History

A 3-year-old sexually intact male mink at a mink farm was found dead and submitted for necropsy. There was no history of prior illness.

Clinical and Gross Findings

Because of the absence of signs of illness, no clinical tests had been performed prior to necropsy. The mink was slightly thin and mildly dehydrated. It had severe periodontal disease, and all 4 canine teeth were broken. The pleural cavity was filled with thick, yellow to brown, creamy fluid (Figure 1). The pulmonary pleural and pericardial surfaces were tan, granular, and proliferative.

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Photographs of the thoracic cavity (A) and the heart and lungs (B) of a mink that was found dead and that had no history of prior illness. Notice the large amounts of thick purulent exudate that fill both sides of the thoracic cavity. The pulmonary pleural and pericardial surfaces are tan, granular, and proliferative. In panel B, the pericardium is indicated by an asterisk.

Citation: Journal of the American Veterinary Medical Association 235, 9; 10.2460/javma.235.9.1049

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Formulate differential diagnoses from the history, clinical findings, and Figure 1—then turn the page →

Histopathologic Findings

Histologic examination of sections of lung tissue revealed that the pleura was markedly expanded by granulating fibrosis and was lined by reactive and proliferative mesothelial cells (Figure 2). There were also large numbers of degenerate and viable neutrophils, amorphous eosinophilic necrotic cellular debris, and fibrin along the surface. In some sections, multifocal bacterial colonies composed of long, slender filamentous rods that were often arranged in a somewhat radiating pattern were detected in the neutrophilic pleural exudate (Figure 3). Staining of sections revealed that these rod bacteria were gram positive and were not acid fast. In some bronchioles, epithelial necrosis was also evident and these airways were filled with large numbers of mononuclear inflammatory cells, neutrophils, and occasional filamentous bacterial colonies.

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Photomicrograph of a section of lung obtained from the mink in Figure 1. Reactive and proliferative mesothelial cells are present along the surface of the markedly fibrotic pleura as well as large numbers of degenerate and viable neutrophils. H&E stain; bar = 50 μm.

Citation: Journal of the American Veterinary Medical Association 235, 9; 10.2460/javma.235.9.1049

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Photomicrograph of a section of lung obtained from the mink in Figure 1 illustrating one of several bacterial colonies that were detected at multiple locations within tissue sections. The colony is composed of long, slender filamentous rods that are arranged in a somewhat radiating pattern. These rods are gram-positive organisms (inset). Main image, H&E stain; bar = 150 μm. Inset, modified Brown and Brenn Gram stain; bar = 60 μm.

Citation: Journal of the American Veterinary Medical Association 235, 9; 10.2460/javma.235.9.1049

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Sections of lung tissue and a swab specimen of the pleural fluid were submitted for aerobic and anaerobic bacterial culture. Moderate numbers of Actinomyces israelii, along with large numbers of Pasteurella multocida were subsequently identified.

Morphologic Diagnosis

Severe, diffuse, chronic active, proliferative, suppurative pleuritis and multifocal, necrotizing, pyogranulomatous bronchiolitis with intralesional filamentous bacterial rods.

Comments

In the mink of this report, the gross and histologic lesions resembled those classically associated with bacterial pyothorax and pleuritis caused by Actinomyces spp, Nocardia spp, or other higher bacteria (ie, bacteria with some characteristics of fungi). Although actinomycosis and nocardiosis in mink have not been reported to the authors' knowledge, the features of the pyothorax in the mink of this report were highly similar to those associated with pyothorax in other animals (often dogs and cats) infected with these species of bacteria. Suppurative pyothorax attributable to Streptococcus group G infection, as reported¹ in cats, was also considered as a likely differential diagnosis.

