Foals at risk as roundworm resistance to common wormer revealed

Intestinal impaction with parascaris
Intestinal impaction with parascaris. © Equine Science Update

A Swedish study has identified pyrantel resistance in the large roundworm (Parascaris spp) of the horse, while fenbendazole still appears to be effective against the parasite.

The researchers also identified the species of worm involved as Parascaris univalens, and not Parascaris equorum.

The large roundworm of horses is found throughout the world, and is a common parasite of foals, with deaths reported in youngsters up to 4 months of age. Older foals develop immunity, and the parasite rarely causes problems in adult horses.

Adult worms live in the small intestine, and migrating larvae may cause a mild cough and nasal discharge. A heavy infection leads to failure to thrive and may cause intestinal impaction or rupture.

A 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. (Foal bodyweight was estimated by stud personnel and rounded up to nearest 50kg to avoid underdosing).

Parascaris univalens egg at first miotic division showing one pair of chromosomes, At 400x magnification. The eggs collected from a farm in northern Sweden. 
A Parascaris univalens egg at first miotic division showing one pair of chromosomes, at 400x magnification. The eggs were collected from a farm in northern Sweden.

Using the faecal egg count reduction test (FECRT) they found evidence of pyrantel resistance in parascarids on Swedish stud farms.

If pyrantel were effective a reduction in faecal egg count after treatment of 94% or more would be expected. This was seen in only four of the 11 tested groups. As many as 42 of the 97 tested foals (43%) excreted eggs 10 to 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.

Based on their findings, the researchers suggest that fenbendazole should be the first choice for treatment of Parascaris infection. They suggest treatment at two and five months of age.

“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.”

• Following DNA tests in 2015, the worm, widely known as Parascaris equorum, was identified as Parascaris univalens, a close relative. Univalens looks identical to, and, in the past, has been confused with 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. The two species are otherwise considered structurally identical. P. univalens was discovered more than 130 years ago.

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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