Lymphome du thymus juvénile sporadique chez une génisse Holstein de six mois. Une génisse Holstein âgée de six mois présentait de la distension récurrente et une masse ferme, principalement du côté gauche, dans la région caudoventrale du cou. L’exploration chirurgicale a révélé une masse encapsulée et vascularisée qui s’étendait de la région sous-mandibulaire à l’entrée du thorax. L’examen macroscopique et histopathologique, de même que l’anamnèse, ont permis d’établir un diagnostic de lymphome thymique sporadique.

(Traduit par Docteur Bisaillon)

Physical examination on day 3 by the producer’s regular veterinarian revealed that the heifer was alert and responsive, with all vital parameters within normal limits. The heifer’s body condition was good (BCS 3.5/5) and weight was estimated to be approximately 181 kg. Penetration of the frontal sinuses was apparent and appeared to be subsequent to dehorning. Other than the large firm mass of the left caudoventral neck region, and recurrent ruminal tympany, no other abnormal findings were noted on physical examination. A localized abscess or inflammation from a possible headgate injury was suspected initially, and the heifer was treated with florfenicol (Nuflor; Schering-Plough, Pointe-Claire, Quebec), 41.4 mg/kg bodyweight (BW), SC in the neck, and ketoprofen (Anafen; Merial, Baie d’Urfe, Quebec), 3.3 mg/kg BW, IV. Dioctyl sodium sulfosuccinate (Anti-gaz emulsion; Vetoquinol, Lavaltrie, Quebec), 300 mL via stomach tube, was also administered to help alleviate and prevent further ruminal tympany. The producer was advised that if no response to medical treatment was noted, the heifer should be brought to the clinic for surgical exploration and possible mass removal.

By day 5, the mass was unchanged and the decision was made to surgically explore the cervical mass. On presentation, the heifer was noted to be severely bloated and ruminal tympany was once again successfully decompressed by using a stomach tube. After being restrained in a headgate within a standing stock, the heifer was sedated with xylazine (Rompun; Bayer, Toronto, Ontario), 0.2 mg/kg BW, IV. Palpation revealed the mass to be extremely firm and located primarily in the left caudal jugular area and extending cranially. The right submandibular area was also firm on palpation but with no evident gross distention. Aspiration of the mass, using a 20 mL syringe with an 18 G needle, produced normal looking blood. A 5-cm craniocaudal incision was subsequently made through skin and subcutaneous tissues to reveal an encapsulated, vascularized mass that intimately involved the esophagus, trachea, jugular veins, carotid arteries, and recurrent laryngeal nerves. The apparent entrapment of the esophagus was suspected to account for recurrent ruminal tympany through the animal’s inability to eructate. Due to the extent of the lesion, the differential diagnoses (thymic tumor, malignant lymphoma), and the poor postoperative prognosis, the heifer was euthanized with sodium pentobarbital (Euthanyl; Vetoquinol, Lavaltrie, Quebec), 79.5 mg/kg BW, IV. The mass was completely excised and a full postmortem examination was carried out later that evening.

The mass was encapsulated, well vascularized, and extended along the entire length of the neck (941 cm) ventrocaudally from the right submandibular region, with the caudal portion being more left sided and extending through the thoracic inlet. At its widest point, the mass measured 14 cm laterally and 11 cm dorsoventrally, weighing 4 kg in total. The mass was firm, fibrous, rubbery, and white, with the texture and color on cross section being consistent with that of the gross appearance (Figure 1). Interestingly, there appeared to be an abscess, 9 cm × 6 cm, on the dorsocaudal portion of the mass; however, it could not be ascertained whether this was a primary or a secondary lesion.

Figure 1 A) Ventral cervical mass excised from a 6-month-old Holstein heifer at necropsy, diagnosed as sporadic juvenile thymic lymphosarcoma. B) Cross section of the same mass illustrating color and texture to be consistent with gross appearance.

Additional necropsy findings were randomly distributed raised white foci, ranging from 0.5 to 1 cm in diameter, on the superficial aspect of the right auricle. On cross section, these lesions did not appear grossly to penetrate the lumen of the right auricle, and did not appear to be mineralized. Further dissection of the heart revealed a lesion at the base of the moderator band in the right ventricle, comparable with those on the right auricle. While lung, liver, spleen, and right kidney appeared to be grossly normal, the left kidney was absent with no organ remnants evident. Other than extreme bloating of the rumen, attributed to xylazine and postmortem change, the gastrointestinal tract appeared normal. Hemolymph nodes, 2 to 5 mm in diameter, were randomly distributed throughout the mesentery, as well as being present retroperitoneally and behind the fascia of the dorsal musculature of the neck. Although there was bilateral penetration of the frontal sinuses, attributed to dehorning, mucus within the sinuses appeared grossly normal upon inspection and no foul odor was evident. Accordingly, sinusitis was considered unlikely. Samples of the mass, heart, lung, liver, spleen, mesenteric lymph nodes, hemolymph nodes, and intestine were collected into 10% buffered formalin and sent for histopathologic examination (Animal Health Laboratory, Guelph, Ontario).

Histopathologic study revealed that both the neck mass and heart lesions were densely infiltrated with relatively uniform, mitotically active, medium-sized cells with round to ovoid hypochromatic and occasionally cleaved nuclei that were surrounded by small amounts of indistinct eosinophilic cytoplasm. The colonic mucosa was edematous. All other samples had no evident significant lesions. Diagnosis, based on histopathologic signalment, presentation, and necropsy, was sporadic juvenile thymic lymphoma.

