Characterization of and factors associated with causes of pleural effusion in cats

Marina Domínguez Ruiz Veterinary Hospital Center Frégis, 43 Ave Aristide Briand, 94110 Arcueil, France

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Florence Vessières Veterinary Hospital Center Frégis, 43 Ave Aristide Briand, 94110 Arcueil, France

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Guillaume R. Ragetly Veterinary Hospital Center Frégis, 43 Ave Aristide Briand, 94110 Arcueil, France

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Juan L. Hernandez Veterinary Hospital Center Frégis, 43 Ave Aristide Briand, 94110 Arcueil, France

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Abstract

OBJECTIVE To characterize and investigate potential associations between causes of pleural effusion and various clinical factors in a large cohort of affected cats.

DESIGN Retrospective case series with nested cross-sectional study.

ANIMALS 380 client-owned cats with a diagnosis of pleural effusion from January 1, 2009, through July 14, 2014, for which the cause of pleural effusion had been fully investigated.

PROCEDURES Electronic medical records were reviewed and data collected regarding cat characteristics, clinical signs, cause of pleural effusion, treatment, and survival status at discharge from the hospital. Variables were examined for associations with causes of pleural effusion.

RESULTS 87 (22.9%) cats died or were euthanized before discharge from the hospital. Congestive heart failure (CHF) was the most common cause (155 [40.8%]) of pleural effusion, followed by neoplasia (98 [25.8%]). Other causes included pyothorax, idiopathic chylothorax, trauma, feline infectious peritonitis, and nontraumatic diaphragmatic hernia. Cats with trauma or feline infectious peritonitis were significantly younger than those with CHF or neoplasia. Cats with lymphoma were significantly younger than those with carcinoma. Cats with CHF had a significantly lower rectal temperature at hospital admission (mean ± SD, 36.9 ± 1.2°C [98.4 ± 2.2°F]) than did cats with pleural effusion from other causes (37.9 ± 1.2°C [100.2 ± 2.2°F]).

CONCLUSIONS AND CLINICAL RELEVANCE Cats with pleural effusion in this study had a poor prognosis; CHF and neoplasia were common causes. Age and hypothermia may be helpful to raise the index of suspicion for certain underlying causes of pleural effusion in cats.

Abstract

OBJECTIVE To characterize and investigate potential associations between causes of pleural effusion and various clinical factors in a large cohort of affected cats.

DESIGN Retrospective case series with nested cross-sectional study.

ANIMALS 380 client-owned cats with a diagnosis of pleural effusion from January 1, 2009, through July 14, 2014, for which the cause of pleural effusion had been fully investigated.

PROCEDURES Electronic medical records were reviewed and data collected regarding cat characteristics, clinical signs, cause of pleural effusion, treatment, and survival status at discharge from the hospital. Variables were examined for associations with causes of pleural effusion.

RESULTS 87 (22.9%) cats died or were euthanized before discharge from the hospital. Congestive heart failure (CHF) was the most common cause (155 [40.8%]) of pleural effusion, followed by neoplasia (98 [25.8%]). Other causes included pyothorax, idiopathic chylothorax, trauma, feline infectious peritonitis, and nontraumatic diaphragmatic hernia. Cats with trauma or feline infectious peritonitis were significantly younger than those with CHF or neoplasia. Cats with lymphoma were significantly younger than those with carcinoma. Cats with CHF had a significantly lower rectal temperature at hospital admission (mean ± SD, 36.9 ± 1.2°C [98.4 ± 2.2°F]) than did cats with pleural effusion from other causes (37.9 ± 1.2°C [100.2 ± 2.2°F]).

CONCLUSIONS AND CLINICAL RELEVANCE Cats with pleural effusion in this study had a poor prognosis; CHF and neoplasia were common causes. Age and hypothermia may be helpful to raise the index of suspicion for certain underlying causes of pleural effusion in cats.

