Cause of Waipio Valley Horse Deaths Undetermined

Posted on Sep 14, 2018 in Main

Sept. 14, 2018

HONOLULU – After extensive tests and observations, an ensemble of veterinarians were not able to determine the specific cause of the deaths of about 13 wild horses from Waipio Valley on Hawaii Island that occurred during June and July 2018. It was noted that infectious diseases were not the cause of the deaths and that the incident may be attributed to possible exposure to a toxicological event.

Veterinarians from the Hawaii Department of Agriculture (HDOA), University of Hawaii Manoa-College of Tropical Agriculture and Human Resources, local practicing veterinarians, researchers from the University of Hawaii Hilo–College of Pharmacy and a cadre of veterinarians, veterinary toxicologists and pathologists across the nation were involved in the attempt to determine why the horses exhibited neurological problems and eventually died.

It is estimated that there are about 50 to 60 wild horses in Waipio Valley that form five separate bands. The reported sick horses were all from one band that resided on the west side of the valley. None of the other bands of horses or domestic trail riding horses in the valley were affected.

“This disease investigation into the cause of the deaths of these wild horses is one of the most extensive in Hawaii in my recollection,” said Jason Moniz, DVM and manager of HDOA’s Animal Disease Control Branch. “In addition to the veterinarians, laboratories and researchers who worked on this case, we appreciate the help of Waipio Valley taro farmers who provided assistance, observations, information and feedback and who showed sincere concern for these horses.”

Farmers assisted the veterinarians in finding one of the sick horses and blood and tissue samples were submitted to laboratories in Hawaii and on the Mainland. All tests for infectious equine diseases were negative.

Equine pathologists found mild inflammation of the horse’s brain, but it was not deemed significant enough to cause the deaths. Two small lesions found in the spinal cord appeared old and were also determined insufficient to cause significant neurologic disease or death. Testing was conducted for rat lungworm disease and although very low levels of DNA were detected, changes in the brain and spinal cord were not consistent with an active rat lungworm infection. Veterinarians did note an incident of a small hydraulic fluid spill which occurred in the area prior to the onset of the illnesses. Measurement of a nerve tissue enzyme was found to be depressed.  This enzyme depression can occur when there has been exposure to organophosphates, other insecticides and some other chemicals.  Certain hydraulic fluids contain organophosphates and are known to cause a delayed neuropathy consistent with what was seen in the affected horses. However, microscopic evaluation of nerve tissue did not detect pathology expected with this type of neuropathy.

One of the most significant abnormal finding was the poor body condition seen in this and other affected horses. There was marked atrophy of muscle particularly the posterior muscles of the horse.  There was also marked fat atrophy of what little fat deposits that were left which is consistent with the poor nutritional condition of the horse.

Tests on fecal material and pathology found in the small and large intestine indicated moderate to heavy levels of parasites which may have contributed to its reduced nutritional condition but alone would not account for the clinical signs seen in these horses and their deaths.

Blood tests indicated there was damage to the horse’s liver and muscles and there was evidence of liver damage consistent with intestinal parasite migration. A microscopic parasite, Sarcocystis spp., was found sporadically in various muscles. Further testing is ongoing to determine the species of this parasite so its significance can be determined.

There are a number of plants growing in Waipio Valley that are toxic to horses and other animals. However, no toxins were detected in the stomach content or liver of this horse. In addition, plants with toxic principles grow throughout the valley and not only in the area where the affected horses resided.

Although an exact cause of the illnesses could not be determined, veterinarians have concerns related to the findings of this investigation and have the following recommendations to safeguard and improve the health of the Waipio Valley wild horses:

  1. When operating equipment with hydraulics on farms and ranches, utilize OP- free hydraulic fluids in case of accidental spills.
  2. Control loose and feral dog populations, the definitive host for certain Sarcocystis species, to prevent Sarcocystis infections in horses.
  3. Control rodent populations to reduce rat lungworm infections in humans and animals.
  4. Consider a plan to treat the Waipio wild horses for intestinal parasites to lower the intestinal parasitic impact on the horses and improve their physical condition.
  5. Consider improving grass production to improve the nutrient level of the Waipio wild horse population and reduce the potential consumption of toxic plants.
  6. The public is encouraged to report any additional health concerns seen in the Waipio Valley horses to the State Veterinarian’s office (808) 483-7100 for further work up.

A detailed report of the tests conducted and the findings is available on HDOA’s website at:

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