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OBJECTIVE To characterize clinical findings, surgical procedures, complications, and outcomes in dogs undergoing extirpation of masses from the cranial mediastinum via video-assisted thoracic surgery (VATS) and establish preliminary guidelines for case selection when considering VATS for thymectomy in dogs. DESIGN Retrospective case series. ANIMALS 18 client-owned dogs that underwent extirpation of a cranial mediastinal mass by means of VATS at 5 academic referral hospitals from 2009 through 2014. PROCEDURES Medical records were reviewed and data extracted regarding signalment, clinical signs, physical examination findings, diagnostic imaging results, surgical approach and duration, cytologic and histologic examination results, complications, outcome, and cause of death, when applicable. RESULTS 16 dogs had a thymoma, 1 had thymic anaplastic carcinoma, and 1 had hemangiosarcoma. Seven had both megaesophagus and myasthenia gravis. Median approximate tumor volume was 113.1 cm 3 (interquartile range, 33.5 to 313.3 cm 3 ). Median duration of VATS was 117.5 minutes (interquartile range, 91.5 to 136.3 minutes). Conversion to an open thoracic surgical procedure was required for 2 dogs, 1 of which died during surgery. Median survival time following VATS for dogs with thymoma and concurrent myasthenia gravis and megaesophagus was 20 days. Dogs with thymoma without paraneoplastic syndrome survived for ≥ 60 days, and none of these dogs died of disease-related causes. CONCLUSIONS AND CLINICAL RELEVANCE VATS appeared to be an acceptable approach for extirpation of masses from the cranial mediastinum in dogs under certain conditions. Dogs with myasthenia gravis and megaesophagus had a poor postoperative outcome.

Neurological signs and serum acetylcholine receptor antibody (AChR-Ab) levels before and after thymectomy were monitored in a 6-year-old male cat with acquired Myasthenia Gravis (MG) as a paraneoplastic syndrome of thymoma. Soon after surgery, the neurological symptoms relapsed, and the cholinesterase inhibitor was administered to control them. The AChR-Ab levels increased postoperatively until 90 days after surgery. This is the first report on long term measurements of serum AChR-Ab levels in a cat with MG. Although thymectomy is valuable for the removal of thymoma, it may not resolve MG symptoms, neurological signs and serum AChR-Ab levels, without medication early after surgery. Also, this case report indicates that the AChR-Ab level might be a guide to detect a deterioration of MG symptoms.

Objective: To report the use of low-pressure carbon dioxide insufflation during video-assisted thoracoscopic surgery for resection of a noninvasive thymoma in a cat with secondary myasthenia gravis., Study Design: Clinical case report., Animal: Client-owned cat., Methods: An 11-year-old castrated male domestic shorthair cat was examined for generalized weakness, voice change, hypersalivation, hyporexia, vomiting, coughing, and gagging. Thoracic ultrasound revealed a cranial mediastinal mass for which cytology was consistent with a thymoma (or lymphoid tissue). Acetylcholine receptor antibody concentration was elevated at 3.16 mmol/L (reference interval < 0.3 mmol/L). Thoracic computed tomography showed two round, contrast-enhancing structures in the cranioventral mediastinum identified as the sternal lymph node and a cranial mediastinal mass (11 × 17 × 24 mm). A presumptive diagnosis of thymoma with paraneoplastic myasthenia gravis was made and surgical resection of both mediastinal masses was recommended., Results: Video-assisted thoracoscopic resection of the cranial mediastinal mass and sternal lymph node were performed with low-pressure carbon dioxide insufflation maintained at an intrathoracic pressure of 2-3 mmHg. The cat recovered from surgery without serious complications. Nineteen months after surgery, the cat developed hind limb stiffness. Thoracic radiographs ruled out a cranial mediastinal mass or megaesophagus. Acetylcholine receptor antibody concentration remained elevated at 2.72 mmol/L., Conclusion: Low-pressure thoracic insufflation facilitated video-assisted thoracoscopic resection of cranial mediastinal masses in this cat., (© Copyright 2016 by The American College of Veterinary Surgeons.)