In healthy dogs and cats, a small volume of fluid (approx 0.1 and 0.3 mL/kg, respectively) within the pleural space lubricates and couples the visceral and parietal pleura to one another, thereby preventing the lungs from collapsing.1–4 Pleural effusion occurs when an underlying disease upsets the normal homeostatic mechanisms within the thorax and allows an abnormal volume of fluid to accumulate in the pleural space.2,3,5

Common types of pleural effusion in cats include transudates (secondary to hypoalbuminemia, fluid overload, or CHF), modified transudates (resulting from CHF or various other obstructive or inflammatory processes), exudates (associated with septic or sterile pyothorax), chylous effusions (idiopathic or secondary to neoplasia, lung lobe torsion, or CHF), hemorrhagic effusions (resulting from trauma, coagulopathies, or vessel erosion), and neoplastic effusions.1–5

Congestive heart failure, neoplasia, pyothorax, and FIP are reportedly responsible for 88% to 100% of cases of pleural effusion in cats.3,5–8 Uncommon causes include peritoneopericardial diaphragmatic hernia, uremia, pulmonary thromboembolism, lung lobe torsion, extension from a perinephric pseudocyst, pancreatitis, glomerulonephropathies, and Aelurostrongylus abstrusus infection.3,5 A retrospective study5 involving 82 cats with pleural effusion revealed high prevalences of intrathoracic neoplasia (28%; 17% of which were mediastinal lymphoma), pyothorax (18%), FIP (18%), and hypertrophic cardiomyopathy (10%). However, other sources3,9 suggest a higher prevalence of CHF-related pleural effusion and lower prevalence of pleural effusion secondary to mediastinal lymphoma.

Given the many causes of pleural effusion, an analysis of the associations between clinical signs at hospital admission and the underlying causes could be useful to guide clinicians in decision-making when treating cats with pleural effusion. The purpose of the study reported here was to use existing medical record data to investigate potential associations between causes of pleural effusion and clinical characteristics of a large cohort of affected cats. We hypothesized that the most common causes of pleural effusion would be CHF and neoplasia and that cats with CHF would have a lower rectal temperature at hospital admission than cats with noncardiogenic causes of pleural effusion.

Materials and Methods

Case selection criteria

Electronic veterinary medical records were reviewed to identify all cats evaluated at the Veterinary Hospital Center Frégis between January 1, 2009, and July 14, 2014, that had a radiographic-, ultrasonographic-, or CT-confirmed diagnosis of pleural effusion during this period. Most of these cats had been referred for investigation of pleural effusion. Cats lacking comprehensive investigation of the cause of the pleural effusion, as noted in the medical records, were excluded from the study.

Medical records review

Information was collected from the medical records regarding cat characteristics (age at hospital admission, breed, sex, and neuter status), vaccination status, outdoor exposure status (kept indoors vs allowed outdoors), chief clinical signs at or reason for hospital admission, infectious disease status (per results of serologic or PCR tests for FIV infection, FeLV infection, or FIP), physical examination findings (eg, rectal temperature, heart rate, and respiratory rate) at admission, CBC and serum biochemical values at admission, pleural effusion analysis results (biochemical, cytologic, bacterial culture, and RT-PCR assay findings for FIP status), diagnostic procedures performed (thoracic radiography ultrasonography, and thoracic CT), surgical procedures performed, final diagnosis, treatments administered and response, and outcome. Hypothermia was defined as a rectal temperature < 38°C (< 100.4°F; reference range, 38° to 39°C [100.4° to 102.2°F]). Information on underlying cause was obtained from the medical record.

Procedures

Pleural fluid was classified by the investigators as transudate, modified transudate, or exudate on the basis of the total protein concentration, total nucleated cell count, and cell type or types (Appendix).1,2,4 Effusions were deemed protein rich when the total protein concentration was ≥ 2.5 g/dL and were further characterized on the basis of findings of cytologic evaluation and biochemical analysis.1,2,4 Septic inflammatory exudates (pyothorax) were characterized by the presence of degenerate neutrophils with phagocytized intracellular bacteria on cytologic evaluation or by bacterial growth on aerobic or anaerobic bacterial culture of pleural fluid.1–5 Effusions were deemed neoplastic when cytologic evaluation revealed cells with signs of malignancy.1–4 Chylothorax was diagnosed when effusions had a white or pink opaque appearance and contained a triglycerides concentration > 100 mg/dL.1–3,10 Effusions were considered cardiogenic in nature when they had a serosanguineous, white, pink, opaque or translucent appearance, and clinically important cardiomegaly or left atrial dilation was identified via thoracic radiography or echocardiography, respectively.

