Fatal extraintestinal toxoplasmosis in a young male cat with enlarged mesenteric lymph nodes

Abstract

A 22-month-old indoor/outdoor neutered male domestic short-haired cat had a history of progressive lethargy, vomiting, and decreased appetite. Abdominal ultrasound revealed an irregular hyperechoic mass in the mid-abdomen. He was unresponsive to symptomatic medical management and was euthanized after 3 days of hospitalization. A diagnosis of disseminated extraintestinal toxoplasmosis was made based on the finding of intracytoplasmic protozoan parasites on histopathological examination of mesenteric lymph nodes, hepatic and intestinal samples, and on immunohistochemistry.

Toxoplasma gondii is an obligate intracellular parasite that can infect many mammalian species, including humans. Felids are the only known definitive hosts capable of passing oocysts into the environment, with outdoor domestic cats representing the most significant group in terms of epidemiology and public health risk. Meat intended for human consumption is a less common source of exposure (1,2). Cats can become infected through ingestion of infective oocysts or intermediate hosts (usually small rodents or birds) infected with Toxoplasma gondii tissue cysts. Less commonly, transplacental or transmammary infection following parasitemia of a pregnant queen can occur (3,4).

Upon ingestion of environmental oocysts or tissue cysts, the infective life stages within these structures are released into the intestinal lumen and subsequently invade enterocytes (3,4). Bradyzoites are released from tissue cysts, and sporozoites are released from oocysts. In the felid definitive host only, bradyzoites undergo numerous stages of asexual and sexual development and environmentally resistant unsporulated oocysts are passed in the feces (4,5). Following 1 to 5 days in a favorable environment of warm air and moisture, oocyts sporulate and become infective for the next host (6). In all hosts, including felids, sporozoites undergo a similar process of asexual replication and produce numerous infective tachyzoites (3–5). Tachyzoites are capable of rapid intracellular replication causing necrosis of infected tissues, and can disseminate via blood or lymph during active infection (7).

Feline toxoplasmosis is most often subclinical, but systemic infections do occur and can be fatal (2,7–9). Antemortem diagnosis is challenging as clinical signs are generally nonspecific and the disseminated form of the disease can be rapidly progressive (2). Affected cats may succumb to infection with T. gondii without any premonitory signs other than depression, lethargy, inappetence, and persistent pyrexia (1). Pneumonia is the principal lesion in feline toxoplasmosis (10) and was documented in all fatal cases observed in a previous case series (2). Other manifestations include hepatitis, pancreatic necrosis, myositis, myocarditis, uveitis, dermatitis, and encephalitis (1). Transmission of the parasite in cats is more efficient through ingestion of tissue cysts compared with ingestion of oocysts (5), and the congenital form of the disease generally leads to more severe clinical rather than subclinical presentations (3,4). In addition, concurrent infection with feline immunodeficiency virus (FIV), feline leukemia virus (FeLV), or feline infectious peritonitis (FIP) virus may complicate infection with T. gondii and possibly activate latent infections (3,5,7). Infected cats undergoing treatment with immunosuppressive drugs such as cyclosporine appear to be at increased risk of developing fatal systemic toxoplasmosis (11,12). The purpose of this case report is to highlight the challenges faced with ante-mortem diagnosis of feline clinical toxoplasmosis.

