The discovery, by the University of Minnesota, raises hopes of a similar microbicidal treatment to block HIV transmission in humans. Several microbicides have been tested, but results have been disappointing.
The study - focusing on a compound called glycerol monolaurate (GML) - is published online by the journal Nature.
GML is a naturally occurring compound widely used as an antimicrobial and anti-inflammatory agent in food and cosmetics. Crucially, it is also cheap, and is likely to protect against other sexually transmitted infections too.
Lead researcher Dr Ashley Haase said that if GML proved to be effective in blocking HIV it could potentially help to save millions of lives. A majority of cases of HIV worldwide are now contracted vaginally, and in Sub-Saharan Africa, where the pandemic is at its most intense, women account for nearly 60% of new infections.
Ten monkeys - five treated with the gel, and five untreated - were injected with doses of SIV large enough to infect 50% of cells. Four hours later, the monkeys were again treated with GML and then given a second dose. The researchers monitored the animals for evidence of SIV infection for two weeks.
If there was no evidence of infection, the procedure was repeated for a second time.
Four of the five untreated monkeys contracted SIV, but the infection was not seen in any of the animals who were treated with the GML gel.
Further research is needed before the results can be extrapolated to humans.
