Acute Cortical Blindness Associated with Occipital Lobe Compression from Simian T Cell Lymphoma Virus Lymphoma in a Baboon (Papio anubis)

Introduction

Simian T cell leukemia virus (STLV) is a deltaretrovirus in the subfamily Orthoviridae that has been identified in and isolated from a wide variety of Old World monkeys, including baboons (Papio spp.).^1–11 Cross-species transmission has been identified in both captive and wild NHP species, with higher infection rates documented in wild populations compared with captive populations.^5,6,12,13 Older female animals appear to be predisposed to developing clinical disease.^2,14–16

STLV is typically transmitted through bodily fluids, including semen, blood, and breast milk, as well as perinatally and can lead to an adult T cell leukemia/lymphoma–like syndrome resembling that of non-Hodgkin lymphoma in humans.^{2,3,13,17–19} While infection is generally asymptomatic, common clinical signs include weight loss, anorexia, weakness, lethargy, dyspnea, generalized lymphadenomegaly, and hepatosplenomegaly.^{1,2,13,16,20–23} Leukocytosis is the most common hematologic abnormality.^{1,2,13,16,17,20,21}

While STLV has been isolated from many NHP species, lymphoproliferative disease secondary to STLV only occurs in species of African origin.^1,5,18 High prevalence and transmission of the virus through normal social interactions with 1%-2% of infected animals developing lymphoma or leukemia may explain why lymphoma is one of the more commonly reported neoplasms of NHPs.^{2,11,13,14,24} Specific to baboons, the most frequently reported neoplasms are of hematopoietic origin, and the most frequent diagnosis is lymphoma or lymphosarcoma.^14,25 Most viral-associated lymphomas in baboons are T cell in origin, which can be confirmed by immunohistochemistry.¹³

While lymphoma in baboons secondary to STLV is a well-known cause of lymphadenomegaly and visceral involvement,^13,14,17,22 neurologic dysfunction from CNS involvement has not been reported. Herein, we described a case of apparent cortical blindness secondary to compression of the occipital cortex by metastatic lymphoma in an STLV-positive baboon (Papio anubis).

Ethical Review

The animal was housed and maintained at The University of Texas MD Anderson Cancer Center Michale E. Keeling Center for Comparative Medicine and Research in Bastrop, TX. The animal was housed in a protected outdoor dome enclosure with an indoor climate-controlled den area as part of a harem breeding group. Animals received regular veterinary health care with sedated physical examinations and TB testing on a semiannual basis and blood work (hematology and chemistry) performed annually. The colony was conventionally maintained with no SPF status, and the viral statuses of animals were not monitored.

Animals had access to ab libitum food (7195; Teklad high-fiber primate diet; Inotiv, Madison, WI) and water with a regular schedule of enrichment items including fruit and vegetables, seeds and nuts, a rotation of enrichment devices, manipulanda, and enclosure structural items. Animal care and husbandry conformed to practices established by AAALACi, the Guide for the Care and Use of Laboratory Animals,²⁶ and the Animal Welfare Act.²⁷ The animal was maintained under a breeding protocol approved by The University of Texas MD Anderson Cancer Center IACUC.

Case Report

A 14-year-old female, captive-bred, experimentally naive olive baboon (P. anubis) used for an in-house breeding program was reported in April 2024 for weight loss and lethargy with frequent bouts of lateral recumbency. Her clinical history included a single enlarged inguinal lymph node on the right side reported a year prior (March 2023) during a semiannual physical examination. Blood for routine hematology and chemistry was not collected at that time. The enlarged lymph node was not observed at either the previous (October 2022) or subsequent (November 2023) semiannual physical examinations. Minor blood abnormalities were noted at both examinations but were not correlated with clinical significance (Table 1). With a normal physical examination, elevated white blood cells and neutrophils during the November 2023 semiannual physical examination were attributed to a stress leukogram response from roundup and sedation (Table 1). Most recently, the animal had been sedated in March 2024, a month prior to the clinical report, for collection of biologic samples of cerebrospinal fluid (2.2 mL) and blood (10 mL) for a research project. At that time, she weighed 18.5 kg, had a body condition score of 3/5, and no clinical abnormalities were noted during her physical examination.