Case description

A 22-month-old indoor/outdoor 4.68-kg neutered male domestic short-haired cat was presented with a 2-week history of progressive lethargy, vomiting, and decreased appetite. On presentation, the cat had a rectal temperature of 39.9°C and a body condition score of 3/9 (WSAVA Guidelines). Mild, generalized discomfort was noted on abdominal palpation but no masses were palpated. The physical examination was otherwise unremarkable. Serum biochemical analysis (Abaxis Veterinary Diagnostics; Union City, California, USA) revealed a moderate increase in alanine aminotransferase [ALT; 279 U/L; reference interval (RI): 20 to 100 U/L], a mild increase in amylase (1333 U/L, RI: 300 to 1100 U/L) and a moderate increase in total bilirubin (19 μmol/L; RI: 2 to 10 μmol/L). Complete blood (cell) count (CBC) analysis (Abaxis Veterinary Diagnostics) revealed no abnormalities other than a marginal decrease in mean corpuscular volume (37.0 fl; RI: 39 to 50 fl) and mean corpuscular hemoglobin (12.4 pg; RI: 12.5 to 17.5 pg). An FIV/FeLV SNAP test (IDEXX Laboratories, Markham, Ontario) was negative and the feline pancreatic lipase (FPL) SNAP test (IDEXX Laboratories) was within the reference range. Differential diagnoses following initial hematological analysis included FIP, hepatitis, cholangiohepatitis, and sepsis. An abdominal ultrasound was performed and an irregular hyperechoic mass was detected in the midabdomen, consistent with enlarged lymph nodes or neoplasia. Neoplasia was added to the list of differential diagnoses.

The cat was hospitalized and treated symptomatically with IV Lactated Ringer’s Solution at 20 mL/h, maropirant (Cerenia; Zoetis, Kirkland, Quebec), 10 mg/kg body weight (BW), subcutaneously (SQ), and buprenorphine (Vetergesic Multidose; Reckitt Benckiser, Mississauga, Ontario), 15 μg/kg BW, PO buccally. Mirtazapine (Apotex, Toronto, Ontario), 3.75 mg, was given orally to stimulate the cat’s appetite. Since systemic or localized bacterial infection had not yet been ruled out, an injection of cefovecin (Convenia; Zoetis), 10 mg/kg BW, SQ, was given. The day following presentation, the cat remained pyrexic and became anorexic. Meloxicam (Metacam; Boehringer Ingelheim, Burlington, Ontario), 0.1 mg/kg BW, SQ, was administered once and resulted in a transient decrease in temperature. Due to concerns of sepsis causing the persisting fever, amoxicillin-clavulanic acid (Clavaseptin; Vétoquinol, Lavaltrie, Quebec), 62.5 mg, PO, was added to the treatment regimen 2 d after initial presentation. Rectal temperature increased to 40.5°C. Three days after presentation, enrofloxacin (Baytril; Bayer Animal Health, Mississauga, Ontario), 5 mg/kg BW, IM and the anti-inflammatory agent dexamethasone (Vétoquinol), 0.15 mg/kg BW, IV, were administered. Repeat CBC and serum biochemistry analysis were also performed. Serum biochemistry at this time revealed a marked increase in ALT (1523 U/L), a marked increase in total bilirubin (34 μmol/L) as well as a mild decrease in Na⁺ (139 mmol/L; RI: 142 to 164 mmol/L), K⁺ (3.1 mmol/L; RI: 3.7 to 5.8 mmol/L) and total proteins (52 g/L; RI: 54 to 82 g/L). Hematological analysis reported only a mild decrease in mean corpuscular hemoglobin (12 pg) and a slight increase in mean platelet volume (15.2 fl; RI: 6.5 to 15 fl). On evaluation of a blood smear there was a mild (< 10%) polychromasia, a moderate (50%) anisocytosis and a marked poikilocytosis (75% acanthocytes). Platelet numbers were normal (8 to 10/oil immersion field); however, all platelets observed were macrothrombocytes. In addition, a left shift was noted on manual differential white blood cell (WBC) counts. At this point treatment with clindamycin (Apo-Clindamycin; Apotex), 12 mg/kg BW, PO, was added. The next morning the cat’s condition had deteriorated and the cat was euthanized.

On necropsy, the liver was pale with an accentuated lobular pattern and the mesenteric lymph nodes were grossly enlarged. The omental fat of the jejunal arcade appeared mildly icteric (Figure 1) and the thoracic cavity was filled with about 6 mL of a clear straw-colored fluid.

Figure 1.

Postmortem examination of the cat affected by fatal systemic toxoplasmosis revealed enlarged mesenteric lymph nodes (A — arrowhead) and mild icterus of the omental fat (A). The liver was pale with an accentuated lobular pattern (B).

