Wild Horse Deaths – Waipio Valley, Hawaii Island

Posted on Jul 25, 2018 in Main

July 25, 2018


Over the past two months or so, wild horses in the Waipio Valley on Hawaii Island have been observed with abnormal neurologic signs. Affected horses have been seen unsteady on their back legs and having difficulty walking.  As many as a dozen horses may have been affected, with estimates of at least 10 eventually dying as of July 17, 2018. Domestic trail horses housed in the same area have not been affected.

Veterinarians from HDOA, UH-College of Tropical Agriculture and Human Resources (CTAHR) and several private veterinarians are working collaboratively to determine the cause of the wild horse deaths.


On July 17, 2018 an affected horse was found lying sternal with her head elevated. She appeared alert but depressed, thin and her rear musculature appeared somewhat atrophied.  She struggled to rise once but fell into a lateral recumbent position.  Three attending veterinarians evaluated her in a recumbent position and a decision was made to euthanize the mare and perform a necropsy.


A necropsy was conducted of an ~5 year old female bay horse. The carcass was significantly emaciated with no fat depot and signs that body stores were being utilized to survive. There was no yellow discoloration of the tissues and the horse was producing urine. The stomach was filled with soft green and orange ingesta. The small intestines contained a small quantity of tan creamy digesta. The large colon and cecum contained a moderate quantity of green fibrous digesta. The small colon contained a small quantity of dark green-formed feces. The left gastrocnemius contained an ~ 1.5cm X 1.0 cm X 0.5cm pale region within the muscle belly. The remainder of the gross examination was unremarkable.


(Updated 9/14/18 in blue)

No new illness or mortalities have been seen in the area where thirteen (13) wild horses from a single band died in June and July 2018.  Two horses from the band remain alive and appear normal.  Six (6) additional horses have moved into the area and are not showing clinical signs that resemble the illness.  Taro farmers in the area are continuing to observe the horses and are relaying their observations to the State Veterinarian’s office.

At this point testing, except for a couple pending tests have been completed.  Laboratory tests have not been able to support a definitive diagnosis.

Further testing that have been completed since 8/7/18:

Infectious diseases:
Virus isolation – negative

Clostridium botulinum A,B,C  (PCR) – Negative

Parasitic diseases:
Sarcocystis spp. –(PCR to determine species) – pending

No toxins found in stomach content of liver

Organophosphates and carbamates – not detected

Cyanotoxin BMAA – Negative

Creeping indigo 3-NPA – Negative

Mineral Panel:
No new tests done

Blood Chemistry:
No new tests done

No new tests done

Histology (Microscopic Diagnoses): (Summary of final examination of tissues)

  1. Meningoencephalitis, lymphocytic, multifocal, subacute, minimal, with perivascular cuffing and rare eosinophils, brain.
  2. Fibrosis, multifocal, chronic, mild, spinal cord at the level of the cranial cervical spinal cord and first thoracic vertebral body.
  3. Enterocolitis, eosinophilic and lymphoplasmacytic, multifocal, chronic, mild to moderate, small and large intestine.
  4. Portal hepatitis, eosinophilic, multifocal, chronic, mild, with an eosinophilic granuloma.
5. Fat atrophy, multicentric, marked.
6. Fibrosis and mineralization, focal, chronic, mild, right gastrocnemius tendon.
7. Protozoal cysts (presumptive etiology = Sarcocystis spp.), multifocal, mild, skeletal muscle.

COMMENT: The underlying cause of the clinical deterioration of this horse could not be determined on microscopic examination. There was significant fat atrophy, indicative of an advanced state of negative energy balance. Gastrointestinal parasitism could have contributed to this state of negative energy balance as strongyles were detected with fecal analyses and inflammatory changes compatible with endoparasitism were identified within the small and large intestine. The eosinophilic portal hepatitis was also likely caused by endoparasitism (migrating parasites). The foci of fibrosis were well-demarcated and a non-specific indicator of previous spinal cord damage. Prior parasite migration is a possible differential for the foci of fibrosis in the spinal cord, and more recent parasite migration could have caused the minimal inflammatory changes in the brain. Life stages of parasites were not identified in the brain and spinal cord, and the severity of inflammation present within the brain was not consistent with a previously reported case of Angiostrongylus cantonensis meningoencephalomyelitis in an American Miniature Horse (Costa 2000).

Electronically signed by:
Travis Heskett, DVM, DACVP
Jenee S. Odani, DVM, DACVP

Tests Pending:
Sarcocystis spp. (PCR to determine species)


(Updated 8/07/28)

No new mortalities have been seen in the area where the band of horses, estimated to be fifteen (15), had been dying.  An estimate of twelve (12) wild horses from this band is believed to have died.  One (1) additional affected horse was euthanized for testing purposes and two (2) from the band are believed to remain alive in the area.  Farmers in the area have observed a handful (4-5) of new wild horses from an adjacent band hanging out in the area now.  Farmers are assisting with monitoring and observations and will report new cases to the Hawaii Department of Agriculture, Animal Disease Control Branch.

