Puerto Rico is reporting its first Zika-related microcephaly case amid an outbreak of the mosquito-borne virus that has hit the commonwealth harder than anywhere else in the United States.

In San Juan, Health Secretary Ana Rius told reporters Friday that a fetus turned over to U.S. health officials had severe microcephaly and tested positive for Zika. Rius declined to say whether the woman involved had an abortion or miscarried.

A health department statement referred to a male fetus that showed "severe microcephaly and calcifications in the brain accompanied by Zika-wide presence of the virus." It said the case was detected early through "robust surveillance systems." The abnormalities were detected via ultrasound, the department said.

The Centers for Disease Control and Prevention, which has a branch in San Juan that is working full-time on Zika, also confirmed the finding as "the first case of Zika virus disease in a fetus in Puerto Rico."

A CDC lab did the testing and then shared the results with local health officials, who informed the attending physician and the family. An agency statement did not provide any additional details, and CDC officials declined to comment.

"This case of Zika virus disease in a pregnancy saddens and concerns us as it highlights the potential for additional cases and associated adverse pregnancy outcomes," the agency said.

Last month, the CDC definitively concluded that the virus causes severe fetal abnormalities. The most pronounced is microcephaly, a rare condition marked by an abnormally small head and a lack of brain development.

With rising temperatures in the weeks ahead, U.S. health officials and organizations are warning about expected local outbreaks in the south and southwest.

"Sadly, this is not likely to be the last case of Zika-caused microcephaly in the United States," said Edward R.B. McCabe, chief medical officer for the March Of Dimes, in a statement.  "As summer approaches, there is a very real threat that Zika virus could gain a foothold in the United States.  If that happens, great numbers of pregnant women and women of childbearing age will be at risk for Zika infection."

Puerto Rico health officials are reporting 925 confirmed Zika cases, including in 128 pregnant women. Unlike the 503 cases reported in the U.S. mainland, virtually all of the island's cases involve local transmission. One person, a man in his 70s, died of complications related to his Zika infection.

U.S. health officials estimate that at least 700,000 Puerto Ricans — about 20 percent of the population — could be infected by the end of the year based on previous outbreaks of dengue and chikungunya. Both are related viral diseases.

Anthony S. Fauci, director of the National Institute of Allergy and Infectious Diseases, said this week that Puerto Rico  “is on the precipice of a really serious disaster.”

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