It is a small risk, but when it is you, no risk is too small

Dr Jean Cozzi, Jean Verdier Hospital

A French team took sperm from mice, heated it to 58 degrees centigrade for half an hour, then used individual sperm for ICSI fertilisation.

Single sperm injection

This technique injects a single sperm into the egg to fertilise it.

ICSI has to be used, as the heating makes the sperm immobile and unable to fertilise the egg in the natural way.

In addition, heating up the sperm destroys a protein which is key to activating the egg during fertilisation, reducing the overall chance of successful implantation from 30% to 13%.

However, the researchers managed to produce two healthy baby mice from heated sperm, and genetic analysis suggested that no damage to the DNA had been suffered.

They are still healthy several months on and researchers plan to examine their genes more closely, and those of their offspring, to check the techniques are safe.

The chances of male parent to baby transmission is small, especially using ICSI, but Dr Jean Cozzi, from Jean Verdier Hospital near Paris, said that the technique could provide reassurance to parents desperate to avoid even the slightest chance.

At the moment, would-be parents are offered a sperm "washing" technique prior to insemination, which removes cells and other detritus which can harbour the virus.

However, transmission of HIV despite this has been reported.

The temperature used by the team is the same used to treat blood products such as plasma for potential HIV contamination, and is generally accepted as sufficient to kill the virus.

Dr Cozzi told BBC News Online: "It is a small risk, but when it is you, no risk is too small.

"We have to go on to make sure it is safe - that is crucial."