A rare form of lymphoma may be the cause of more deaths than horsemen realize

by Denise Steffanus

WHEN the sad news first broke that champion sprinter Lost in the Fog had lymphoma of the spleen, the public was bewildered. How could this be possible in such a robust four-year-old colt who, as recently as June 3, had battled with Kelly's Landing through the six-furlong Aristides Breeders' Cup Handicap (G3) at Churchill Downs to win by 1 1/4 lengths?

At the University of California at Davis veterinary school, clinicians theorized that the two football-size tumors they found--one in Lost in the Fog's spleen and the other near his spine--had been developing over the past four months, but perhaps as long as a year. A third tumor the size of an egg was found in the ligament supporting his spleen. The verdict was the worst possible: nothing could be done but to make the colt as comfortable as possible until his condition deteriorated to the point where euthanasia would be the humane route.

Fairfield Bain, D.V.M., who is on staff at Woodside Equine Clinic in Ashland, Virginia, while on sabbatical from the medicine department of Hagyard Equine Medical Institute in Lexington, is a board-certified veterinary pathologist. According to Bain, lymphoma most commonly strikes horses between the ages of four and ten.

"The cause remains unknown; no viral cause has been found, as it has in other animals and some forms of the disease in humans," Bain said. "The most common signs associated with lymphoma are weight loss, usually associated with intestinal involvement or the effects of cancer on the body's metabolism. Lymphoma can involve a variety of areas of the body, both in the abdomen and in the chest cavities, as well as nodules in the skin or around the eyelids."

Rarely detected

A review of scientific literature sheds little light on this unusual form of equine cancer. Other forms of cancer, such as skin melanomas, are common in horses, especially gray ones.

In 2002, Matthew Schoessler, D.V.M., then a graduate student at the University of Minnesota's college of veterinary medicine, discovered a large tumor in the spleen of an Arabian mare upon necropsy. He and fellow graduate students Kari Lundsford and Kevin Prestia wrote a paper titled "You Ain't Spleen Nothing Yet" about the rare cancerous growth they had found.

"The reason we chose this [for our presentation] was because it was pretty interesting because it wasn't something you run across every day," said Schoessler, who now is a practitioner at Triple Oaks Equine and Bovine in Cambridge, Minnesota.

Schoessler theorized that even if a lymphoma, also called a lymphosarcoma, were limited to a horse's spleen and the organ were removed, over time the disease most likely would crop up someplace else in its body.

"This horse was basically seeding her entire body with lymphosarcoma, and lymphosarcoma likes to get into the other lymph nodes and other areas of the body," he said of the unusual case.

Schoessler believes lymphosarcoma of the spleen may be more prevalent than reported because many horses that die or are euthanized are presumed to have common disorders, such as colic, and owners often do not want to necropsy them.

"I don't think it's caught very often," he said. "There may be lots of it out there that we never see."

In 1992, Gordon Brumbaugh, D.V.M., and his colleagues in the department of large-animal medicine and surgery at Texas A&M University, reported finding large lymphosarcomas in the spleens of three horses in Texas. All three horses were examined at Texas A&M's large-animal hospital after showing symptoms of anorexia, weight loss, and lethargy. All three horses had to be euthanized.

"It's such an uncommon thing," he said. "There is just so little known about it, there's no way to predict or know what would happen if [a splenectomy] could be performed to remove the problem."

Brumbaugh said he knows of no horse that has had its entire spleen removed, although he said he has seen partial splenectomies, but the damage to the spleen in those cases was from other causes, not lymphosarcoma.

Research needed

John Robertson, V.M.D., Ph.D., director of the Center for Comparative Oncology at the Virginia-Maryland Regional College of Veterinary Medicine, has devoted much of his efforts toward learning more about cancer in horses. Supported in part by the Take the Reins Foundation, based in Virginia, Robertson is conducting ongoing research into the most common type of cancer in horses, malignant melanoma, which primarily affects gray horses.

According to Robertson, it is not unusual for a tumor to grow as rapidly as those found in Lost in the Fog.

"A lot of the behavior and growth characteristics are dependent upon the individual tumor," Robertson said. "So it really is not uncommon to see tumors that grow in a short period of time."

