There were seven cases of the defect called anencephaly in 2013 in Yakima, Benton and Franklin counties for a rate of 8.7 cases per 10,000 births for the three-county area, while the average rate for the years from 2010-12 was 8.4 cases per 10,000 births - both at least four times the national rate of 2.1.
Health investigators are checking records and interviewing families and doctors, but so far there's no common factor, the Yakima Herald-Republic reported.
The explanation might be a combination of social, economic, genetic and environmental factors, said epidemiologist Mandy Stahre, who is assigned to Washington state by the Centers for Disease Control and Prevention.
"You can't ignore all the risk factors that we see within this population and some of the reasons why we have the higher rates," she said Tuesday in an interview during a visit to Yakima. "What we've been trying to target is if there's something other than that, but you can't ignore the social determinants of health: health disparities, high rates of obesity, lack of access to healthy foods, lack of access to prenatal care. ... When you add all that together, you can't say that that's not contributing."
Anencephaly is a fatal defect that occurs when the protective neural tube fails to close completely around the spinal cord at the base of the brain.
The Health Department has been pulling records, interviewing families and talking with providers since the area was first flagged as abnormally high in January 2013. Investigators have mapped the cases, asked the mothers what they ate and where they bought groceries, talked about pesticides, and looked at family history of birth defects.
The cases were spread throughout an area of hundreds of square miles. They did not show up seasonally, as would probably be the case if they were tied to pesticide exposure on farms and orchards, Stahre said. They're not clustered in any one socio-economic or ethnic group, nor in any specific geographic area within the three counties. Most cases occurred in families on public water, which is regularly tested for nitrate levels. They're not focused near the Hanford site, nor did they start showing up only after the Fukushima nuclear disaster in Japan.
"About 60 percent of our cases were Hispanic, but the population in this area is about 50 to 60 percent Hispanic. It reflected the population here," Stahre said.
"A large proportion of the women were on Medicaid, but (the population) reflects that as well. It wasn't something that made us go, 'Ah, right here, poverty is the No. 1 thing.' It's probably a contributing factor."
To help look for answers, the Health Department is holding two listening sessions in May, one in Yakima County and one in Benton-Franklin counties. Then in June, a new committee will meet for the first time to talk about next steps. The committee will include birth defect experts from the CDC, researchers, local health care providers, public health officials and community leaders.
"We want to know what are the next steps we should do," Stahre said. "Should we go back (to previous years)? Should we expand it to the entire state? These are questions we want the committee to weigh in on, to find the best way to use our resources moving forward."