A recent Swedish study has identified pyrantel resistance in
the large roundworm (Parascaris spp) of the horse. What’s more, the study found
that the worm, widely known as Parascaris equorum, may be more correctly
identified as Parascaris univalens, a close relative.
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| Intestinal impaction with parascaris |
Migrating larvae may cause a mild cough and nasal discharge.
Adult worms live in the small intestine. A heavy infection leads to failure to
thrive and may cause intestinal impaction or rupture. Deaths have been reported
in foals up to 4 months of age. Older foals develop immunity, and the parasite
rarely causes problems in adult horses.
A new study by Frida Martin and colleagues at the Swedish
University of Agricultural Sciences, Uppsala, Sweden investigated anthelmintic
resistance in parascarids on Swedish stud farms.
Foals with faecal egg counts of 150 epg or more were
included in the study. They were treated with either pyrantel or fenbendazole
at the manufacturers’ recommended dose. (Foals’ bodyweight was estimated by
stud personnel and rounded up to nearest 50Kg to avoid underdosing).
Using the faecal egg count reduction test (FECRT) they found
evidence of pyrantel resistance in parascarids on Swedish stud farms.
If pyrantel were effective you would expect a reduction in
faecal egg count after treatment of 94% or more. This was only seen in four of
the 11 tested groups. As many as 42 of the 97 tested foals (43%) excreted eggs
10–16 days after treatment, indicating that in those foals the pyrantel had not
been effective.
In contrast, fenbendazole still appeared to be effective against
Parascaris spp. on most of the farms involved in the study.
The researchers also identified the species of worm involved
as Parascaris univalens, and not Parascaris equorum.
P. univalens looks identical to, and, in the past, has been
confused with, its sister species P. equorum. The difference between the two is
only apparent at a cellular level using karyotyping (the visual depiction of
all the chromosomes in a cell.) P. univalens possesses a single pair of
chromosomes, whereas P. equorum has two.
This finding agrees with studies around the world that are
increasingly finding Parascaris univalens rather than P equorum.
Based on these findings, the researchers suggest that
fenbendazole should be the first choice for treatment of Parascaris
infection. They suggest treatment at 2
and 5 months of age.
They add: “these data suggest that P. univalens is likely
the main species now observed in equines and that perhaps the designation
Parascaris spp. should be used unless cytological characterization has
confirmed the species.”
For more details, see:
Resistance to
pyrantel embonate and efficacy of fenbendazole in Parascaris univalens on
Swedish stud farms
Frida Martin,
Johan Höglund, Tomas F.Bergström, Oskar Karlsson Lindsjö, EvaTydén
Veterinary
Parasitology (2018) Vol 264, pp 69-73
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