The non-hyphal, zoosporic, chytrid fungus Batrachochytrium dendrobatidis (Bd) is a recently emerged pathogen that causes the infectious disease chytridiomycosis (Stadler 2013, Stockwell et al 2012, Densmore and Green 2007). This pathogen has caused widespread and dramatic population declines in both wild and captive amphibians worldwide (Murphy et al 2015, Stadler 2013, Stockwell et al 2012, Densmore and Green 2007). Often a primary condition, the chytrid fungus can also serve as an opportunistic organism causing secondary fungal infection in immunocompromised individuals.
|Transmission||Contact with contaminated water, moist or wet substrates, or infected animals.
Any wound or abrasion of the skin can cause the infection to become easily established.
Crayfish may act as carriers.
|Clinical signs||Red or discolored skin, excessive skin shedding or dysecdysis, abnormal behavior such as absence of fear when approached and captured, occasional neurologic deficits like abnormal posture or loss of righting reflex, or sudden death. Skin lesions are most common on the digits and ventrum as well as the inguinal and pelvic regions.|
|Diagnosis||A presumptive diagnosis can be made by cytology, and diagnosis can be confirmed by histology, immunohistochemistry, or molecular identification (PCR) (Baitchman and Pessier 2013, Densmore and Green 2007). Morphologic tests (cytology and histopathology) are most appropriate for clinically significant infections that are associated with high numbers of organisms (Baitchman and Pessier 2013, Standler 2013). PCR testing is needed to diagnose low-intensity or subclinical infections (Baitchman and Pessier 2013, Stadler 2013).
An itraconazole bath is currently considered the treatment of choice for B. dendrobatidis. Itraconazole 0.005% (50 mg/L) is diluted with 0.6% saline or amphibian Ringer’s solution used as 5-minute bath once daily x 6-10 days (Stadler 2013, Jones et al 2012). Alternatively, lower concentrations of itraconazole 0.0025% (25 mg/L) for 5 minute baths for 6 days have also been successful in eliminating Bd (Brannelly 2014, Stadler 2013, Brannelly 2012). Lower drugs levels caused fewer side effects and less treatment-associated mortality.
Optimum growth of Bd occurs between 17°C and 25°C (62.6-77°F) (Baitchman and Pessier 2013). A useful adjunct treatment in the management of Bd is to elevated environmental temperatures of 37°C (98.6°F) for 16 hours in thermotolerant species (Stadler 2013, Pessier 2012, Retallick and Miera 2007, Woodhams et al 2003).
Recent studies show that sodium chloride (NaCl) has fungicidal properties that reduce the mortality rates of infected hosts in captivity (Stockwell et al 2015, Stockwell et al 2012). Frogs exposed to 3-4 ppt NaCl were found to have significantly higher survival rates (Stockwell et al 2012).
Chloramphenicol baths have also been reported as a treatment for chytridiomycosis (Baitchman and Pessier 2013, Young et al 2012). Three terminally ill green tree frogs (Litoria caerulea) with heavy Bd infections were cured using a combination of continuous shallow immersion in 20 mg/L chloramphenicol solution for 14 days, parenteral isotonic electrolyte fluid therapy for 6 days, and increased ambient temperature to 28°C (82°F) for 14 days (Young et al 2012). Chloramphenicol stock solution is made by adding 200 mg of chloramphenicol powder (Chloramphenicol C0378; Sigma-Aldrich) to 1 L of hot water. One part of the stock solution is diluted in 9 parts water to make a 0.002% treatment solution. To make 100 ml of 0.002% chloramphenicol treatment solution, add 10 ml of 200-mg/L stock solution to 90 ml water (Baitchman and Pessier 2013). Treatment solution is changed daily.
No treatment course should be considered effective without confirmation via post-treatment PCR. The highest level of confidence is achieved through three PCR tests performed over a 14-day period after treatment (Baitchman and Pessier 2013, Pessier 2012).
|Prevention||Isolate affected amphibians.|
|Control||Batrachochytrium is easily killed on equipment and in the environment with a wide range of disinfectants including bleach, Virkon, and quarternary ammonium compounds. Chytrid fungus is also destroyed by heat and desiccation, however the organisms can survive and remain infective for long periods in a moist environment (Baitchman and Pessier 2013).
