A little-known mosquito-borne tropical virus that scientists believe could be connected to a serious birth defect reached the mainland United States this week, one of what is expected to be an increasing number of cases.

Officials in Harris County, Texas, said that a middle-aged woman who had traveled to El Salvador was diagnosed with Zika virus, after she developed a rash, fever and joint pain.

Experts told the Guardian that the case is “deeply” concerning, because of the possibility that local mosquitoes could contract the disease along the American Gulf coast, where the subtropical climate could be conducive to the disease’s spread.



“For every case that’s been picked up there might be a dozen more across the state of Texas, or maybe 100 more that we’re not diagnosing,” said Dr Peter Hotez, dean of the National School of Tropical Medicine, an expert on subtropical virology, at Baylor College of Medicine in Texas.

Health officials in Harris County, which already boasts robust mosquito monitoring programs, said the disease is so new that no rapid test for mosquitoes is currently available, and one isn’t expected for several months. The officials also did not know how long after infection a patient may be able to transmit the disease to local mosquitoes.

Dr Umair A Shah, executive director of the Harris County department of public health, said, “It’s probably not a case of if we get Zika in our native mosquitoes, it’s probably a case of when we get Zika in our native mosquitoes.”

Zika is a subtropical virus transmitted by the Aedes aegypti and Aedes albopictus mosquitoes, part of a group of diseases known as arboviruses, short for arthropod-borne viruses.

The virus was first recognized in Uganda in the Zika forest in the 1940s. Though it causes fever, rash and joint pain, it is was not considered a menace until a connection between microcephaly and Zika was discovered in Brazil.

Virologists said Zika has already reached “epidemic” proportions in El Salvador, where doctors were able to isolate the virus in some children suffering from microcephaly.

“We’re deeply concerned about the Zika starting a transmission cycle here in south Texas,” said Dr Nikos Vasilakis, an arbovirus researcher at University of Texas Medical Branch in Galveston. “Not long ago, dengue and yellow fever were endemic in the area, [in] Houston, Galveston … Both the climate conditions and human density exist to allow circulation of the virus,” Vasilakis said, naming related viruses.

Vasilakis, who studied Zika with the Brazilian health ministry in December 2015, said that an estimated 1.5 million people have been infected in Brazil, possibly leading to the recent spate of an estimated 3,000 cases of the ordinarily rare birth defect. By the Centers for Disease Control’s estimate, the rate is 10 times the average for cases of microcephaly. Brazilian health authorities cited just 147 cases of microcephaly in 2014, and 167 cases in 2013, according to the New York Times.

“What we’re looking at now [in Brazil] is the avalanche. In the clinics we see anywhere from 10 to 15 cases a day [of microcephaly],” said Vasilakis. “Last year, we’ve seen about 3,000 cases of microcephaly, which is an extremely, extremely sad condition.”

But little is known about Zika and its connection to microcephaly. The CDC reported that “some samples” from children born with microcephaly tested positive for Zika, while “several” did not.

“Although we don’t have a direct connection yet, we have a lot of indirect evidence, and some direct right now, to point that Zika is responsible for that,” said Vasilakis. Nevertheless, the CDC recommends pregnant women avoid being bitten by mosquitoes, as did Harris County, though the county purposely omitted the disease’s possible connection to microcephaly in a press release.



“The Gulf coast is uniquely vulnerable to arboviruses because we have at least three different species [of mosquitoes] that transmit arboviruses,” said Hotez. “The other is the subtropical climate, and the third – the one that people don’t typically appreciate – is poverty.

“When you go into the poor areas of Houston, you see absent screens, absent air conditioners, and tires along the side of the road filled with water,” an ideal breeding ground for mosquitoes, Hotez said. “It looks like the global health movie you show [medical] students.”

Once a little-known virus, epidemiologists are now working feverishly to determine from where and how the virus came to Brazil, and how it came to be so easily spread. Some scientists theorized that it was introduced to Brazil during the 2014 World Cup in Rio, while others believe that the disease was introduced from French Polynesia or Chile’s Easter Island.

The first Brazilian case was reported in May 2015, according to the CDC. By New Year’s Eve 2015, the CDC reported that Puerto Rico had the first locally acquired case of Zika, meaning it was contracted within the US territory rather than being brought in by a traveler to the island. The disease has also been reported in Mexico, Caribbean nations and Latin American countries, Vasilakis and the CDC said.

“This is quite a large epidemic, so another question is how did this get so big so fast? And no one has the answer,” said Hotez. “There’s nothing really published, most of what we’re going on are World Health Organization alerts.”

Hotez suspects that a genetic mutation in the virus may be behind its more recent rapid spread, but frustratingly, scientists don’t have any conclusive answers.

“Zika is a very obscure virus,” Hotez said. “Up to 2007 there were less than 20 documented cases of Zika infection in humans. After 2007, in south-east Asia, Micronesia, we started seeing the first reports of large human epidemics, so we don’t really know what is happening … We don’t really know why this is happening, why the global spread is taking place that fast.”

Both Hotez and Vasilakis recommend intense surveillance to stop the disease from spreading within the US, and that a vaccine should be developed.