When the clinical abnormalities were reported in April 2024, on the initial cage-side veterinary examination, the animal was observed ambulating normally and had mild to moderate enlargement of the right side of the muzzle. The animal was sedated for further investigation of the clinical presentation. On sedated physical examination (ketamine at 10 mg/kg IM; Ketaset ketamine HCl injection; Zoetis, Parsippany, NJ; NDC 54771-2013-01) the animal had lost 4.5 kg and was in poor physical condition with a body condition score of 1.5/5 (Figure 1). The mandibular lymph nodes were enlarged bilaterally and swelling in the right side of the muzzle was confirmed. The liver was moderately enlarged on palpation with the margins extending ∼5 cm past the caudal ribs. The right axillary and inguinal lymph nodes were markedly enlarged. An aspirate of the right inguinal lymph node was obtained for cytology. The lymph node aspirate was stained with Wright-Giemsa and examined by light microscopy. The aspirate was markedly cellular, and the most cells were medium to large round cells with moderate amounts of dark blue cytoplasm interpreted as neoplastic lymphocytes. These cells had a round nucleus with hyperchromatic chromatin and one variably distinct round nucleolus (Figure 2A). Within the population of medium to large lymphocytes were scattered multinucleated round cells with light blue cytoplasm and 3-6 round to oval nuclei with variably distinct nucleoli (Figure 2A). Mitotic figures were noted in the neoplastic lymphocyte population (Figure 2A). A cytologic diagnosis of T cell lymphoma was confirmed.

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Blood collected for hematology and serum chemistry revealed moderate anemia with mildly decreased red blood cell and hemoglobin values (Table 1). Reticulocytosis indicated a regenerative response (Table 1). Other hematologic abnormalities included mild leukopenia as well as significant thrombocytopenia, lymphopenia, and basophilia (Table 1). Blood chemistry abnormalities included mild decreases in creatinine, ALT, and total protein with mild to moderately elevated BUN, bilirubinemia, and hypercholesterolemia (Table 1). Significant changes included severely increased ALP as well as hypertriglyceridemia (Table 1). Electrolyte derangements included mild to moderate hyponatremia and hypochloridemia (Table 1). The animal was treated symptomatically for potential infection with penicillin G procaine (150,000 IU IM; 300 IU/mL Norocillin; Norbrook, Lenexa, KS; NDC 55529-0021-02) and compounded extended-release meloxicam (0.6 mg/kg SC; 2 mg/mL; meloxicam in polymer injection solution; Wedgewood Pharmacy, Swedesboro, NJ). The animal recovered uneventfully from sedation and was pair-housed overnight in her den.

The following day, during cage-side veterinary observations, the animal appeared to have vision loss evidenced by lack of eye contact with staff, and difficulty in locating food items tossed close to her. In addition, the animal was observed bumping into the side of the enclosure and moving cautiously. Otherwise, the animal appeared to be active and was foraging for food. Swelling remained present on the right side of the muzzle.

Two days after the initial examination, animal technicians reported that the baboon had increased difficulty in locating food, decreased appetite, and significant difficulty in moving around the den. During a cage-side veterinary examination, the animal was recumbent and displayed signs of discomfort, including grunting and rolling side to side on her dorsum. Reduced visual acuity with no reaction or eye contact was observed when approached by staff or when exposed to negative reinforcement (capture net). The animal was sedated (10 mg/kg ketamine IM) for further investigation. Further weight loss of 0.5 kg was noted, and the animal’s physical condition remained poor. The right axillary and inguinal lymph nodes were subjectively larger than previously noted, with new enlargements of the left inguinal and axillary lymph nodes. Mucous membranes were pale pink, and the animal was mildly hypothermic. Based on a poor prognosis, poor clinical condition, and a quality-of-life discussion with the staff present, the animal was humanely euthanized and submitted for a postmortem examination.

The initial working diagnosis was an infection of unknown origin as confirmatory test results were pending. The timeline from initial reporting to euthanasia was ∼48 hours. Initial differential diagnoses included meningitis and septicemia.

