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DISCOVERY!
Irwin D. Mandel, Editor
KEY WORDS: HIV, saliva, transmissions, protection.
Received January 18, 2001; Accepted April 12, 2001
INTRODUCTION
The oral transmission of HIV is infrequent. In comparison, sexual transmission of HIV is frequent. Studies of this dichotomy have led to new concepts of mucosal transmission of HIV and of salivary defenses. One emerging hypothesis may better explain the epidemiology of HIV and salivary protection, and potentially lead to prevention of sexual transmission.
INCIDENCE OF ORAL TRANSMISSION OF HIV
There are now 36 million chronically infected carriers of HIV worldwide. At the outset of the epidemic, there was concern that HIV might be transmitted casually from the oral secretions of HIV carriers during kissing, dental treatment, biting, and aerosolization. Later it became clear that oral transmission is actually a rare event (Rogers et al., 1990; Gooch et al., 1993; Moore et al., 1993). Correspondingly, the incidence of infected saliva in HIV carriers is low-only 1%-although all carriers have infectious virus in their blood (Barr et al., 1992; Moore et al., 1993; Coppenhaver et al., 1994). Saliva of infected individuals usually contains only non-infectious components of HIV, indicating that saliva may cause a breakdown of HIV and infected leukocytes (Goto et al., 1991; Baron et al., 1999). In comparison, 20% of seminal fluids of carriers contain infectious HIV (Milman and Sharma, 1994; Zhu et al., 1996). Thus, the rarity of casual oral transmission correlates with the low infectivity of virus in saliva compared with high vaginal transmission (27%) and high infectivity in seminal fluid (European Study Group on Heterosexual Transmission of HIV, 1992; Royce et al., 1997).
MECHANISMS OF MUCOSAL TRANSMISSION
In blood and secretions such as the seminal fluid and milk of chronically infected carriers, most infectious HIV occurs in infected leukocytes and a smaller fraction as cell-free HIV (Escaich et al., 1991; Simon et al., 1993; Bouscarat et al., 1996). In fact, after the acute HIV infection subsides, the infectious cell-free virus in plasma and secretions declines, but viral RNA with low infectivity remains. Low infectivity of cell-free HIV in body fluids and secretions has been attributed either to defective virus or to neutralization of cell-free HIV by antibody produced after the initial acute infection (Daar et al., 1991; Ariyoshi...