Thursday, December 20, 2018

Parascaris resistance and identity

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. 
Intestinal impaction with parascaris
The large roundworm of horses is found throughout the world. It is a common parasite of foals.
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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