Pathologic Findings

At necropsy, the animal was thin, and the lymph nodes throughout the body were diffusely and severely enlarged. The inguinal lymph node chain was 3 × 2.5 × 1.2 cm; on cut section, corticomedullary distinction was inapparent, and the tissue was pale tan with multifocal areas of hemorrhage and necrosis. The spleen was diffusely enlarged with multifocal pale tan to mottled red, soft, raised nodules up to 3 cm in diameter. The liver was diffusely enlarged with rounded margins and had similar appearing nodules scattered throughout the lobes. Bilaterally, the kidneys had similar nodules in the cortex. The bone marrow in the femur was diffusely tan and firm. On dorsal midline at the occipital lobe were two 2.5 × 3.5- and 1.5 × 1.2-cm gelatinous masses that extended from the dura and compressed the underlying cerebral cortex (Figure 2B). The dura was focally adhered to the calvarium. No additional gross lesions were noted in the CNS or ophthalmic tissues. Samples from inguinal lymph node and the brain mass were snap-frozen and stored at −80 °C for ancillary testing. Tissues were collected into 10% neutral buffered formalin according to institutional protocols and were then routinely processed for histology. Based on the clinical presentation of blindness, special emphasis was placed on evaluating tissues involved in sight, including both eyes with optic nerve, optic chiasm, and the occipital cortex where the tumor was located.

Extending rostrally and caudally from the dura was an infiltrative neoplasm composed of round cells arranged in sheets on preexisting stroma (Figure 2C). The predominant neoplastic population was composed of round cells with moderate amounts of eosinophilic cytoplasm. The nuclei were round with hyperchromatic chromatin and one round nucleolus. A distinct subpopulation of neoplastic cells included large cells with abundant eosinophilic cytoplasm and 2-5 round to oval nuclei (Figure 2D). Anisocytosis and anisokaryosis were moderate. The neoplasm had multifocal areas of hemorrhage. The underlying neuroparenchyma was spongiotic and had a laminar band of neuronal necrosis. Immunohistochemistry for CD3, CD20, and IBA-1 was used to characterize the population. The neoplastic population including the giant cells had strong membranous staining with CD3 confirming T cell origin (Figure 2D, inset). There were no additional abnormal findings in the rest of the brain, and both eyes were histologically normal.

After the necropsy and lymph node aspirate confirmed lymphoma, an archived serum sample was submitted to Charles River Research Animal Diagnostic Services (Wilmington, MA) for confirmatory STLV testing. All other viral results were negative. Results from Charles River confirmed that the baboon was positive for STLV by anti-immunoglobulin multiplexed fluorometric immunoassay and equivocal for STLV by an indirect fluorescent antibody assay. To confirm the diagnosis, DNA was extracted from frozen inguinal lymph node and the brain mass using a Qiagen DNeasy blood and tissue kit (Qiagen, Hilden, Germany) according to the manufacturer’s protocol. To serve as a negative control, DNA from a section of formalin-fixed paraffin-embedded histologically normal brain was extracted using the Qiagen formalin-fixed paraffin-embedded kit (Qiagen, Hilden, Germany) according to the manufacturer’s protocol. Oncostatin M was used as a housekeeping gene to estimate copy number per cell and was performed according to a protocol published by Bruce et al.²⁸ Quantitative PCR (qPCR) for STLV and cytomegalovirus using in-house primer sequences were run with template DNA from the inguinal lymph node, brain mass, and normal brain. STLV amplified in the inguinal lymph node and brain mass with similar Ct values of 24.41 and 24.14, respectively. The qPCR from the histologically normal brain was negative for STLV, and all samples were negative for cytomegalovirus.

Discussion

In this case, the diagnosis of STLV-associated lymphoma was confirmed postmortem based on serology results and qPCR for STLV in the lymphoma lesions. Neoplastic lymphocytes were confirmed as T cell origin based on positive immunohistochemical staining for CD3 with concurrent negative staining for CD20 and IBA-1. The presence of STLV DNA was confirmed in both the inguinal lymph node and the brain tumor via qPCR. The Ct values for housekeeping gene, oncostatin M, were 22.83 in the brain mass and 22.16 in the inguinal lymph node. This indicates that the housekeeping gene was present at greater concentrations than the proviral DNA (ratio, 0.74 and 0.50 for the brain mass and lymph node, respectively). However, the normal tissue did not amplify with primers for STLV, confirming that the positive results are not the result of blood contamination and confirm that STLV DNA was in the neoplastic tissue.

