Horse owners in the Aachen and Düren areas of North Rhine-Westphalia, as well as some in the Belgian province of Lüttich, have seen their animals develop muscular problems and die, sparking fears that someone was poisoning them.
Now vets say they suspect the animals had developed atypical myopathy, which is a fatal disease linked to eating seeds from sycamore trees. It appears that only those horses which were being kept in fields had been affected - those kept inside were not.
The Westdeutsche Allgemeine Zeitung reported that more than 50 horses had died from the disease.
Johannes Hörmeyer, a vet from the Aachen Veterinary Office, recommended horse owners to check whether there were sycamore trees in or near their fields.
Jessika Cavalleri from the Horse Clinic at Hannover's Veterinary University said: "This is a very dangerous illness which is usually deadly."
It would seem the illness comes in waves, with many cases surfacing in 2005 and 2009. Cavalleri said this could be related to weather conditions or changing toxin levels in the trees.
Dozens of horses are said to have died in North Rhine-Westphalia over the last few weeks, but the disease does not have to be registered with authorities so Cavalleri said it was impossible to say how many horses had been affected.