A board-certified veterinary diagnostic imaging specialist had previously performed and interpreted the results of thoracic radiography, ultrasonographic examination (thoracic and abdominal), and thoracic CT examination. A board-certified veterinary internal medicine specialist had previously performed and interpreted the results of the echocardiographic examinations. A board-certified veterinary clinical pathologist or a board-certified pathologist had previously performed and interpreted the results of cytologic and histologic evaluation, respectively. Reports from these specialists were reviewed and data extracted.

Feline infectious peritonitis was suspected when a cat had a protein-rich exudate plus at least one of the following: positive results of RT-PCR assay testing for feline coronavirus in pleural fluid samples, positive results of serologic testing for antibody against feline coronavirus, bicavitary effusion (thoracic and abdominal), hyperglobulinemia, or hyperthermia. Cats were serologically tested for anti-FIV antibody and FeLV antigen by means of a commercial test.a Because no FIV vaccine is available in Europe, a positive FIV result could not have been attributable to vaccination-induced antibody.

Statistical analysis

Statistical analyses were performed with statistical software.b,c Normality of data distribution for continuous variables was evaluated with the Shapiro-Wilk test. We elected to perform selected statistical analysis to avoid the risk of a type I error and limit nonrelevant findings. Analysis of variance followed by the Scheffe test for post hoc comparisons was used to investigate possible associations between age and cause of pleural effusion. Rectal temperature at hospital admission was compared between cats with and without CHF and age was compared between cats with carcinoma and cats with lymphoma with the Student t test. The χ2 test was used to compare proportions of Maine Coons with pyothorax versus cardiac disease, Birmans with versus without FIP, and Siamese with versus without tumors. The Pearson χ2 test of independence was used to determine whether sex, neuter status, or unilateral (vs bilateral) disease was associated with cause of pleural effusion. For all analyses, values of P ≤ 0.05 were considered significant.

Results

Cats

Four hundred sixty-five cats with pleural effusion were identified in the electronic veterinary medical database. Eighty-five (18.3%) cats were excluded from the study because the cause of the pleural effusion had not been investigated (via cytologic evaluation of pleural fluid or diagnostic imaging). The remaining 380 cats were included in the study.

Of the included cats, 191 (50.3%) were castrated males, 37 (9.7%) were sexually intact males, 124 (32.6%) were spayed females, and 28 (7.4%) were sexually intact females. Thirteen breeds were represented, including domestic shorthair (n = 321 [84.5%]), Birman (13 [34%]), Maine Coon (11 [2.9%]), Siamese (9 [2.4%]), Chartreux (8 [2.1%]), Persian (5 [1.3%]), British Short-hair (3 [0.8%]), Norwegian Forest Cat (3 [0.8%]), Abyssinian (2 [0.5%]), Turkish Angora (2 [0.5%]), Balinese (1 [0.3%]), Egyptian Mau (1 [0.3%]), and Sphinx (1 [0.3%]). Median age at hospital admission was 9 years (range, 6 months to 21 years; mean ± SD, 8.7 ± 4.8 years).

Outdoor exposure status was recorded for 233 (61.3%) cats, of which 119 (51.1%) were kept indoors and 114 (48.9%) were allowed outdoors. Vaccination status was recorded for 134 (35.3%) cats, of which 92 (68.7%) were fully vaccinated (against feline panleukopenia virus, herpesvirus, calicivirus, rabies virus, and leukemia virus) and 42 (31.3%) were unvaccinated or partially vaccinated.

Chief clinical signs

Chief clinical signs at or reasons for hospital admission were recorded for all 380 cats. Signs of difficulty breathing were reported by owners for 172 (45.3%) cats, anorexia or dysorexia for 76 (20.0%), lethargy for 41 (10.8%), coughing for 17 (4.5%), vomiting for 16 (4.2%), hind limb paresis or paralysis for 16 (4.2%), lateral recumbency for 7 (1.8%), road traffic accident for 6 (1.6%), and fall from heights for 5 (1.3%). Other miscellaneous clinical signs or reasons were reported for 24 (6.3%) cats.

