Evidence of West Nile Virus Found in Breast Milk

Health authorities said Friday that they found evidence of West Nile virus in breast milk from a nursing mother, prompting concerns of a new transmission pathway for the disease.

Although the Michigan woman’s child has not become sick, authorities fear that the infection could be transmitted in breast milk just like HIV and some other viruses.

That would make a fourth route of transmission. The Centers for Disease Control and Prevention already had concluded that the virus can be transmitted not only by mosquito bites, but also via organ transplants and blood transfusions.

CDC officials also said that the deadliest outbreak of West Nile virus in the United States--an epidemic that has sickened 2,339 Americans and claimed 116 lives this year--appears to have peaked and the number of new cases should begin to decline in the coming weeks.

Cooler weather ends the mosquito season, reducing human exposure to the virus, according to Dr. Lyle Petersen of the CDC. Tropical Storm Isidore probably also washed away a significant portion of mosquito eggs and larvae when it flooded parts of Louisiana and Mississippi this week, further reducing the threat in those states.

The woman in whose breast milk the virus was found is thought to have been infected via a transfusion.

The 40-year-old woman gave birth Sept. 2 and received transfusions that day and the next. Although she developed a fever, she was sent home Sept. 4. She developed flu-like symptoms, however, and was rehospitalized for three days two weeks later, suffering what is now a confirmed case of West Nile virus, said Dr. Matthew Boulton, a Michigan state epidemiologist.

Researchers have not proved that she was infected by a transfusion, but tests of other blood from the same donor show that it was contaminated by the virus, Boulton said. He added that Michigan officials have also studied a case involving a 47-year-old man who developed West Nile disease after a blood transfusion. In that case also, the virus was found in the donor blood, strengthening the conclusion that transfusions are a potential source of the disease.

The Michigan woman breast-fed her baby for two weeks, but stopped on the advice of her physician when she was hospitalized. Subsequent tests showed the presence of genetic material from the virus in the milk. The baby has not developed any symptoms, however, and is healthy. Michigan officials are now checking the baby’s blood, however, to determine whether the virus was transmitted to the child.

But Petersen cautioned that the CDC has received no confirmed reports of West Nile virus transmission via breast milk. In fact, only four babies under the age of 1 year are known to have been infected. “This would suggest that the risk of West Nile virus infections from breast milk is going to be low, if there is any risk at all,” he said.

Petersen urged that women not forgo breast feeding unless they have a confirmed case of West Nile virus, and even then they should consult with their physician before doing so.

“If [a woman] has severe disease and cannot breast-feed easily and provide sufficient nutrition to her child, we certainly in that case recommend supplemental feeding,” he said. “On the other hand, breast feeding has many beneficial effects ... and the decision to discontinue breast feeding is a big one.”

CDC currently recommends a halt to breast feeding only if the mother is infected by HIV or human T-cell leukemia virus type 1.

This year, there have been confirmed reports of West Nile infections in 32 states, including one in California, and the District of Columbia.

Petersen said the data suggest that the epidemic peaked in the Southern states in August and within the last two weeks in Northern states.