Pulmonary Paecilomyces lilacinus Infection in a Cat

Introduction

Atypical fungal infections with organisms such as Paecilomyces sp., Fusarium sp., and Acremonium sp. represent a diagnostic and therapeutic challenge, because cytological and histological identifications are rare. Their appearance can mimic more common pathogenic organisms such as Candida sp., Sporothrix sp., and Aspergillus sp. Documented as an etiological agent in systemic and disseminated disease, Paecilomyces (P.) lilacinus is a ubiquitous, opportunistic, hyaline, nonpigmented fungus.^1–4 The organism is common in the soil and decaying organic matter; however, flowers, water, saline, sheep wool, Ixodes ricinus ticks, and lotions have also been documented as sources of infection.^5–10

Originally classified in the genus Penicillium as Penicillium lilacinum, the organism was classified into the separate genus Paecilomyces in 1974 and has since been documented to infect humans, animals, and reptiles.^{5,6,9,11–15} The two major pathogenic species of Paecilomyces are P. lilacinus and P. variotii; of these, the former is more frequently cited as an etiological agent and also appears the most resistant to antifungal therapy.^4,7 These fungi are ubiquitous; therefore, they usually produce disease in immunocompromised or chronically ill, debilitated humans and animals. These include transplant patients, those exposed to long-term corticosteroid treatment, and patients on chemotherapy.^4,5,16,17 Paecilomyces lilacinus has also been implicated in nosocomial infections in hospitalized and postoperative patients.^6,7 Paecilomyces lilacinus has been previously identified in the eye, lungs, lymph nodes, skin, spleen, brain, vertebral column, and kidney; the skin and alimentary and respiratory systems are considered important portals of entry.^1,4,6,13 In spontaneous disease, P. lilacinus often gains access to the body via inoculation in mucosal or submucosal tissues.⁶

Diagnosis of respiratory disease initially involves thoracic radiographs and hematology. However, without obtaining diseased tissue for histopathology and culture, diagnosis of fungal infection may be difficult.

The purpose of this paper is to highlight the clinical presentation and importance of a thorough workup in diagnosing a rare pulmonary fungal infection. In addition, this case represents the first veterinary literature report of pulmonary P. lilacinus infection in an animal that was not immunocompromised or traumatized.

Case Report

A 6-year-old, 4.12-kg, spayed female domestic shorthair cat was examined because of an intermittent, nonproductive cough and wheezing of 3 to 4 months’ duration. The wheezing episodes reportedly ranged from three to six times daily and lasted approximately 1 to 2 minutes. The cat had been in the owners’ care since she was obtained as a kitten, and she was vaccinated against feline leukemia, herpesvirus-1, calicivirus, panleukopenia, and rabies viruses. The cat lived primarily indoors with one other cat, although both cats had infrequent access to a garage. The cat showed no evidence of systemic illness other than the intermittent coughing and wheezing. Her activity level decreased only during these episodes. No history of trauma or previous illness was reported.

Upon presentation to the referring veterinarian, the cat was dyspneic, with audible wheezing and increased bronchovesicular sounds on auscultation. A complete blood count revealed leukocytosis (16,200/μL; reference range 3500 to 16,000/μL) with mild neutrophilia (10,200/μL; reference range 2500 to 8500/μL) and monocytosis (648/μL; reference range 0 to 600/μL). Serum biochemical values were all within normal limits. Thoracic radiographs revealed a questionably enlarged cardiac silhouette, along with bronchointerstitial and mild alveolar patterns in the right middle and caudal lung lobes. The cat was referred for further diagnostic evaluation.

The initial physical examination revealed increased bronchovesicular lung sounds, although no wheezes were audible. An enzyme-linked immunosorbent assay Snap test for feline leukemia virus antigen and feline immunodeficiency virus antibody^a was negative for both. Thoracic radiographs were repeated to provide comparison to those performed by the referring veterinarian. Atelectasis of the right middle and caudal lung lobes was present, with probable hyperinflation of the accessory lung lobe. The pulmonary arteries, bronchi, and veins associated with these lung lobes could not be identified to the periphery of the lung margins, possibly because of vascular and/or bronchial obstruction [Figures 1A, 1B]. Differentials for intraluminal obstruction included neoplasia, mucous plugs, or foreign body. The owner was provided with options to have the cat undergo either bronchoscopy or lateral thoracotomy in an attempt to identify and/or remove the pathology involving the right middle and caudal lung lobes. The owner elected bronchoscopy, which was performed the following day using a 3.8-mm bronchovideoscope.^b

The cat was premedicated with glycopyrrolate^c (0.01 mg/kg) and buprenorphine^d (0.01 mg/kg). Propofol^e (6.0 mg/kg) was used for induction, and anesthesia was maintained with isoflurane gas^f (1.5% to 2.0%). The cat was intubated with a 4.5-mm Murphy endotracheal tube that was removed for bronchoscopy, because the lumen of the endotracheal tube was not sufficiently large to accommodate passage of the endoscope. End-tidal carbon dioxide level, blood oxygen saturation, blood pressure, heart rate, respiratory rate, and temperature were monitored continuously. The cat was placed on intermittent positive-pressure ventilation prior to bronchoscopy, and 100% oxygen was administered through the bronchoscope once the procedure was initiated.

