The Zika outbreak in the Americas has raised concerns around the world, not only because the virus spreads quickly and has been linked to birth defects, but also because so little is known about the fever disease that normally causes only mild symptoms.

Here are five of the problems that researchers are still trying to solve.

SEXUAL TRANSMISSION

US health authorities have reported a case of sexual transmission of the virus that is normally spread by mosquitoes. However, WHO spokesman Christian Lindmeier said it was still unknown how highly infected a person must be to pass on the virus through blood or semen. "Are we talking about extremely exceptional cases in which other factors also play a role, or are minor infections sufficient?," Lindmeier said. However, case numbers in the Americas would likely be much higher already if low concentrations of Zika virus in humans were sufficient for transmission through body fluids, he added.

INFECTIONS FROM MOTHER TO CHILD

Medical experts are trying to find out why the outbreak has coincided with a spike in babies born with abnormally small brains and heads, a condition known as microcephaly. The possible mode of transmission inside the womb is still being studied. It is also unclear if the problem affects mothers who were stung by mosquitoes before conception or during pregnancy, according to Lindmeier. "Research in this field is still at a very early stage, and we are working intensely on his," he said.

MOSQUITOES

Aedes aegypti, known as the yellow fever mosquito, is considered the main culprit in the current outbreak, but according to WHO the virus can also be transmitted by Aedes albopictus, known as the tiger mosquito. Aedes albopictus has been spotted in a larger number of European countries in recent years than Aedes aegypti. However, German virologist Christian Drosten said additional experiments in were needed to prove that Aedes albopictus can carry Zika. "The question will be whether European mosquitoes can become infected under realistic conditions and whether they can pass it on," the Bonn University scientist said. In any case, the European climate prevents mosquitoes from breeding at rates seen in tropical regions.

MICROCEPHALY MONITORING

Brazilian authorities are investigating 3,670 suspected cases of babies with small heads, but scientists have struggled to prove a direct link to Zika. A main reason for this is the lack of a standard medical definition for this malformation. "Standardization is one of the key requirements" that is currently promoted by WHO, because it allows health authorities to compare cases, Lindmeier said.

CAUSES OF MICROCEPHALY

Another reason for the lack of clarity is that microcephaly can be caused by various viruses, toxins and genetic conditions, including rubella and herpes infections, alcohol consumption by the mother, and Down syndrome. In addition, Brazil had seen a slight annual rise of microcephaly cases over the past years, even before the Zika outbreak, according to WHO. German epidemiologist Christina Frank said that a combination of Zika virus and another factors might cause the current microcephaly spike, such as antibodies against another virus. "In addition, the Zika virus could in theory be completely innocent. There could be another factor that coincides with the Zika outbreak and that is the real cause, for example a medicine," said Frank, who works at the Robert Koch Institute in Berlin.

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