Physical examination findings

Physical examination findings were recorded for all 380 cats, of which 277 (72.9%) had respiratory distress. Rectal temperature at hospital admission was recorded for 254 (66.8%) cats, with a mean ± SD value of 37.7 ± 1.6°C (99.9 ± 2.9°F). Hypothermia was identified in 124 (48.8%) cats with a recorded rectal temperature, of which 67 (54.0%) had CHF.

Diagnostic imaging results

Thoracic radiograph reports were available for 304 (80.0%) cats. Pleural effusion was identified in 299 (98.4%) of these cats, of which 275 (92.0%) had bilateral effusion and 24 (8.0%) had unilateral effusion. Abnormalities of the lungs were identified in 38 (12.5%) cats and included a diffuse alveolar pattern, bronchial and peribronchial pattern, nodular pattern, pulmonary mineralization, lobar atelectasis, and pulmonary mass. Other thoracic abnormalities included cardiomegaly (n = 16 [5.3%]), diaphragmatic hernia (9 [3.0%]), thoracic mass (6 [2.0%]), and pneumothorax (1 [0.3%]). In 5 (1.6%) cats, no pleural effusion was visible on thoracic radiographs.

Thoracic ultrasonography was performed for 128 (337%) cats, and pleural effusion was confirmed in all. Five of these cats had no pleural effusion identified via radiography. The pleural effusion was bilateral in 119 (93.0%) cats and unilateral in 9 (7.0%) cats. Thoracic masses were identified in 32 (25.0%) ultra-sonographically imaged cats.

Echocardiographic findings were recorded for 183 (48.2%) cats, of which 127 (69.4%) had visible pleural effusion and 21 (11.5%) had pericardial effusion. Congestive heart failure was considered the cause of pericardial effusion in 18 (86%) affected cats. Heart disease was considered the cause of pleural effusion in 141 (77.0%) echocardiographically imaged cats. Three (1.6%) cats had mild heart disease identified that could not explain the origin of the pleural effusion. Hypertrophic cardiomyopathy or left ventricular hypertrophy secondary to hyperthyroidism was the most common form of heart disease (n = 72 [50.7%]), followed by unclassified restrictive cardiomyopathy (53 [373%]). Dilated cardiomyopathy (8 [5.6%]) was less common. Other cardiac abnormalities identified via echocardiography included arrhythmogenic right ventricular cardiomyopathy (n = 4 [2.8%]) and congenital cardiac defect (4 [2.8%]).

Abdominal ultrasonography was performed for 70 (18.4%) cats, and abdominal effusion was identified in 41 (59%) of these cats. Ultrasonographically identified abnormalities included abdominal masses (n = 10 [14%]), lymphadenopathy (10 [14%]), hepatic venous congestion (5 [7%]), hepatomegaly (4 [6%]), and splenomegaly (3 [4%]). Other abdominal abnormalities included bilateral renal enlargement, small intestine wall thickening, generalized steatitis, mesenteric nodules, and signs of pancreatitis.

Thoracic CT was performed for 8 (2.1%) cats, for which neither thoracic radiography nor thoracic ultrasonography allowed identification of the cause of pleural effusion. Pleural effusion appeared bilateral via thoracic CT in 7 cats and unilateral in the remaining cat. Other thoracic abnormalities included diffuse pulmonary nodules, bronchiectasis, and pulmonary mineralization (n = 1); diffuse pulmonary nodules and lymphadenopathy (1); diffuse alveolar pattern (1); soft tissue mass in the left caudal lung lobe (1); and fluid-filled cavitary lesion in the right caudal lung lobe (1). No abnormalities other than pleural effusion were recorded for the remaining 3 cats.

Clinicopathologic findings

Pleural fluid samples from 199 (52.4%) cats were analyzed for total protein concentration, total nucleated cell count, and cytologic characteristics. Effusions were classified as septic exudate (n = 55 [27.6%]), neoplastic (50 [25.1%]), chylous (30 [15.1%]), sterile exudate (29 [14.6%]), transudate (23 [11.6%]), and modified transudate (12 [6.0%]).