Unlabelled: HISTORY AND PRESENTATION: A 12 year old, 4.2 kg, domestic long hair, castrated male cat was presented with regurgitation, inability to retract the claws, general weakness, cervical ventroflexion and weight loss. A thymic mass was evident on radiographs. Acetylcholine receptor antibody titer was positive for acquired myasthenia gravis (MG). Thymectomy via midline sternotomy was scheduled. ANESTHETIC MANAGEMENT:  Oxymorphone and atropine were administered subcutaneously as premedication, and anesthesia was induced with etomidate and diazepam given intravenously to effect. The cat's trachea was intubated and anesthesia was maintained with isoflurane in oxygen, and continuous infusions of remifentanil and ketamine. Epidural analgesia with preservative-free morphine was administered prior to surgery. Postoperative analgesia was provided by oxymorphone subcutaneously, interpleural bupivacaine, and fentanyl infusion. Postoperative complications included airway obstruction, hypoxemia and hypercapnia., Follow-Up: The cat was discharged 3 days after surgery. Discharge medications included pyridostigmine and prednisone. Nine days after surgery, the cat had a significant increase in its activity level, and medications were discontinued. Histopathologically, the mass was consistent with a thymoma. Approximately 6 weeks later the cat became weak again and pyridostigmine and prednisone administration was resumed., Conclusion: The perioperative management of patients with MG for transsternal thymectomy is a complex task. The increased potential for respiratory compromise requires the anesthesiologist to be familiar with the underlying disease state, and the interaction of anesthetic and non-anesthetic drugs with MG. Careful monitoring of ventilation and oxygenation is indicated postoperatively., (© 2011 The Authors. Veterinary Anaesthesia and Analgesia. © 2011 Association of Veterinary Anaesthetists and the American College of Veterinary Anesthesiologists.)

A 7 yr old Labrador retriever initially presented for severe halitosis, mild ptylism, and depigmentation of the nasal planum. Erythema multiforme was diagnosed based on clinical signs and dermatopathology. Treatment was initiated but the condition did not resolve. Six months later, the dog was diagnosed with a mediastinal mass. Trucut biopsy was performed and histopathologic diagnosis was consistent with a thymoma. A median sternotomy was performed, the thymoma was excised, and the dog recovered well. Four months postoperatively, there were no longer any obvious erythema multiforme lesions and the skin condition was controlled without medication. Erythema multiforme may be a paraneoplastic disorder associated with thymoma in the dog, similar to thymoma-associated exfoliative dermatitis in the cat. Clinical signs of erythema multiforme may warrant performing thoracic radiographs or thoracic CT to rule out thymoma as an underlying cause.

Thymoma-associated exfoliative dermatitis was suspected in a cat with a cranial mediastinal mass. The dermatopathy resolved with surgical removal of a thymoma. The cat manifested neurologic signs consistent with myasthenia gravis 7 wk after surgery. Exfoliative dermatitis and post-thymectomy myasthenia gravis in the same cat has not been reported previously.

An 8 years old male persian cat with a diagnosis of myasthenia gravis was scheduled for transternal thoracotomy. An anterior mediastinal mass, suspected to be a thymoma, had to be resected. Progressive paraparesis and reduced ocular reflexes and menace response were the main clinical features. At the preoperative examination the cat appeared free of significant myocardial or respiratory diseases. The possibility of compromised respiratory function due to muscle weakness was addressed by the choice of a balanced anesthesia protocol without the use of muscle-relaxants. Intravenous induction was followed by intubation, administration of isoflurane in oxygen and ventilatory support. Thorough cardiorespiratory monitoring was performed during anaesthesia. Epidural morphin was given to reduce the amount of inhalation agent required to maintain anaesthesia and supplemental intravenous analgesia was given. At the end of the surgery, intrapleural bupivacaine was administered to help controlling poststernotomy pain, while reducing the need for systemic analgesics. Although rapid returning of swallowing reflex and spontaneous breathing followed the disconnection from the anaesthetic circuit, the cat needed to breath oxygen enriched air to maintain a normal hemoglobin saturation in the early postoperative phase.