Samples of the liver, small intestine, mesenteric lymph nodes, and pancreas were submitted for histopathology at the Animal Health Laboratory (AHL; Guelph, Ontario). Thick sections (5 μm) were stained with routine hematoxylin and eosin (H&E) staining methods. Histologically, within the liver, there was multifocal-to-regionally extensive hepatocellular necrosis accompanied by significant hemorrhage as well as accumulation of fibrin, chromatin, debris, and macrophages. Some hepatocytes contained multiple 3 to 4 μm diameter round-elongate basophilic bodies within their cytoplasm, resembling protozoan parasites. In addition, the mesenteric lymph nodes were extensively necrotic with few remaining resident lymphocytes and macrophages. Similar basophilic bodies were noted in the cytoplasm of extracapsular macrophages. In the small intestine, crypts were elongated and lined by flattened, jumbled, and basophilic enterocytes. Surface enterocytes were similar in morphology, some with erosion and adhesion of villi. The lamina propria contained increased numbers of plasma cells and macrophages, and occasional cells in the lamina propria contained similar basophilic bodies.

Immunohistochemistry for Toxoplasma gondii was performed at the AHL: 4-μm thick sections were collected from the original blocks, subjected to heat-induced epitope retrieval (pH 8), treated with a rabbit polyclonal antibody against T. gondii (Biogenex, Fremont, California, USA), a goat anti-rabbit horse-radish peroxidase-labeled polymer secondary antibody, DAB chromagen (Ventana, Tucson, Arizona, USA), and counter-stained with hematoxylin. As shown in Figure 2, the liver sample was immunoreactive for Toxoplasma antigen. To rule out concurrent infection with feline coronavirus, immunohistochemistry was performed: 4-μm thick sections were collected from the original blocks, treated with protease 1 (Ventana) antigen retrieval, a monoclonal antibody against feline coronavirus (Custom Monoclonals International, Sacramento, California, USA), a goat anti-rabbit alkaline phosphatase-labeled polymer secondary antibody, Fast Red chromagen (Ventana), and counterstained with hematoxylin. No significant immunoreactivity was identified in sections of mesenteric lymph node.

Figure 2.

A — Hematoxylin and eosin stained section of liver tissue showing necrosis of hepatocytes and intracytoplasmic basophilic bodies (arrowhead) resembling protozoa. B — Immunohistochemical staining of Toxoplasma gondii antigen showing a positive (brown) signal accentuating individual small zoites (asterisks) and a larger cyst-like structure (arrowhead) in the field shown.

Discussion

Antemortem diagnosis of feline clinical toxoplasmosis is difficult and relies on Toxoplasma antibody measurements (especially IgM) and/or clinical response to treatment effective against T. gondii infection, usually clindamycin. Diagnostic confirmation requires detection of tachyzoites, either by cytology or polymerase chain reaction (PCR) of tissue samples or body fluids (3,4). Histopathology and immunohistochemistry confirmed the presence of T. gondii structures postmortem in this cat.

Feline clinical toxoplasmosis was not included in the initial list of differential diagnoses in this case, although it should have been. Feline infectious peritonitis was included due to the young age of the cat, the nonspecific clinical signs including unexplained pyrexia, and consideration of a previous case report of FIP presenting with a massive enlargement of a mesenteric lymph node (13). Although FIP is an appropriate differential diagnosis, it should have been considered the least likely. The highest prevalence of FIP is among purebred cats, usually in multicat environments or catteries. Hematology and biochemistry in these cases typically reveal a lymphopenia and hyperglobulinemia (14), both of which were not reported in this case. The increase in total bilirubin is the one clinicopathologial finding most suggestive of FIP (15). While there are limitations to feline coronavirus serology, a negative antibody titer could have ruled out FIP early on in this case (14). Sepsis, similarly, should not have been considered a likely differential diagnosis. There was no defined focus of infection identified on ultrasound, no characteristic leukocyte changes, and no consistent clinical signs (e.g., hypoglycemia) other than pyrexia (16).

