09/15/2006 12:00AM

Cure remains elusive goal


The type of internal cancer diagnosed in sprint champion Lost in the Fog - lymphoma - is less common than some other kinds of cancer, veterinarians say. But it occurs often enough that a few researchers have been working on treatments for it, including chemotherapy.

But researchers say treatments for inoperable systemic lymphoma are more likely to provide a horse with a longer period of quality life, rather than a cure for the disease.

The exact incidence of lymphoma is difficult to determine, partly because lymphoma can be difficult to diagnose in a living horse without access to magnetic resonance imaging or similar technology. But some veterinary articles have put the rate of occurrence at less than 1 percent of the general horse population.

It is more unusual than skin cancer in horses, which often is associated with - but not limited to - gray horses.

"The most common cancer in horses is the equine sarcoid, which is a tumor of the skin," said Dr. Catherine Kohn, a specialist in internal medicine and professor in the Ohio State University veterinary school's Department of Clinical Sciences. "It's seen pretty frequently, and there are some other skin tumors that are somewhat common, like melanomas in gray horses. Those tend to be tolerated pretty well in horses, though in some cases they can be fatal."

Other types of tumors are rare in horses, said Dr. John Robertson, the director of the Center for Comparative Oncology at Virginia-Maryland Regional College of Veterinary Medicine. "Horses seem to lead a charmed life as far as developing things like lymphoma, gastro-intestinal tumors, and lung tumors," he said. "There's something unique about horses resisting many of the common forms of cancer seen in humans and other animals."

Researchers at Ohio State, led by oncologist Dr. Guillermo Couto, have long been interested in horses with systemic cancers like lymphoma, Kohn said.

"We have used combinations of systemic chemotherapeutic drugs in an effort to control lymphoma," she said. "In some cases, we've been lucky enough to cause remission."

Remission is unusual but, Kohn said, chemotherapy can help prolong a horse's good quality of life - the goal in Lost in the Fog's case.

"Typically, with lymphoma in horses, you'll have involvement of multiple organs," Kohn said, "so you need a systemic cancer therapy. How well you control the disease depends on how widespread and how aggressive the tumor is.

"Luckily, in the last 20 years, there's been a lot of work on the horse genome and on the identifying characteristics of antigens on blood cells like lymphocytes that have helped us to categorize what the tumors are."

The University of California at Davis, where Lost in the Fog's oncology team is based, is a center for equine cancer research. UC-Davis's Dr. Peter Moore and Dr. Jeff Stott have developed a method of pinpointing lymphoma types through antibodies, a strategy that helped them identify Lost in the Fog's particular lymphoma as B-cell lymphoma. Such detailed diagnoses are important and can help veterinarians tailor their medical treatments to the specific type of cancer.

UC-Davis's Dr. Alain Theon has developed "a standard protocol of combination chemotherapy that includes doxorubicin" that has had some success, the university said in a statement issued Sept. 2 about Lost in the Fog. But the main hope in such high-dose treatments is remission, not a complete cure. And there are potential side effects.

"Allergic reactions, bone marrow suppression, gastrointestinal upsets, adverse impacts on fertility, and adverse effects on the heart are all potential complications that might need to be addressed," the statement said.

Even so, Theon's research has indicated "that horses tolerate high-dose chemotherapy much better than do many species," the statement said.

As they did early in Lost in the Fog's case, veterinarians can also try to shrink tumors by using corticosteroids such as dexamethasone in the hopes they might then be able to remove the tumors surgically. But there are risks with long-term steroid use in horses, Kohn said, including increased chances of the development of laminitis, an often-fatal disease of the hoof wall.

In the meantime, Lymphoma treatment in horses remains a work in progress.

"We do not have a lot of well-established protocols," said Robertson. "We have not done large clinical studies. . . . There are a very small number of people who specialize in oncology in horses."