Sweden’s efforts to combat the growth of drug resistance in horse parasites may be aiding the comeback of the most dangerous of the large strongyles, research suggests.
Frequent deworming programs since the 1970s have reduced the prevalence of Strongylus vulgaris, but it has come at a price.
There is now widespread benzimidazole resistance among small strongyles and ivermectin resistance has developed, especially in species of Parascaris, the equine roundworm.
To slow the progression of drug resistance in Sweden, deworming drugs were made prescription-only medicine in 2007 and selective therapy principles were introduced. At this time, only 1% of horse farms in the country submitted faecal samples on a regular basis for diagnositic parasite checks.
The change, driven by an European Union directive, meant that only horses with high egg excretion or clinical signs of helminth infection were treated instead of the blanket treatment of all horses on a farm.
The change led to a more than 50% reduction in anthelmintic drugs sold for use in horses and livestock in the subsequent 10-year period.
Eva Tydén and her colleagues set out to investigate the prevalence and risk factors associated with S. vulgaris infection in Sweden 10 years after the introduction of a selective therapy regime.
Sweden has an estimated 350,000 horses, which is comparable with the number of dairy cows. About 80% of the horse farms are close to cities, where extensive grazing areas are limited, allowing accumulation of parasite eggs in paddocks.
A total of 529 faecal samples from 106 farms were collected during March to June in 2016 and 2017. An online questionnaire was used to collect information about deworming routines.
Strongyle faecal egg counts (FEC) were performed using the McMaster method, and the presence of S. vulgaris was determined using a molecular-based test on individual larval cultures.
The study team, writing in the journal Veterinary Parasitology: X, found there has been a three-fold increase of S. vulgaris prevalence in horses in Sweden since 1999.
They found that 28% of the horses were infected, and 61% of the 106 farms had infected horses present.
The risk of a horse having an S. vulgaris infection was 2.9 times greater on farms that only performed faecal egg counts as compared to farms that complemented strongyle FECs with larval cultures or dewormed regularly, 1 to 4 times per year, without prior diagnosis.
The researchers found no association between the prevalence of S. vulgaris and strongyle FEC levels, horse age, geographical region or signs of colic.
The prevalence of S. vulgaris was 25% in horses shedding 150 eggs per gram or less (a low rate of shedding).
“Thus horses with low strongyle FECs that are left untreated could be an important source of S. vulgaris infection,” they said.
“This may be an important reason for the approximately three-fold increase in S. vulgaris prevalence since 1999 in Sweden. However, our combined results indicate that selective therapy based on a combination of strongyle FECs and larval cultivation was not associated with an increased risk of S. vulgaris infection.
“Still, S. vulgaris needs to be monitored continuously and should be taken into careful consideration when the treatment frequency is reduced.’
The authors said the three-fold increase in the prevalence of S. vulgaris appeared to be at least partly associated with parasite analyses based on only FEC without S. vulgaris diagnostics.
The combination of strongyle FECs and larval cultures overcame this, delivering results comparable to regular blanket treatment 1 to 4 times per year.
“The only risk factor for infection with S. vulgaris in our study was selective therapy based on strongyle FECs alone. A key message to horse owners and veterinarians is the importance of including specific diagnostics for S. vulgaris even in situations when the excretion of strongyle eggs is low or below the detection limit in individual horses.”
Prevalence of Strongylus vulgaris in horses after ten years of prescription usage of anthelmintics in Sweden
Eva Tydéna, Heidi Larsen Enemark, Mikael Andersson Franko, Johan Höglund, Eva Osterman-Lindd.
Veterinary Parasitology: X, Volume 2, November 2019, 100013