An earlier tentative antemortem diagnosis of toxoplasmosis could have also been based on detection of antibodies; however, Toxoplasma antibodies can be found in both healthy and sick cats (4). A high IgM antibody titer or a 4-fold change in IgG titer is most consistent with clinical toxoplasmosis (5); however, positive serology must always be combined with the presentation of clinical signs for a more reliable diagnosis (6). Nonetheless, serology is a noninvasive and useful tool and could have led to earlier and more appropriate therapy in this cat. The use of dexamethasone also likely worsened the cat’s condition (8), which could have been prevented had toxoplasmosis been suspected and FIP ruled out upon initial hospitalization. Antemortem fecal detection of toxoplasmosis is generally not reliable as oocysts closely resemble the oocysts of other coccidian species and definitive hosts only shed for a short time (1 to 2 wk). During this period of oocyst shedding the host typically does not display clinical signs, further limiting the usefulness of fecal testing during active infection (4,5).

Hematological and biochemistry parameters during the acute phase of clinical toxoplasmosis are nonspecific and may include a nonregenerative anemia, neutrophilic leukocytosis, lymphocytosis, monocytosis, and eosinophilia, in addition to a hypoproteinemia and hypoalbuminemia (6). The results of a cross-sectional epidemiological study and case series performed in Finland revealed that marked elevations in the liver enzyme alanine aminotransferase (ALT) > 1000 U/L is a consistent finding in cats with generalized toxoplasmosis (2). The ALT value increased to 1523 U/L from 279 U/L 3 days after presentation in this case, leading the clinician to eventually suspect toxoplasmosis and initiate clindamycin therapy late in the course of the disease. Clindamycin was the fourth antibiotic used in this case and unfortunately was introduced too late to save the cat’s life.

The histopathological lesions of necrotizing hepatitis, lymphadenitis, and enteritis are consistent with most cases of fatal systemic or extraintestinal toxoplasmosis (8). However, since pneumonia is a known principal lesion in clinical feline infections (9) and neurological manifestations are common (3,4), lung and brain samples should also have been collected in this case. Pulmonary involvement is suspected, given the pleural effusion detected on postmortem evaluation and the extent of dissemination observed. Fine-needle aspiration and cytological examination of the enlarged mesenteric lymph nodes could have revealed tachyzoites as documented in a previous case report (9), leading to a presumptive diagnosis of toxoplamosis ante-mortem. Thoracic and peritoneal effusions are, however, more commonly rewarding during acute disease (4). Thoracic radiographs could have served as an additional aid in diagnosis since a diffuse interstitial to alveolar pattern with a mottled lobar distribution is a well-documented finding in acute toxoplasmosis with pulmonary involvement (6). This cat did not display any respiratory signs to prompt thoracic radiographs, but imaging would have revealed the thoracic fluid suitable for sampling and should have been considered in this case.

From 2008 to 2014, the Animal Health Laboratory (Guelph, Ontario) saw 9 cases of feline extraintestinal toxoplasmosis, including the case herein. Interestingly, 8 of these cases were in young male domestic cats (17). This is the first report of fatal disseminated toxoplasmosis in a young, apparently immunocompetent cat, presenting with enlarged mesenteric lymph nodes. Since this cat was known to be a hunter, a newly acquired T. gondii infection was likely the source of tachyzoite dissemination. Reactivation of a latent tissue cyst by immunosuppression is also possible (3,4), and a concurrent undiagnosed condition leading to an immunocompromised state cannot be ruled out.

The purpose of this case report is to demonstrate that disseminated toxoplasmosis can be commonly misdiagnosed. Toxoplasmosis should be considered as a differential diagnosis in cats with consistent clinical signs and appropriate therapy should be instituted. Pet owners should be made aware of both the public health risk and the feline health risk associated with feline toxoplasmosis. The infection is largely preventable by not feeding raw meat to cats and by keeping them indoors (2,3,5). Although the disease can be rapidly fatal, treatment, if instigated early enough, is often successful (4).