BROWNSVILLE, Texas -- In less than 36 hours last spring, three children were born without brains at Valley Regional Medical Center here.
Two of the babies were stillborn. The third hung on for three days, doomed by a gruesome, fatal defect that leaves infants with an open skull and only the rudiments of a brain.
The deaths from the rare defect, known as anencephaly, puzzled Margaret Diaz, an occupational health specialist. She thought the three cases could have been a statistical fluke. Then, she had a chance conversation with a radiologist.
He had recently performed ultrasound examinations on seven pregnant women. Each, he said, was carrying a child without a brain.
Doctors soon learned of at least 10 more cases, most of them clustered in this city of 98,000 along the Rio Grande. The outbreak here and in surrounding Cameron County may be the '' largest ever in the United States.
Across the river in Matamoros, Mexican health officials are worried, too.
Two anencephalic children were delivered at the general hospital in 1990, but 10 were born last year.
Dr. Diaz's alarm has prompted full-blown investigations by the U.S. Centers for Disease Control, three Texas agencies and a local group of lawyers, doctors and chemists. So far, they have few answers.
"We think something terrible happened to cause this, but we don't know what it is," said William Lipps, a Brownsville chemist assisting the CDC.
But some here have their suspicions. Long uneasy about the heavy pollution in their sister city of Matamoros, Brownsville residents now fear that an environmental time bomb has gone off.
Like other Mexican border towns, Matamoros is struggling under the residue left by years of unchecked industrial growth.
It is dominated by U.S.-owned companies that came south over the last three decades for cheaper labor, favorable trade rules and lax enforcement of environmental laws. Today, Matamoros is an ugly sprawl of industrial plants and shacks housing Mexican workers. Its open sewers contain toxic wastes and human refuse. Its factories spew fumes and leak chemicals.
While CDC experts are considering environmental factors in their investigation of the outbreak, they say that the inquiry is in its early stages and that they have no evidence linking the deaths to the chemical stew in Matamoros.
"It would be a tremendous medical breakthrough if we could find what caused just half of those Texas cases," said Dr. Gregg Sylvester, a Johns Hopkins-trained epidemiologist who is leading the investigation. "I suspect that the causes of anencephaly may involve a multitude of factors and that we won't find a single cause."
Even as the medical detectives search for answers, the tragedy is unfolding. Another anencephalic baby was born at Valley Regional on Christmas Day.
To the parents of the children, the horrible nature of the defects is devastating. "Many of the patients were poor Mexican-American women who had no idea what to expect," said Vikki Barton, the head nurse in the hospital's nursery.
"It is very shocking when you're expecting a normal baby."
Doctors know that anencephaly occurs in the early days of pregnancy as the brain and spine are formed. But they don't know what triggers the defect.
A British study last year established that folic acid -- a vitamin found in green leafy vegetables, bread, rice, citrus fruit and nuts -- is critical in the first weeks of pregnancy. The study showed that adding folic acid to the diets of high-risk mothers sharply reduced their chances of producing anencephalic babies.
Researchers have also shown that exposure to solvents -- chemicals used to clean or mix in paint, plastics and electronics factories -- has been linked to central nervous system defects. A 1979 Finnish study tied such defects to a mother's exposure to solvents.
More recently, epidemiologists at the Texas Department of Health found in a 1990 study that men who used solvents faced higher risks of fathering anencephalic children.
If those men bore Spanish surnames, the risk was even higher, as much as four times greater than that of men not exposed to solvents.
The findings about solvents raise suspicions because preliminary test results show residues of heavy concentrations of organic solvents in the soil around Matamoros and in the Rio Grande.
Investigators in the Texas outbreak are theorizing that exposure to some chemical or other environmental or genetic factor prevented the Brownsville mothers from having sufficient folic acid in the crucial first weeks of pregnancy, said Dr. James E. Cheek, a CDC epidemiologist.
The scientists are also trying to discover if there is a predisposition to anencephaly among Mexican-Americans, since the defect occurs in Mexico at nearly six times the U.S. rate. Even so, other factors may be involved because no anencephalic outbreaks have occurred in similar Mexican-American border populations in California, New Mexico or Arizona.