Bronchoscopy confirmed narrowing of the right main-stem bronchus with no obvious intra- or extraluminal obstruction. Slight pressure on the collapsed bronchus resulted in minimal opening of the airway and allowed passage of the bronchoscope. However, upon passing the scope beyond the collapsed lumen, the cat developed a pneumothorax. The scope was immediately removed, and the cat was reintubated. Thoracentesis yielded 2 liters of air from the right side of the chest, without resolution of the pneumothorax. A thoracostomy tube was placed at the right seventh intercostal space. Air was manually evacuated from the pleural space until the thoracostomy tube could be attached to a continuous suction unit.^g Thoracotomy was recommended to identify and correct the cause of the pneumothorax; however, this option was not permitted by the owner. Thoracic radiographs were taken to evaluate proper placement of the thoracostomy tube, and they confirmed bilateral pneumothorax that was worse in the right hemithorax than in the left. Multifocal cavitated lesions within the right caudal lung lobe and extensive pneumomediastinum with gas extending into the retroperitoneal space were seen [Figure 2].

After recovery from anesthesia, the cat was found to have hind-limb paralysis. The cause for this paralysis was not known. A saddle thrombus or vascular event was suspected but ruled out, because the cat’s footpads were pink, nail beds were warm, and femoral pulses were palpable. A complete neurological examination revealed loss of deep pain in both hind limbs and the presence of paraspinal pain at the fourth to fifth lumbar vertebrae. Radiographs of the spine were within normal limits. The cat was closely monitored in the intensive care unit for the next 2 days. Buprenorphine^d (0.02 mg/kg intravenously q 6 hours) and bupivacaine/bicarbonate^h (0.5 mg/kg via intrathoracostomy tube q 4 hours) were provided for analgesia, and meloxicamⁱ (0.2 mg/kg subcutaneously [SC] once; 0.1 mg/kg SC q 24 hours) was administered. Crystalloid fluid therapy with potassium chloride supplementation was provided to ensure adequate hydration.

After 2 days, the thoracostomy tube was inadvertently dislodged from the thoracic cavity. Almost immediately, subcutaneous emphysema was noted as air escaped through the chest wall defect. Air was evacuated using a 19-gauge butterfly catheter inserted into the subcutaneous space as the cat would become severely dyspneic; however, dyspnea could not be adequately controlled with this measure. Dyspnea was attributed to both the pneumothorax and subcutaneous emphysema. At this point, the owner consented to surgical intervention in an attempt to alleviate the cat’s dyspnea and to correct the underlying cause of the severe pneumothorax. The cat was quickly prepared for surgery, which was performed within approximately 45 minutes.

A right lateral thoracotomy was performed at the fifth intercostal space. A rent was identified in the parenchyma of the right caudal lung lobe near the hilus. The right caudal lung lobe had diffuse atelectasis with multifocal, small, palpable, light grey nodules on the visceral pleura and in the parenchyma, suggestive of either a neoplastic or inflammatory process. The right middle lung lobe was also atelectatic, and the right cranial lung lobe exhibited similar but milder lesions at its caudal margin. A TA-30 stapling device^j was used to occlude the hilus of the right caudal lung lobe, which was resected. A thoracostomy tube was placed in the right hemithorax. The cat recovered from surgery without complication. The pneumothorax resolved immediately postoperatively, and the thoracostomy tube was removed 2 days later without respiratory compromise. Postoperatively, all signs of respiratory disease resolved. The cat’s bilateral hind-limb paralysis improved as she regained deep pain and eventually motor function. Conscious proprioception returned within 2 weeks after discharge.

Representative sections of lung were fixed in 10% neutral-buffered formalin, routinely processed, and embedded in paraffin. Four- to 5-μm tissue sections were stained with hematoxylin and eosin (H&E), Ziehl-Neelsen, periodic acid-Schiff (PAS), and Grocott methenamine silver (GMS). Sections of lung stained with H&E revealed pyogranulomatous and eosinophilic bronchointerstitial pneumonia with areas of perivascular fibrosis. Rarely, the bronchial mucosa was ulcerated, and inflammatory cells were present within the lumen of the bronchiole. In some regions, hyperplasia of type II alveolar epithelial cells was present in addition to flooding of alveolar lumens by epithelioid macrophages admixed with occasional lymphocytes, plasma cells, and neutrophils. Occasionally, mild resolving arteritis was also present. The Ziehl-Neelsen and PAS stains were negative, whereas the GMS stain revealed extremely rare, small (4 to 7 μm), predominately extracellular, occasionally budding yeast cells [Figure 3].

