Array of tests zero in on disease afflicting wild horses in Waipio Valley

KAILUA-KONA — Around a dozen wild horses in Waipio Valley have died inexplicably and in similar fashion over the last two months.

Now, test results are streaming into the Hawaii Department of Agriculture (HDOA) from labs across not only the state but the country, which are painting a clearer picture of symptoms and helping investigators close in on the mysterious affliction.

As of Monday, HDOA estimated 15 wild horses had suffered or were suffering from an unidentified disorder of the nervous system leading to neuropathy, or nerve damage. This damage initially presented in the hind limbs and altered the animals’ gaits, ultimately robbing from them the ability to move and resulting in their deaths.

Of the 15 animals officials believe afflicted, a dozen have died and a thirteenth, which was dying, was euthanized for testing purposes. Jason Moniz, program manager of HDOA’s Animal Disease Control Branch, wrote in an email to West Hawaii Today that two from the affected band of horses are believed alive and roaming the valley.

“No new mortalities have been seen in the area,” Moniz wrote in the email, the information from which has also been posted on the HDOA website. “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.”

Dr. Tim Richards, a member of the Hawaii County Council and a longtime veterinarian on Hawaii Island, examined the animal from which brain and spinal tissue were procured for more extensive testing. He told WHT in July he’d never seen a clinical presentation like that before.

Dr. Kelleyerin Clabaugh, a private veterinarian who conducted initial blood tests on sick horses at the behest of valley residents and horseback tour operators concerned about contagion impacting domestic horses, echoed Richards’ uncertainty about the uniquely debilitating and deadly disease.

The results

Many concerned citizens brought up the possibility of rat lungworm (RLW) on social media once they heard of the epidemic. Susan Jarvi, Ph.D., with The Daniel K. Inouye College of Pharmacy at the University of Hawaii at Hilo, said she’d treated horses afflicted with RLW before and conducted tests to that effect on the samples procured from Waipio Valley at her Hawaii Island lab.

Moniz wrote that a “very low quantity of RLW DNA” was detected in the euthanized horse’s brain, cerebral spinal fluid, heart and lung.

“Correlation with histopathologcial findings are not present at this point,” he said. “Further histopathological exams will be completed before a final diagnosis will be made.”

Labs also detected sarcocystosis, caused by species of sarcocystis, which can manifest in conditions that involve weight loss and muscle loss in the hind limbs of horses. One type of sarcocystis can cause equine protozoal meyloencephalitis (EPM), but other tests on the animals have ruled out 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,” Moniz wrote. “Further evaluation of tissues will be conducted and additional test(s) will be sought out to rule this in or out as a cause of the illness.”

Clabaugh, in her initial assessment last month, said symptoms indicated a potential toxicological cause. Moniz wrote that tests looking for organophosphates and carbamates, commonly used pesticides that can result in neuropathy, are still ongoing.

Other pending tests include those for botulism and to identify plant material present in the animal’s stomach at the time of its euthanasia. Histopathology, or the study of how diseases may have changed tissue, also remains underway.

Blood chemistry tests ordered and conducted weeks before at Claubaugh’s request were repeated and showed elevated enzymes that pointed to both muscle and liver damage, Moniz wrote. Test results for the infectious disease dubbed eastern equine encephalitis came back negative.

No new tests were conducted involving cytology or a mineral panel.

Histology

Also included in the email and on HDOA’s website is a summary of microscopic diagnoses, otherwise known as histology, from the preliminary study of the horse’s tissue. The work remains ongoing, Moniz said, but an in-depth list of what tests have shown so far can be accessed by visiting https://hdoa.hawaii.gov/ai/main/wildhorses/ on the HDOA website.

Moniz summed up the findings in his email and on the website.

“The inflammatory changes within the brain were minimal, and their clinical significance is uncertain,” he wrote. “The degree of inflammation was not consistent with a previously reported case of Angiostrongylus cantonensis (Rat lungworm) meningoencephalomyelitis in the literature.”