CHICAGO (Reuters) - Prior infection with West Nile or dengue - two viruses closely related to Zika - can make Zika symptoms worse, U.S. researchers said on Thursday.

The findings in mice, published in the journal Science, confirm studies in cells suggesting that prior infection with dengue could worsen the effects of Zika.

That could explain higher rates of severe Zika side effects, such as the birth defect microcephaly, in areas such as Brazil, where dengue is common. It also raises concerns about current or experimental dengue vaccines by Sanofi, Takeda Pharmaceutical Co, GlaxoSmithKline and others because they could inadvertently make Zika infections worse.

In the study, a team of researchers at the Icahn School of Medicine at Mount Sinai Hospital in New York injected Zika-infected laboratory mice with very low amounts of antibodies against dengue or West Nile.

They found that mice injected with antibodies from either virus were more likely to die from Zika or have more severe symptoms than mice exposed to Zika alone.

Symptoms were worst among Zika-infected mice that were given dengue antibodies, with only 21 percent of the mice surviving. That compared with survival rates of 93 percent among mice infected with Zika alone.

Zika-infected mice that got the dengue antibodies also developed severe neurological symptoms, including paralysis of several limbs and, in some cases, total body paralysis, symptoms that have also been seen in rare cases of adults infected with Zika.

The findings illustrate a process called antibody-dependent enhancement already seen in people infected with one of the four strains of dengue. This occurs when proteins made in response to the first infection make it easier for a related virus to enter host cells.

Given the high rates of dengue antibodies in regions most affected by Zika, the findings “suggest that pre-existing immunity to dengue may have contributed to the rapid spread of Zika in the Americas, possibly associated with increased viremia and clinical symptoms, including microcephaly,” the researchers wrote.

The findings may also have implications for people who develop Zika in the United States, where more than 3 million people have been infected with West Nile virus, said Mount Sinai microbiologist Jean Lim, one of the study’s authors.

As for the potential increased risk due to vaccines, Lim suggested that companies may consider combining a dengue and Zika vaccine to protect against both illnesses at once.