Diabetes-prone (DP)-BB rats spontaneously develop diabetes and are widely used as an animal model for the study of type 1 diabetes. Since DP-BB rats develop diabetes before or at the time of breeding, such rats used for breeding need to be protected against diabetes development by the transfer of regulatory T cells obtained from diabetes-resistant (DR)-BB rats, by insulin treatment or by thymectomy. Thymectomy of juveniles is not commonly used to protect DP-BB rats, and we investigated whether breeding with thymectomized DP-BB rats was a realistic alternative to the two other methods. No differences in pregnancy rates, numbers of pups per litter or growth rates of pups were found. Moreover, no differences were found in diabetes development in the offspring. Protection of juvenile DP-BB rats by thymectomy is comparable to the other established procedures, is simple and safe, and the rats recover well from the procedure. Breeding with thymectomized animals will reduce the number of animals needed, and it improves the well-being of the animals because it reduces the negative side effects associated with the other procedures such as episodes of hypo and hyperglycaemia. Therefore, although thymectomy is an invasive procedure, we would like to recommend weanling thymectomy as the first choice for the protection of DP-BB rats for breeding purposes.

In this study the role of the thymus in the development of sessile T cell populations resident in spleen and lymph nodes (LN) was contrasted with the development of recirculating T cell populations trafficking between blood and lymph. Extensive analysis of the composition and the rate of growth of the secondary lymphoid tissues and recirculating lymphocyte pool coupled with neonatal thymectomy revealed that the sessile and recirculating T cell populations showed different degrees of thymic dependency and increased in size at different rates, suggesting these two populations might be under separate homeostatic control. Neonatal thymectomy also resulted in a much greater depletion of CD8+ and gammadelta TCR+ T cell subsets compared with CD4+ T cells in the sessile and recirculating T cell pools, and greatly reduced the number of T cells homing to peripheral lymph nodes compared with those homing to the gut.

A 10-year-old, neutered male cocker spaniel-cross experienced regurgitation, dry retching, and weight loss. A large, mediastinal mass and dilatation of the esophagus were seen on thoracic radiographs. Cytological, histopathological, immunohistochemical, and serological findings were consistent with a lymphoepithelial thymoma and focal, esophageal myasthenia gravis. Surgical removal of the mass resulted in rapid resolution of the megaesophagus and a decrease in serum acetylcholine-receptor antibody concentration. The dog was clinically normal until the thymoma recurred six months postoperatively. Clinical signs, diagnostic evaluation, management, and treatment of a dog with thymoma and megaesophagus are described.

Further studies were carried out on the monoclonal antibodies (mAbs) from the First International Swine CD Workshop which react with gamma delta Null T-lymphocytes, defined by the binding of mAb w020/141 (MAC320) as Swine Workshop Cluster number SWC6. Studies were also carried out on several other mAbs from the same workshop which identify other CD antigens, but whose binding is not restricted exclusively to gamma delta Null T-lymphocytes. The first group consists of 11 mAbs (w021, w022, w059-w065, w105 and w117) and the second group of 18 mAbs (w008, w026, w056, w067-w071, w080, w091-w094, w110, w111, w118, w119 and w121). All mAbs were characterised by binding to peripheral blood lymphocytes (PBL) from normal, sham-thymectomized (STx) and thymectomized (Tx) pigs of the Babraham SLAb/b line and by their overlap, using two-colour immunofluorescence with biotinylated mAb MAC320 (w020/141), which identifies all gamma delta Null cell T-lymphocytes and the Null cell subpopulation identified by MAC319 (w021). The Null cell-specific mAbs were also used in inhibition studies of MAC319 and MAC320 binding and by staining PBL with pairs of mAbs together with either MAC319 or MAC320. Based on these data we suggest a putative relationship of the Null cell subsets defined by these mAbs with each other.

