HIV transmission throughbreastfeeding is believed to occur because:

• HIV has been found in breast milk, and especially in colostrum.
• Transmission has been reported both in individual cases and in prospective studies.
• Infants' oral and gastro–intestinal mucosa are significantly more vulnerable than adults' and their immunity is less well developed.
• Blood, as well as breast milk may transmit the virus if the nipples are cracked or bleeding.

Researchers have suggested that there is a very high likelihood of transmission when the mother first becomes HIV–infected whilst breastfeeding (Van de Perre). In this situation, the risk of transmission from mother-to-child has been estimated to be about 30% (Dunn).

The breastfeeding and HIV International Transmission Study, which analysed data from over 5,000 children found that over a period of 6.8 months of breastfeeding, the best estimate is that a little over a third (36%) of infections were due to breas-feeding. As the overall infection rate was 25%, this suggests that 9%-10% of infants will be infected with HIV from seven months of breast-feeding.

Information from a number of studies (including the European Collaborative Study) has been used to estimate that the additional risk of transmission through breastfeeding for mothers who were already HIV–positive during pregnancy is an extra 14%.

Longer-term follow-up of mother-infant pairs involved in the African PETRA study has highlighted the potential for breast-feeding to undo the benefit which may be gained through the use of anti-HIV therapy. PETRA found that a short AZT/3TC regimen could reduce perinatal transmission, but within two years of birth, the risk of of HIV infection in infants exposed to treatment was no different than in untreated children - the treated children had effectively 'caught up' through the ongoing risk posed by exposure to breast milk (Gray).

Factors thought to increase the risk

A number of factors are thought likely to influence the chances of infection:

• Infection of the mother during the period of breastfeeding, resulting in temporarily higher levels of HIV in breast milk.
• The vulnerability of the child's oral and gastrointestinal mucosa and consequently their lesser immune function.
• Mixed-feeding practices where breast milk is supplemented by water, fluids, cereals, juices, etc.

Can the risks be reduced?

The risk of transmitting HIV during breastfeeding will be eliminated if the mother does not breastfeed. This includes not putting the baby to the breast immediately after delivery.

It must be remembered that in some countries where safe water is not available, the risk of other life threatening conditions from formula feeding may be far higher and more immediate than the risk of HIV from breastfeeding. The high cost of formula may also be prohibitive, and in many societies, formula-feeding is stigmatised and may identify women as HIV-positive.

Whether or not to breastfeed

This decision should be made on the basis of local circumstances.

The World Health Organization has recommended that in situations where infectious diseases are not a primary cause of infant death, a safer alternative to breastfeeding should be recommended. Therefore, in Europe and the USA, HIV–positive mothers are encouraged not to breastfeed because of the increasing likelihood of HIV transmission to the infant. Women at risk of becoming infected are also recommended not to breastfeed in case infection occurs during the period of breastfeeding.

In the UK the Department of Health recommends that it is `prudent' not to breastfeed, and has also issued instructions concerning the donation of breast milk. Women who wish to donate breast milk should follow the same self–exclusion criteria as for blood donors, and breast milk banks are advised to pasteurise all donations by heating.

For women in countries where there is unsafe water and formula-feeding is therefore dangerous, the benefits are likely to outweigh the risks. Factors to consider include the following:

• Other forms of feeding can introduce other infections into the child's body.
• Breast milk contains immunoglobulin A, which hinders infection by viruses and micro–organisms through the walls of the gut.
• Maternal antibodies contained in breast milk protect the child against many common infections.
• Breastfeeding may be psychologically satisfying to both child and mother, and a convenient and cheap source of nourishment.

However, these concerns must be balanced against the negative impact that breastfeeding may have on maternal health. A large Kenyan study found that breastfeeding women were three times more likely to die in the two years following delivery than women who formula fed their infant. Maternal mortality necessarily impacts on infant mortality - in this study, infants whose mother died during the period of follow-up were themselves eight times more likely to die consequently (Nduati).

A study involving women in Durban, South Africa, found an increased transmission risk associated with mixed-feeding - the use of supplementary liquids in addition to breast milk, compared to exclusive breastfeeding. It is suggested that mixed- feeding may have exposed babies to both allergens, causing inflammation and damage to gut mucosal barriers, and HIV, which in turn led to a higher infection rate (Coutsoudis 2000).

These data present health care workers with a significant challenge. Mixed-feeding is a very common practice in resource-poor nations. Only 26% of the women in the Durban study who chose to exclusively breastfeed their infants did so. A separate study involving women in rural Kwazulu Natal similarly found exclusive breastfeeding of very young infants to be uncommon. Of 124 infants followed for the first 16 weeks of life, 46% received supplementary fluid within the first 48 hours. 69% of mothers of these infants described a concern that their baby was unsatisfied as the reason for this. By 16 weeks, only 5% of babies were exclusively breastfed (Bland).

