Lymphosarcoma with Virus-Like Particles in a Neonatal Foal

Abstract

Lymphosarcoma is a common neoplasm affecting all species of domestic animals. The first report of lymphosarcoma in a domestic animal was in a horse in 1853 [9]. A recent review of lymphosarcoma in the horse cited 9 1 cases [9]. At least seven other cases have been reported 11-3, 6, 10, 141. In cases in which age was reported, only three of 30 horses were less than 4 years old; two were in foals 2 and 4 months old [9]. A 10-year-old Quarter Horse mare, which had previously delivered three normal live foals, gave birth to a weak and ataxic foal. It was examined when about 3 hours old; Actinobacillus equi septicemia was suspected. The foal was treated symptomatically with intravenous chloramphenicol and fluids containing glucose and electrolytes. Colostrum was milked from the mare and force-fed to the foal. The foal died about I hour after being treated and was necropsied within 2 hours of death. The abdominal lymph nodes were enlarged. Tissues fixed in formalin were lung, spleen, abdominal lymph node, kidney, liver and several sections of small intestine. Thymus was not submitted. The normal architecture of the lymph node was replaced by a uniform population of discrete cells (fig. I) . These cells had large, round to oval nuclei with vesicular chromatin and a thin rim of cytoplasm. Nucleoli were prominent and mitotic figures were numerous. The nuclei of a few of these cells were pyknotic. Similar cells were in the lymph node capsule and perinodal fat. The germinal centers in the spleen were composed of uniform cells similar to those in the lymph node. These areas contained many more cells with pyknotic nuclei than were in the lymph node. The interstitium of the renal cortex (fig. 2) and medulla, as well as the lamina propria and muscularis of the small intestine (fig. 3), were infiltrated by cells resembling those in the lymph node. These cells were interpreted to be neoplastic lymphoblasts. Centrolobular hepatocytes were vacuolated, but neoplastic cells were not seen in the liver. Based on findings of light microscopy, lymphoblastic lymphosarcoma was diagnosed. Sections of formalin-fixed lymph node were cut into I-millimeter cubes and processed for electron microscopy. Ultrathin sections showed virus-like particles budding into the smooth endoplasmic reticulum (fig. 4) and free in the cytosol near the nucleus of neoplastic lymphoblasts (fig. 5). These particles contained a core, shell and nucleoid-features that are compatible with those of C-type oncornavirus 141. A blood sample from the mare was heparinized and lymphocytes isolated by the Ficoll-Hypaque density gradient centrifugation were washed and suspended ( 106/ml) in RPMI-1640 medium with 10% inactivated fetal calf serum. Phytohemagglutinin was added to a final concentration of 5 pg/ml and the cells incubated at 37' C in an atmosphere with 5% COn for 48 hours [7]. The cells were then harvested and processed for examination in an electron microscope. Virus particles could not be demonstrated in the cultured lymphocytes. The classification and characteristics of lymphosarcoma in various species of domestic animals have been reviewed [8]. Although the number of reported cases of lymphosarcoma in horses was insufficient to make accurate observations on its epidemiology, presenting signs vary, depending on the organ system or systems affected. Gastrointestinal signs of recurrent colic or diarrhea may occur [ 13, 141. Emaciation, chronic cough and peripheral lymphadenop-