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Infection & disease

Streptobacillary pleuritis in a koala (Phascolarctos cinereus)

Russell, EG & Straube, EF 1979, Journal of wildlife Diseases, vol. 15, no. 3, pp. 391-394.

Streptobacillus moniliformis pleuritis was observed in a koala. There were granulomatous lesions observed, which occur when the immune system of an animal walls off foreign substances with macrophages due to being unable to eliminate the substance. A culture was made and inoculated in mice and rats, with the organism determined to be pathogenic for mice but not rats.

  The lesions were observed to be confined to the thorax of the koala, with yellow-brown granular cells and fluid observed having seeped from the pleura (the membrane surrounding the lungs). The right lung had severe pleuritis, while the left lung was normal, with a moderately enlarged heart observed. Also in the right lung was extensive atelectasis (collapsing of a section of the lung) and emphysema (destruction and enlargement of lung sections). The alveolar septa’s (the separations between the alveoli) architecture was difficult to discern and a decrease in cell numbers consistent with single alveolar cell death was visible. Eosinophilic material (a type of white blood cell) was present throughout the bronchioles, alveolar ducts and atelectatic alveolar spaces, with macrophages also present (a phagocytic white blood cell). The visceral pleura had a layer of loose granulation tissue with a significant mononuclear and epithelioid cell presence. Multiple gram-negative bacterial colonies were present on the pleural tissue. The infection seemed mostly limited to the right lung, with the left lung having low macrophage levels, the heart showing only superficial degermation, and the liver being mildly congested. Tiny translucent colonies were observed on microaerophilically incubated and anaerobic plates after 24 and 48 hours, respectively, with no colonies present on aerobically incubated plates. Upon further investigation, the organism was observed to be a small, gram-negative rod with long filamentous elements. It appeared granular, not encapsulated and non-motile. Its glucose metabolism was fermentative, producing acid but not gas. After inoculation with the Streptobacillus moniliformis, a rat was unaffected and euthanised after 2 weeks. All organs were cultured and found to negative for S. moniliformis. Intravenous inoculation of mice resulted in death within 5-9 days and cultures from the blood, liver and lungs returned positive for the bacteria. Intraperitoneal inoculation of mice caused death within 11-14 days, but only came back positive for the blood cultures, with the tissue cultures coming back negative.

  Previous studies have showed that Streptobacillus moniliformis is the cause of streptobacillary rat-bite fever in humans, although no sufficient histological description was written, hence a difficulty in comparison. Bacterial pneumonia has been observed in koalas previously, with pleural fluid and lung collapse also noted. Bacteriological studies were not performed, however, so direct comparison is again difficult.

  Through understanding the diseases that affect and kill koalas, better veterinary treatment can be administered, resulting in reduced mortality rates.

 

Summarised by Laura Wait

 

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