A case study of a stillborn baby whose Brazilian mother was infected with Zika raises suspicions that the virus may be capable of doing more damage to fetal tissue than previously thought, researchers said on Thursday.

The study showed the baby's brain was absent, a condition known as hydranencephaly. Instead of tissue, the brain cavities were filled with fluid. The baby also had abnormal pools of fluid in other parts of its body.

The case, published in the journal PLOS Neglected Tropical Diseases, is the first to link Zika virus with damage to fetal tissues outside the central nervous system.

So far, birth defects associated with the rapidly spreading Zika virus have been almost entirely confined to Brazil and linked to microcephaly, a condition in which babies are born with abnormally small heads.

Brazil has confirmed more than 580 cases of microcephaly and is investigating more than 4,100 suspected cases.

Although Zika has not been proven to cause microcephaly, scientists say the evidence is growing stronger. On Feb. 1, the World Health Organization declared Zika a global health emergency. The WHO estimates Zika could eventually affect as many as 4 million people in the Americas and may spread to parts of Africa and Asia.

The new study was led by Yale University tropical disease expert Dr. Albert Ko along with Dr. Antonio Raimundo de Almeida of Roberto Santos General Hospital in Salvador, Brazil.

Ko said the study's findings are hard to generalize because they are on just one case, but they are unusual. In addition to microcephaly, he noted that the fetus had no brain tissue left. "It was just fluid."

Fluid also filled the lungs, abdomen and other tissues. These resulted from a condition known as hydrops fetalis in which the fetus loses the ability to manage body fluids. The fetus also had arthrogryposis, a condition in which joints don't move normally and may be stuck in place.

Ko has worked with Brazilian colleagues to understand the Zika outbreak since shortly after the first cases of the mosquito-borne virus were reported in the country in early 2015.

There is no vaccine or treatment for Zika, which causes mild fever, rash and red eyes. An estimated 80 percent of people infected have no symptoms, making it difficult for pregnant women to know whether they have been infected.

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The researchers describe a 20-year-old Brazilian woman who showed no signs of Zika infection or other related viruses, such as dengue and or chikungunya. Her pregnancy appeared normal through the first trimester, and she tested negative for other potential causes of microcephaly such as HIV, hepatitis C, rubella and toxoplasmosis.

During a routine ultrasound in her 18th week, doctors noticed the fetus was severely underweight. By the 30th week, the fetus had severe microcephaly and a range of other birth defects. At 32 weeks, the fetus had died and doctors induced labor.

Ko said the case suggests that the virus may be associated with stillbirths, which doctors should be looking for, especially in pregnant women who may not show signs of Zika infection.

"We can't really prove there's a causal association, but it raises concerns," he said.