In today’s VETgirl online veterinary CE blog, we discuss pleural effusion in cats. If you’ve heard me lecture before, I always say that the top 3 differentials for pleural effusion in cats are: CHF, CHF, and cancer. But is it true?

Cats in respiratory distress are challenging and make even specialists tachypneic! The diagnostic workup or even just the cat’s stress level from the unfamiliar environment of the hospital can send these little sympathetically overcharged creatures into crisis. Diagnostics for dyspneic animals typically includes a TFAST ultrasound/scan, therapeutic/diagnostic thoracocentesis, chest radiographs, baseline blood work (including T4, pro-BNP, etc.) and potentially echocardiography or thoracic ultrasonography. More invasive diagnostics include CT (under anesthesia) once the patient has been more stabilized. However, all of these modalities involve a bit more handling than some feline patients can tolerate in their clinical condition. Our veterinary community strives to find less invasive ways to obtain a clinical picture of pets on admission to the hospital, and to gain quick prognostic information that can help guide our conversations with pet owners before embarking on potentially costly medical assistance. For these reasons, Ruiz et. al out of France wanted to evaluate the common etiologies for cats with pleural effusion in a study called Characterization of and factors associated with causes of pleural effusion in cats. The authors retrospectively evaluated cats treated for pleural effusion to investigate any common clinical features with the various causes of feline pleural effusion.

In this study, a total of 380 medical records of cats were evaluated between 2009 and 2014. These cats were diagnosed with either unilateral or bilateral pleural effusion by way of radiographs, ultrasound, or CT imaging. Patient clinical factors investigated included age, breed, sex, neuter status, vaccination status, indoor vs. outdoor lifestyle, FIV and FeLV status, rectal temperature, heart rate, and respiratory rate. Pleural fluid was classified as transudate, modified transudate, or exudate and effusions deemed protein rich if the total protein concentration exceeded 2.5 g/dL. The pleural effusions were evaluated by cytologic and biochemical testing means to diagnose the cats with pyothorax, chlyothorax, and neoplasia. Pyothorax was diagnosed by presence of intracellular bacteria within degenerative neutrophils on cytologic evaluation or by positive aerobic or anaerobic culture. Neoplasia was diagnosed based on cytologic review of the pleural effusion. Chylothorax was diagnosed with triglyceride concentration > 100 mg/dL. FIP diagnosis was achieved by exhibiting a protein-rich exudate and clinical hyperthermia, or positive RT-PCR for coronavirus from pleural effusion, or positive serologic testing for feline coronavirus antibody. Interestingly, fluids were deemed cardiogenic in nature when they appeared serosanguineous, white, pink, opaque, or translucent and were coupled with diagnostic imaging to suggest “clinically important cardiomegaly” or left atrial dilation. The authors did not mention whether further biochemical testing was performed on the fluid to determine if cardiac abnormalities were the primary cause of the pleural effusion rather than some other type of underlying medical condition.

Of the 380 cats, 72.9% exhibited respiratory distress. Not surprisingly, the chief clinical complaint among these cats was dyspnea (45%). Other clinical complaints included anorexia (20%), lethargy (10.8%), coughing (4.5%), vomiting (4.2%), hind limb paresis or paralysis (4.2%), and lateral recumbency (1.8%). Hypothermia (defined as rectal temperature <100.4˚F/<38˚C), was identified in 66.8% of the cats for which a rectal temperature was recorded. Of the hypothermic cats, 54% (67) had CHF. Cats with CHF (mean 9.6 year) and non-lymphoma neoplasia (mean 12.1 year) were older than cats with other causes of pleural effusion except for uremic pleuritis (10.7 year) or idiopathic chylous effusion (mean 10.3 year), whereby cats with FIP or trauma were much younger (mean 2.1 year and 3.2 year respectively).

Almost all cats had thoracic radiographs taken during hospitalization. Most cats with radiographically evident pleural effusion had bilateral effusion (92%). Thoracic abnormalities seen were categorized as involving the lung (12.5%), cardiomegaly (5.3%), diaphragmatic hernia (3.0%), thoracic mass (2.0%), and pneumothorax (0.3%). Five cats without radiographic pleural effusion were later confirmed to have pleural effusion via thoracic ultrasound. Of the cats that received thoracic ultrasound, most exhibited bilateral pleural effusion (93%). Thirty-two cats that received thoracic ultrasonography were found to have thoracic masses.

