New Hope Stables in Hertford in Perquimans County reports no one knows what made the horses sick.
A Facebook post by the stable says many known infectious diseases have been ruled out, and that they are not ruling out the possibility that the horses were poisoned.
The horses were all tested for coronavirus and other diseases, and all the tests came back negative.
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The post also said the stable is working with the N.C. Department of Agriculture to determine the cause. The horses fell ill at the end of September.
The Oaks Equine and Farm Service is helping the stables treat the remaining sick horses. They are also accepting donations to help cover the costs; you can reach them at 757-365-4887.
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EDITOR'S NOTE: The video attached to this story is from a previous story about a mosquito-borne horse illness.