The 2005 Retrovirus Conference heard that the ‘Mashi’ trial, whose name means ‘milk’ in Tswana, included a phase which randomised babies to receive either formula feed, or breastfeeding plus AZT, up to seven months of age. All of the babies were given AZT until they were one month old.

There was little difference in the ultimate fate of the children in this trial. By the age of 18 months, 14% of formula-fed and 16% of breastfed babies were HIV-positive, including those who had HIV at birth.

However at seven months, when AZT was stopped, there was a larger difference, with 6% of formula-fed babies being positive, and 9% of those breastfed.

Set against this were higher mortality rates in the formula-fed babies. Nine per cent of formula-fed babies had died by eight months as opposed to 5% of breastfed babies. However by 18 months the ‘mortality and morbidity’ rates were similar: 14% in formula-fed and 16% in breastfed babies.

The original study design had also randomised mother / baby pairs to receive nevirapine or placebo at birth. The drug was given to the mothers during labour and to the infants no more than three days after birth.

This increased the difference between formula feeding and breastfeeding, with a 3% difference between breast and formula feeding in the placebo arm, but a 12% advantage to formula feeding by seven months in the nevirapine arm. Only 3% of babies given both formula feed and nevirapine were HIV-positive by seven months. This advantage disappeared during the second phase of the trial when all babies, but only half of the mothers received nevirapine. This seems to argue for continued measures to reduce the viral load in breast milk, especially in the immediate post-natal period.

One encouraging finding was that mothers were adherent to formula feeding. Only 9% of these mothers said they had sometimes breast-fed, despite 75% of trial participants having no mains electricity, 69% no fridge, and 45% no source of water on their premises. Formula-feeding mothers reported giving 95% of AZT doses to their babies, and breast-feeding mothers 86%.

In contrast only 18% of women in the breast-feeding arm said they exclusively breast-fed up to 18 months and only 50% to seven months.

The study had previously reported a 44% nevirapine resistance rate in mothers.

References

Bland RM et al. Longitudinal infant feeding study: constraints to exclusive breast feeding. 13^th International Conference on AIDS, Durban, abstract WeOrC497, 2000.

Bulterys M et al: Detection of HIV–1 in Breast Milk, Eighth International Conference on AIDS, Amsterdam, abstract ThC 1524, 1992.

Coutsoudis A. Method of feeding and transmission of HIV-1 from mothers to children by 15 months of age: Prospective cohort study from Durban. 13^th International Conference on AIDS, abstract LbOr6, Durban,  2000.

Dunn et al: AIDS 7: 1064–1066, 1994.

Dunn DT et al: Risk of HIV–1 transmission through breast feeding, The Lancet 340: 585–588, 1992.

European Collaborative Study: Children born to women with HIV–1 infection: natural history and risk of transmission, Lancet 339: 1007–1212, 1992

Gray G et al. The PETRA study: early and late efficacy of three short ZDV/3TC combination regimens to prevent mother-to-child transmission of HIV-1. 13^th International Conference on AIDS, Durban, abstract LbOr5, 2000.

Landesman S et al. Obstetrical factors and the transmission of HIV from mother to child, NEJM 334:25: 1617–1623, 1996.

Nduati R et al. Impact of breastfeeding on maternal mortality among HIV-1 infected women: results of a randomised clinical trial. 13^th International Conference on AIDS, Durban,  abstract WeOrC495, 2000.

Newell M et al: HIV–1 infection in mothers and babies. AIDS Care 2(3) 205–211.

Palmer G: Breast–feeding: the debate, WorldAIDS, Nov 1992.

Read JS et al. Breastfeeding and late postnatal transmission of HIV-1: an individual patient data met-analysis (Breastfeeding and HIV International Transmission Study). XIV International AIDS Conference, Barcelona, abstract TuOrB1177, 2002.

Sperling RS et al: Maternal viral load, zidovudine treatment and the risk of transmission of HIV type 1 from mother to infant, NEJM 335(22): 1621-1629, 1996.

Thior I et al. Breastfeeding with 6 months of infant zidovudine prophylaxis versus formula feeding for reducing postnatal HIV transmission and infant mortality: a randomized trial in Southern Africa. Twelfth Conference on Retroviruses and Opportunistic Infections, Boston, abstract 75LB, 2005.

Van De Perre P et al: Postnatal transmission of human immunodeficiency virus from mother to infant – a prospective cohort study in Kigali, Rwanda, NEJM 325 (9): 593–598, 1991.

Ziegler JB et al: Postnatal transmission of AIDS – associated retrovirus from mother to infant, Lancet i: 896–898, 1985.