Consensus among the investigators is the cause of the illness is in something the horses are eating, such as toxins from mold spores. Tissue samples from a euthanized horse have been sent to Cornell University for evaluation. The affliction appears to be fatal.
See this report in the Hawaii Tribune-Herald dated 07/23/18.
RELATED: Mysterious Illness Affecting Wild Horses on Big Island.
Western Horse Watchers Association 