Third degree atrioventricular block was diagnosed in 4 dogs with acquired myasthenia gravis (serum acetylcholine receptor antibody titer > 0.6 nmol/L). All 4 dogs had megaesophagus. Two dogs also had mediastinal thymomas, which were resected. One dog with thymoma received a permanent pacemaker at the same time that it underwent thymectomy; the other did not develop third degree atrioventricular block until 3 months after thymectomy. Both dogs with thymoma died of severe aspiration pneumonitis within 3 months after surgery. The third dog received a permanent pacemaker and was treated with pyridostigmine bromide, but also died of aspiration pneumonitis 1 month after the pacemaker was implanted. The fourth dog was treated with prednisolone and pyridostigmine bromide and improved, but did not become clinically normal. Because third degree atrioventricular block as well as myasthenia gravis can cause signs of weakness, acquired myasthenia gravis should be considered in dogs with idiopathic cardiac conduction disturbances. Likewise, an ECG should be evaluated in dogs with acquired myasthenia gravis.

Sixty-one Japanese white rabbits underwent total thymectomies. Eight of the animals died of intraoperative hemorrhage or respiratory failure, and twelve died of postoperative hypovolemia, dehydration, malnutrition or wound infections. Forty-one rabbits survived and were used as a total thymectomy model. The whole procedure of our technique is described in detail so that even those with no experience of performing thymectomies can follow it. Some tactics especially how to deal with the thymic vessels and the pleurae, are key points. Our technique permits safe and perfect total thymectomies to be carried out on rabbits.

Treatment of chickens as embryos and in the first six days after hatching with mg amounts of anti-TCR2 (alpha beta T cell antigen receptor) antibody plus thymectomy 6 to 8 days after hatching suppressed by more than 90% the TCR2 and TCR1 (TCR gamma delta) subpopulations in the blood. Thymectomy alone resulted only in a profound reduction (greater than 90%) of the absolute number of TCR1 cells, while the absolute number of TCR2 cells was reduced only by 65%. We were interested in the regulation of the immunoglobulin synthesis in these T cell deficient chickens. We determined serum IgG, IgM and IgA levels and found a profound selective reduction (by about 90% on average) of the serum IgA level in anti-TCR2 antibody treated plus thymectomized chickens. In chickens which had been thymectomized only, the serum IgA level was reduced on average only by about 60%. Serum IgG and IgM levels were not reduced in any group. Thus, there was a positive correlation of the serum IgA level and the absolute number of TCR2 cells in all groups. These results indicate that T cells which play a role in IgA specific isotype switching or in IgA B cell differentiation belong to the TCR2 subpopulation.

The aim of this study was to develop an operation for total thymectomy suitable for nonhuman primates. The thymus was removed from 24 macaque monkeys from fetal to adult life by a cervicothoracotomy and blunt gauze dissection. Mean wet weight of the thymus rose from 150 mg (range 50-250 mg) in fetal animals to 2.5 gm (range 0.3-4.2 gm) in prepubertal monkeys. Nonhuman primate thymic weight fell steadily and uniformly with age by over 300% when plotted as a function of body weight. This surgical technique appears to be a valid method of total thymectomy in macaque monkeys.

Partially purified thymus products were used to evaluate the maturation of T lymphocytes in immunodeficient mice. Three different bovine thymic extracts, designated thymic extracts A, B, and C, were successful in increasing the longevity of conventionally raised nude mice. Daily injection of bovine thymic extracts A, B, and C and mouse thymus extracts failed to mature a population of T lymphocytes and restore the capacity to reject heart allografts. Preincubation of normal syngeneic bone marrow with thymic extract B in vitro before injection into nude mice also failed to reconstitute the host's ability to reject heart grafts. The number of antibody plaque-forming cells of sheep red blood cells in lethally irradiated, bone marrow-reconstituted mice could be increased by preincubating the bone marrow cells with bovine thymic extract fraction B before injection followed by daily injections. A similar but less marked increase in plaque-forming cells was obtained by the daily injection of bovine thymic extract fraction C. Complete functional maturation of T lymphocytes in immunodeficient animals appears to require more than thymic extract stimulation of bone marrow cells or pre-T lymphocytes.

