What's knocking a horse off its feet?

Horses pull carts, jump hurdles, and carry everyone from jockeys to preteen girls. They are pretty good on their feet. So when a horse falls for no apparent reason, people notice.

That's what happened three years ago to Ranger, a retired Thoroughbred racehorse living at Manor College's Jenkintown campus.

"The students kept telling me that he would fall asleep suddenly and almost fall down before catching himself," said veterinarian Amy Bentz, an adjunct professor who teaches large-animal courses in Manor's Veterinary Technology Program.

Horses commonly sleep (non-REM) standing up. Their special "stay apparatus" - which locks their joints as they sleep - keeps them propped while snoozing. Furthermore, sleep onset in horses is not typically sudden. Surely, the students were mistaken, Bentz thought.

But one day, Bentz was jolted by a thud in Ranger's stall.

"All of a sudden, he fell to the floor," she said.

The 21-year-old chestnut gelding collapsed onto his left side, eyes shut and out cold. Bentz was unable to rouse him. After about a minute, he awoke and stood.

Had Ranger suffered a seizure? Stroke? Sudden cardiac event? Did he have an orthopedic problem? Bentz examined him but found nothing wrong.

Ranger had had a waxing-and-waning hoof abscess for several months, but it had not hampered his ability to stand or walk.

Yet the ordinarily outgoing horse was keeping to himself lately. He would stand statue-still in the pasture, head slung low and eyes half-closed.

Ranger's hair coat appeared dull, coarse, and a touch wavy. As temperatures rise, equine hair coats shed. But it was early spring, and the weather was already warm. Odd, Bentz thought.

Back to that apparent sleep attack: Was something amiss in Ranger's nervous system? Bentz pondered the causes of neurological problems in horses. These include infectious diseases like West Nile virus, a brain tumor, and toxins. Perhaps Ranger had consumed poisonous plants in the field?

His droopy behavior, hair coat changes, hoof infections: Could these somehow be connected?

Certainly, hypothyroidism can delay normal hair coat shedding and render a patient sluggish. But it's uncommon in horses.

Diabetes impairs the immune system and could explain Ranger's hoof infections. But diabetes is also rare in horses and brings other symptoms that Ranger was not exhibiting.

Routine blood work came back normal. So Bentz ordered a special assay called a thyrotropin-releasing hormone (TRH) stimulation test. It was positive.

Ranger had pituitary pars intermedia dysfunction (PPID), with possible episodes of narcolepsy, somnolence, or even arrhythmia.

PPID - the equine form of Cushing's syndrome, or hyperadrenocorticism - is caused by benign growth of the pituitary gland, a pair of marble-size lobes at the base of the brain that pilots the endocrine system.

Swelling pushes it into overdrive, and various hormones flood the body. One is cortisol, which plays a key role in immune function, cardiac activity, nerve conduction, muscle tone, and such.

Affected horses tend to drink and urinate excessively, eat more, and become potbellied. Like Ranger, they are given to lethargy, shaggy hair coats, and frequent infections.

The drug pergolide - originally developed for humans with Parkinson's disease - is used to treat PPID. Administered daily for life, pergolide halts the overproduction of pituitary hormones. Barn hygiene, quality feed, regular vaccination, parasite control, and hoof care are also recommended in these immune-compromised patients.

Typically diagnosed about age 20, PPID is seen more commonly today than years ago because better health care and management practices enable horses to live longer.

Ponies and heavy-bodied breeds like Morgans, paso finos, and American quarter horses seem more prone to PPID than leaner horses like Thoroughbreds.

Narcolepsy, a rare brain disorder characterized by sudden sleep attacks, stems from biochemical abnormalities in the central nervous system.

Did PPID trigger Ranger's apparent narcoleptic spells? The two conditions rarely occur together, and so their relationship is unclear.

In Ranger's case, the enlarged pituitary gland may have been pressing on the sleep-wake center of the brain, which runs between the hypothalamus and the brain stem. But this is speculative.

Perhaps the falling was caused by excessive sleepiness, which PPID patients can experience as a result of their abnormal hormone levels. Or maybe a coincidental heart arrhythmia, not uncommon in older animals.

Ranger responded well to pergolide. Within a month of starting the medication, his hair coat regained its luster. His bounce returned. And the mystery tumbles seemed to stop.

Though Ranger died a year later of unrelated natural causes, he flourished in the interim.

"He looked like a young horse again," said Bentz. "The years just melted off his face."