Of the 183 cats that received echocardiograms, 21 had pericardial effusion with CHF as the primary cause of the pericardial effusion for 86% (18) of these cats. Heart disease was though to be the cause of pleural effusion in 141 (77%) of the 183 cats that received echocardiograms. The most commonly implicated heart diseases were HCM and left ventricular hypertrophy secondary to hyperthyroidism. Other heart diseases implicated include dilated cardiomyopathy, arrhythmogenic right ventricular cardiomyopathy, and congenital cardiac defects. Abdominal ultrasounds were performed in 70 cats with pleural effusion, and revealed concurrent abdominal effusion in 59% of these cats. Abdominal abnormalities identified on ultrasound included abdominal masses, lymphadenopathy, hepatic venous congestion, hepatomegaly, splenomegaly, renal enlargement, small intestinal wall thickening, steatitis, and pancreatitis. Thoracic CT was performed on only 8 cats that had no radiographic or ultrasonographic explanation for their pleural effusion. All but one of the cats had bilateral pleural effusion with unilateral effusion seen in one cat.

Pleural effusions from 199 cats were classified as septic (27.6%), neoplastic (25.1%), chylous (11.6%), sterile exudate (14.6%), transudate (11.6%), and modified transudate (6%). FIP was diagnosed in 3 cats via antibody titers against feline coronavirus, and in 9 additional cats via pleural effusion RT-PCR. Since hyperthyroidism is known to cause cardiac abnormalities that can result in pleural effusion, 155 cats had total thyroxine concentrations measured with 20 (12.9%) of the 155 cats with CHF showing hyperthyroid results. Of the 66 cats tested for FIV and FeLV status, 5 (8%) were positive for FIV and 5 (8%) were positive for FeLV, but no cat was seropositive for both diseases concurrently. Of note, 4 of the FeLV positive cats that had neoplasia all had mediastinal lymphoma. Thorascopy was performed in 4 cats, and thoracotomy was performed in 22 cats, revealing viral pneumonia, chronic bronchitis, mediastinal lymphoma, mesothelioima, vasculitis, and fibrinonecrotic leuropneumonia.

The most common causes for pleural effusion in all 380 cats were found to be CHF (n=155, 40.8%) and neoplasia (n=98, 25.8%). Other causes included pyothorax, idiopathic chylothorax, trauma, FIP, nontraumatic diaphragmatic hernia, vasculopathy, uremic pleuritis, hypoproteinemia, and vitamin K antagonist toxicity. For neoplastic effusions, the most common neoplastic process was lymphoma, followed by carcinoma, sarcoma, mesothelioma, and mast cell tumor. Of note, cats with lymphoma were found to be significantly younger than those with carcinoma. Male cats were overrepresented in the group of cats with CHF with no association found in neuter status. As other studies have concluded, cats with CHF were found to have significantly lower rectal temperatures on admission. Although this study did not find statistic significance, there was a trend observed whereby Maine Coons were overrepresented in the CHF and pyothorax groups and Siamese were overrepresented in the neoplasia group. Interesting, there was significance seen in the overrepresentation of the Birman breed with FIP although the authors do not offer a suggestion as to why this breed would be overrepresented. As FIP is of viral origin, one concern would be if breeding facilities/housing increased risk for spread of this virus.

Unfortunately, 87 of the 380 (23%) cats died within the hospital. The authors vaguely suggest that the following causes of pleural effusion carry mortality rates in descending order: vasculopathy, FIP, uremic pleuritis, non traumatic diaphragmatic hernia, neoplasia, CHF, pyothorax, trauma, idiopathic chylothorax. The limitation of this study is that due to its retrospective nature, treatment outcomes for these cats could not be described.

So, what do we take away from this VETgirl podcast? Cats with pleural effusion have a poor prognosis. And yes, congestive heart failure is the most common cause of feline pleural effusion. A tip-off to the clinician at admission may be the coupled findings of pleural effusion and low rectal temperature. Additionally, in cats with CHF and/or signs of left ventricular hypertrophy, clinicians should pursue thyroid testing. When in doubt, make sure you feel comfortable approaching and working up the fragile, dyspneic cat with pleural effusion!

CHF Congestive heart failure
CT Computerized tomography
FIV Feline immunodeficiency virus
FeLV Feline leukemia virus
FIP Feline infectious peritonitis
HCM Hypertrophic cardiomyopathy
RT-PCR Reverse transcriptase polymerase chain reaction
TFAST Thoracic focused assessment with sonography for trauma

1. Dominguez Ruiz M, Vessières F, Ragetly GR, Hernandez JL. Characterization of and factors associated with causes of pleural effusion in cats. JAVMA 2018;253(2):181-187.

  1. Hi my name Kim and I had a six month I’d kitten who sadly died of pulmonary effusion on Sunday. But my concern is that on the Friday before he went in for his ten day check after having been castrated and was given the ok but my concern is that he should of had a pre check before the op and would they have picked up the fact he had fluid around the heart and should they have treated it first as we had said I think he has problems with his breathing. Before the op.

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