A high incidence of bovine papular stomatitis (BPS) occurred in neonatal calves following neonatal thymectomy and antilymphocyte globulin treatment, sham thymectomy, treatment with normal horse globulin and in untreated calves. The source of BPS virus was not identified but was suspected to be latent in the calves and activated by thymectomy although no experimental evidence directly supported this conclusion. The potential for activation of a latent BPS infection was indicated by an apparent relationship between the stress of surgery or foreign protein inoculation and the severity of lesions. Subsequent bovine viral diarrhea virus infection did not result in recrudescence.

The effect of immunosuppression on Mycoplasma hyopneumoniae infection was evaluated by comparing data from infected, thymectomized, and antithymocyte serum-treated pigs (group 1) with data from infected (group 2) and noninfected (group 3) healthy pigs. After groups 1 and 2 pigs were inoculated intranasally with M hyopneumoniae, mycoplasmas tended to multiply slightly more in the lungs and bronchial lymph nodes of group 1 pigs than that of group 2 pigs. Organisms were also isolated from the spleen of 1 of 3 group 1 pigs. Pneumonia developed in group 2 pigs and was characterized by massive peribronchial, peribronchiolar, and perivascular lymphoid hyperplasia and exudate consisting mainly of polymorphonuclear leukocytes in the alveoli and lumina of the bronchioles and bronchi. In group 1 pigs, perivascular and peribronchiolar cuffings by lymphocytes were less prominent, and the extent of intraluminal exudate was severe and widespread. Bronchial lymph nodes from group 2 pigs had marked hyperplasia of germinal centers and paracortical areas. In group 1 pigs, germinal centers were hyperplastic, whereas in the paracortical areas, depletion of lymphocytes was evident. Seemingly, cell-mediated immune mechanisms are important in the development of pneumonic lesions in enzootic pneumonia of pigs.

Using the cytochemical demonstration of alkaline phosphatase (AP) two different populations of lymphocytes are seen in the short-tailed mouse Clethrionomys glareolus: AP-positive and PA-negative lymphocytes. The aim of this study was to find out as to whether in this species AP represents a cytochemical marker for differentiating T- and B-lymphocytes. For this purpose, we analysed the frequency of AP-positive lymphocytes in the peripheral blood, bone marrow, thymus, lymph node, spleen, and Peyer's patches of normal adult animals 3 months after neonatal thymectomy, and at varying times following administration of intraperitoneal injections of a maximum dose of 16-methylenprednisolone. In normal mice the frequency of AP-positive lymphocytes in peripheral blood and bone marrow corresponds to that of T-lymphocytes in other species. In the thymic cortex, nearly 100% of the lymphocytes show an AP-positive reaction. In the peripheral lymphatic organs these cells are prevailing in the T-dependent areas (paracortex of lymph node, periarteriolar lymph sheet of spleen, and serosa-near region of Peyer's patches). Neonatal thymectomy as well as administration of corticosteroid cause a significant loss of enzyme-positive lymphocytes in the organism. This T-cell reduction becomes particularly evident in B-dependent regions, because in these areas AP-positive (T-) and AP-negative (B-) cells are usually intermingled, and this provides the necessary condition of T/B-interaction, which is of immunological significance. In T-dependent zones, populated mainly by AP-positive cells, neonatal thymectomy results in overall atrophy, while T-cell depletion, expressed as a percentage, is less pronounced within these atrophied areas. Based on these findings the conclusion may be drawn that in Clethrionomys glareolus committed thymocytes, recirculating T-lymphocytes as well as T-helper cells show an AP-positive reaction in contrast to prethymic T-cells, T-suppressor cells and B-cells, which do not express this enzyme. The histochemical demonstration of lymphocytic AP therefore permits an easy visualisation and quantification of T-lymphocytes in Clethrionomys glareolus.

