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02 Emydomyces testavorans Infection

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Known Species Affected:
Actinemys marmorata
Clemmys guttata
Emydoidea blandingii
Heosemys spinosa
Malaclemys terrapin
Podocnemis unifilis
Pseudemys floridana
Trachemys scripta elegans


(White et al., 2025; Winter et al., 2025; Woodburn et al., 2021) Cause of disease

Emydomyces testavorans infection is one recently recognized cause of ulcerative shell disease in captive or wild turtles. Unlike the broader category of shell disease, which other bacterial pathogens can also cause, E. testavorans infection refers specifically to the disease caused by this fungal pathogen. E. testavorans infection often presents with shell ulcers, and in severe cases, it may also invade underlying tissues [4].

E. testavorans belongs to the order Onygenales. Other well-known fungi in this order recognized as reptile pathogens include Paranannizziopsis, Nannizziopsis, and Ophidiomyces (i.e., Ophidiomyces ophiodiicola is associated with snake fungal disease). These have been described as keratinophilic fungi, meaning a keratin-loving fungi that can break down and utilize keratin as its source of nutrition. Additionally, some are also known to act as environmental saprophytes. Keratin is a key component of the skin and shell. This functional trait enables them to penetrate the outermost layer of the skin and shell, leading to ulcerative lesions. An environmental saprophyte survives in soil or aquatic habitats by utilizing dead and decaying organic matter as a nutrient source. The precise environmental presence and natural habitat of fungi within the order Onygenales have not yet been fully characterized [4,11,12]. Overall, exposure to E. testavorans, combined with shell trauma or compromised immunity, can facilitate infection.

Looking at the microscopic characteristics of members of the order Onygenales, their hyphae (i.e., microscopic thread-like structures that make up the fungus body) can invade and degrade keratinized tissue, and arthroconidia (i.e., spores produced by breaking apart hyphae) aid in environmental transmission and persistence. However, E. testavorans can be distinguished from other members of the order Onygenales by genetic variation and morphological features [12].

History

In 2009, biologists in Washington state reported cases of suspected ulcerative shell disease. While shell disease in captive turtles is often associated with poor water quality and sanitation, these cases were notable because they occurred in free-ranging turtles, many of whom were not exposed to unsanitary environments. The investigation of persistent lesions in turtles has persisted for years and has found that captive turtles are highly vulnerable [3].  

In 2019, Woodburn et al., formally described the fungus E. testavorans and highly associated it with ulcerative shell disease. The majority of the samples used in this discovery were from Washington, western pond turtles (Actinemys marmorata). The name Emydomyces testavorans was derived from Greek and Latin roots, and its literal meaning is “turtle fungus that devours the shell” [12].
      • Emydo means freshwater tortoise
      • myces means fungus
      • testa means shell
      • vorans means devouring
Following its initial identification, E. testavorans was confirmed in additional regions, including Illinois [10]. By 2021, the first documented emergence in California had been reported [5].

Clinical Signs and Progression

E. testavorans infection most often contains clinical signs associated with shell damage, including flaking, pitting or indentations, ulcerations, pliability (i.e., structural weakness), and discoloration. While skin lesions are relatively uncommon in E. testavorans infection, documented cases range from minor flaking and crusting to pronounced ulcerations [1,3,11]. However, some individuals may be asymptomatic carriers who do not exhibit signs of disease but still harbor the pathogen [5].

While some of these signs are not exclusive to E. testavorans infection, disease progression may result in additional clinical signs such as lethargy and emaciation, ultimately compromising quality of life. In severe cases, lesions can penetrate from the shell to the coelomic cavity, leading to osteolysis and decreased bone thickness. Additionally, cysts may be present in the superficial layers of the skin [4,9]. Although superficial cystic lesions may be observed on the skin, histologically, these lesions are characterized by epithelial inclusion cysts lined by keratinized stratified squamous epithelium that can accumulate keratin debris, and, in advanced cases, the underlying bone may become necrotic (Figure 1) [4,9,11].

