(NEW YORK, NY) -- The higher salt content of seminal fluid and breast milk may overcome the ability of saliva to fight HIV infection, according to researchers at the University of Texas Medical Branch in Galveston. The finding helps to explain how semen and breast milk transmit the virus via the mouth.

Salivary transmission of HIV is relatively rare, Dr. Samuel Baron and colleagues explain. Saliva is thought to protect against HIV-transmitting white blood cells because of its unique low salt concentration, which helps to kill these cells, the authors note in the February issue of The Journal of Infectious Diseases. However, HIV is successfully transmitted via seminal fluid, breast milk and colostrum, white cell-rich fluid secreted in the first days of breastfeeding.

To further investigate how HIV-infected seminal fluid and milk overcome the protective effects of saliva, Baron's team conducted a series of laboratory studies using mixes of samples of saliva and milk, colostrum, and seminal fluid.

The researchers found that as the percentage of saliva was reduced, and the percentage of seminal fluid, milk or colostrum was increased, the ability of saliva to inactivate HIV-infected white blood cells decreased.

``When saliva was reduced to only one third of the mixture, with the remaining two-thirds being the saltier substances, then saliva was no longer protective against HIV,'' Baron commented in a University press release.

``Under normal circumstances, there is just about one-fifth of a teaspoon of saliva in the mouth,'' he continued. ''Deposited semen typically would equal more than four times that volume and mother's milk much, much more. So it's easy to see that saliva can't kill the infected cells because there's just too much of the saltier substances.''

Baron also believes these findings have implications for the actual cause of HIV infection. ``There is very good evidence that infected white blood cells are the main transmitting agent of HIV in the vagina and the rectum,'' he continued.

While many ``physicians still believe that HIV is transmitted by free virus'' from HIV-infected individuals, ''most cell-free virus is not infectious, probably because HIV carriers make sufficient antibodies, which bind to the virus and hamper its ability to infect other cells,'' Baron explained.

Conversely, ``HIV-positive milk and semen are highly infectious, probably because these fluids contain white blood cells that are infected by HIV and the antibodies can't get inside these cells to attack the virus,'' he added.

Baron's group is currently trying to develop anti-HIV gel that can mimic the protective effects of saliva in the vagina and rectum, environments that are both more salty than the mouth. SOURCE: The Journal of Infectious Diseases 2000;181.