Zika In Dallas: Waiting for Transmission

Eventually the other shoe will drop and a Texan will catch Zika from a mosquito. It hasn't happened yet, as it has in Miami, so public health authorities in Texas and Dallas County are in a holding pattern.

Miami was always going to be one of the first cities in the United States to see local, mosquito-borne transmission, Zika researcher and University of Texas professor Sahotra Sarkar says. The city's geography and its status as a hub for South and Central American travel make it especially vulnerable to viruses that start their journeys south of the border.

"[The Miami outbreak] fits completely within the timeline we expected," Sarkar says. "Miami has the highest risk of all U.S. cities for Zika being brought by air travel, so yes, it was completely expected that it would have the first U.S. outbreak."

In March, Sarkar and a group of researchers developed a map showing the relative risks of U.S. cities for a local Zika outbreak based on both international travel and the prevalence of mosquito types that can potentially carry Zika in the cities researched. Miami and other cities in Florida including Fort Lauderdale, were the first sites where Sarkar and company expected infections.

After that initial group, cities in Texas, starting with Houston and followed by Dallas, are the next group at most risk.

Although the number of cases in Florida remains small — six people had caught the virus from local mosquitoes as of Sunday, according to the Centers for Disease Control — any increase in domestic transmission can put Dallas at a higher risk, due to Dallas/Fort Worth International Airport's status as a major hub for domestic travel.

The more people carrying Zika to whom Dallas mosquitoes are exposed — remember adult Zika carriers often exhibit mild or no symptoms — the greater chance that local transmission begins in North Texas. "As the number of cases increases, say by the end of August or so, that's an additional thing we have to factor in that the [transmission] models don't do yet," Sarkar says.

So far, however, there hasn't been a big enough change in conditions to change Dallas' level of risk for an outbreak, Sarkar says.

Dallas is home to the Aedes aegypti mosquito, widely thought to be Zika's most effective carrier and the Aedes albopictus mosquito, a heartier mosquito that is more willing to travel north and appears in greater numbers in Dallas. Albopictus has not been confirmed as a Zika carrier, but Sarkar and his researcher group believe it could be a carrier for the disease, albeit an ineffective one, due to albopictus' ability to transmit Dengue fever, which is also primarily carried by Aedes aegypti mosquitoes.

"[Because the Aedes aegypti mosquito becomes less prevalent the farther north one goes], it does mean that by the time you get to North Texas, the chances of Zika establishing local cycles goes down — it goes down for the rest of the country. Now if both mosquito species are equally good at transmitting the virus, then the entire country and even Canada and much of Europe are at risk," Sarkar said in March.

Each of the six cases in Miami is believed to have been transmitted by an Aedes aegypti mosquito, Sarkar says, but there has still been very little significant research into the transmission potential of Aedes albopicitus.

So far, according to Sarkar, the response to Zika around the country has been inadequate, thanks largely to Congress failing to pass additional funding to combat the virus prior to its summer recess. What money Dallas County and the state of Texas does have, Sarkar says, should be used primarily to educate Texans about what they should do to fight the disease themselves.

"The first thing that needs to be done is a massive campaign to ensure that the public does what it can to prevent standing water from accumulating anywhere," he says.

There is only so much spraying can do — and more needs to be done, Sarkar says — but standing water is far and away the biggest catalyst for mosquito breeding. Both spraying and education efforts do cost money, he emphasizes, and if spending on Zika continues at current levels, the National Institutes of Health will run out of Zika money by the end of August and the CDC — which gave Texas a little more than $700,000 to fight Zika last week — will run dry in September.

According to the governor's office, the state is focusing on delaying local transmission and, most of all, preventing Texas women who are pregnant from getting the virus and passing on birth defects to their unborn children. All pregnant women in Texas are eligible to get free mosquito repellent from their pharmacist with a doctor's note, and Governor Greg Abbott put out this new Zika education video on Sunday. 
Dallas County has seen 22 residents test positive for Zika. Twenty-one of those individuals caught the virus during travel to the Caribbean or South America. One person contracted the virus in Dallas county after having sex with someone who was already infected. The county is distributing kits with condoms, mosquito-killing water dunks and mosquito repellent to anyone who comes home with the virus.

The next big test, Sarkar says, will occur when those who have traveled to the Olympics in Brazil return to the United States. Although it's winter in Rio de Janeiro and mosquito season is over, there is still a high chance of people returning with Zika after having sex with Brazil natives, according to Sarkar, because the virus can be sexually transmitted for as much as eight weeks.