Strongylus vulgaris wild horse reservoir

Wild horse populations may act as a source of infection for domestic horses, according to a recent report.

Strongylus vulgaris, (also known as the blood worm) is the cause of verminous endarteritis. The larvae migrate from the horse’s intestines into the surrounding blood vessels and continue to various organs around the body, causing damage as they go.  The resulting thrombi can block blood flow and result in tissue damage and colic.

The parasite has been well controlled by macrocyclic lactones – so much so that from being the most important equine parasite of 30 years ago it is now rarely a problem in domestic horses.

Researchers from the University of Sydney and the University of Tasmania examined 289 faecal samples collected from six separate wild horse herds in south east Australia.

Andrea Harvey and colleagues found a wide range of total strongyle egg counts (ie both “large” strongyles such as S vulgaris and “small” strongyles – such as the cyathostomins) Eggs of large and small strongyles look the same. Appearance cannot be used to differentiate between Strongylus vulgaris and other strongyle-type eggs.

Most of the faecal samples contained S vulgaris DNA. The research team report: “A high prevalence of S. vulgaris DNA in faecal samples was demonstrated across all six populations, with an overall predicted prevalence of 96.7%.”

Over 89% of samples had more than 500 eggs per gram, classing them as “high shedders”.

The researchers suggest that vigilance is required when adopting wild horses, and when domestic horses graze in on land also grazed by wild horses. 

They advise that it would be wise to monitor horses for S. vulgaris using larval culture or DNA testing.

But it's not all bad news. The population of worms in the wild horses also acts as a reservoir of small strongyles that have not been exposed repeatedly to anthelmintics and may be able to play a role in management of anthelmintic resistance.

For more details, see:

Wild horse populations in south-east Australia have a high prevalence of Strongylus vulgaris and may act as a reservoir of infection for domestic horses

Andrea M.Harvey, Maira N.Meggiolaro, Evelyn Hall, Ellyssia T.Watts, Daniel Ramp, Jan Šlapeta

International Journal for Parasitology: Parasites and Wildlife. (2019) Vol 8, pp 156-163

https://doi.org/10.1016/j.ijppaw.2019.01.008