MU vet finds new way to treat horse’s tumor

A new therapy has been performed successfully on five horses since 2000.
Monday, February 7, 2005 | 12:00 a.m. CST; updated 10:10 a.m. CDT, Monday, July 14, 2008

In 2000, Rose Pasch noticed that her American saddle horse, Dixie, was keeping her right eye closed and ooze was coming out of it. At once, Pasch called her local veterinarian, who found Dixie’s eyelid tumor.

“He took the growth off four or five times, but it just kept coming back,” Pasch said.

Dixie had squamous cell carcinoma, the most common form of cancer in an equine eye and the second most common tumor in horses overall. The tumor is especially prevalent in light-colored horses and is defined as “perioccular,” meaning around the eye, because it usually affects the eyelid. If not treated, it could metastasize and kill the horse.

There is no universal standard therapy for removing squamous cell carcinoma. Without treatment, Dixie’s condition could have been fatal. Simply removing the eyelid of a horse is not an option because the lid performs many important functions, such as spreading tear film, removing debris and protecting the eye.

In most cases, losing the eyelid means losing the eye because horses do not have enough facial skin to have a skin flap operation performed.

Losing an eye is grave because horses’ eyes are positioned far to the sides of the head. Eye loss would cause an entire blind side to half of their body. Besides being a serious disability to the animal, it also presents a certain danger and liability to the owner.

Rose was determined to help her horse and soon traveled to the University of Missouri’s Veterinary Medical Teaching Hospital to see whether anyone could help her beloved 18-year-old horse.

Ophthalmology Section Head Elizabeth Giuliano examined Dixie.

“When I first met Dixie, I wasn’t sure I would be able to save her eye because I wasn’t sure we had a whole lot of options available left to us. But her owner was very motivated, very dedicated, and she really came to us in search of anything we could offer,” Giuliano said. “Then a light bulb went off in my head, and I wondered if we could try photodynamic therapy on this horse.”

There were two major problems with performing photodynamic therapy on Dixie. First, systemic injection of the drug is impractical in an animal that weighs almost 1,000 pounds because the amount of the agent would be excessive.

“We have no idea what would happen if we gave these drugs systemically to a horse. There are no pharmacokinetic, drug distributions or toxicological studies of horses undergoing photodynamic therapy,” Giuliano said.

The second problem is that the photosynthesized drug can sensitize the skin. Like a person with fair skin, Dixie could become sun-sensitive and the effects of the drug could last from weeks to months. Trying to keep a horse confined to a shady stall for long periods of time predisposes the animal to potentially life-threatening respiratory and gastrointestinal problems.

Giuliano wanted to perform surgery, but only an additional treatment could keep Dixie’s tumor from returning. After brainstorming with Dudley McCaw, who was performing photodynamic therapy on small dogs and cats at the hospital, Giuliano came up with a novel idea.

“We developed the idea of taking an experimental photodynamic agent and instead of giving it to the horse systemically, I injected the drug locally into the tumor bed and went ahead and treated her,” Giuliano said.

The innovative procedure saved Dixie’s eye, and she has made a full recovery.

Giuliano was the first veterinarian to perform this specific approach to photodynamic therapy, and the MUVeterinarian Medical Teaching Hospital is the only place where it has been performed.

“If it weren’t for Elizabeth we wouldn’t have Dixie,” Rose said.

Since Dixie was the first horse to undergo the experimental therapy, the hospital performed the operation for free. New clients, however, pay about one-third of the total cost, which ranges from $500 to $800. A grant by the Morris Animal Foundation is covering the other two-thirds.

“Morris Animal Foundation chose to fund this study based on the advisory board’s high marks for scientific merit,” Heidi Jeter, Morris Animal Foundation’s Publications & Media Specialist said.

Giuliano has successfully performed the innovative therapy on five Missouri horses in the past three years.

“We’d like to solicit more cases to see more horses that are affected by this and see if we can help them,” Giuliano said.

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