Infectious diseases:
Eastern Equine Encephalitis – confirmed negative

Parasitic diseases:
Rat lungworm (RLW) – very low quantity of RLW DNA detected from the brain, cerebral spinal fluid (fluid around the brain and spinal chord), heart and lung. (Correlation with histopathologcial findings are not present at this point, further histopathological exams will be completed before a final diagnosis will be made)

Sarcocystosis – a protozoan parasite Sarcocystis spp. was found in several muscle tissues examined microscopically.  (Sarcocystis neurona causes Equine Protozoal Meyloencephalitis (EPM).  EPM has already been ruled out with other tests for EPM.  There are other species of Sarcocystis however, one in particular, Sarcocystis fayeri can cause illness similar to what has been seen in the Waipio horses.  Further evaluation of tissues will be conducted and additional test will be sought out to rule this in or out as a cause of the illness.)

Tests are ongoing to evaluate for organophosphates and carbamates

Mineral Panel:
No new tests done

Blood Chemistry:
Test redone – Elevated enzymes indicate liver and muscle damage

No new tests done

Histology (Microscopic Diagnoses): (Summary of preliminary examination of tissues, further evaluation underway)

Meningoencephalitis, (Inflammation of the brain and the membrane covering it) lymphocytic, multifocal, subacute, minimal, with perivascular cuffing and rare eosinophils, brain. (Two types of cells from the immune system seen as part of the inflammatory process)

Fibrosis (scaring), focal, chronic, mild, spinal cord at the level of the first thoracic vertebral body.

Enterocolitis, eosinophilic and lymphoplasmacytic, multifocal, chronic, mild to moderate, small and large intestine.

Fat atrophy, multicentric, marked. (Indicates horse in poor body condition, severe weight loss)

Fibrosis and mineralization, focal, chronic, mild, right gastrocnemius tendon and muscle surrounding the larynx

Protozoal cysts (presumptive etiology = Sarcocystis spp.), multifocal, mild, skeletal muscle.

COMMENT: The inflammatory changes within the brain were minimal, and their clinical significance is uncertain. The degree of inflammation was not consistent with a previously reported case of Angiostrongylus cantonensis (Rat lungworm) meningoencephalomyelitis in the literature.   The focus of fibrosis in one segment of the spinal cord was small and non-specific, but indicative of prior damage. The enterocolitis was most compatible with gastrointestinal parasitism, and the fat atrophy was consistent with the grossly described reduced nutritional condition of this horse. Additional levels and special stains are pending, as well as additional review by other pathologist.

Tests Pending:
Organophosphate and carbamate
Plant identification of stomach content
Histopathology ongoing


(Updated 7/25/28)

Infectious diseases:
Equine Herpes virus 1 (EHV-1) – negative
Eastern Equine Encephalitis (EEE) – negative
West Nile Virus (WNV) – negative
Rabies – negative
Brucellosis – negative

Parasitic diseases:
Intestinal parasites (worms) – moderately high levels
Toxoplasma gondii- negative
Equine Protozoal Meyloencephalitis (EPM) – negative

Arsenic – negative
Thallium – negative
Cadmium – negative
Lead – negative
Mercury – negative

Mineral Panel:
Copper – mild deficiency

Blood chemistry:
Elevated liver enzymes
(The blood chemistry is being redone before further interpretations will be made)

Cerebral spinal fluid – non-specific central nervous system pathology detected
(Non-specific = possibility of viral, protozoal, fungal or toxological conditions causing pathology.  However, other tests for common viral and protozoal conditions that cause signs seen in these horses are negative)

Tests Pending:
Rat lungworm
Other toxicology
Additional chemistry


At this point of the investigation typical infectious and contagious neurological diseases of horses have been confirmed negative. Fecal egg counts performed on stool samples indicate a moderate to high level of intestinal parasitism. However, gross findings at necropsy did not detect damage caused by intestinal parasites. Toxic heavy metals have been tested for and have not been detected. A mild copper deficiency has been detected. Copper deficiencies in pasture-raised livestock is not uncommon in Hawaii. This copper deficiency would not be expected to cause the abnormalities seen in the affected horses. There are indications that the liver of this horse may have suffered significant damage. Confirmation and characterization of the liver damage will come from microscopic evaluation of the liver tissue expected sometime next week.

Conclusive information at this point on the euthanized horse:

  • No infectious or contagious diseases detected
  • Moderate to high intestinal parasite level found
  • Significant liver damage suspected
  • Mild non-specific pathology of the central nervous system detected

Additional tests are pending at veterinary diagnostic laboratories at Cornell, Kansas State University, University of Hawaii and the Hawaii Department of Agriculture’s Animal Industry Division.


Any sick wild horses in the area should be reported to the HDOA Animal Disease Control Branch at (808) 887-6059 on Hawaii island or the 24-hour number on Oahu at (808) 837-8092.

Further updates will be posted at this site.