Kiss that frog: amphibian answer to mosquito peril
[AFP 2006-02-21]

Feb 21 7:21 PM US/Eastern

A bottle-green Australian frog may hold the key to a next-generation mosquito repellent, according to a scientific paper due to be released.

Scientists are marvelling over secretions exuded by the dumpy tree frog (Litoria caerulea), a species that inhabits forests in northern Australia and New Guinea.

Using a small electrical current, they gave a gentle zap to a frog, causing smooth muscles in its glistening skin glands to contract and secrete the fluid that covers its skin.

The secretions were washed off with distilled water and applied to the tails of lab mice.

The mice were then exposed to dozens of Culex annulirostris mosquitoes -- an aggressive Aussie mozzie notorious for transmitting encephalitis among other diseases.

Mice which had been give the frogs' secretions remained bite-free for up to 50 minutes. Those which had been given DEET, the chemical that is typically used in commercial mosquito repellant, were protected for up to two hours.

But the luckless rodents that had been selected as "controls" and were given neither frog secretion nor DEET, lasted just 12 minutes before their tails were bitten.

Two other Australian species, the desert tree frog (Litoria rubella) and Mjoberg's toadlet (Uperoleia mjobergi), were found to have a mosquito repellent odour from their skin, although their secretions were not tested on mice.

The dumpy tree frog and its slimy secret are not in themselves considered a substitute to DEET, the repellent originally formulated for the US army after World War II.

But the discovery highlights the potential of the unsung properties of amphibian skin, the paper says.

"Many aspects of frog chemical ecology remain unexplored," it points out.

The paper appears in Biology Letters, which is published on Wednesday by Britain's Royal Society, the de-facto British academcy of sciences.

Frogs and toads have long been known to exude toxic or malodorous chemicals on their skin as a form of protection against fungus and insect pests and to ward off predators.

Previous research has uncovered that these secretions can also be powerful painkillers and hallucinogens. Work is now unfolding to synthesise such molecules so that they can be reproduced pharmaceutically.