By Gene Emery

NEW YORK (Reuters Health) - Pregnant women infected with Ebola may be contagious for days before they show symptoms, a group of doctors is warning in the June 18 New England Journal of Medicine.

The warning is based on a single case and the woman did not actually spread the deadly infection to anyone. But laboratory tests revealed high levels of the virus when no symptoms were present.

Usually people aren't considered to be contagious until they start to feel ill.

The reason pregnant women may be an exception may have to do with the way pregnancy affects the woman's body, the doctors say. Her immune system becomes more tolerate of the fetus, whose tissues would normally be considered foreign.

As a result, her symptoms may take longer to become obvious.

"The unique immunologic status of pregnant women might alter disease presentation and progression," the doctors write in a letter to the Journal.

Their theory is based on the case of a 31-year-old pregnant woman in Liberia who sought medical attention because she thought her membranes had ruptured prematurely.

She reported having no contact with anyone with the disease and showed no signs of fever, which is usually one of the first symptoms of Ebola.

But an initial precautionary blood test showed a high viral load. The result was so unusual the doctors had her blood tested again. She tested positive once more.

It wasn't until the evening of her third day of care that symptoms started to appear. She died seven days after admission, her baby still in the womb. Pregnant women seldom survive a case of Ebola.

In addition to the blood, the virus showed up in samples from the urine, vagina and mouth, but not skin. The levels "in the various samples suggest that she may have been able to transmit (Ebola virus) for several days before the onset" of symptoms," said the team, led by Dr. Emma Akerlund of Medecins san Frontieres (Doctors Without Borders) in Brussels, Belgium.

"This case highlights the challenges that clinicians may face in assessing pregnant women for possible infections, including (Ebola), and the potential risk for health care staff," they said.

The corresponding author, Dr. Livia Tampellini, also of Medecins sans Frontieres, did not respond to a request for an intervew.