Six of the 380 (1.6%) cats were serologically tested for FIP status; of these cats, only 3 had positive results, with high antibody titers against feline coronavirus (1:2,560, 1:2,560, and 1:1,200). Pleural fluid samples from 16 (4.2%) cats were tested for FIP status via RT-PCR assay, with positive results obtained for 9 cats. On the basis of serologic titer or virus RT-PCR detection in pleural fluid, FIP was suspected in 12 (3.2%) cats. Serum total thyroxine concentration was measured in 155 (40.8%) cats, revealing hyperthyroidism in 20 of 155 (12.9%) cats with CHF and 20 of 73 (27%) cats with left ventricular hypertrophy specifically.

The FIV and FeLV status were recorded for 66 (17.4%) cats. Five (8%) of these cats were seropositive for FIV antibody and 5 (8%) cats were seropositive for FeLV antigen. No cats had positive results for both FIV and FeLV testing. Of cats tested for FIV antibody seropositivity was identified in 2 cats with CHF, 2 cats with pyothorax, and 1 cat admitted for trauma. Of the cats tested for FeLV antigen, seropositivity was identified in 4 cats with neoplasia (all of which had mediastinal lymphoma) and 1 cat with CHF.

Thoracoscopy and thoracotomy

Thoracoscopy was performed for 4 (1.1%) cats for diagnostic purposes. Thoracotomy enabled a diagnostic or treatment procedure for 22 (5.8%) cats. Histologic evaluation of samples obtained via thoracoscopy or thoracotomy was performed for 6 cats, and diagnoses included viral pneumonia (n = 1), chronic bronchitis (1), mediastinal lymphoma (1), mesothelioma (1), vasculitis (1), and fibrinonecrotic pleuropneumonia (1).

Cause of pleural effusion

The cause of the pleural effusion was determined for all 380 cats. The 2 most common causes were CHF (n = 155 [40.8%]) and neoplasia (98 [25.8%]). Other causes included pyothorax (55 [14.5%]), idiopathic chylothorax (24 [6.3%]), trauma (16 [4.2%]), FIP (12 [3.2%]), nontraumatic diaphragmatic hernia (8 [2.1%]), suspected vasculopathy (5 [1.3%]), suspected uremic pleuritis (3 [0.8%]), hypoproteinemia (2 [0.5%]), and vitamin K antagonist toxicosis (2 [0.5%]). Congestive heart failure was diagnosed on the basis of results of pleural fluid analysis and echocardiography for 142 of 155 (91.6%) affected cats and results of pleural fluid analysis and thoracic radiography for 13 (8.4%) affected cats. Neoplasia was diagnosed on the basis of results of cytologic pleural fluid evaluation for 52 of 98 (53.0%) affected cats, fine needle aspiration and cytologic evaluation of a mass or organ for 44 (44.9%) cats, and histologic evaluation for 2 (2.0%) cats. Type of tumor was identified in all situations and included lymphoma (n = 50 [51.0%]), carcinoma (41 [41.8%]), sarcoma (3 [3.1%]), mesothelioma (3 [3.1%]), and mast cell tumor (1 [1.0%]).

Associations with causes of pleural effusion

Cats with trauma (mean ± SD age, 3.2 ± 3.2 years) or FIP (2.1 ± 1.8 years) were significantly (P < 0.001) younger than those with CHF (9.6 ± 4.7 years) or neoplasia (10.2 ± 4.4 years; Figure 1). Cats with CHF were slightly but significantly (P < 0.001) younger than those with neoplasia. Cats with lymphoma (mean ± SD age, 8.3 ± 4.4 years) were significantly (P < 0.001) younger than those with carcinoma (11.8 ± 3.4 years). Cats with CHF (mean ± SD age, 9.6 ± 4.7 years) or nonlymphoma neoplasia (12.1 ± 34 years) were generally older than those with pleural effusion attributable to other causes, except for suspected uremic pleuritis (10.7 ± 3.2 years) and idiopathic chylothorax (10.3 ± 4.5 years).

