Wild horses in southeastern Australia are a reservoir of one of the most pathogenic equine parasites, according to researchers.
Modern drenches have vastly reduced the threat posed by Strongylus vulgaris, considered the most dangerous of the large strongyles, or large redworms.
Damage is caused by the larval stages of the parasite, which migrate in the arteries that supply the large intestines. The larvae can cause endarteritis (artery inflammation) and thrombosis, and may result in blockage of the arteries and intestinal infarctions.
The introduction of macrocyclic lactones as a drench in the 1980s dramatically reduced problems caused by migrating S. vulgaris larvae. On the other hand, cyathostomines (small strongyles) remain a problem in most, if not all, domestic horse populations, particularly due to their increasing resistance to anthelmintics.
Andrea Harvey and her colleagues hypothesised that wild horse populations unexposed to dewormers would have a high prevalence of S. vulgaris.
Australia has more than 400,000 wild horses, the largest wild equid population in the world, scattered across a range of different habitats.
While artery problems and colic due to migrating S. vulgaris larvae is now absent or unreported in domestic horses in Australia, wild horses may pose a risk for its re-emergence, the researchers said.
A total of 289 faecal egg counts were performed across six remote wild horse populations in south-east Australia, of varying densities, herd sizes and habitats.
Total strongyle egg counts ranged from 50 to 3740 eggs per gram, with an average of 1443.
They found that 89% of the faecal samples had egg counts above 500, classifying them as high level shedders.
There were significant differences in average total strongyle egg counts between different locations, habitats and population densities.
The occurrence of S. vulgaris was not predictable based on the counts of total strongyle eggs or small strongyle eggs.
A high prevalence of S. vulgaris DNA in faecal samples was demonstrated across all six populations, with an overall predicted prevalence of 96.7%.
“This finding is important,” the study team reported in the International Journal for Parasitology: Parasites and Wildlife, “because of the ample opportunity for transmission to domestic horses.
“The high prevalence of S. vulgaris suggests vigilance is required when adopting wild horses, or when domestic horses graze in environments inhabited by wild horses.”
Appropriate veterinary advice is required to minimise disease risk due to S. vulgaris, they said.
Monitoring horses for S. vulgaris remained prudent, they added.
“Gastrointestinal parasites in wild horse populations may also serve as parasite refugia, thus contributing to integrated parasite management when facing emerging anthelmintic resistance.”
The full study team comprised Harvey and Daniel Ramp, from the University of Technology Sydney; Maira Meggiolaro, Evelyn Hall and Jan Šlapeta, from the University of Sydney; and Ellyssia Watts, from the University of Tasmania.
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, and Jan Šlapeta.
International Journal for Parasitology: Parasites and Wildlife, Volume 8, April 2019, Pages 156-163