Bilateral exophthalmos in a Holstein cow with lymphosarcoma

Abstract

A 4-year-old Holstein cow presented with severe bilateral exophthalmos. A complete blood cell count revealed late stage lymphoma; the agar gel immunodiffusion test for enzootic bovine leukosis was positive. The cow was euthanized 1 wk after presentation. Necropsy revealed generalized lymphadenopathy and tumors in most organs. Final diagnosis was lymphosarcoma.

A 4-year-old Holstein cow was presented because of bilateral exophthalmos. During the previous 5 mo, milk production had decreased and generalized weight loss had been observed. Four months earlier, during her 2nd lactation, she had aborted a “circular mass of tissue” at 6 mo of gestation. Milk production had declined steadily after the abortion. The cow had been rebred approximately 35 d before presentation. Lactation was subsequently terminated due to poor production, and the cow was moved to a dry cow lot, 10 d prior to examination. During this time, the cow experienced an episode of respiratory distress that appeared to be responsive to an IM injection of oxytetracycline (Liquamycin LA-200; Pfizer, London, Ontario) administered by the owner. Two days later, during a routine herd health visit, the cow was examined because of bilateral exophthalmos (day 1).

Physical examination revealed severe bilateral exophthalmos with mydriasis and amaurosis. No anterior chamber opacity or aqueous flare was noted in either eye; however, exposure keratitis was evident. Both prescapular and submandibular lymph nodes were palpably enlarged, and moderate brisket edema was present. The cow was mildly depressed and had a decreased appetite; wet mud and manure, visible on her sides, suggested recent recumbency. Her gait appeared normal with no obvious neurological deficits. Body condition score was 2/5. On the basis of the history and the limited physical examination, a tentative diagnosis of bovine leukosis was made. The differential diagnoses included hypovitaminosis A, malignant catarrhal fever, lymphadenitis, chronic frontal sinusitis, and idiopathic retrobulbar abscessation or inflammation.

On day 2, the cow was found in sternal recumbency, quiet, alert, and responsive, but despite encouragement would not rise. The rectal temperature was normal (38.8°C); the heart rate (84 beats/min) and respiratory rate (40 breaths/min) were mildly elevated. Vulvar mucous membranes were slightly pale, but the capillary refill time was normal (< 2 s). No oral lesions were present. The eyes appeared to be nonpainful, but they could not be retropulsed. A blood sample was collected from the coccygeal vein for an enzootic bovine leukosis (EBL) agar gel immunodiffusion (AGID) test, a complete blood cell (CBC) count, and serum protein evaluation (Animal Health Laboratory, University of Guelph, Guelph, Ontario). On day 4, the cow was still recumbent, extremely weak, and in severe respiratory distress. Recumbency was attributed to severe generalized musculoskeletal weakness rather than neurological deficits. Severe melenic and watery diarrhea, tachycardia, submandibular edema, and moderate bilateral jugular vein engorgement were observed. The cow was euthanized and transported to the Animal Health Laboratory for postmortem examination.

At necropsy, there was prominent enlargement of the mesenteric lymph nodes; the mandibular, deep cervical, prescapular, abomasal, sublumbar, and pelvic lymph nodes were also enlarged. On cut surface, all nodes appeared uniformly pale grey, with little to no cortical-medullary differentiation. Retrobulbar tissue from both orbits was diffusely enlarged and pale, but it contained no discrete masses or nodules. The pericardium of the heart appeared normal and there was no excess fluid. All of both left and right auricles and approximately 20% of the myocardium of each atrium were replaced by irregular, firm, multifocal, scirrhous nodular tissue. A single, large, irregular scirrhous nodule ran transluminally through the interventricular septum, and there was a nodule in the right ventricular wall. All vessels of the heart and those surrounding it appeared normal on gross examination.

Both multifocal and coalescing irregular flattened nodules protruded from the mucosa of approximately 30% of the luminal surface of the cardiac and fundic regions of the abomasum. The mottled, necrotic, black-red lesions showed varying degrees of mucosal ulceration, ranging from mild erosions to deep ulcers. The small intestine was diffusely red-brown and thickened, and contained watery ingesta. The liver was pale, firm, and approximately 40% enlarged, with no obvious lesions.

A single scirrhous nodule at the caudal pole of the right kidney extended outward from the renal capsule, with no grossly visible involvement of the kidney parenchyma. The bladder contained at its apex a single irregular necrotic nodule, which protruded from the serosal surface. On the luminal surface, 3 soft, flattened nodules mushroomed from the mucosa. Irregular nodules were visible on the serosal surface of both uterine horns, and 2 similar nodules were located on the luminal surface of the right horn. The uterine mucosa appeared otherwise normal on gross examination. A 2-cm conceptus with associated chorioallantoic membranes was present in the right horn near one of the nodules. The ovaries were normal, with a corpus luteum present on the right ovary.

Many soft nodules adhered to the costal pleura and projected into the pleural cavity, closely associated with, but not penetrating, the intercostal muscles. On cut surface, all nodules contained some necrotic tissue. The lungs, mammary glands, and spinal cord were normal on gross examination.

Histopathologic examination revealed that lymph nodes, myocardium, abomasal wall, bladder, uterus, periorbital fat, and skeletal muscle were all extensively infiltrated by medium- to large-sized lymphocytic and lymphoblastic-type cells with a low mitotic index. The liver showed marked fatty infiltration.

