Zika outlook less dire as disease wanes in Southern Hemisphere


“The sources of last year’s epidemic are largely gone,” said Dr. Peter Hotez, dean of the National School of Tropical Medicine at Baylor and Texas Children’s Hospital and a world-renowned Zika expert. “There’s less virus circulating in the Southern Hemisphere to be imported and introduced into places like Texas and Florida.”

Top officials at the Centers for Disease Control and Prevention, Harris County’s public health department and a leading Zika expert at the University of Texas Medical Branch at Galveston echoed Hotez’ outlook.


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Dr. Ben Beard, a Zika specialist at the CDC, said the agency is “encouraged” about the epidemic’s declining numbers and “guardedly” optimistic for the United States this year.

Nonetheless, experts continued to call for vigilance because the virus will continue to circulate for some time, despite the drop in activity. They also noted Zika is unpredictable, frequently popping up in new places.

In particular, they mentioned the threat from Mexico, which is among the nations reporting a significant drop in activity but imported the disease to Brownsville late last year. State health officials in December confirmed six cases of mosquitoes transmitting the Zika virus there, and Texas became the second U.S. state after Florida to experience local spread of the disease.

“I can’t go as far as the others in expressing optimism,” said Dr. John Hellerstedt, commissioner of the Texas Department of State Health Services. “It’s very good news that so many countries are reporting decreasing numbers, but I think that the Texas threat is great still. I fear that the 2017 season will pick up where the 2016 left off.”

He said the situation around the border may not reflect the improvement seen elsewhere in Mexico.

Hellertedt said he was surprised there have not been any locally transmitted Zika cases in Texas so far this year. There have been 12 travel-related cases.

Zika emerged as a global threat in 2007, then started spreading extensively in 2015 and 2016, reaching at least 58 countries and territories, most in the Southern Hemisphere. The only reported cases in Harris County to date have involved people who acquired the virus while traveling to regions, typically in south and central America, where there were outbreaks. Seventy-five such cases were imported to Harris County in 2016.

The virus is not considered fatal, but it has been linked to thousands of babies born with microcephaly, a condition in which an infant is born with an abnormally small head and brain, which can result in seizures, developmental delays, intellectual disability, feeding problems and hearing and vision problems. Among pregnant women in the continental United States with a confirmed Zika infection, one in 10 had a baby with such brain damage or other serious birth defects, according to a recent CDC report.

Zika’s decreased activity in the Southern Hemisphere was reported in a May 25 update of the Pan American Health Organization. The update shows much less transmission in the Caribbean, the source of last summer’s Florida cases; Central America and Mexico; and South America, with the exception of Peru, Ecuador and Argentina, new sites of activity. Puerto Rico declared an end to its Zika outbreak this week, though the move was criticized by some experts as premature and the CDC still recommends pregnant women not travel there.

Experts attributed such waning to herd immunity that has occurred as so many people in affected countries became infected, then developed immunity to the virus. After outbreaks, herd immunity makes transmission of the virus inefficient.

The experts compare Zika’s progress to chikungunya, a virus also transmitted by Aedes albopictus and Aedes aegypti mosquitoes.

“Hopefully, we’re starting to see the same situation with Zika as chikungunya a few years ago,” said Scott Weaver, director of UTMB’s Institute for Human Infections and Immunity and chairman of the Global Virus Network’s Zika Task force. “It burned out as people became immune after infection.”

Weaver said he is “cautiously” optimistic about Zika this summer, though he questioned the reported decline in activity in Mexico. He said there’s “very limited surveillance and diagnostic testing going on there.”

Hellerstedt said because of its lack of exposure to the virus, Texas would be susceptible if Zika is reintroduced around the border. He stressed the lack of a scientific tool to predict with any degree of certainty how significant the disease threat will be in 2017.

Hotez said the “cautiously pessimistic” view about the season involves the fact that there already was local transmission of Zika in Texas and that there is not enough active surveillance to know if the virus is still around. He noted Zika last year began spreading in mosquitoes in south Florida three months before officials detected a local infection.

Still, Hotez expressed relief that the Zika probably will not be brought into Florida or Texas this summer from the Caribbean, Central America and most of South America. He said the bigger question is whether the virus’ wane is permanent or temporary, as is the case with dengue fever and West Nile, both of which increase in numbers periodically.

In any case, public health officials still urge people to heed precautions.

“I would agree with a cautiously optimistic outlook,” said Dr. Umair Shah, executive director of Harris County Public Health. “The fact we’re now starting to see the curve slow down in countries where there was significant amount of transmission the past year is encouraging, but that doesn’t mean we’re out of the woods yet. People shouldn’t suddenly put their guard down.”

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