Over a six-year period, 45 babies in Yakima, Benton and Franklin counties were born with anencephaly, a neural-tube defect that is uniformly fatal.

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YAKIMA — After a nearly two-year investigation, the state Department of Health has concluded that there was no sole factor to blame for a cluster of fatal birth defects in Yakima and neighboring counties between 2010 and 2016, according to a report released last week.

In that six-year period, according to the department, 45 babies in Yakima, Benton and Franklin counties were born with anencephaly, a neural-tube defect that is uniformly fatal. Anencephaly affects fetuses within the first few weeks of pregnancy, and causes them to be born missing pieces of their skull and brain and without a fully developed neural sac to protect the spinal cord. Babies born with anencephaly die within hours or a few days.

“Neither the descriptive epidemiology of the anencephaly-affected pregnancies, the medical records-based case-control study of pregnancies from January 2010-January 2013, interviews of mothers of (neural tube defect)-affected pregnancies, nor investigation of community concerns identified a preventable cause for most of the NTD-affected pregnancies,” the department’s report said.

After consulting with public-health officials involved in researching the cause of the cluster, the Department of Health suspended its investigation in late 2016 and is now focusing on surveillance, outreach and prevention, the report said.

The average national rate of the defect is 2 out of every 10,000 babies. The Yakima-Benton-Franklin rate was four times that, with more than 8 of every 10,000 babies affected between January 2010 and September 2016.

A major factor identified by public-health officials, both in this investigation and in established literature on anencephaly, is a mother’s folic-acid intake prior to and in the first several weeks of pregnancy.

Women in the Yakima-Benton-Franklin county area whose pregnancies were affected by neural-tube defects showed low folic-acid intake compared with women in the rest of Washington, the report said.

Folic acid is found in leafy green vegetables, citrus fruits, beans and — because white flour is fortified with folic acid — grain-based food like bread, rice, cereal and pasta.

In 2016, the Food and Drug Administration approved folic-acid fortification of corn masa flour, which public-health officials hope will improve folic-acid intake among Hispanic women who use masa flour in cooking.

That’s important because Hispanic women are more likely to be affected by anencephaly: The report shows that between 2010 and 2015, the rate of anencephaly was 9.6 per 10,000 births among Hispanic or Latina women, 50 percent higher than the rate of 6.4 per 10,000 births among non-Hispanic white women.

Hispanic women accounted for roughly two-thirds of anencephaly cases in the cluster, but only about 56 percent of total births for that time frame.

Responding to concerns from community members, the Department of Health spent part of its investigation looking into whether nitrates in drinking water played a role in the birth defect cluster. Investigators mapped mothers’ addresses to assess whether they were on private or public water systems, then looked up available water-testing results for the months immediately preceding and following conception in cases of anencephalic births.

Investigators determined that even on private water systems, nitrate levels “continued to be well below 5 mg/L,” the level at which nitrates in the water are considered dangerous.

“In summary … drinking water nitrate levels were not elevated among women with anencephaly- or NTD-affected pregnancies in the three-county area,” the report said.

Investigators also looked into community concerns of pesticide exposure, but found no history of pesticide drift involving the chemicals most likely to influence anencephaly. Mapping mothers’ residential proximity to fields that use pesticides didn’t turn up a smoking gun, either, as a majority of both women with healthy pregnancies and women with anencephalic births in the three-county area live near agricultural operations.

The possibility that radiation from Hanford or from the Fukushima nuclear disaster might have contributed to the cluster was similarly dismissed. Several other factors were also considered in the investigation.

Since summer 2014, the Department of Health has been working with March of Dimes and other health-care partners on outreach to increase prenatal use of folic acid supplements, “because this is the only known way to effectively prevent (neural-tube defects),” the report said.

Folic-acid intake affects the health of a pregnancy before most women are even aware they’re pregnant, so the outreach efforts are aimed at all women of childbearing age.