28-year-old, male, American alligator, (Alligator mississippiensis).This alligator was housed with 10 others at a rescue facility in the northeastern United States. During the winter months, the animals were kept indoors in a house, living on wood floors, with access to an unspecified water source; there was reportedly visible mold in this environment. The alligators were not induced to hibernate during this time, and were housed at an ambient temperature of 74oF (23oC). This alligator was found dead with no premonitory signs.

Gross Description:  

An adult, 70.5 kg male American alligator was presented in good nutritional condition with abundant fat stores. There was an extensive adhesion of the right lung to the right dorsal body wall which enclosed approximately 75 mL of pale tan, thin liquid with grey to pale tan particulates. The pleural surfaces of both lungs had discrete to coalescing areas of firm, brown to pale tan discoloration which were more extensive in the right lung and extended into the parenchyma. On cut section there were extensive regions of parenchyma replaced by firm, white, crumbly material (caseous necrosis).  Additionally, multiple subpleural cavitations and air spaces in the right lung were lined by white, slightly fuzzy, fungal mats. Numerous small, whitish foci up to 0.3 cm diameter were present in the liver, and similar individual foci were present in the heart, spleen, and kidney.

Histopathologic Description:

Up to 80% of the parenchyma is effaced by necrosis that extends to the pleural surface. Necrotic areas are characterized by variable loss of architectural detail with accumulation of fibrin, hypereosinophilic cellular debris, edema, hemorrhage, and inflammatory cells composed predominantly of heterophils and macrophages. Within the necrotic areas, there are numerous, lightly basophilic to transparent fungal hyphae that are 2-6 microns in diameter, septate, and parallel-walled with occasional right angle branching. At the air-tissue interface in a couple of large airways there are scattered hyphae that give rise to dense clusters of ampulliform conidiogenous cells measuring up to 3 microns in diameter, each with a single to several terminal round conidia measuring 1-3 microns in diameter. Large numbers of bacteria are also frequently admixed. Rarely, in areas of dense fungal growth, there are a few translucent, variably shaped, anisotropic crystals (oxalate crystals). Frequently, vessels are occluded by fibrin thrombi, and their walls contain necrotic debris, fibrin, moderate numbers of degenerate heterophils and occasional fungal hyphae (vasculitis). The parenchyma adjacent to necrotic areas is variably expanded by edema, mixed inflammatory cells, and reactive fibroblasts.

Morphologic Diagnosis:  

Lung: severe, subacute, multifocal to coalescing, fibrinonecrotizing and hetero-philic pneumonia and pleuritis with vasculitis, fibrin thrombi and intralesional bacteria and fungal hyphae and conidia, consistent with Beauveria bassiana.

Lab Results:  

Fungal culture and identification: A swab from the lung was initially plated on potato dextrose agar (PDA) and inhibitory mold agar (IMA) with and without antibiotics and then incubated at 30oC for 14 days; this yielded heavy growth of white fungal colonies on both plate types. On PDA, the bottom of the colony had an orange to pink tinge, whereas on IMA the bottom of the colony was red. Cultures were submitted to the Fungus Testing Laboratory, University of Texas Health Science Center in San Antonio, Texas for identification. Combined phenotypic characterization and DNA sequencing of ITS and TEF targets identified the fungus as Beauveria bassiana.


Beauveria bassiana, alligator

Contributor Comment:  

This is a case of mycotic pneumonia in an American alligator caused by Beauveria bassiana.  Identification of this organism was based on the morphology of the fruiting bodies (conidiogenous cells and conidia) on H&E; its phenotypic characteristics in culture; and DNA sequence analysis, all of which differentiated it from other common agents of fungal pneumonia, particularly Asper-gillus species. Beauveria bassiana is a ubiquitous soil saprophyte that is entomopathogenic, i.e. pathogenic to insects due to an affinity for chitinous exoskeletons. As such, it has been widely used for more than 100 years as biocontrol of pest insects.5 Though widespread in the environment, its upper temperature limit is around 30oC, and it is inactivated within hours or days when exposed to sunlight.9 Due to the temperature limitations, B. bassiana rarely causes infections in mammals but is an opportunistic pathogen of reptiles, with previous reports in captive American alligators,2 chelonians,3,6  and in cold-stunned Kemp’s Ridley sea turtles.62 In the current case, the temperature of the indoor enclosure was reportedly kept at 23oC, well within the temperature range of B. bassiana. High levels of fungus in the environment and poor ventilation were also probably involved in this case, as mold was reportedly visible in the enclosure where this group of alligators was housed. Other predisposing factors for fungal pneumonia in captive reptiles include additional husbandry-related issues, such as humidity, hygiene, and nutrition, immunosuppression, overuse of antibiotics, and concurrent disease.5 Shortly after diagnosis of this case, a second alligator from the same group died naturally, but a necropsy was not performed. Transmission is thought to occur from inhalation or ingestion of fungal spores from the environment, and the lung appears to be the primary site of infection. Hematogenous dissemination of the infection from the lung to other tissues, such as liver and spleen, occurred in this case as in previous cases.2,6 Beauveria bassiana produces several toxic compounds including oxalic acid, which promotes the formation of oxalate crystals within affected tissues;9 only a few crystals were seen in this case.