Figure 1—
Figure 1—

Mean age of cats with pleural effusion categorized by whether the underlying cause was CHF (n = 155), neoplasia (98), pyothorax (55), idiopathic chylothorax (24), trauma (16), FIP (12), nontraumatic diaphragmatic hernia (8), suspected vasculopathy (5), or suspected uremic pleuritis (3). Error bars represent SD. *†‡Values with different symbols differ significantly (P < 0.001). Values with no symbols do not differ significantly from any other value.

Citation: Journal of the American Veterinary Medical Association 253, 2; 10.2460/javma.253.2.181

Compared with the distribution for all other causes of pleural effusion combined (116/228 [50.9%]), males were significantly (P = 0.01) overrepresented among cats with CHF (112/155 [72.3%]). No association was identified between neuter status and cause of pleural effusion. Cats with CHF had a significantly (P < 0.001) lower rectal temperature at hospital admission (mean ± SD, 36.9 ± 1.2°C [98.4 ± 2.2°F]) than those with pleural effusion due to other causes (37.9 ± 1.2°C [100.2 ± 2.2°F]). The proportion of cats with bilateral pleural effusion was significantly (P < 0.001) larger for those with pyothorax (42/55 [76.4%]) than for others (260/302 [86.1%]).

Although a high proportion (7/11 [64%]) of Maine Coons had cardiomyopathy, this proportion was not significantly (P = 0.19) greater than that for all other breeds combined (148/155 [95.5%]). In addition, a high proportion of Maine Coons had pyothorax (4/11 [36.4%]) relative to other breeds (51/55 [92.7%]); however, the difference was not significant (P = 0.059 with Yates correction). Although a high proportion of Siamese had neoplasia (3/9 [33.3%]), this proportion did not differ significantly (P = 0.31) from the proportion for all other breeds combined (95/98 [96.9%]). When the association between breed and all causes of pleural effusion was investigated, a significant (P = 0.001) association was identified between the Birman breed and FIP (3/13 [23.1%]).

Treatment

Of the 155 cats with CHF, 97 (62.6%) received some of the following medications: furosemide, angiotensin-converting-enzyme inhibitors, clopidogrel, diltiazem, pimobendan, spironolactone, β-adrenoceptor antagonists, aspirin, nitric oxide, heparin, amlodipine, morphine, digoxin, altizide, carbimazole. Fifty-eight (37.4%) of these cats also had thoracentesis performed.

Of the 98 cats with neoplasia, 75 (76.5%) were treated with thoracentesis and a medical approach (27 received glucocorticoid treatment and 13 received multidrug chemotherapy involving cyclophosphamide, vincristine, doxorubicin, and prednisolone). Eleven (11.2%) cats were treated only medically (8 cats received glucocorticoid treatment, 2 cats received chemotherapy, and 1 cat received NSAIDs). Two (2.0%) cats had bilateral thoracostomy tubes placed, 1 (1.0%) cat underwent exploratory surgery, and 1 (1.0%) cat underwent exploratory thoracoscopy. Ten (10.2%) cats received no treatment.

Of the 55 cats with pyothorax, 28 (51%) received thoracostomy tube drainage (22 bilaterally and 6 unilaterally). Six (13%) cats underwent exploratory surgery when they failed to respond to drainage (n = 3) or a thoracic lesion was identified via imaging (3). All these cats subsequently underwent lung lobectomy. Twenty-seven (49%) cats were treated solely by thoracentesis and antimicrobials (amoxicillin and metronidazole).

Of the 24 cats with idiopathic chylothorax, 15 (62%) underwent thoracentesis and received medical treatment (prednisolone and a low-fat diet), and 9 (38%) underwent surgical treatment (thoracic duct ligation, pericardectomy, and pleural omentalization). Thoracostomy tubes were placed to provide drainage in 8 of the 9 cats following surgery. Of the 16 cats with trauma, 12 received supportive care (IV fluid therapy, analgesia, and supplemental oxygen), 2 were treated with thoracentesis alone, and 1 underwent thoracentesis followed by surgical treatment to repair a diaphragmatic hernia.

