Fast Facts on the Chytrid Fungus

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.

Chytridiomycosis
Incubation14-70 days
TransmissionContact 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 signsRed 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.
DiagnosisA 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).

  • Suitable cytology samples are gentle skin scrapings from the feet and ventral body or shed skin fragments, which are either examined as a wet mount or air dried on a microscope slide and routinely stained (Baitchman and Pessier 2013). Shed skin fragments can be collected from the animal or from the environment (Baitchman and Pessier 2013).

  • Histopathology samples, generally obtained at necropsy, should include at least three sections of skin from the ventral body such as the pelvic patch, legs, and feet) (Baitchman and Pessier 2013)

  • Molecular identification (PCR) is performed with skin swabs or pieces of skin (Stadler 2013). Use fine-tipped swabs, not wooden-handled; gently swab the skin 20 to 30 times (Stadler 2013).

  • Samples for PCR testing are best sent to reliable labs, preferably those at university, such as the University of Florida Zoo Medicine Infectious Disease Lab. Cheap labs advertised on the Internet can often report either false positives or negatives.

  • Other laboratories performing PCR testing include: Amphibian Disease Lab, Pisces Molecular, Research Associates Laboratory, and Zoologix Laboratories.

TreatmentITRACONAZOLE

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.

TEMPERATURE

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).

SODIUM CHLORIDE

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

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.

POST-TREATMENT PCR

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).
PreventionIsolate affected amphibians.
ControlBatrachochytrium 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