Specific pathogen-free chickens (bursectomized and x-irradiated (SBx-X), thymectomized and x-irradiated, x-irradiated, as well as nontreated) were inoculated (in the right tibiometatarsal joint) at the 4th week after hatching with a synovitis-derived Mycoplasma synoviae strain. Differences were not observed in recovery rate of mycoplasmas from tissues among these groups. The SBx-X chickens which were killed at 2 and 4 weeks after the intraarticular inoculation or which died during this period and bacteremia and severe synovitis with edema and heterophil infiltration in synovial fluid and in area from synovial membrane to subcutaneous tissue. Swelling of left leg joints or foot pads were observed mainly in the SBx-X group. A significant difference in the mortality rate and severity of airsacculitis was observed between SBx-X and other groups. The thymectomized and x-irradiated chickens did not show edema, synovial fluid, or swelling in the joint area. However, small lymph follicles composed of lymphoid and plasma cell were formed locally in synovial membranes of various joints. These results indicated that bursa-dependent lymphocytes are correlated with resistance to develop the lesions and that thymus-dependent lymphocytes are necessary for macroscopic lesions to develop.

Twelve cats were thymectomized at 5 weeks of age. Six of these cats were inoculated at 8 weeks of age and 6 at 4 months of age with the Rickard (R) strain of feline leukemia virus (FeLV), which produces a high incidence of thymic lymphosarcoma. Two groups of age-matched nonthymectomized cats were inoculated with the same FeLV-R stock. Thymectomy prior to FeLV infection had no influence on the induction of viremia or the incidence of lymphosarcoma. In the FeLV-inoculated nonthymectomized cats, lymphosarcoma developed in the thymus. In the thymectomized cats, lymphosarcoma developed in the intestine, mesenteric lymph nodes, and bone marrow, but the malignant lymphoblasts had surface markers characteristic of feline T lymphocytes. It was concluded that the presence of the thymus per se is not required for infection and oncogenesis by FeLV and that feline T lymphocytes may be transformed after peripheralization to other tissues.

The nude mouse was used to evaluate transplantation of feline tumors and subsequent doxorubicin chemotherapy. Three mammary gland adenocarcinomas and one salivary gland adnoecarcinoma were readily transplanted. All were successfully passed at least once. The two sarcomas, a fibrosarcoma and a malignant hemangioendothelioma, failed to grow when transplanted. Growth rates for the adenocarcinomas appeared to be specific for individual tumors. Generally, tumor weights increased five to eight times by days 40 to 50. Structural changes were not observed histopathologically in tumors transplanted several times. When doxorubicin was given intraperitoneally (1 mg/kg of body weight) weekly for 6 weeks to nude mice with transplanted feline mammary gland adenocarcinoma, there was no significant difference (P less than 0.05) in weights of tumors of experimental (n = 150 and control (n = 10) mice.

Thymectomy of 5-week-old kittens was accomplished by left parasternal thoracotomy, ligation of the left internal thoracic artery, and removal of the thymus from the mediastinum by traction and blunt dissection. Thymectomy had no adverse clinical effects, and neither the response of peripheral blood lymphocytes to phytomitogens nor the proportions of circulating lymphocytes bearing T- or B-cell markers differed substantially between thymectomized cats and nonthymectomized control cats. Skin allograft rejection times of the thymectomized cats (3 weeks after operation) were slightly longer than those of littermate nonthymectomized controls. We conclude that removal of the thymus in preweanling kittens does not have important physiologic or immunologic consequences.

A method for removal of the thymus in the newborn rat employing measures aimed at reducing the mortality incurred by the surgical procedure, cannibalism and infection is described in detail. Tranquilisation of the dam, sterile technique, magnification of the operative site for thymus aspiration, warming to 37 degrees C during the recovery phase, and application of pheromones to the offspring prior to their reunion with the dam, all proved important in increasing the survival rate.

