Pansteatitis in Wild American Alligators (Alligator mississippiensis) Open Access

Crocodilians possess strong innate immune systems (Siroski and Soledad 2020), and only limited pathogens and diseases are known to cause extensive mortality in these animals (Huchzermeyer 2003). Nevertheless, some crocodilian mass mortalities have occurred (Whitaker et al. 2008; Woodborne et al. 2012). Pansteatitis is a nutritional or environmental disorder that causes ceroid accumulation and fat inflammation (Lane et al. 2013). Records of this disease exist for captive caiman (Caiman crocodilus; Frye and Schelling 1973), captive American alligators (Alligator mississippiensis; Larson et al. 1983), and captive and wild crocodiles (Crocodylus spp.; Huchzermeyer 2003). An outbreak of pansteatitis in Nile crocodiles (Crocodylus niloticus) occurred in South Africa in the Olifants River Gorge from 2008 to 2012, causing mortality of at least 216 individuals (Woodborne et al. 2012; Lane et al. 2013). Since then, there have been no further reports of pansteatitis in wild crocodilians.

American alligators occupy fresh and brackish waters throughout the southeastern US. Though recorded in captive alligators (Wallach and Hoessle 1968; Wallach 1970; Larson et al. 1983), no records exist of pansteatitis or steatitis-related mortality in wild alligators. On 3 August 2023, South Carolina Department of Natural Resources (DNR) personnel found a dead adult American alligator on the bank of a freshwater impoundment at the Tom Yawkey Wildlife Center, Georgetown County, South Carolina, US. The position of the alligator suggested that it had exited the impoundment before dying on the bank. The DNR staff observed no odor or bloating associated with the carcass, despite direct sun exposure and an ambient air temperature of 30 C, indicating recent mortality. The carcass was immediately stored at 1 C for 16 h until examination.

At necropsy, the alligator (male, 2.42-m total length) showed no observable external injuries or lesions. The body condition appeared fair, with no observed muscular atrophy and decreased coelomic adipose stores. Incision into the lateral tail revealed large, hard, yellow-brown, granular masses of variable size within the muscle tissue (Fig. 1A), similar to pansteatitis lesions reported in Nile crocodiles (Lane et al. 2013). Similar masses were found in the mandibular muscle, intramuscular adipose tissue along the rib cage, within the body cavity, and around the coelomic viscera. The coelomic fat body was not located during necropsy, potentially due to a reduction in size associated with pansteatitis (Lane et al. 2013). A sample of masses and surrounding muscle from the tail was collected, initially frozen, and thawed and subsampled for preservation in 10% buffered formalin. The formalin-preserved sample was processed for routine H&E staining and examined by bright-field microscopy (Wolfe 2019).

Microscopically, the masses consisted of lobules of well-formed, mature adipocytes infiltrated by large numbers of epithelioid, eosinophilic to foamy macrophages consistent with granulomatous inflammation (Fig. 2A, B). The macrophages dissected between individualized adipocytes exhibited a faded to absent, peripherally oriented nucleus, irregular, collapsed cell membranes, and pink wispy cytoplasm, indicative of fat necrosis (Fig. 2B). Necrotic adipocytes rarely contained basophilic granular mineral debris within the cytoplasm. Ceroid was often present within affected regions, staining deep pink to magenta with Ziehl-Neelsen and periodic acid–Schiff stains (Fig. 2C, D).

A second, unrelated case was observed in Mississippi, US, during the 2022 public recreational alligator hunting season. Hunters brought a large adult male alligator to a processing facility for skinning and butchering. The processor observed extensive yellow-brown masses throughout the tail muscle of the alligator (Fig. 1B). Unaware of pansteatitis, the processor proceeded, removing masses and affected tissues from edible tissues. Although the gross appearance and consistency of the masses were consistent with pansteatitis and similar to those observed in the South Carolina alligator and wild crocodiles, no samples were collected from the animal. Thus, pansteatitis was not confirmed microscopically, and other differential diagnoses, such as infectious agents, remain possible considerations for the lesions in this animal. However, assuming the lesions in this case represent pansteatitis, it would suggest pansteatitis may occur subclinically in wild alligators throughout the range.

