vital.

Given resistance issues, strategies for sustainable control of equine internal parasites are urgently needed, including pasture management, they write.

The study team comprised Hedberg-Alm, Johanna Penell, Miia Riihimäki and Eva Tydén, all with the Swedish University of Agricultural Sciences; Eva Osterman-Lind with the National Veterinary Institute in Sweden; and Martin Nielsen, with the Maxwell H. Gluck Equine Research Center, which is part of the University of Kentucky.

Hedberg-Alm, Y.; Penell, J.; Riihimäki, M.; Osterman-Lind, E.; Nielsen, M.K.; Tydén, E. Parasite Occurrence and Parasite Management in Swedish Horses Presenting with Gastrointestinal Disease—A Case–Control Study. Animals 2020, 10, 638.

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

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