Thymoma is an uncommon canine neoplasm of thymic epithelial cells. It is seen in various breeds but may occur more frequently in German Shepherd Dogs. Middle-aged or older dogs can be affected and no sex predilection exists. A paraneoplastic syndrome of myasthenia gravis, nonthymic malignant tumors, and/or polymyositis occurs in a significant number of dogs with thymoma. Clinical signs are variable and are related to a space-occupying cranial mediastinal mass and/or manifestations of the paraneo-plastic syndrome. Dyspnea is the most common presenting clinical sign. Thoracic radiographs usually show a cranial mediastinal mass. Lymphoma is the main differential diagnosis. A definitive diagnosis may be made by closed biopsy but is more likely to be confirmed by thoracotomy. Thymomas may be completely contained within the thymic capsule or may spread by local invasion or metastasis. A staging system allows for an accurate prognosis and a therapeutic plan. Surgical removal of encapsulated thymomas may result in long-term survival or cure. Invasive or metastatic thymomas carry a guarded prognosis. Manifestations of the paraneoplastic syndrome complicate treatment. Adjuvant radiation and chemotherapy may be of value for advanced cases; however, adequate clinical trials have not been done in the dog.

The investigations were carried out on male Wistar rats. 12 h after birth newborn rats were subjected to thymectomy, and subsequently observed for the period of 8 weeks. The studies of the rat hypothalamus revealed that supraoptic nuclei (NSO) contained numerous cells which dyed dark. The cells were deprived of nuclei, or else they were shrunk. Similar cells were encountered in the medial eminence (EM). Measurements of cellular nuclei within NSO and NPV demonstrated that their size diminished following thymectomy what after t-test calculations yielded a statistically significant value.

Neonatally thymectomized and intact Holstein calves were inoculated intradermally with bacille Calmette Guérin mycobacteria at 4 to 6 months of age. The immune response of the calves was evaluated by in vitro lymphocyte stimulation, with a purified protein derivative of Mycobacterium bovis, and by a cervical skin test. Eight of te calves had been previously infected at 7 to 15 weeks of age with bovine viral diarrhea virus. The cutaneous and in vitro immune responses were not different in neonatally thymectomized calves, whether or not they had been previously infected with bovine viral diarrhea virus. However, a prominent histologic difference in the nature of the primary granulomatous lesions was observed in the thymectomized calves. These injection sites were characterized by few lymphocytes and an increased number of multinucleated giant cells. These observations were interpreted as manifestations of a deficient thymus-dependent effector response in the thymectomized calves.

Effect of immunosuppression on pathogenicity of herpesvirus of turkeys (HVT) in chickens was studied. In three experiments, newly hatched chickens of two genetic strains were surgically thymectmized and gamma-irradiated and then exposed to HVT. Lymphoid cells of thymectomized-irradiated chickens had reduced in vitro mitogenic response; this result indicated that the immunosuppressive regimen caused suppression of T cell function. The immunosuppressed chickens had more circulating HVT and had a slightly higher frequency of microsocpic lesions in peripheral nerves than the intact control chickens, although the intensity of lymphoproliferation remained very mild and no gross lesions or clinical disease was noted during the 5-week observation period. The absence of significant lesion development in an immunosuppressed host indicated that pathogenic potentials of HVT may be limited.

Intradermal injection of phytohemagglutinin (PHA) in the wattles of chickens elicited marked swellings often accompanied by induration. Histologically, this reaction was characterized by a perivascular accumulation of lymphocytes and macrophage migration in the central layer of the wattles. Heterophilic infiltration was observed mostly at early hours and waned thereafter. Sometime basophils were prominently located around the vessels. These responses were significantly decreased in cases of neonatal thymectomy. Therefore, the PHA skin test was considered to be a thymus-dependent response. The use of the PHA skin test in chickens may provide useful information for the evaluation of thymus-dependent function.

Adult mice were thymectomized using a retrosternal approach. The sternothyroid muscle was divided, the sternum was elevated, and the lobes of the thymus were removed by aspiration. The technique was used to thymectomize about 300 mice of several strains. The incidence of thymic remnants was less than 1% and losses from surgery were less than 10%.