Strongyle faecal egg counts in Swiss horses: A retrospective analysis after the introduction of a selective treatment strategy

Lüthin, S.; Zollinger, A.; Basso, W.; Bisig, M.; Caspari, N.; Eng, V.; Frey, C. F.; Grimm, F.; Igel, P.; Lüthi, S.; Regli, W.; Roelfstra, L.; Rosskopf, M.; Steiner, B.; Stöckli, M.; Waidyasekera, D.; Waldmeier, P.; Schnyder, M.; Torgerson, P.R. and Hertzberg, H. (2023). Strongyle faecal egg counts in Swiss horses: A retrospective analysis after the introduction of a selective treatment strategy. Veterinary parasitology, 323, p. 110027. Elsevier 10.1016/j.vetpar.2023.110027

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The standard parasite management of horses based on regular anthelmintic treatments, now practiced for decades
has resulted in a worrying expansion of resistant helminth populations, which may considerably impair
control on the farm level. The aim of the present study was to obtain a retrospective (year 2010 – 2016)
nationwide analysis of faecal egg count (FEC) data from the Swiss adult horse population, related to horse age
and geographic region. Thirteen labs provided a total of 16,387 FEC data of horses aged four to 39 years
(average: 13.6 years). The annual number of performed FEC tests increased from 38 to 4,939 within the
observation period. Independent of the annual sample size the yearly patterns of the FEC were very similar.
Seventy-eight percent (n = 12,840) of the samples were negative and 90 % (n = 14,720) showed a FEC below 200
strongyle eggs per gram (EPG) of faeces. The annual mean strongyle FEC ranged between 60 and 88 EPG with a
total mean of 75 EPG. Horses aged 4–7 years showed a significantly (p < 0.00001) higher mean FEC compared
with the other age groups, differences were not significant among the older horses. Based on ZIP codes, samples
were allocated by 70.0 %, 6.0 % and 0.2 % to the German-, French- and Italian-speaking regions of Switzerland,
respectively. With 222 EPG the mean FEC in the French part of Switzerland was significantly higher (p < 0.05)
than in the German-speaking region (60 EPG). Eggs of Parascaris spp., anoplocephalids and Strongyloides westeri
were found in 0.36 %, 0.32 % and 0.01 % of the samples, respectively. Based on 3,813 questionnaire feedbacks
from owners in 2017 covering a total of 12,689 horses, sixty-eight percent (n = 8,476) were dewormed without
diagnosis, two percent (n = 240) were not dewormed at all, whereas for 30 % (n = 3,721) the selective
anthelmintic treatment (SAT) concept was applied. The SAT implementation rate differed significantly (p <
0.0005) between regions, with 33 %, 20 % and 25 % for the German-, French- and Italian-speaking areas,
respectively. The rate of horses spending 16–24 h on pasture per day was significantly higher in the Frenchspeaking
region compared to the German-speaking part of Switzerland (p < 0.0001). In addition, pasture hygiene was practiced at a significantly lower rate in the French-speaking part compared to the German- and
Italian-speaking regions (both p < 0.0001). Overall, the shift towards the SAT-concept represents a very
promising development with respect to mitigating the further spread of anthelmintic resistance.

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