Transmission and Epidemiology

Although E. testavorans has been shown to significantly impact captive turtles, wild populations remain at risk due to the potential introduction of infected captive individuals through owner release [1]. In wild populations, the pathogen has been detected in coastal regions of both eastern and western United States [11]. Several factors have been associated with the development of this condition, including preexisting wounds or shell damage, malnutrition, and inadequate living conditions in captive sea turtles, such as improper temperature, lighting or humidity [4].

E. testavorans has been primarily documented to affect aquatic chelonians including northwestern pond turtles (Actinemys marmorata), red-eared sliders (Trachemys scripta elegans), and coastal plain cooters (Pseudemyces floridana) [1,10]. In particular, Washington has experienced an increasing prevalence of E. testavorans infection within northwestern pond turtles [1]. This has raised concern because the species is listed as endangered in Washington, increasing its conservation priority [2]. Another species of concern is the red-eared sliders, which is invasive in many regions [9]. Red-eared sliders have been shown to carry two distinct fungal pathogens that cause another emerging infectious disease, sea turtle egg fusariosis (STEF). This suggests that as an invasive species, red-eared sliders may act as potential vectors, introducing infectious pathogens such as E. testavorans into new populations [7].

E. testavorans appears to be primarily transmissible through direct contact with infected individuals [8]. Potential reservoirs may also include water, soil, contaminated equipment, or other environmental substrates due to the possibility of E. testavorans being an environmental saprophyte [12].

Diagnosis

Initial identification of E. testavorans infection can be made through physical examination of the turtle’s skin and shell. However, since other potential causes are possible, additional testing is required to confirm presence of the pathogen, E. testavorans [10]. Deoxyribonucleic acid (DNA) extracted from the shell swabs, shell scrapes, and cloacal swabs samples, is amplified using polymerase chain reaction (PCR) to enable identification of the fungal pathogen [9]. Additional testing involves the use of diagnostic imaging, such as computed tomography (CT) scans, to identify the level of severity and internal shell or bone lesions (Figure 2) [2,3,11]. Tissue biopsies provide histological evidence of fungal infection through the presence of hyphae or conidia [2]. E. testavorans infection may additionally be characterized by epithelial inclusion cysts from skin and shell samples [11].  

Treatment and Prevention

Although further research is required, studies have indicated that antifungal medications, such as itraconazole, voriconazole, and terbinafine, may be effective in the treatment of E. testavorans infection [12]. For example, while voriconazole has shown toxic effects in a single snake species, additional studies are necessary to determine its safety profile across other reptile species [6]. Terbinafine, however, has been proposed as a preferable treatment option due to its minimal reports of toxicity in multiple species and its accumulation in keratinized tissue [2].

As mentioned before, there are additional factors that can contribute to the development of this disease. Therefore, it is important to prevent the emergence of E. testavorans infection by ensuring appropriate husbandry and environmental conditions are provided as it is crucial for the health of captive turtles. This includes proper temperature, lighting, humidity conditions, and providing proper nutrition [4].  Additionally, regularly disinfecting equipment and the environment is necessary to reduce pathogen transmission. Quarantining infected individuals is also a crucial preventative measure to take. While these methods cannot entirely prevent spread of disease, they can help minimize its emergence. Continuous surveillance and monitoring are also crucial for maintaining control over disease transmission.

Further Research

As a recently discovered pathogen, additional research is required to fully establish the association between E. testavorans fungus and shell lesions and disease [3,11]. Further research should also focus on treatment, including evaluating pharmacokinetic differences between females and males and determining whether nutrition has an impact [2]. Additionally, there is no specific vaccine available to prevent the disease. Other areas in need of research to support population conservation include transmission and epidemiology [1]. While biologists and researchers continue to deepen their understanding of the disease, it is important that active observation and monitoring of the E. testavorans pathogen in turtle populations be maintained.

