Frogs Australia Network

Chytrid fungus

Many recent frog declines have been attributed to diseases. The infectious disease responsible for many of these deaths is Chytridiomycosis caused by the chytrid fungus, Batrachochytrium dendrobatidis. This disease is a global epidemic that has been associated with frog deaths in Australia, New Zealand, Europe, North America, Central America, South America and Africa.

The disease was reported in Australia in the 1990s, however, scientists using museum specimens were able to show it has been present in this country at least since 1978. It was probably introduced to Australia through the trade in exotic amphibian pets (Axolotyls) and frogs arriving in rainforest plants and agricultural produce.

What does it do to frogs and tadpoles?

Chytrid fungus attacks the parts of a frog's skin that contain keratin (a tough, fibrous protein that forms the basis of hair, nails, horns etc). The skin of a sick frog then gets thicker and sometimes looks a bit different.

The fungus is thought to kill frogs either by releasing fatal toxins that are absorbed by the frogs semi-permeably skin, or through damaging the skin so badly that the frog's water and electrolyte balance as well as their respiration are affected. The fungus also damages the nervous system, affecting the frog's behaviour.

Infected frogs may have the following symptoms:

loss of skin from the arms, legs and belly
are sluggish, and have no appetite
trembling
sit out in the open, not protecting themselves by hiding
have their legs spread slightly away from themselves, rather than keeping them tucked close to its body.

Note: Some frogs display no symptoms and die suddenly

Tadpoles can carry the fungus, but since they only have keratin in their mouths they are not killed until they start to turn into frogs. Sometimes the fungus can have a localized effect on the tadpole’s mouth parts, causing them to fall out or be damaged.

How is it spread?

Chytrid fungus is probably transferred by direct contact between frogs and tadpoles, or through exposure to infected water. The disease may not kill frogs immediately, and they can move to other areas before they die, spreading fungal spores to new ponds and streams.

Apart from the movement of infected frogs the fungus is also spread by:

Frogs hitchhiking in boxes of fruit or vegetables (also referred to as ‘banana box’ frogs)
Wet soil on the shoes of bushwalkers who walk in an infected frog area and then a non-infected area
Flooding
Water birds
Car tyres in the wet weather
Releasing frogs and tadpoles in a different place to the one where they came from
Moving pond plants from infected water

Frog populations at risk

Frogs from high altitude regions are thought to be more affected by chytrid fungus as experiments have shown that infected frogs are more likely to die at lower temperatures. Equally frogs that live in streams are more likely to be infected as the disease is waterborne.

Combination of threats

Some researchers have suggested that the appearance of chytrid fungus in a frog population make it less able to cope with other threatening processes. The death of adult frogs from the disease in conjunction with predation from introduced fish, habitat fragmentation and pollution would increase the likelihood that an entire population could go extinct.

For further information visit:

James Cook University – Amphibian diseases home page
http://www.jcu.edu.au/school/phtm/PHTM/frogs/ampdis.htm

New South Wales National Parks and Wildlife Service - Frog Chytrid Fungus
http://www.nationalparks.nsw.gov.au/npws.nsf/Content/Frog+Chytrid+fungus