FILE - In this file photo, an Aedes aegypti mosquito known to carry the Zika virus, is photographed through a microscope at the Fiocruz institute in Recife, Pernambuco state, Brazil. (Felipe Dana/AP)

District health officials mishandled Zika testing for hundreds of residents last year, including two pregnant women who were incorrectly told they did not have the virus when in fact they were infected.

The mistakes, made public Thursday by city officials, have prompted retesting for the Zika virus of specimens from more than 400 people, including nearly 300 pregnant women who may have mistakenly been told they didn’t have the mosquito-borne viral infection.

Officials blamed the botched tests on a mathematical error by lab workers.

The tests involve specimens collected by private physicians, hospitals and other health-care providers from men and women between July 14 and Dec. 14 that were analyzed by the District’s public health lab. The tests included 294 for pregnant women.

The specimens from the pregnant women have been shipped to the Centers for Disease Control and Prevention for retesting; the remaining samples for 115 nonpregnant women and men were sent to other public health labs approved by the CDC, said Jenifer Smith, head of the D.C. Department of Forensic Sciences, which oversees the city’s public health laboratory.

So far, officials have received 62 results from the specimens retested by the CDC; two tested positive for Zika. Physicians for those two women were notified Wednesday.

Smith said she could not say if those women or any of the others whose tests were botched have given birth.

“The information is best gained through those [health-care] providers,” Smith told reporters. “We just have talked to the provider. It’s very helpful in this situation to have patients working with the physicians who are trained to communicate this information.”

She said results for all retested specimens are expected within three to four weeks.

The revelations shocked pregnant women and their doctors.

“Oh my God, that is crazy, that is crazy, I mean, I am terrified,” said Melissa Levitt, 35, an attorney who is due to give birth to her second child March 26 and was tested for Zika in August. “You go and take this test and you wait on pins and needles to get the results. And then you’re ecstatic when there’s no evidence of Zika exposure. And now, it’s eight months later and you’re telling me this test was not done properly? That’s terrifying.”

Levitt and her husband were living in Florida last year and moved to the D.C. area to escape the threat of Zika in Miami.

Rita Driggers, medical director of Sibley Memorial Hospital’s maternal-fetal medicine division, said she has about 20 pregnant patients who had Zika testing during the five months in question. She has not been contacted about any of their results, she said.

While doctors know that diagnostic testing for Zika is not foolproof and conduct regular ultrasounds if a patient could be at risk, botched results from the public health lab mean some patients “could have been given false reassurances,” Driggers said.

“The sad thing is, if [the test results] were indeed positive for Zika,” the women might have had time to consider other options, such as terminating pregnancies, she said. But these errors mean officials “took away options for patients.”

The District appears to be the only jurisdiction in the country with this problem, CDC officials said. The city’s public health lab has been processing tests for Zika since January 2016.

“There doesn’t seem to be the kind of widespread problem with [this test] at the other public health labs,” said Wendi ­Kuhnert-Tallman, who heads the CDC’s Zika lab task force.

While District officials did not elaborate about what exactly went wrong inside the public health lab, they described a complicated test that involved multiple calculations and said the problem was essentially a math mistake.

The trouble was discovered by Anthony Tran soon after he was hired late last year to run the public health lab.

Tran said he was reviewing the methodology used for one of two Zika tests performed by the lab when he spotted something wrong: All 409 of the secondary Zika tests returned were negative, something that seemed statistically suspect.

“The error was a technical formulation and calculation error that I discovered after I got here,” he said.

“These are complex procedures, multilayered, and the reason it kind of takes constant vigilance,” Smith said. “Dr. Tran was hired to help me identify issues that may exist in the lab, and he’s doing exactly what he’s supposed to do.”

The city contacted the CDC in January and got help with troubleshooting. Kuhnert-Tallman said staff turnover at the D.C. lab was among the problems.

Accurate testing for pregnant women is particularly urgent because the virus can cause severe defects in developing fetuses, especially early in the pregnancy. Determining whether someone has been infected with Zika is enormously difficult because most infected people don’t have symptoms. Only 1 in 5 infected individuals experiences the most common symptoms: fever, joint pain, rash and conjunctivitis (pinkeye).

Researchers have concluded that a Zika infection during pregnancy is linked to a distinct pattern of birth defects that they are officially calling congenital Zika syndrome. They include severe microcephaly, characterized by abnormally small head size and often an underdeveloped brain, vision problems and joints with limited range of motion. In some cases, babies who were born with normal-size heads developed microcephaly five months to a year after birth, according to a CDC study released in November.

Zika, which can be transmitted sexually, usually remains in the blood of an infected person for about a week. One kind of test can detect the virus if it is conducted within that time frame. After that, making a diagnosis is more difficult, and a secondary test looks for the presence of antibodies that humans produce in response to the virus.

D.C. officials said the new test results will be faxed to all health-care providers who submitted specimens from their patients during the time period in question.

Health officials in the District have been encouraging all pregnant women to be tested for Zika if they or their sexual partners have traveled to Zika affected areas.

There had been 1,047 Zika-
infected pregnancies in the United States as of Feb. 7, according to the CDC. These include 43 babies born with birth defects and five pregnancies that resulted in stillbirths, miscarriages or abortions.

There have been more than 5,000 Zika cases reported in the U.S. mainland, including 31 in the District.

The D.C. Department of Forensic Sciences also oversees the D.C. crime lab, which halted DNA forensic testing from April 2015 until March 2016 amid questions about the integrity and independence of advanced DNA analysis at the lab. The $220 million D.C. Consolidated Forensic Laboratory was barely three years old at the time and was touted as a model government crime lab run independently of police or prosecutors.