The equine lungworm is Dictyocaulus arnfieldi. It has an unusual quirk: it is well adapted to donkeys and mules, which often carry the worm without showing any signs, but it tends to cause a noticeable cough in horses sharing the same pasture.
The donkey connection
Donkeys are the natural reservoir. They can shed lungworm larvae onto pasture for long periods while appearing perfectly healthy. Horses grazing alongside them pick up the larvae and may develop a persistent cough — even though the worms frequently fail to fully mature in the horse.
Mixed grazing is the main risk factor
If horses and donkeys share grazing, treat the whole group as one unit for worming and ask your vet about a strategy that covers the donkeys, who are usually the hidden source.
Signs in horses
The classic sign is a chronic cough that worsens with exercise, sometimes with increased respiratory rate and effort. Because the worms often don’t reach full maturity in horses, faecal larval counts can be falsely negative — so history and grazing companions matter as much as test results.
Treatment in horses
Lungworm in horses responds well to several dewormers. Fenbendazole and the macrocyclic lactones (such as ivermectin and moxidectin) are commonly used. Treating the donkeys on the property is just as important as treating the affected horse, to remove the source of contamination.
Use the calculator for a fenbendazole estimate, and ask your vet about a whole-herd plan.
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