First case of Zika virus contracted in US mainland, confirmed in Texas, is only second documented example of virus being passed through sexual contact

Officials in Texas have reported the first case of Zika contracted in the US mainland, and said that the virus was sexually transmitted. If confirmed, the case would be only the second documented example of the virus being passed between humans through sexual contact.

Officials at the Centers for Disease Control and Prevention confirmed the Zika infection, while Dallas County officials said they performed the “public health follow-up” to determine that the infection was sexually transmitted.



In an email to the Guardian, the CDC also warned for the first time that pregnant women or those hoping to become pregnant should “consult with their healthcare professional if their partner has had exposure to Zika virus”.

“Based on what we know now, the best way to avoid Zika virus infection is to prevent mosquito bites AND to avoid exposure to semen from someone who has been exposed to Zika virus or has been ill from Zika virus infection,” the CDC wrote. It is unclear how long the virus may linger in men’s semen after virus symptoms subside.

The CDC spokesperson further added, “We have not made recommendations regarding sexual transmission of Zika virus.” Just a few days earlier, the CDC had emphasized that current science only allowed for the “plausibility of spread” through sexual contact, but that mosquitoes were still the most likely source of infection.



“There is one reported case of Zika virus through possible sexual transmission,” Dr Anne Schuchat, CDC deputy director, said in a press briefing on Zika on 28 January. “That gives you the plausibility of spread, but the science is clear to date that Zika virus is primarily transmitted to people through the bite of an infected mosquito.”

In Dallas, the Zika-infected individual is believed to have returned to the US already infected by the disease. The second infected person was a sexual partner of the traveler, but did not travel themselves.

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“Now that we know Zika virus can be transmitted through sex, this increases our awareness campaign in educating the public about protecting themselves and others,” said Zachary Thompson, Dallas County health and human services director. “Next to abstinence, condoms are the best prevention method against any sexually transmitted infections.”

The only other known case of Zika spread through sexual contact is between a Colorado State University mosquito-borne virus researcher, Brian Foy, and his wife, a nurse, in 2008. Foy reported in an academic article that both himself and a research partner were infected with Zika in Senegal, and that he believed he passed the disease to his wife upon his return.

In another case in 2013, researchers reported that a Tahitian man in French Polynesia sought treatment for Zika-related symptoms, and Zika was isolated from his semen.

Zika is part of a family of mosquito-borne viruses which includes West Nile virus, an illness likely better known to Americans, which is passed through the same Aedes aegypti mosquito. Symptons of Zika tend to be milder than those of viruses in in the same family, and an estimate 80% of people infected as asymptomatic.



The Zika virus became a major concern for public health authorities after it was connected with a massive increase in cases of microcephaly in Latin America. Microcephaly is a debilitating, life-long birth defect that leaves children with abnormally small heads, and a host of severe developmental and physical problems.

On Tuesday the Brazilian Health Ministry said the suspected and confirmed cases of newborn with microcephaly linked to the Zika virus in Brazil had increased to 4,074 as of 30 January, up from 3,718 a week earlier.

The World Health Organisation declared Zika a public health emergency on Monday, and the CDC warned pregnant women against traveling to countries with large-scale Zika infections in January. There is no vaccine for Zika.

“It’s probably uncommon, but we can’t say for sure [that Zika is being trasmitted through sex],” said Dr Peter Hotez, of Baylor College of Medicine in Houston, Texas. Hotez is the dean of the school of tropical medicine. “Because how do you know whether a man and woman who are living in the same room together in Latin America, if they weren’t bitten by the same mosquito?” Most cases, he said, come to light when one partner has traveled and the other hasn’t.

Professor Peter Horby, a professor of tropical medicine and infectious diseases at Oxford University, said that the report of sexual transmission was “not entirely unexpected, but it is certainly unwelcome”.

“It adds weight to the evidence that sexual transmission is a real risk, and raises many questions and dilemmas,” he said. “This highlights our ignorance of this virus and the need for an urgent, comprehensive and coordinated research response.”

Professor Luis Cuevas, a professor of international health and epidemiology at the Liverpool School of Tropical Medicine, said more research was needed to determine how long the virus could be infectious in sexual secretions. “Given this potential risk, although still considered rare, current advice in the UK is that individuals with a confirmed Zika infection should use condoms for six months,” he said.

To date, only imported cases of Zika infection have been reported in America, with the exception of the US territory of Puerto Rico. Many virologists, however, expect at least some limited spread of the disease in US Gulf Coast states.



The virus was discovered in the Zika forest in Uganda in the 1940s. It was once incredibly rare, but became suddenly endemic in some regions of Africa, before spreading to southeast Asia, French Polynesia and most recently Latin America.