Fresh lung sections were submitted for mycological culture. The fungal organism recovered in culture was submitted to a fungus-testing laboratory for identification. The morphological features of the isolate were examined after 7 days of incubation at 25.0°C on potato flakes agar prepared in house. Colonies were floccose, had a moderate growth rate, and were mauve-colored with a colorless reverse [Figure 4A]. Microscopic features included long (400 to 600 μm), slightly roughened conidiophores giving rise to whorls of divergent phialides bearing chains of ellipsoidal conidia (2 × 3 μm) [Figure 4B]. Based on these morphological characteristics, the isolate was identified as P. lilacinus.

Treatment with itraconazole^k (5 mg/kg PO q 12 hours) was begun as soon as yeast cells were identified in lung tissue. Upon receipt of the final diagnosis, the owner was instructed to continue administering itraconazole for a total of 6 months. This recommendation was based upon previous reports suggesting susceptibility of Paecilomyces species in vivo and in vitro to triazole antifungal therapy.^5,6,11,12,18 Follow-up evaluation consisted of periodic physical examinations. Follow-up radiographs were not performed, as clinical signs did not recur. The last contact made with the owner was 8 months after discharge, at which time it was determined that the cat had never exhibited clinical evidence of coughing, wheezing, or dyspnea since discharge.

Discussion

Paecilomyces lilacinus has previously been identified as an opportunistic pathogen of immunocompromised patients; however, a few reports in the recent literature, and the case in this report, support its pathogenicity in healthy patients.^{7–9,18–21} Paecilomyces lilacinus can be isolated as an airborne laboratory contaminant; however, contamination was ruled out in this case, as P. lilacinus fungal elements were demonstrated within granulomas.^1,4–7,9,16 Animals that develop an infection with this organism often have prior mucosal or submucosal damage, allowing dissemination of the organism.⁶ The source of infection in this case is unknown. This cat lived primarily indoors and lacked evidence of historic or concurrent underlying diseases that could have resulted in immunocompromise. The fungus was suspected to primarily be inhaled or hematogenously spread, based on the localization of clinical signs to the lungs and absence of cutaneous lesions; however, an open wound that was not visible to the owner could not be ruled out. The fact that this cat was ostensibly healthy makes this case challenging, because the veterinary and human literature provide few reports of infection in otherwise healthy individuals.^7,9,18–21

Obtaining a diagnosis of paecilomycosis after histopathology and culture allowed for proper treatment of this cat’s respiratory disease. The accurate identification of Paecilomyces sp. is important, because these fungal organisms often are resistant to commonly used antifungal agents, including amphotericin B, griseofulvin, and 5-fluorocytosine.^{1,6,11,12,15,22–24} Previous studies within the human and veterinary literature suggest that the most efficacious compounds against Paecilomyces sp. appear to be the triazole group of antifungal agents.^5,6,11,12,22 Although antifungal susceptibility testing was not performed in this case, historic literature supports the choice of itraconazole as antifungal therapy. In addition, itraconazole is better tolerated in the cat than the other azoles within this class of drugs, with less evidence of hepatotoxicity, anorexia, and/or vomiting.⁶

Even though medical therapy is warranted in cases of paecilomycosis, the optimal treatment is often surgical resection of diseased tissue,^11,22 which can be both diagnostic and therapeutic. Without surgical intervention, the diagnosis of P. lilacinus infection may not have been obtained in this case.

Differentiation of the fungal elements of P. lilacinus from the morphologically similar yeast cells of Sporothrix schenckii sp. and Candida sp. on histological examination can be challenging.¹¹ A definitive diagnosis can only be established through mycological culture.^9,21,23 Paecilomyces lilacinus has been shown to exist in various forms in tissue where it has the ability to sporulate—a process termed “adventitious sporulation.”^21,23 Phialides may be present with markedly constricted points of septation on filaments, often leading to the misdiagnosis of candidiasis and other more common fungi prior to culture results.^7,11,17,23 The presence of pseudohyphae and large numbers of yeast lacking an oval morphology suggest a Candida sp., while small, extracellular, “cigar-shaped” cells may suggest sporotrichosis. Reported cases of hyalohyphomycosis caused by P. lilacinus identified budding, round, ellipsoid structures in tissue in addition to hyphae, suggesting the difficulty in properly identifying this organism.^11,16 One report identified the morphological characteristics of Paecilomyces × sp. as phialoconidia that range from 1.4 to 3.5 × 15.4 μm in size.²³ This finding is consistent with the morphological findings of fungal elements measuring 4 to 7 μm in this case.

In this case, the presence of only the yeast form of P. lilacinus without hyphae on histological sections was unusual and a diagnostic challenge, as few reports in human and veterinary literature identify conidia in the absence of hyphae on histopathology.^{7,11,15,23–25} Moreover, the presence of conidia seen on histopathological examination that were morphologically similar to those produced in mycological culture further supports P. lilacinus infection in this case.

Conclusion

Paecilomyces lilacinus is an opportunistic fungus that is rarely reported in veterinary and human medicine as a cause of disease. Although this organism has been extensively reported as an opportunist pathogen in immunocompromised patients, it may be occurring more frequently in otherwise healthy patients. Although a rare pathogen, P. lilacinus should not be disregarded, as it has the capability of causing a severe, systemic fatal disease.