BY JONATHAN KAMINSKY

OLYMPIA Wash. Wed May 14, 2014 6:51pm EDT

(Reuters) - An alarming number of babies in a rural swath of central Washington state over the last four years have been born with a rare and fatal defect that leaves them without part of their brain and skull, and scientists are stumped.

In a three-county area that includes the city of Yakima, 26 babies born between 2010 and 2013 suffered from anencephaly, which occurs early in pregnancy when the fetal neural tube does not close, according to Juliet VanEenwyk, an epidemiologist with the Washington State Department of Health.

The number of cases is four times the national average, VanEenwyk said, and health officials haven't been able to work out the cause.

"That is the $64,000 question we’re trying to solve," VanEenwyk told Reuters. "Our best guess is that it’s happening for a lot of different reasons."

At a community meeting held in Sunnyside, Washington, on Tuesday, health officials were peppered with questions, VanEenwyk said.

An initial investigation launched after local health workers sounded the alarm in 2012 did not yield a clear cause for the cluster of cases, she said.

Common risk-factors for fetal neural tube disorders, such as obesity in the mother, were not higher among those affected, compared with others living in the area, she said.

Scientists have also been struck by the fact that spina bifida, another disease caused by neural tubes failing to fully close, has not impacted families in central Washington at higher rates. Spina bifida is generally twice as prevalent as anencephaly.

"That’s one of the things that’s really strange about this," said VanEenwyk.

In June, the state health department will convene an advisory group made up of experts, officials and locals to determine how to move the investigation forward.

That investigation may take the form of in-depth interviews with women whose babies were affected as well as testing for elevated levels of pesticides. The latter would be tricky, VanEenwyk said, since pesticides tend to move quickly throughout the body.

Another area of possible exploration is the amount of folic acid, a B vitamin that helps prevent fetal neural tube disorders, present in the affected pregnant mothers, VanEenwyk said.

In the three-county area, 60 percent of pregnant women - 10 percent above the statewide average - do not take a folic acid supplement, VanEenwyk said, although that figure did not appear to be higher among women whose babies were born with the defects.

(Editing by Edith Honan and Gunna Dickson)