Of the 12 cats with FIP, 11 were treated with thoracentesis and glucocorticoid drugs, and 1 was treated with interferon-ω. Of the 8 cats with a nontraumatic diaphragmatic hernia, 3 were treated with thoracentesis and supportive care, and 5 underwent surgical treatment of the hernia. All 3 cats with suspected uremic pleuritis were treated with thoracentesis, of which 2 also received glucocorticoid drugs. Of the 5 cats with vasculopathy, 3 were treated with thoracentesis, and 2 received diuretic and glucocorticoid drugs. Both cats with hypoproteinemia were treated with thoracentesis, of which 1 also received glucocorticoid drugs. Both cats with vitamin K antagonist toxicosis received vitamin K1 treatment, 1 was also treated with thoracentesis, and neither received blood products. The size of the surgically treated group was too small to allow statistical comparisons with the medically treated group.

Outcome

Overall, 22.9% (87/380) cats failed to survive to discharge from the hospital. Mortality rates for cats by cause of pleural effusion were as follows: vasculopathy, 40% (2/5); FIP, 33% (4/12); suspected uremic pleuritis, 33% (1/3); nontraumatic diaphragmatic hernia, 25% (2/8); neoplasia, 24% (23/98); CHF, 23% (35/155); pyothorax, 22% (12/55); trauma, 19% (3/16); idiopathic chylothorax, 17% (4/24); and vitamin K antagonist toxicosis, 0% (0/2).

Discussion

Findings of the present study suggested that cats with pleural effusion had a poor prognosis, given that 22.9% of cats died or were euthanized following diagnosis and prior to discharge from the hospital. Congestive heart failure was the most common cause of pleural effusion in this study, with a prevalence of 40.8%, followed by neoplasia (25.8%), pyothorax (14.5%), and idiopathic chylothorax (6.3%). Feline infectious peritonitis accounted for only 3.2% of cases. These findings differ from those of the previously mentioned retrospective study,5 in which CHF accounted for only 10% of cases and FIP for 18%. These differences could be attributable in part to differences in cat management practices within and outside the home.

Cats with CHF (mean ± SD age, 9.6 ± 4.7 years) or nonlymphoma neoplasia (12.1 ± 34 years) were older than those with pleural effusion attributable to other causes, with the exception of cats with suspected uremic pleuritis (10.7 ± 3.2 years) and idiopathic chylothorax (10.3 ± 4.5 years). These findings corroborate those of the previous retrospective study,5 in which cats with cardiomyopathy had a mean age of 8.1 years and those with pulmonary neoplasia had a mean of age of 14 years. Although significant in some comparisons, the differences in age between cats with CHF, neoplasia, idiopathic chylothorax, diaphragmatic hernia, suspected uremic pleuritis, vasculopathy, and pyothorax appeared too small to assist in clinical decision-making. However, the difference in mean age between cats with trauma (3.2 years) or FIP (2.1 years) and cats with CHF (9.6 years) or neoplasia (10.2 years) could be of clinical relevance. We also found that males were overrepresented among cats with CHF, compared with their distribution in cats with other causes of pleural effusion, whereas other studies5,11–15 have shown that sexually intact males are overrepresented. These results supported our supposition that sex and age may be useful in ranking differential diagnoses, but we would not recommend that clinical decisions be based on signalment data alone.

Concurrent disease may influence the development of pleural effusion. For example, hyperthyroidism was identified in 13% of cats with CHF and 25% of cats with left ventricular hypertrophy. Previous studies3,16 have highlighted the relationship between hyperthyroidism and the development of left ventricular hypertrophy or CHF.

In the present study, FeLV infection was identified in 4 of 13 cats with neoplasia that had been tested for the viral antigen. All these FeLV-positive cats had a diagnosis of mediastinal lymphoma. Interestingly, a similar connection between FeLV and mediastinal lymphoma was identified in the previous retrospective study,5 in which 62% of cats with FeLV infection had mediastinal lymphoma. Prior to the introduction of FeLV testing and vaccination control measures in the early 1980s, > 70% of cats with lymphoma were seropositive for FeLV exposure.9 The lower prevalence of positive results of FeLV testing in cats with mediastinal lymphoma (4/13) identified in this study as well as in other studies3,9 emphasized the importance of FeLV testing, quarantining of cats with unknown status, and FeLV vaccination programs. An increased risk of mediastinal lymphoma has been reported for Siamese versus other breeds, regardless of FeLV status.3,9 In the present study, 3 of 9 Siamese had thoracic neoplasia, 2 of which had mediastinal lymphoma and 1 of which had carcinoma. However, no association between neoplasia and this breed was identified (P = 0.31).

