In 2012, a one-year-old boy in Belgium acquired HIV through his mother's breast milk, a rare but not impossible form of transmission in industrialised countries, doctors say.
"Breast milk transmission has been recognized as an efficient mode of mother to child transmission of HIV for almost 30 years," said senior author Dr. Philippe Lepage.
Read: Where does Aids come from?
This transmission is common for mothers who were already infected before giving birth in developing countries, but it's extremely rare in industrialised countries, where HIV positive women are encouraged not to breastfeed, Lepage said.
Lepage is the head of the department of paediatrics at the Hopital Universitaire des Enfants Reine Fabiola in Brussels, Belgium.
Healthy at time of birth
In this case, according to a report in the journal Paediatrics, the boy's mother was tested for HIV during pregnancy and soon after delivery. The tests were negative both times.
His parents, both from the Democratic Republic of Congo, and three siblings were all healthy at the time of his birth.
But at one year of age the child was hospitalised for bronchitis, where he and his mother were diagnosed with HIV.
"If mothers become HIV infected while they're breastfeeding, their babies are at very high risk of getting the infection," said Jean Humphrey, director of the Zvitambo Institute for Maternal Child Health Research in Harare, Zimbabwe.
She speculated that in this case, the father contracted the infection after the child was born and gave it to his wife.
Read: South Africans not faithful and not using condoms
More screening wouldn't help prevent this kind of transmission, since by and large the mother will have already given the infection to her child by the time she tests positive, she said.
"The real way to prevent this kind of infection is to make women who are breastfeeding understand that having unprotected sex while breastfeeding is putting their babies at very high risk," Humphrey said.
In Africa, it has been suggested that all breastfeeding women use condoms during sex to drive home the message of prevention, she said.
"Sometimes, here, men take greater notice of the fact that sleeping around puts their baby at risk compared to putting their wife at risk," she said.
The only way to prevent this kind of transmission is strong counselling of both parents, Humphrey said.
Lepage and his co-authors suggested the same solution in the case report.
"These interventions should focus on parents from high-HIV frequency countries and also on health care professionals counseling pregnant women who might be unaware of the possible risk associated with breastfeeding," he told Reuters Health by email. "Counselling should address condom use and also include education on the high risk of HIV postnatal transmission after heterosexual exposure during breastfeeding."
For a pregnant woman who knows she has HIV, whether or not she will breastfeed depends on where in the world she lives, which has been one of the most tumultuous public health policy decisions in the past 20 years, Humphrey said.
Read: Child 'cured' of HIV remains virus free
As Lepage noted, in North America, Europe and Australia, HIV positive women do not breastfeed, since safe alternative formulas are available.
For a time women in Africa were also encouraged to formula feed, but that policy led to a wave of diarrhoea deaths, Humphrey said.
"In developing countries, breastfeeding is so important to prevent infections," she said.
"Now there are drug regimens in all African countries that reduce breast milk transmission rates to under 2 percent if it's done right," she said.
With drugs that lower transmission rates, the benefits of breastfeeding now outweigh the risks in Africa, she added.
"Still it's extremely emotive, giving different policies on breastfeeding to poor women and to wealthy women," she said.
Could legalising sex work stop HIV spread?
Bringing HIV testing and prevention to the people
Improper condom use puts gay men at increased HIV risk
Image: Mother is breastfeeding from Shutterstock
See breaking news and the hottest health tips before anybody else by joining South Africa’s biggest and best health community, like Health24 on Facebook now!