JPC Diagnosis:  

Lung: Pneumonia, necrotizing, multifocal to coalescing, severe, with innumerable fungal hyphae and large colonies of mixed bacilli, American alligator, Alligator mississippiensis.

Conference Comment:  

This case provided conference participants the unique opportunity to describe lung pathology in an American alligator, an uncommonly seen species at the Joint Pathology Center. Prior to the discussion of this case, the conference moderator led a review of the normal functional anatomy and physiology of alligator lungs, which was poorly understood until relatively recently.1,7  Research performed at the University of Utah indicates the external and internal morphology of alligator lungs is strikingly similar to the avian respiratory system, although in contrast to birds, alligators lack intra-abdominal air sacss.1,7 Alligators have a highly efficient unidirectional style of breathing, originally thought to be unique to avian species as a consequence of the high oxygen demands of flight.7 However, unlike birds, alligators use a diaphragm to pull air into the lungs. The air then travels one direction through bronchi which branch into numerous smaller parabronchi and continues further into alveolar-like spaces, called faveoli. Gas exchange then takes place within these faveoli, and the air then flows out of the lung via in a one-way loop and valve system.7 Unidirectional breathing is much more efficient than the mammalian bellows-style breathing because there is no alveolar mixing of inspired and expired air. Research is ongoing to elucidate the exact mechanism of unidirectional air flow in alligators and other reptiles, as it was thought that air sacs were necessary for unidirectional air flow breathing.1,7 Reported cases of fungal pneumonia in reptiles caused by Beauveria bassiana are rare and typically involve extensive multifocal necrosis or granulomatous nodules with high numbers of fungal hyphae in the lungs with dissemination to the multiple abdominal organs, as present in this case.2,3,6 Infection occurs after inhaling or ingesting fungal spores from the environment and development of disease in reptiles has been associated with low environmental temperatures and poor husbandry of captive reptiles.  As mentioned by the contributor, the fungus will not grow at mammalian physiologic temperatures (37)2,3,6, although it has been very rarely reported to cause fungal keratitis in people associated with contact lens wear and prior treatment with corticosteroid eye drops.4 The association of this fungus with low environmental temperatures and cold-shocked reptiles in previously reported cases prompted the conference moderator to discuss brumation in ectothermic animals. Brumation is a time of dormancy in reptiles in response to colder winter weather (~21oC), and is similar, but not identical, to hibernation in mammals. During periods of brumation, reptiles have a markedly decreased metabolic rate, but do not fall into a deep sleep, and can regularly emerge to drink and bask during warm days. Additionally, reptiles typically do not eat during periods of brumation.8


1. Farmer CG. Similarity of Crocodilian and avian lungs indicates unidirectional flow is ancestral for Archosaurs. Integr Comp Biol. 2015; 55(6):962-971.
2. Fromtling RA, Jensen JM, Robinson BE, Bulmer GS. Fatal mycotic pulmonary disease of captive American alligators. Vet Pathol. 1979; 16:428-431.
3. Gonzalez Cabo JF, Espejo Serrano J, Barcena Asensio MC. Mycotic pulmonary disease by Beauveria bassiana in a captive tortoise. Mycoses. 1995; 38:167-169.
4. Lara Oya A, Medialdea ME, et al. Fungal keratitis due to Beauveria bassiana in contact lenses wearer and review of published reports. Myco-pathologia. 2016; 181:745-752.
5. Murray MJ. Pneumonia and normal respiratory function. IN Mader, DR, ed. Reptile Medicine and Surgery, St. Louis: W.B. Saunders Company; 1996: 402-403.
6. Pare JA, Jacobson ER. Mycotic diseases of reptiles. IN Jacobson ER, ed. Infectious Diseases and Pathology of Reptiles, Boca Raton: CRC Press; 2007: 538-539.
7. Sanders RK, Farmer CG. The pulmonary anatomy of Alligator mississippiensis and its similarity to the avian respiratory system. Anat Rec. 2013; 295(4):699-714.
8. >Wilkinson A, Hloch A, et al. The effect of brumation on memory retention. Sci Rep. 2017; 7:40079.
9. Zimmermann G. Review on safety of the entomopathogenic fungi Beauveria bassiana and Beauveria brongniartii. Biocontrol Sci Technol. 2007; 17(6):553-596.

Click the slide to view.

1.1 Lung and air sac, alligator.

1.2 Lung and air sac, alligator.

1.3 Lung, alligator.

1.4 Lung, alligator.

1.5 Lung, alligator.

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