Scientists have developed a new kind of “antisense” drug that has produced promising results in laboratory trials involving the ebola and marburg viruses, two of the most lethal known.

The drug works by blocking the critical genes the viruses use to replicate quickly inside the body to give patients valuable time to mount their own immune defence.

Tests on laboratory animals have shown the antisense drugs are effective at fending off ebola and marburg, which cause rapid and intense fever and internal bleeding fatal in about 90 per cent of cases.

There are at present no effective vaccines against either of the viruses, which have caused particular concern because of the possibility of their being used in biowarfare.

The drugs are composed of short strands of nucleic acids which form a sequence complementary or opposite to the sequence found in the genes of the viruses.

The researchers, from the United States Army Medical Research Institute of Infectious Diseases, report in the journal Nature Medicine that an antisense drug called AVI-6002 resulted in a survival rate of better than 90 per cent in laboratory mice and guinea pigs exposed to the Ebola virus.

When the scientists tested the drug on monkeys, three out of five survived.

A similar drug, AVI-6003, proved even more successful against the marburg virus, with every one of the monkeys surviving a viral attack, the study showed.

Ebola and Marburg are considered so dangerous the scientists had to wear space-suits and breathe filtered air to protect them as they did their experiments.