Bovine lymphoma, lymphosarcoma, or leukosis is the most common type of neoplasm found in dairy cattle (1) and is classified into an enzootic type, enzootic bovine leukosis (EBL), and a sporadic type, sporadic bovine leukosis (SBL). The sporadic type is known to have 3 forms; the cutaneous form, juvenile/calf lymphosarcoma, and the thymic/adolescent form. While EBL is noted to occur primarily in adult cattle, is highly prevalent (especially in dairy herds compared with beef cattle due to disease transmission in conjunction with housing), and is known to have a viral etiology, SBL occurs in younger animals (< 3 y of age), is rare, and has no known etiology or viral association.

Enzootic bovine leukosis is caused by the Bovine leukemia virus (BLV), an exogenous C-type oncovirus of the Retroviridae family. It is transmitted via biological material containing B-lymphocytes, such as blood, milk/colostrum, or saliva, either horizontally or iatrogenically (reused rectal sleeves, hypodermic needles). It is believed that insect vector transmission also plays a role in BLV transmission (1,2). Once infection has occurred, the majority (80% to 90%) of animals become BLV-seropositive (2,3). Of these seropositive animals, approximately 30% will develop a persistent lymphocytosis, and less than 5% develop clinical disease or lymphosarcoma (3,4). With lymphosarcoma, the most common clinical findings are superficial lymphadenopathy, abomasal ulceration, cardiac lesions (primarily right-sided), and occasionally spinal lesions (2,10). Clinical disease is due to a variety of factors, such as age, infection prevalence within the herd, and genetic resistance and susceptibility (1,2,5). Calf management is a major risk factor for disease spread through prolonged cow-calf contact postparturition, feeding of infected milk/colstrum, and use of contaminated instruments (gouge dehorners, ear tagging equipment, tattooing, hypodermic needles, castration equipment [2]). The majority of economic losses from EBL are due to the imposition of import restrictions; however, losses also occur with the costs of control and eradication programs (2). While no treatment or BLV vaccine is currently available, screening is best accomplished by the use of either a serum or milk enzyme-linked immunosorbent assay (ELISA) (2,6).

Sporadic bovine leukosis has no known etiology and does not appear to have any association with BLV. Compared with EBL, it has a much smaller prevalence, only affecting 0.5 to 1.2 out of every 100 000 head of cattle (7). While all forms are rare, of the 3 forms, the cutaneous form is the most commonly reported and the thymic form, as reported here, the least (9). The cutaneous form primarily affects cattle aged 1 to 3 y and manifests primarily as skin nodules. While it may regress, it often returns as a generalized lymphosarcoma and carries a fatal prognosis (9). The calf/juvenile form occurs in calves less than 6 mo of age and presents as a generalized lymphadenopathy, followed by death 2 to 8 wk after the onset of the disease, subsequent to weight loss, depression, and weakness (2,9). The thymic/adolescent form has been reported more commonly in beef than dairy calves, occurs in animals aged 6 to 24 mo old, and is characterized by a large, firm swelling of the ventral neck region (1,2,4,5). Presentation and histopathologic findings of the heifer described here were consistent with the thymic form.

While the prevailing symptom of thymic lymphosarcoma is a ventral neck mass, a variety of other clinical signs are often present, the majority of which relate to the space occupying nature of the lesion. Esophageal compression results in ruminal tympany due to the inability to eructate, as seen in this heifer, and it has also been postulated that the ruminal tympany may also be due to vagal nerve irritation and dysfunction, subsequent to mediastinal infiltration by the tumor (8). Dyspnea, anorexia, and weight loss occur, as well as cervical and submandibular edema in conjunction with jugular venous distention (2,8). Generalized lymphadenopathy is uncommon with the SBL form (9). These clinical signs are often similar to those seen in right-sided heart failure, and it is important to differentiate these 2 diseases. It was interesting to note that the heifer did not show any of the above signs, other than the ventral neck region swelling and ruminal tympany.

Calves with lymphosarcoma have also been noted to have leukemia (1); however, no blood sample was taken from the heifer prior to euthanasia, so the leukogram, as well as other hematological parameters, are unfortunately unknown. On immunohistochemical staining of thymic lymphomas, T-cell populations have been reported to be predominant as opposed to the primarily B-cell origin of EBL (8). This brings up the possibility of a correlation between immunodeficiency and SBL (1). A familial thymic lymphosarcoma has also been reported in Holstein cattle in France, postulating a possible genetic predisposition to the disease (7). As with EBL, SBL has no treatment currently available and invariably proves fatal. In the thymic form, death is usually due to acute cardiac and respiratory failure, secondary to ruminal tympany.

With bovine leukosis being of major economic importance, disease control is important. Control based on eradication programs, maintenance of a closed seronegative herd, and prevention of calf infection by using good management techniques are ideal. Screening of herds is recommended by using either serum or milk ELISA, with differentiation between EBL and SBL best accomplished with either ELISA or PCR (6). Subsequent to this case, the producer opted to screen for BLV during a herd health examination. Serum agargel immunodiffusion (AGID) (Animal Health Laboratory) was used. Of the 39 head tested, only 5 animals were negative. If this was a representative sampling, 87% of the herd is seropositive for BLV. While BLV is not related to SBL, it was an interesting finding nonetheless. Screening of the dam of the heifer was not possible, as she had been sent to slaughter prior to testing.

Many thanks to both Campbellford Veterinary Services and Dr. Danny Butler for their assistance with this case. CVJ