The chief clinical signs at or reasons for hospital admission in the present study were difficulty breathing and anorexia or dysorexia. Coughing was recorded for only 3.5% of cats, as corroborated by previous research.3 However, other reports3,17,18 describe coughing in up to 30% of cats with chylothorax and pyothorax. Coughing might be associated with pleural effusion, but might also be related to other abnormalities, such as concurrent bronchopulmonary infection, neoplasia, airway compression due to a cranial mediastinal mass, or cardiomyopathy.3,6 Coughing has also been associated with cardiomyopathy.14

The 2 most common clinical signs at hospital admission for cats in the study reported here were respiratory distress and hypothermia. Rectal temperature at hospital admission was significantly lower in cats with CHF-associated pleural effusion than with other causes of pleural effusion. These findings are in agreement with those of 2 other studies.11,13 Although the actual difference in rectal temperature between cats with and without CHF was too small to assist clinical decision-making, hypothermia at initial evaluation could help clinicians when ranking their list of differential diagnoses for cats with pleural effusion.

Hypothermia in cats with CHF might be related to peripheral hypoperfusion resulting from reduced cardiac output. In humans with CHF, low body temperature at hospital admission (< 36°C [< 96.8°F]) is associated with a higher mortality rate (51%) during hospitalization than the rate for humans without CHF.19 In cats with arterial thromboembolism, low rectal temperature at hospital admission is associated with a lower rate of survival to discharge.20 However, the mechanism of this association remains unknown. One possibility is that excessive secretion of angiotensin II results in hypothermia.19 The decrease in thermogenesis associated with cachexia in humans with severe CHF is another mechanism possibly underlying the association between low body temperature and worse outcomes.19 Additional research is necessary to determine whether rectal temperature at hospital admission in cats with pleural effusion is clinically relevant and useful for prognostication, as reported for humans with CHF and cats with arterial thromboembolism.19,20

A high proportion of cats with identified pericardial effusion in the present study had CHF, which is in line with previously reported findings.21,22 However, in another retrospective study,23 FIP was the most common cause of pericardial effusion.

The present study had several limitations related to its design. Owing to the retrospective approach, some information could not be collected, such as previous treatments, treatment response, and consistent follow-up information. Finally, an association between rectal temperature at admission and long-term prognosis could not be determined. Additional research is required to determine survival times after diagnosis for cats with each cause of pleural effusion and to further investigate the possible association between low rectal temperature and outcome for cats with CHF.

Acknowledgments

No financial support for this study or the associated report was received from any public, commercial, or not-for-profit funding agency. The authors declare that there were no conflicts of interest.

The authors thank Drs. Harry Swales, Rachel Rowell, and Cristoforo Rico for technical assistance.

ABBREVIATIONS

CHF

Congestive heart failure

FIP

Feline infectious peritonitis

RT-PCR

Reverse transcription polymerase chain reaction

Footnotes

a.

SNAP FIV/FeLV combo test, Idexx, Hoofddorp, The Netherlands.

b.

Systat, version 11.0, Systat Software Inc, Chicago, Ill.

c.

OpenStat, version 11.9.08, Softonic, Ames, Iowa.

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Appendix

Criteria used for classification of pleural effusion in cats on the basis of pleural fluid characteristics.

ClassificationTotal protein (g/dL)Total nucleated cells (cells/μL)Predominant cell typeTriglycerides (mg/dL)
Transudate< 2.5< 1,000Mesothelial cells and mononuclear phagocytes
Modified transudate2.5–3.51,000–7,000Mononuclear cells
Exudate> 3.0> 7,000Sterile exudate: nondegenerate neutrophils Septic (pyothorax) exudate: degenerate neutrophils

Septic (pyothorax) exudate: degenerate neutrophils
Chylous≥ 2.5≥ 1,000Small lymphocytes> 100
Neoplastic≥ 2.5≥ 1,000Neoplastic cells

= Not applicable.

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