Causes of pansteatitis are multifactorial, resulting in free radical injury to adipocytes. Free radicals may be produced by ischemia, pro-oxidizing toxins, or consumption of rancid unsaturated fatty acids (Juan-Sallés and Boyer 2021). Alternatively, insufficient oxidant scavenging capacity due to nutritional deficiencies and a lack of antioxidants (vitamin E and selenium) results in free radical injury (Juan-Sallés and Boyer 2021). These deficiencies result from decreased antioxidants in the diet or compromised metabolism of antioxidants during digestion. Lipid-oxidizing products, along with subsequent inflammation, generate an oxidative-positive feedback loop, leading to lipid peroxidation, cell death, granulomatous inflammation, and fat saponification. Clinically, reptiles with pansteatitis may exhibit lethargy, reduced movement and swimming (in crocodilians, particularly reduced tail mobility), and buoyancy abnormalities (Juan-Sallés and Boyer 2021). Reduced mobility or swimming leads to emaciation, decreased immune function, and ultimately death.

In South Carolina, the Tom Yawkey Wildlife Center is adjacent to Winyah Bay, the fourth largest estuary by discharge rate in the eastern US. The Mississippi alligator came from the Ross Barnett Reservoir, Madison and Rankin counties, Mississippi, a large lake serving as a drinking water reservoir for the city of Jackson. Both sites have been anthropogenically altered, including the introduction of contaminants from a variety of sources (Dash et al. 2015; Hale et al. 2017). Past mortality of Nile crocodiles with pansteatitis was associated with the consumption of pansteatitis-affected fish and potential influence from mining runoff and eutrophication (Woodborne et al. 2012; Huchzermeyer et al. 2013; Lane et al. 2013). However, pansteatitis-affected fish populations have not been recorded in South Carolina or Mississippi, and specific contaminants have yet to be linked to this disease in alligators. Thus, the pathogenesis of pansteatitis observed in wild alligators in this study remains unknown. Future studies should perform lipidomic assessment and identify environmental contaminants within blood and tissues to investigate the underlying causes of pansteatitis in wild alligators.

Pansteatitis has not previously been documented in wild American alligators. The disease is presumed to be rare or underreported in these populations. Alligators and other crocodilians readily bioaccumulate contaminants in muscle tissue (Campbell 2003) and can also be affected by pathogens and parasites such as Salmonella, Staphylococcus, and Trichinella spp. (Fasulkova et al. 2024). Human consumption of crocodilian meat may present a health hazard from exposure to contaminants and pathogens (Tipton et al. 2017; Fasulkova et al. 2024). Managers may wish to consider passive surveillance for pansteatitis in wild populations as an indicator of the presence of this disease in aquatic systems where alligators are found and subsequently hunter harvested. Informing game processors how to identify pansteatitis and creating a reporting framework may provide a low-effort means of surveilling pansteatitis in alligators.

We thank J. Dozier and the staff at the Tom Yawkey Wildlife Center for assistance with alligator carcass collection and storage. We thank R. Singh for necropsy assistance, S. Smith from Red Antler Processing, Yazoo City, Mississippi, for the record of pansteatitis in the Mississippi alligator, and J. Myburgh for examining our photographs of diseased alligator muscle. Funding was provided by the USDA National Institute of Food and Agriculture (SC-1700590), Nemours Wildlife Foundation, and the South Carolina Water Resources Center. This research was supported, in part, by a Nemours Wildlife Foundation Graduate Research Fellowship (to M.B.). Any opinions, findings, conclusions, or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the Nemours Wildlife Foundation. This article represents Technical Contribution (7366) of the Clemson University Experiment Station and Nemours Wildlife Foundation Contribution (002).