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Biology (Year 12) - Specific Disease Studies

Ben Whitten

What causes chytridiomycosis? Chytrid fungus (Batrachochytrium dendrobatidis). This disease affects amphibians (frogs) in particular, and is known to decimate many frog populations. Where is the chytrid fungus usually found? Chytrid fungi typically live in water or soil, although some are parasites of plants and insects. They reproduce asexually and have spores that 'swim' through the water. Only the amphibian chytrid fungus is known to infect vertebrate species. Amphibians (frogs) are also considered a reservoir for chytridiomycosis.

How is the chytrid fungus commonly contracted between hosts? Chytrid fungus is often transferred via direct contact between frogs and tadpoles, or through exposure to water which has been infected by the fungus. The disease usually doesn’t kill amphibians immediately, and they can swim or hop to a number of different areas before they die, spreading fungal spores to new ponds and streams, hence increasing the spread of the disease.

What is the typical pathology (signs, symptoms and progress) of chytridiomycosis? Amphibians such as frogs are highly reliant on their skin for processes such as oxygen and carbon dioxide exchange (respiration), water absorption, pathogen defence and electrolyte transportation, therefore the impact of the disease is extensive. As the pathogen invades the skin, the outer keratin layers are damaged, disrupting respiration, salt regulation and osmoregulation. The skin thickens and hardens. As respiration, water and electrolytes decrease, so does the activity level of the frogs. Signs of the disease include lethargy, extension of the hind legs away from the body, and abnormal behaviour such as sitting in the sun instead of the shade.

What is the life cycle of chytridiomycosis?

  1. Zoospores are produced and released from a mature zoosporangium (thallus) discharge tube

  2. Zoospores swim a short distance or are carried by water currents

  3. Zoospores encounter a new susceptible host and attach to and penetrate a skin cell – this is the portal of entry

  4. The cells invade the skin, absorbing nutrients; a new zoosporangium develops via asexual reproduction

  5. The zoosporangium matures and zoospores are produced

  6. Chytridiomycosis damages the frog’s skin causing reduced respiration and osmoregulation, leading to death

What are the treatments, prevention and control measures in Australia? The use of antifungals and heat-induced therapy. Itraconazole is the antifungal treatment of choice in treating Bd. Temperature-controlled laboratory experiments are used to increase the temperature of an individual past the optimal temperature range of Bd. Experiments, where the temperature is increased beyond the upper bound of B. dendrobatidis optimal range of 25 to 30 degrees show its presence will dissipate within a few weeks and individuals infected return to normal. The following precautions should be taken if you intend visiting frog habitats in the wild, including nature ponds in parks or gardens:

  • Only touch frogs when absolutely necessary. Remember to use disposable gloves, sample bags and sterile equipment.

  • Clean and dry all equipment and wet or muddy footwear before and between visiting frog sites. This may include cleaning the tyres of your vehicle before visiting known high-risk sites where threatened frog species may live.

  • Never move a frog from one area to another.

  • Carry cleaning utensils and a disinfectant for use between sites.

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