Given the proliferative nature of the pleura, the differential diagnoses included mesothelioma, although this was considered unlikely. Another less likely differential diagnosis (based on the granulomatous appearance of the pleura) was mycobacteriosis. Cytologic examination of pleural fluid samples can help to differentiate between these possible causes, and findings may be helpful when a diagnosis is needed quickly for treatment of affected live domestic animals. For the mink of this report, cytologic examination of a pleural fluid sample was not performed, although sections of lung tissue and a swab specimen of the pleural fluid were submitted for aerobic and anaerobic bacterial culture. Bacterial cultures revealed moderate numbers of A israelii along with large numbers of P multocida in the lungs and the pleural exudate.

Actinomyces spp are saprophytic, typically anaerobic, gram-positive, non–acid-fast organisms and are commensals of the oropharynx in humans and other animals.^2,3 These bacteria are also present on the mucous membranes of the urogenital tract.² In veterinary medicine, diseases associated with Actinomyces spp in goats, sheep, cattle, wild ruminants, horses, swine, dogs, cats, monkeys, rabbits, squirrels, hamsters, marsupials, birds, and other animals have been reported.² Intrathoracic actinomycosis and nocardiosis most commonly develop in dogs and cats; blood-stained purulent exudate (that has a so-called tomato soup–like appearance) often accumulates within the thorax.⁴ Yellow flecks called sulfur granules are often present within the exudate,^2,4 but were not detected in the exudate in the mink of this report.

Intrathoracic actinomycosis is not a contagious disease, and possible causes include bite wounds to the thoracic region, penetrating foreign bodies, extension from an adjacent structure (eg, rupture of a pulmonary abscess, bronchopneumonia, or parapneumonic spread), and migration of inhaled or ingested grass awns.^2,4–6 Inhaled grass awns are barbed and can penetrate the pulmonary airways, migrate through the pulmonary parenchyma, and pierce the pulmonary pleura^4,6; bacteria on the awns then can reproduce within the thoracic cavity. Occasionally, plant awns can be identified during careful dissection of the pleura in affected animals.⁶

In the mink of this report, P multocida was also cultured from the lung tissue and from a swab specimen of the pleural exudate. Pasteurella multocida is a commensal of the oropharyngeal region and is especially common in carnivores.⁷ Infection usually develops following transfer of the organism via ingestion, inhalation, or bite wounds, and many infections are likely endogenous.⁷ Mixed bacterial infections are common causes of pyothorax in dogs and cats,⁸ and in 1 study,⁵ Pasteurella spp were most commonly cultured from samples obtained from affected cats. Pasteurella multocida certainly could have had a role in the development of pyothorax in the mink of this report.

The source of the bacterial infection in the mink of this report was not determined, but on the basis of the severe periodontal disease, it was likely derived from the oral cavity. Periodontal disease is a common finding in farm-raised mink. Any animal with dental disease is at greater risk for hematogenous spread of bacteria from the oral cavity to multiple organs or for transfer of bacteria to the lungs via inhalation or aspiration of oropharyngeal contents. In humans with actinomycosis, pulmonary lesions are usually caused by aspiration of material from the oropharynx.³ In 1 study⁵ in cats, it was determined that the most common cause of pyothorax in this species is aspiration of oropharyngeal flora with colonization and invasion of lung tissue and subsequent direct extension of the infection from the bronchi and lungs to the pleural surface. In humans with anaerobic lung infections, bronchopleural fistulas with extension to the pleural space have been reported.^5,9 Because of the bronchiolar lesions and the presence of bacteria within the bronchioles in the mink of this report, we speculate that infection resulted from inhalation or aspiration of bacteria in the oropharyngeal region. There was a lack of vascular lesions such as vasculitis and fibrin thrombi formation, and no bacteria within vessels were detected, which would be expected with a hematogenous route of infection. In addition, the culture of > 1 type of bacteria is more supportive of inhalation rather than hematogenous spread as the route of infection.⁵ It is plausible that subsequent extension of the bacteria in the affected airways to the pleural surface led to the seeding of bacteria within the thoracic cavity. Although trauma and migration of foreign material cannot be entirely ruled out, there was no external evidence of trauma and plant awns were not identified in the mink.