Tadpoles and larval salamander are asymptomatic to chytrid infection, and are a source of spared of the disease. Tadpoles should not be released into the wild form one area to the other.
References and further reading
Baitchman EJ, Pessier AP. Pathogenesis, Diagnosis, and Treatment of Amphibian Chytridiomycosis. Vet Clin North Am Exot Anim Pract 16(3):669-685, 2013.
Brannelly LA. Reduced itraconazole concentration and durations are successful in treating Batrachochytrium dendrobatidis infection in amphibians. J Vis Exp 85:2014.
Brannelly LA, Richards-Zawacki CL, Pessier AP. Clinical trials with itraconazole as a treatment for chytrid fungal infections in amphibians. Dis Aquat Organ 101(2):95-104, 2012.
Densmore CL, Green DE. Diseases of amphibians. ILAR Journal 48(3):235-254, 2007.
Jones ME, Paddock D, Bender L, et al. Treatment of chytridiomycosis with reduced-dose itraconazole. Dis Aquatic Organ 99(3):243-249, 2012.
Murphy BG, Hillman C, Groff JM. Chytridiomycosis in dwarf African frogs Hymenochirus curtipes. Dis Aquat Organ 114(1):69-75, 2015.
Pessier AP. Diagnosis and control of amphibian chytridiomycosis. In: Miller RE, Fowler ME, editors. Zoo and wild animal medicine: current therapy, vol. 7. St Louis (MO): Elsevier Saunders; 2012. p. 217–223.
Retallick RW, Miera V. Strain differences in the amphibian chytrid Batrachochytrium dendrobatidis and non-permanent, sub-lethal effects of infection. Dis Aquat Organ 2007;75(3):201–207.
Stadler C. Chytridiomycosis disease. American Association of Zoo Veterinarians Infectious Disease Committee Manual. May 30, 2013. Available at http://c.ymcdn.com/sites/www.aazv.org/resource/resmgr/IDM/IDM_Chytridiomycosis_Disease.pdf. Accessed on July 5, 2015.
Stockwell MP, Storrie LJ, Pollard CJ, et al. Effects of pond salinization on survival rate of amphibian hosts infected with the chytrid fungus. Conserv Biol 29(2):391-399, 2015.
Stockwell MP, Clulow J, Mahony MJ. Sodium chloride inhibits the growth and infective capacity of the amphibian chytrid fungus and increases host survival rates. PLoS One 7(5):e36942, 2012.
Woodhams DC, Alford RA, Marantelli G. Emerging disease of amphibians cured by elevated body temperature. Dis Aquat Organ 2003;55(1):65–67.
Young S, Speare R, Berger L, Skerratt LF. Chloramphenicol with fluid and electrolyte therapy cures terminally ill green tree frogs (Litoria caerulea) with chytridiomycosis. J Zoo Wildl Med 43(2):330-337, 2012.
Ellison AR, Tunstall T, DiRenzo GV, et al. More than skin deep: functional genomic basis for resistance to amphibian chytridiomycosis. Genome Biol Evol 7(1):286-298, 2014.
Holden WM, Fites JS, Reinert LK, Rollins-Smith LA. Nikkomycin Z is an effective inhibitor of the chytrid fungus linked to global amphibian declines. Fungal Biol 118(1):48-60, 2014.
Jani AJ, Briggs CJ. The pathogen Batrachochytrium dendrobatidis disturbs the frog skin microbiome during a natural epidemic and experimental infection. Proc Natl Acad Sci USA 111(47):E5049-E5058, 2014.
Harkewicz K, Pollock C. Fast facts on the chytrid fungus. LafeberVet Web site. Available at https://lafeber.com/vet/fast-facts-on-the-chytrid-fungus/