WASHINGTON (AFP) - Zapping testicles with ultrasound can reduce sperm counts and might be used in the future as an inexpensive, reliable and reversible male contraceptive, according to US researchers.

Scientists from the University of North Carolina School of Medicine, who conducted experiments on male rats, issued their findings on Saturday in the journal Reproductive Biology and Endocrinology.

They said they were convinced that the method, if studied further, could be used on humans.

The team led by Doctor James Tsuruta found that by rotating high frequency

(3MHz) ultrasound around the testicles, they were able to cause depletion of germ cells throughout the testes, the reports said.

The best results were seen using two ultrasound sessions of 15 minutes, two days apart.

Saline was used to provide conductivity between the ultrasound transducer and the skin, and the testes were warmed to 37 degrees Celsius (almost 99 degrees Fahrenheit), the study noted.

The procedure reduced the sperm count to zero, the researchers said.

Fertile men in normal conditions have more than 39 million sperm when they ejaculate.

The World Health Organization has defined low sperm concentration as less than 15 million sperm per milliliter.

"Unlike humans, rats remain fertile even with extremely low sperm counts,"

Tsuruta said. "However, our non-invasive ultrasound treatment reduced sperm reserves in rats far below levels normally seen in fertile men."

He said that further studies will be required to determine how long the contraceptive effect lasts and if it is safe to use multiple times.