Although viral serology was not performed for the colony, the colony arrived at the Keeling Center in 2017 from a facility with known positive STLV animals allowing the reasonable assumption to be made that STLV was endemic in the population.¹⁹ No additional animals of outside origin were introduced to the colony after arrival. Due to the conventional status of the baboon colony, serology for disease tracking was not part of the routine healthcare screening processes; historical seroprevalence for the animal and the colony is unknown after 2017.

While the baboon exhibited classic clinical signs of STLV-associated lymphoma, including enlarged regional lymph nodes, hepatomegaly, anorexia, lethargy, and weight loss, the hematology results showed mild leukopenia and significant lymphopenia. Although not performed in this case, and while it does not occur as prominently in baboons as in humans, a blood smear may have identified circulating multilobulated neoplastic lymphocytes, which can be highly indicative of adult T cell leukemia/lymphoma.¹³ Fine-needle aspiration of the lymph node for cytologic examination provided a perimortem diagnosis and is an important diagnostic tool for identifying lymphoma cases.

The acute and rapid visual decline was an unusual clinical sign, and the cause is attributed to metastatic foci compressing the occipital lobe, which is an area of the brain crucial for processing visual information.²⁹ To our knowledge, this is the first documented case of neurologic clinical signs associated with STLV lymphoma in an NHP. While STLV has not been directly linked to neurologic disease before, a related deltaretrovirus, bovine leukemia virus, is a known cause of spinal lymphoma that can present as hindlimb paresis or paraplegia in cattle.³⁰ A study of CNS samples from bovine leukemia virus–seropositive cattle with neurologic symptoms found that bovine leukemia virus has no specific tropism for the CNS, but metastatic foci can be found in the brain or spinal cord.³¹ The study went on to speculate that the high prevalence of bovine leukemia virus–seropositive rates and low incidence of CNS lymphosarcoma may be explained by the difficulty of the virus crossing the blood brain barrier.³¹ This speculation is supported by the work of Afonso et al³² that demonstrated that human cerebral endothelial cells are susceptible to HTLV with infection resulting in alterations to the blood–brain barrier and lymphocyte migration. In addition, although rare in humans, HTLV-associated myelopathy, also known as tropical spastic paraparesis, can develop in specific groups of people, but this clinical presentation does not involve compression of the spinal cord.^33,34 However, HTLV RNA has been observed in the white matter of the spinal cord and cerebellum in human cases, which suggests that the virus may be able to cross the blood–brain barrier.^35,36

The pathogenesis of malignant transformation of lymphocytes during deltaretrovirus infection, regardless of the host, is underlaid by Tax proteins, which are an activator of viral gene transcription that can induce immortalization.³⁰ In a study in France, naturally infected STLV-1 baboons (P. anubis) were found to have STLV-1 viral Tax proteins on immunohistochemistry of spinal cord, cerebellum, and cortex, primarily localized to NeuN⁺ neurons with presence also in Iba1⁺ microglial cells.³⁷ These cases support the ability of members of the deltaretrovirus family to cross the blood–brain barrier and infect tissues of the nervous system.

In humans, the most common ocular complication associated with HTLV is uveitis, which presents as ocular inflammation.^36,38,39 Other ocular manifestations of HLTV are malignant infiltration of the eye and opportunistic infection of the eye with cytomegalovirus.^34,38,40 While none of these conditions was present in this case, these conditions can result in sight-threatening problems and should be considered in future similar NHP cases.^34,38

Conclusions

This case provides evidence that STLV can cause metastatic lymphoma outside of the hematopoietic organs, lungs, and kidneys, to include CNS involvement, which, to our knowledge, has not been reported previously in cases of baboons with lymphoma. Examination of the brain and spinal cord should be included in all suspected cases of STLV-associated lymphoma in baboons, as well as clinical evaluation for neurologic dysfunction including vision impairment, ataxia, and sensory derangements.