(introduction...)

Mother-to-child transmission (MTCT) of HIV, which can occur during pregnancy, delivery or breastfeeding, is responsible for more than 90% of HIV infection in children worldwide (UNAIDS/WHO, 1998). The present review focuses on HIV-1. Both HIV type 1 (HIV-1) and HIV type 2 (HIV-2) can be transmitted from mother to child, but HIV-2 is transmitted much less frequently, as it is less pathogenic than HIV-1 (Adjorlolo-Johnson et al., 1994; Andreasson et al., 1993; Morgan et al., 1990).

The remaining 10% of paediatric infections are attributed to transfusion with contaminated blood and blood products, use of contaminated medical equipment, other practices that cut or pierce the skin, or sexual contact (MAP, 1998; UNAIDS/WHO, 1998; Tovo et al., 1988).

HIV infection in women

Most children acquire the virus through transmission from an HIV-infected mother, therefore, the incidence of paediatric HIV reflects that of HIV infection in women of childbearing age. In areas of high seroprevalence, a significant number of children are at risk.

Mother-to-child transmission (MTCT) of HIV focuses attention on women, but the use of the term MTCT is not to imply blame, whether or not a woman is aware of her own infection status. A woman can acquire HIV through unprotected sex with an infected partner, by receiving contaminated blood, or through exposure to unsterile instruments or medical procedures. HIV is often introduced into the family through the woman's sexual partner, often the father of her child.

The prevalence of HIV varies considerably from region to region. Women and children in sub-Saharan Africa are disproportionately affected, with eight in every 10 HIV-infected women worldwide, and nine in every 10 newly infected children living in this region (MAP, 1998; UNAIDS/WHO, 1998). In West and Central Africa, HIV prevalence in pregnant women currently reaches 10-15% in some urban areas and 1-5% in others. Prevalences in East Africa are higher at 15-25% in urban areas and 5-10% in rural areas, while in Southern Africa antenatal seroprevalences of 20-30%, and in some places even as high as 40%, have been reported (MAP, 1998; UNAIDS/WHO, 1998). In the Caribbean, Central America and South America, HIV-1 seroprevalence rates currently range from 0.1% - 5.0%. Asia is experiencing a rapidly growing epidemic with seroprevalence rates in big cities of Cambodia, India and Thailand currently ranging from 1-5% (UNAIDS/WHO, 1998).

Rates of mother-to-child transmission

Estimates of the rate of mother-to-child transmission of HIV in cohorts of women who have not received any preventive treatment (such as antiretrovirals) range from 15-25% in industrialized countries to 25-45% in developing countries (Msellati et al., 1995). The highest rates of MTCT have been found in women in Africa (Kind et al., 1998; Maguire et al., 1997; Ometto et al., 1995; Lallemant, Le Coeur et al., 1994; Roques et al., 1993; European Collaborative Study, 1992; Blanche et al., 1989).

Differences in study methods, the composition of the populations studied, and the prevalence of co-factors of transmission may explain some of these differences. However, it is likely that much of the increased rate of transmission seen in women in sub-Saharan Africa is associated with breastfeeding,¹ where many women breastfeed for about 2 years (The Working Group on Mother-to-Child Transmission, 1995; Ryder and Behets, 1994; Dabis et al., 1993).

¹ Many women who breastfeed do not breastfeed exclusively. Other fluids (juices, milks, teas) and foods may also be given to the infant. In many studies looking at HIV transmission and breastfeeding no differentiation is made between women who "exclusively" or "partially" breastfeed. In this document, unless otherwise stated, "breastfeeding women" will often include both women who "exclusively" or "partially" breastfeed.

In an attempt to quantify the relative contribution of intrauterine and intrapartum transmission of HIV in non-breastfed infants, a working definition of timing has been proposed (Bryson et al., 1992).

In utero infection. In this, a child is classified as having been infected during pregnancy (in utero) if HIV-1 genome is detected within 48 hours of delivery by polymerase chain-reaction test (DNA-PCR) or viral culture.

Intrapartum infection. Acquisition of infection is assumed to have occurred during delivery (intrapartum) if these diagnostic tests were negative in a sample taken during the first 48 hours after delivery, but became positive in subsequent samples taken within 7-90 days of delivery.

Following this classification, a French study estimated that of the infants infected with HIV, 35% of the non-breastfed infants studied were infected before birth and 65% were infected late in pregnancy or during delivery (Rouzioux et al., 1995). A recent review indicated that in women who did not breastfeed their infants, about one-third of MTCT infection was acquired during the intrauterine period. In women who did breastfeed their infants, less than a quarter of all MTCT was acquired during the intrauterine period (Newell, 1998).

Table 1. Percentage HIV infection acquired by different routes *

*These rates are observed in the absence of interventions to reduce MTCT