Page 31 - AIDSBK23C

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time of delivery through the birth canal, or through breast milk in infancy and childhood.
Intrauterine transmission accounts for 15 to 20% of infections, while 45 to 50% occur during
labor and delivery. Postpartum, up to 24 months, following birth, 30 to 40% of HIV
transmission mother-to-child can occur. HIV-1 transmission to the fetus may occur through the
placenta or by swallowing large amounts of infected amniotic fluid in utero or through contact
with blood and vaginal secretions during delivery. Postpartum, HIV in milk from breastfeeding
can cross mucosal surfaces in the gastrointestinal tract. Ingested fluids and cells with HIV can
pass through the neonatal stomach that lacks an acid environment and reach the intestine. The
passage of cell-free virus across placental trophoblasts is restricted, so transmission of HIV may
rely on breaches of the placental barrier or on direct infection of placental cells or transcytosis of
cell-associated virus. HIV can be detected in syncytiotrophoblasts, Hofbauer cells, and placental
macrophages in the placenta during both early and late stage of pregnancy.[172,173]
The probability of breast-milk transmission of HIV-1 is calculated to be 0.00064 per liter
ingested and 0.00028 per day of breast-feeding. Breast-milk infectivity is significantly higher for
mothers with more advanced disease with higher prenatal HIV-1 RNA plasma levels and CD4
cell counts. The probability of HIV-1 infection per liter of breast milk ingested by an infant is
similar in magnitude to the probability of heterosexual transmission of HIV-1 per unprotected
sex act in adults.[174]
Vertical transmission of HIV-1 from mother to child from breast-feeding has been
estimated to occur in 14% to 16% of women who breast-feed with an established maternal HIV-
1 infection and in 29% with acute maternal HIV-1 infection. The risk for HIV-1 transmission
from an infected mother to an infant through breast-feeding is increased with the duration of
breast-feeding and with increased maternal viral load.[175] The risk for transmission of HIV-1
is also increased with presence of mastitis or breast abscess.[176] Most cases of HIV-1
transmission through maternal milk occur early during breast-feeding. HIV-1 can be detected in
over half of breast milk samples from infected mothers.[169,172,177]. Replication of HIV-1
within mammary epithelial cells has been demonstrated, and is increased by hormonal
stimulation in pregnancy.[178] HIV and HIV-infected macrophages can transmigrate across
fetal oral mucosal squamous epithelium. HIV-infected macrophages and, to a lesser extent,
lymphocytes can transmigrate across fetal intestinal epithelia.[37]
Perinatal transmission leading to congenital AIDS occurs, on average, in one fourth of
babies born to untreated HIV-1 infected mothers who appear well, and in two thirds of mothers
with HIV related disease or prior vertical transmission. The most significant maternal risk factor
for perinatal transmission is the HIV-1 viral load, but there is no safe threshold. Additional
maternal factors cited for congenital HIV-1 transmission are: a low CD4 lymphocyte count, p24
antigenemia, prematurity, and placental chorioamnionitis or funisitis. Parity, race, mode of HIV
acquisition, and sex of the baby do not appear to be significant factors in the vertical
transmission of HIV.[179,180]
The likelihood for vertical HIV-1 transmission is markedly reduced, down to only 1%, by
employment of multiple strategies: antiretroviral prophylaxis during pregnancy and in the
intrapartum period, elective caesarean section, and neonatal antiretroviral prophylaxis. Mother
to child transmission of HIV is further reduced by avoidance of breast-feeding. For patients with
access to health care in developed nations, such strategies are possible, but not for disadvantaged
patients.[181] A reduced duration of breast-feeding for uninfected children born to HIV-infected
mothers living in low-resource settings has been associated with significant increases in