Readings

1. Brunner, T. J., Wong, A. D., Adamovicz, L., Simmons, A., Ospina, E., & Keller, K. A. (2024). Emydomyces testavorans DNA is detected from shell swabs of companion chelonians. Journal of Exotic Pet Medicine, 50, 49-53. https://doi.org/10.1053/j.jepm.2024.05.005

2. Cerreta, A. J., Reinhart, J. M., Forsythe, L. R., O'Connor, M. R., Tang, K. N., Cox, S., & Keller, K. A. (2023). Bioencapsulation is a feasible method of terbinafine administration in Emydomyces testavorans-infected western pond turtles (Actinemys marmorata). American Journal of Veterinary Research, 84(1). https://doi.org/10.2460/ajvr.22.08.0138

3. Haman, K., Hallock, L., Schmidt, T., Holman, E., & Murphie, B. (2019). Shell disease in northwestern pond turtles (Actinemys marmorata) in Washington State, USA. Herpetological Review, 50(3), 495-502. https://research.ebsco.com/c/m3ubsp/viewer/pdf/74n34toltn

4. Hastain, S. A., Buchy, J. M., Dombrowski, D. S., Womble, M. A., Armwood, A. R., & Gruber, E. J. (2023). What is your diagnosis? Ulcerative shell lesions from a diamond-backed terrapin (Malaclemys terrapin). Veterinary Clinical Pathology, 53(2), 164-166. https://doi.org/10.1111/vcp.13291

5. Lambert, M. R., Hernandez-Gomez, O., Krohn, A. R., Mutlow, A., Patterson, L., Rosenblum, E. B., Timmer, M., Willis, J., & Bushell, J. Turtle shell disease fungus (Emydomyces testavorans): First documented occurrence in California and prevalence in free-living turtles. Ichthyology & Herpetology, 109(4), 958-962. https://doi.org/10.1643/h2020151

6. Lindemann, D. M., Allender, M. C., Rzadkowska, M., Archer, G., Kane, L., Baitchman, E., Driskell, E. A., Chu, C. T., Singh, K., Hsiao, S. H., Sykes, J. M., & Cox, S. (2017). Pharmacokinetics, efficacy, and safety of voriconazole and itraconazole in healthy cottonmouths (Agkistrodon piscivorus) and massasauga rattlesnakes (Sistrurus catenatus) with snake fungal disease. Journal of Zoo and Wildlife Medicine, 48(3), 757-766. https://doi.org/10.1638/2016-0179.1

7. Martínez-Ríos, M., Martín-Torrijos, L., & Diéguez-Uribeondo, J. (2022). The invasive alien red-eared slider turtle, Trachemys scripta, as a carrier of STEF-disease pathogens. Fungal Biology, 126(2), 113-121. https://doi.org/10.1016/j.funbio.2021.11.003

8. Rato, J., Xavier, R., Harris, D. J., Banha, F., & Anastácio, P. (2024). A comprehensive review of disease-causing agents in freshwater turtles: Implications for conservation and public health. Diversity, 16(3), 171. https://doi.org/10.3390/d16030171

9. White, A., Lim, T., Culver, E., Timmer, M., Bushell, J., Muttlow, A., Aymen, J., Hussain, L., Haman, K., Lambert, M. R., & Hernández-Gómez, O. (2025). Emydomyces testavorans infection and shell disease prevalence in threatened northwestern pond turtle populations of the San Francisco Bay Area, USA. Herpetological Conservation and Biology, 20(3), 457-465. https://www.herpconbio.org/Volume_20/Issue_3/White_etal_2025.pdf

10. Winter, J. M., Moorhead, K., Grochowski-Grum, K., Anchor, C., Landolfi, J. A., Adamovicz, L. A., & Allender, M. C. (2025). Health assessment of non-native red-eared sliders (Trachemys scripta elegans) and their impact potential on native species. PLoS One, 20(10), e0333786. https://doi.org/10.1371/journal.pone.0333786

11. Woodburn, D. B., Kinsel, M. J., Poll, C. P., Langan, J. N., Haman, K., Gamble, K. C., Maddox, C., Jeon, A. B., Wellehan, J. F. X., Ossiboff, R. J., Allender, M. C., & Terio, K. A. (2021). Shell lesions associated with Emydomyces testavorans infection in freshwater aquatic turtles. Veterinary Pathology, 58(3), 578-586. https://doi.org/10.1177/0300985820985217

12. Woodburn, D. B., Miller, A. N., Allender, M. C., Maddox, C. W., & Terio, K. A. (2019). Emydomyces testavorans, a new genus and species of onygenalean fungus isolated from shell lesions of freshwater aquatic turtles. Journal of Clinical Microbiology, 57(2), e00628-18. https://doi.org/10.1128/jcm.00628-18