Sweden changed its horse deworming strategy. Here are the results

Sweden’s controlled approach to deworming treatments for horses is working, the findings of a study show.

Routine calendar-based deworming was used for several decades in Sweden. However, like many countries, concerns grew over rising levels of resistance to deworming drugs (anthelmintics).

The emergence of drug-resistant worm populations stimulated the introduction of alternative control strategies to reduce the frequency of antiparasitic treatments.

Although deworming without prior faecal analysis is still permitted in Sweden, treatment decisions are now usually based on the results of individual faecal analyses, with 200 strongyle eggs per gram often used as the threshold for deworming.

Using the results of faecal analyses for treatment decisions means that individual horses that contribute substantially to pasture contamination are treated as opposed to all horses.

Researchers Eva Osterman-Lind, Mia Holmberg and Giulio Grandi, all with the National Veterinary Institute in Uppsala, have described the outcome of that strategy, based on a 10-year study of horses from equestrian premises across Sweden from 2008–2017.

In 2006, a parasite-monitoring programme for equestrian premises (with at least eight horses) was initiated and marketed by the institute to enable parasite control based on targeted selective treatment.

The aim is to achieve an effective control of parasites on horse premises, and simultaneously slow down the progression of resistance by avoiding unnecessary treatments and encouraging the adoption of various pasture hygiene methods.

Dung removal at a rate of twice per week has been shown to be very effective at reducing the number of infective strongyle larvae of horses and donkeys on pastures.

In line with the institute’s practices, a veterinarian in the parasitology laboratory communicates the results of the faecal analyses and provides the owner or operator of each premises with specific advice about deworming treatment and pasture management.

A practising veterinarian connected to the equestrian property then evaluates the advice and prescribes the deworming medication.

Laboratory methods used in the programme were chosen and developed on the basis of diagnosing the most important intestinal parasites (Strongylus vulgaris, cyathostomins, Anoplocephala perfoliata and Parascaris species) at a price that horse owners find reasonable.

Faecal samples from individual horses were collected and submitted to the institute by owners of horses or equestrian premises in the spring and autumn for laboratory testing.

Between March 1, 2008, and June 30, 2017, a total of 43,330 faecal samples collected from 26,625 horses on 935 equestrian premises were analysed.

Yearly, the number of faecal samples ranged from 3240 to 4946 and were submitted from 244 to 324 equestrian premises.

Larval cultures showed that the vast majority of larvae belonged to cyathostomins, the study team reported in the journal Animals. The proportion of individuals that tested positive for S. vulgaris each year in 2013–2017 varied from 4 to 11%.

Between 53% and 61% of horses per year had counts of up to 200 eggs per gram. The prevalence of S. vulgaris was not correlated with high egg shedding. In contrast, horses with up to 200 eggs per gram were significantly more often positive for S. vulgaris than horses with more than 200 eggs per gram.

In all, between 4 and 11% of individual horses tested positive for S. vulgaris and 3–10% were shedding tapeworm eggs. There were recurrent high and low egg shedders.

Three-quarters of horses with S. vulgaris appeared to have been recently introduced into their respective herds, the researchers found.  “Thus, it is assumed that newcomers are often responsible for the introduction of the parasite to equestrian premises.

“This should drive increased awareness of the importance of treating new horses and checking the treatment efficacy before letting them out on grazing areas.”

The authors noted that the proportion of S. vulgaris-positive premises increased when individual samples rather than pooled samples were used.

Based on the results of S. vulgaris diagnostics and strongyle egg-shedding level, 59% of the horses did not need to be dewormed.

The occurrence of S. vulgaris was significantly higher in horses under five years old than in horses aged five years or more.

The authors noted that, in the mid-1990s, when routine treatments were performed three to four times per year, the occurrence of S. vulgaris on Swedish equestrian premises was still 14%.

The results showed that a minority of horses in an equestrian premise excrete most of the eggs, which is in line with the “20/80 principle” — that is, approximately 20% of the horses shed 80% of the eggs.

Consequently, fewer than 50% of the horses in the programme are dewormed after each sampling occasion, increasing the chance of slowing down the development and spread of anthelmintic resistance.

In the present study, 62% of the horses were recurrent low or high egg-shedders, which is of practical relevance, as the institute recommends that the former do not need to be sampled in the autumn and the latter often need to be dewormed more frequently.

In groups of adult horses, the institute does not generally recommend non-specific/pre-emptive anthelmintic treatments for all horses but suggests only selective treatments.

However, to reduce the occurrence of S. vulgaris, which is still prevalent in Swedish equestrian premises, non-specific treatments are recommended when summer pastures are infected by S. vulgaris.

In this case, since 2019, it was recommended that macrocyclic lactones be given to all horses in October and March for two consecutive years before returning to the selective treatment programme.

The treatment in October was aimed at killing larvae acquired during the grazing season while the second treatment in March was aimed at removing any residual migrating pre-adult stages since it is known that not all the developmental stages are equally susceptible to anthelmintic treatments.

To follow up on the effect of these treatments and diagnose tapeworm and cyathostomins, it is still recommended that equestrian premises sample their horses before the grazing season.

Compulsory treatments of young horses and horses at risk of infection for various parasites are considered in some guidelines, they noted. The national recommendation in Sweden is to deworm all foals against Parascaris species (fenbendazole at 8–10 weeks of age and 16–18 weeks of age) and strongyles (macrocyclic lactones in the autumn).

From the second year of life, it is suggested that faecal analyses should be included as part of the parasite control programme.

The authors said their study shows that routine-based deworming interventions can be greatly reduced when they are based on faecal analyses.

“The constant presence of a potentially lethal parasite as S. vulgaris is somewhat worrying and the effect of recommendations of blind treatments for newcomers and extra treatments in infected equestrian premises will be monitored,” they said.

“Moreover, the monitoring programme itself has continuously generated updated figures on the occurrence of equine intestinal parasites and made equestrian premises aware of the effectiveness of their efforts to manage parasites.”

In addition to autumnal tapeworm diagnostics, an annual faecal egg count reduction test (FECRT) could be one way to further improve the monitoring programme, they said.

Osterman-Lind, E.; Holmberg, M.; Grandi, G. Selective Anthelmintic Treatment in Horses in Sweden Based on Coprological Analyses: Ten-Year Results. Animals 2023, 13, 2741. https://doi.org/10.3390/ani13172741

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