Lost in the Fog - DVM
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Lost in the Fog
UC-Davis veterinarians battle a rare case of lymphoma; racing icon euthanized in mid Sept.


Lymphoma can involve a variety of areas of the body, both in the abdomen and in the chest cavities, as well as skin nodules or around the eyelids. Lost in the Fog, though he presented with extensive internal tumors, showed no signs of debilitating disease.

According to John Robertson, VMD, PhD, director for the Center for Comparative Oncology, Virginia Tech CVM, horses usually present with "a relatively short history within weeks to months of growth of firm nodules on the head, neck and chest," with other variable clinical signs that include anorexia, inability or unwillingness to work, depressed, fever, dyspnea and weight loss. Some horses with primary cutaneous lymphosarcoma "show no signs other than multiple, non-painful swellings in the skin or subcutis and peripheral lymphadenopathy."

The long-term prognosis for lymphoma is usually poor. The treatment rationale is to shrink the tumor enough so it can be removed surgically.
Lost in the Fog showed none of these signs, as he raced, seemingly healthy, other than his poor racing performance. Until his discomfort the week of Aug. 14, it is assumed he had presence of lymphoma for several months without obvious clinical signs.

Lymphoma is rare in the horse, so blood work rarely provides a specific diagnosis, but 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. Diagnosis of internal forms of lymphoma, such as the case with Lost in the Fog, is usually made by imaging, with ultrasound being most important.

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.

"My experience reflects germ cell tumors as being the most common aggressive intra-abdominal tumor in otherwise healthy looking young (3-6 yrs) horses (still rare though)," says Claude Ragle, DVM, dipl. AVCS, Washington State University CVM. "Diagnosis can be a challenge from biopsy only. Even though the aggressive biologic behavior of the tumor is evident, getting the "fine print" of the cell type and origin can be frustrating."

Peter Moore, BVSc, Ph.D., dipl. ACVP, lymphoma pathologist specialist, definitively typed the tumor as a B-cell lymphoma using clonality and cell surface markers that only he has antibodies for in horses. The histopathology report states, "The diagnosis of B-cell lymphoma was based upon the positive immunoreactivity with CD11a (leukocyte marker), CD21 (B-cell marker), and CD79a (B-cell marker) on sections of frozen tissue and the results of clonality (clonal rearrangement of IGH - immunoglobulin heavy chain gene). The sample was negative for T-cell markers on frozen sections (CD3, CD4 and CD8)."

Splenic involvement

The fact that Lost in the Fog's lymphoma primarily involved the spleen (considered Stage IV) is of interest to his lack of performance and to the issues involved with his treatment.

The spleen, situated against the left abdominal wall, just caudal to the stomach, is a unique organ that involves blood filtration as well as acting as an immune organ. The equine spleen also serves as a reservoir for blood cells.

"The horse spleen is a giant bag of extra blood," explains Kenneth McKeever, Ph.D., FACSM, associate professor, Equine Science Center, Rutgers University. "When the horse runs, its spleen contracts (as the muscles surrounding the spleen contract) and forces a fresh supply of oxygen-rich red blood cells into its blood stream, effectively pumping extra oxygen into its cardiovascular system virtually on demand."

When at rest, about 35 percent of the horse's total blood volume is comprised of red blood cells. Horses increase their red blood cell numbers to more than 65 percent of blood volume during a race. This greatly increases the horse's racing ability to carry needed oxygen.

With splenic involvement, it is no wonder. Lost in the Fog ran out of gas in his last several races.

Not only was he probably experiencing severe discomfort with the jockey perched on his back (because of the tumor along his spine), but his spleen's ability to dump red blood cells for extra oxygen and energy during racing performance also was probably impaired.


Source: DVM360 MAGAZINE,
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