Horses are continuously exposed to potential carcinogens. Although certain tumor types, such as those affecting the skin (squamous cell carcinoma, sarcoid, and melanoma), are more prevalent and are readily visible and palpable, other cancers can be lurking in more “distant” body systems.
Lymphomas are cancers that can cause extensive damage to multiple regions of the body before a veterinarian identifies them. They also can cause rapid deterioration and death. Lymphomas are mostly classified into three categories and include: multicentric/multisystemic, intestinal, and cutaneous.
Multicenteric/multisystemic lymphoma typically involves the lymph nodes only, however it can invade more distant sites such as the mediastinum (chest cavity). This might cause a severe pleural effusion (excess fluid accumulation in the pleural cavity, the fluid-filled space that surrounds the lungs), coughing, and shortness of breath. Such horses might suffer loss of appetite, weight loss, ventral edema, intermittent pyrexia (fever), and exercise intolerance. Thoracic ultrasonography, radiography, evaluation of cell-types of lower respiratory tract secretions using a microscope, and/or ultrasound-guided biopsy to detect cancer cells might be useful in finding evidence of a tumor. It might be a diagnostic challenge to find cancer cells, and this could depend on the tumor’s degree of exfoliation and invasiveness or if there’s a large quantity of pleural fluid on thoracocentesis (sampling of the pleural spaces). Unfortunately, most horses with either primary or metastatic (spreading) pulmonary cancer have advanced disease, and although clinicians can stabilize them in a veterinary hospital, euthanasia might be warranted. If there is no evidence of distant spread, the veterinarian might consider chemotherapeutics, but this could depend on how advanced the disease is and financial constraints. The tumor may affect the horse’s pericardium (the sac surrounding the heart) and the heart itself. Fluid within the pericardial sac could compress and impair the heart’s filling ability, leading to right-sided congestive heart failure. Clinical signs include depression, pyrexia, decreased appetite, and rapid breathing. An echocardiogram (also called an “echo”) showing increased pericardial fluid could be diagnostic, as could an electrocardiogram (ECG) that displays changes in electric conductance. A veterinarian may make a diagnosis based on pericardiocentesis (sampling of the pericardial fluid to check for abnormal cells) in conjunction with an antibiogram and culture for secondary bacterial infection. A temporary pericardial lavage device for drainage and repeated flushing may be useful. However, the prognosis for neoplasia affecting the heart or pericardium remains grave in horses. Should lymphoma affect a horse’s myocardium (cardiac muscle), it could interfere with contractility, resulting in the development of arrhythmias. Exercise intolerance, depression, weakness, and respiratory distress are common signs, possibly with cyanosis (a bluish tinge to skin or mucous membranes), edema (fluid swelling) of the ventrum (midline on belly) and lungs, and pyrexia. A veterinarian can perform an echocardiogram and can measure elevations in cardiac-specific biomarkers that are present in the blood. Immediate rest, antiarrhythmic therapy, inotropes (agents that alter muscular contractions), and diuretics can be therapeutic, but the prognosis for this condition, called infiltrative neoplastic myocardial disease, is extremely grave.
Intestinal lymphoma typically affects the small intestine. The horse might have hypoproteinemia (low protein blood levels) and display ventral edema, recurrent colic, and weight loss. Diagnostic tools include endoscopy, rectal examination, abdominal ultrasonography, peritoneal fluid (from the abdominal cavity) cytology, and biopsy. Also, affected horses might have abnormal glucose absorption test results due to small intestinal malabsorption. Affected horses tend to present in an advanced disease stage with spread of tumor cells to other organs (such as liver and spleen). The prognosis is severely guarded, with horses generally living less than six months following diagnosis.
Cutaneous lymphoma could appear as solitary or multiple skin nodules, which grow larger and then become smaller and might show an association with seasonality, hormonal balance and steroid use. Depending on the location, these tumors have the potential to impair lymphatic drainage, blood flow, or cause lameness..