Chytrid Fungus in Frogs

For many years Chytrid fungi were thought to be free living organisms that would only infect vascular plants and a few invertebrates. But then in the 1990s a species known as Batrachochytrium dendrobatidis or Bd was discovered in dead and dying frogs. Since then the fungus has been sweeping through frog populations all over the world. It causes a disease known as chytridiomycosis which attacks keratin in a frog's skin.

Frogs use their skin for a number of functions including respiration so the disease causes breathing difficulties. It also affects the nervous system which creates behavioural changes.

Signs that a frog may have chytridiomycosis include:-

• Discoloured skin
• Abnormal sloughing of skin
• Lethargy
• Failure to seek shelter
• Abnormal posture - hind legs spread away from the body

The origin of the virulent pathogen is unknown, but many scientists suspect that it came from the Xenopus clawed frog in southern Africa. The earliest case of chytridiomycosis was found in Xenopus laevis in 1938.

The mechanism by which the fungus attacks frogs is only now starting to be revealed. In research carried out by a team of scientists mainly from Australia skin samples were taken from healthy and sick green tree frogs to get a handle on the fungi's modus operandi.

They came to the conclusion that Bd kills its amphibious victims by changing their electrolyte balance which results in cardiac arrest. The balance of electrolytes such as sodium and potassium is essential for the normal function of cells and organs of many living things.

In frogs the electrolytes pass through the skin, but in the diseased animals the researchers found that the compounds did not pass through it as readily.

Then the scientists took blood and urine samples from the frogs and observed that the concentrations of sodium and potassium were much lower in diseased frogs. In fact the potassium concentration was down by as much as 50%.

In other animals this kind of electrolyte reduction is known to cause cardiac arrest.

To further reinforce their hypothesis the scientists took electrocardiogram readings of the frogs' hearts a few hours before they died. They detected serious changes in the rhythm.

When the frogs were given drugs to restore their electrolyte balance their health was restored, although this was only for a few hours or days, as they all died in the end.

Although many frog experts find this evidence compelling, the exact mechanism by which the fungus wreaks its havoc is as yet unknown. Some speculate that it impairs electrolyte transfer by releasing a toxin, or attacking individual cells.

Although the research is a major step forward in understanding the fungus, it will not provide immediate benefit to frogs in the wild. This may come when the genetics of how the fungus disrupts electrolyte balance is revealed.