Detection of internal tumors remains a challenge. Because the horse is so large, modalities such as diagnostic ultrasonography, magnetic resonance imaging (MRI), and computed tomography (CT) scans, which are employed in other species, often do not penetrate deeply enough to be useful. Often, the first indication a horse has a tumor is when the mass interferes with bodily functions or causes discomfort, as was the case with Lost in the Fog, who was admitted to UC-Davis with colic symptoms.

"Very, very frequently the frustration we deal with is that we find these tumors because they have caused clinical signs, just as they have in Lost in the Fog, at a time when the tumor is at a point where we can't do anything about it," Robertson said. "That is, they've grown for a while, and we've been unable to detect them by physical examination, and certainly there are no blood tests right now that are available to detect these things. And because horses are so big, it's very difficult to image them using things like MRI and CT to find it."

"Leukemia is rare in the horse," Bain said. "So blood work rarely provides a specific diagnosis, but it does often show secondary effects, such as evidence of inflammation, anemia, and alterations in the serum proteins. Some immunoglobulins, specifically IgM, are lowered in some lymphoma patients.

"The specific diagnosis is made by microscopic examination of a needle aspirate or biopsy of a mass. Further study of the cell types using immunohistochemistry will likely help characterize the tumor and help determine its cause and how it will respond to treatment."

Poor prognosis

Bain said chemotherapy has been used to treat cancerous tumors in horses and that it may achieve periods of suppression of the cancer, but the long-term prognosis remains poor.

"Our experience with treating these internal masses is so poor that there is no standard therapy," Robertson said. He added that neither radiation therapy nor chemotherapy has been widely used in the horse.

"The frontline therapy for horses that have cancer has been surgery, depending on the type of neoplasm," he said. But Robertson said he was confident in the ability of UC-Davis to provide the best care possible.

"When I heard that Lost in the Fog was going to Davis, I was very happy because they're just a top-notch institution, one of the best in the world," he said. "And if their surgeons felt that there was something that could be done to resect this, or if they felt there was some type of chemotherapy, they certainly would have advocated this. But, again, our collective frustration in dealing with cancer in horses is that we don't have good answers and good treatment. We really need to work a lot harder at it."

Chemotherapy is designed to kill fast-growing cells, but it does not differentiate between cancer cells and other fast-growing cells in the body. So while the clinician attempts to impede the cancer, healthy areas of the body can be affected.

"Fortunately, effects of chemotherapy are not as dramatic in the horse as they are in the human--hair loss, etc.," Bain said. "But monitoring of the blood work is most important, as these drugs will affect rapidly dividing cells, usually those in the bone marrow, resulting in a low white blood cell count and creating a susceptibility to secondary infections--pneumonia, diarrhea. And the specter of laminitis always looms with such problems in the sick horse."

No clear cause

In other species, lymphoma of the spleen can be caused by a virus, but in horses, no such correlation has been drawn. In fact, researchers are baffled as to why some horses develop these tumors and others do not. Robertson said that tumors of the spleen are prevalent in some purebred dogs, but he could not speculate if the same were true of purebred horses.

"We know that there are several breeds of dogs that have a very high incidence of splenic tumors, and those breeds would be German shepherds and golden retrievers," Robertson said. "The fact that they're purebred animals that are related in their pedigree suggests there is some genetic or genomic elements that are important."

Robertson agreed with Schoessler that the number of internal tumors that remain undetected in horses is far greater than the few that are found.

"That's undoubtedly the reality," he said. "Unfortunately, I think we, as veterinarians, have been a little complacent about dealing with neoplasms in horses because we consider them to be uncommon and because we consider some of them, like skin tumors, to be kind of, 'Well, horses get them and you don't worry about them.' That, indeed, is not the reality of how to deal with them.

"Let people know that horses do get cancer and that there are people out there who are working on a daily basis to try and detect it and figure out how to treat it," Robertson said. "We haven't done enough up until now, but we intend to pick up the pace."

Denise Steffanus is a contributing editor of Thoroughbred Times who writes frequently on veterinary and farm management topics.