The CBC count revealed anisocytosis (1+); thrombocytosis (797 × 10⁹/L; reference range, 160 to 650 × 10⁹/L) with occasional platelet clumps; moderate leukocytosis (26.2 × 10⁹/L; reference range, 5.05 to 13.3 × 10⁹/L); moderate mature neutrophilia (12.31 × 10⁹/L; reference range, 1.7 to 6.0 × 10⁹/L); moderate lymphocytosis (12.84 × 10⁹/L; reference range, 1.8 to 8.1 × 10⁹/L); and mild monocytosis (0.79 × 10⁹/L; reference range, 0.1 to 0.7 × 10⁹/L). A blood smear revealed a prominent population of large lymphocytes with noticeable chromocenters and occasional nucleoli. Total serum protein was 86 g/L (reference range, 60 to 80 g/L). The EBL AGID test was positive (Vita-Tech Canada, Markham, Ontario), confirming a diagnosis of end-stage bovine lymphosarcoma.

Enzootic bovine leukosis, also known as bovine lymphosarcoma or bovine leukemia, is a malignant neoplastic disease of the lymphoreticular system in cattle (1,2,3,4). It is caused by the bovine leukemia virus (BLV), an oncogenic type C retrovirus that integrates as a provirus into the host DNA. The disease is characterized by chronic low-grade viremia and a long latency period (1,2,3,4,5). The virus is distributed worldwide, with its reservoir in persistently infected cattle. The prevalence in Canadian dairy cattle is 10% to 25%, and the within-herd infection rate is commonly 80% or more (1,3,4,6,7). In a recent study, VanLeeuwen et al (7) reported that 20.8% of dairy cattle in the Canadian maritime provinces (Nova Scotia, New Brunswick, and Prince Edward Island) were EBL-positive, with 70% of herds having at least 1 EBL-positive cow. Large economic losses result due to death of cattle, slaughter condemnation, loss of export privileges, and veterinary costs.

The virus is highly cell associated and horizontally transmitted under conditions of close contact via blood or tissue containing infected lymphocytes (1,2,3,4,5,8). Poor hygiene during herd management and veterinary procedures (vaccination, dehorning, castration, rectal palpation, tattooing, and eartagging) increases the risk of disease transmission (1,2,3). In utero transmission occurs with a frequency of 3% to 10% via placental transfer of infected lymphocytes from dam to fetus (1).

The pathogenesis of EBL is extremely complicated. Genetic susceptibility plays an important role and is evident in the fact that so few cattle are clinically affected (1,2,3,4). Infected cattle are permanent carriers of BLV. Most infections are asymptomatic, with some animals developing transient viremia without seroconversion. The virus cannot be isolated from these cattle 3 to 4 mo postinfection. Approximately 30% of infected cattle seroconvert and may develop persistent lymphocytosis within 3 mo to several years after infection (1,2,4,8). Such cattle show evidence of BLV infection, only on peripheral blood analysis. Leukosis is a rare manifestation of infection, with about 1% to 5% of infected cattle developing the neoplastic disease (1,2,3,4,8). Both T-lymphocytes and B-lymphocytes may be infected with BLV; however, the resulting tumors are thought to be composed solely of B-lymphocytes (1,4,5,8). Lymphosarcoma may not become evident clinically for many years after infection. As in this cow, tumors usually occur in animals more than 3 y of age, with peak prevalence occurring between 5 and 8 y of age.

Clinical signs of EBL may vary widely and depend upon the location of the tumors, but once clinical signs appear, the disease usually follows a rapid course, as in this case. Initially, there may be an unexplained decline in milk production, loss of body condition and appetite, pallor, and generalized muscle weakness (4,8). Superfi cial lymph node enlargement, as observed in this case, occurs in 75% to 90% of cases, and rectal palpation often reveals visceral node involvement (4,6,8). Neural lymphomatosis with posterior paresis, a common clinical presentation of EBL (4), was not observed in this case. Some less common clinical signs of EBL occurred in this cow, including respiratory involvement due to retropharyngeal lymph node enlargement, diarrhea, right-sided congestive heart failure, and diffuse lymphocytic infiltration of periorbital tissues causing exophthalmos. Exophthalmos, which often progresses to exposure keratitis, occurs bilaterally in 13% and unilaterally in 9% of cattle with EBL (4,6,8,9); the prognosis is more guarded when the exophthalmos is bilateral rather than unilateral (6,9). Lymphosarcoma is the most common cause of orbital tumors in cattle (6,9). As in this case, most cattle with orbital lymphosarcoma have concurrent diffuse tumor involvement and die within weeks of presentation.

Definitive diagnosis of BLV infection in this case was based on serum AGID. A serum enzyme-linked immunosorbent assay (ELISA) for BLV infection is also available. Both methods detect BLV antibodies to the gp51 antigen, with AGID being the gold standard (1,2,10). A recent study showed that some ELISAs are just as specific and more sensitive than the AGID test (10). Both BLV-ELISA and BLV-AGID are now recognized as acceptable and official tests for use in Canada (10).

For control of BLV, calves should be serotested no earlier than 6 mo of age (1,2,8), as false-positives may occur in younger animals due to circulating colostral BLV antibody (4). Pregnant cattle should be tested at least 6 wk prior to parturition to avoid false-negatives due to immunoglobulin shifts into colostrum. Herd additions should be serotested, isolated for 1 to 2 mo, and then retested (1). If seropositive animals are identified, 1 or 2 strategies may be used. The ideal strategy is to cull or slaughter positive animals; alternatively, positive animals may be segregated (1,8). No serotesting strategies were employed in the herd from which this case originated.

At present, the possibility of an effective vaccine for BLV is remote, and there is no effective or practical treatment for bovine lymphosarcoma or BLV infection. Euthanasia is recommended when clinical lymphosarcoma is diagnosed, as the course of the disease is rapid and will inevitably result in animal suffering.