Zika virus infections are way down in New York and the rest of the U.S.

Steve Orr
Democrat and Chronicle
Aedes albopictus mosquito, which can transmit the Zika virus.

Zika, the virus that sparked widespread concern last year? It’s on the wane, at least for now.

The number of people infected with Zika is down substantially this year in upstate and the rest of New York, reflecting a precipitous drop in mosquito-borne virus infections in Puerto Rico and other overseas locations.

New York has reported 155 Zika infections at last count, a figure that’s 56 percent lower than at the same point last year. Only 30 of them have been in upstate New York, including just one case in Monroe County. Monroe ended last year with 22 Zika cases, far more than any other upstate county.

The same scenario is playing out elsewhere in the United States, for the same basic reason — the Zika infection rate has cratered in some one-time hot spots in just 12 months’ time. That has made it considerably less likely that Americans will pick up the virus while traveling abroad this summer.

And with fewer Americans bringing the virus home with them, local transmission of Zika in the United States has become much less likely.

In fact, not a single case of local Zika transmission has been found so far this year.  

Last year there were 224 people in South Florida and southern Texas who physicians said contracted the virus from the bite of local mosquitoes.

“That’s an encouraging sign. People’s fear is that it had the potential to spread here,” said Dr. Paul Graman, Clinical Director of the Infectious Disease Division at Strong Memorial Hospital and Golisano Childrens' Hospital in Rochester, NY.

It is worth noting that the U.S. peak mosquito season is only now underway. The first locally transmitted cases in Florida didn't appear last year until July.

Experts caution that Zika remains a threat to people who live offshore or are traveling there, and said local transmission of the virus in the United States could restart.

Carlos Varas, a Miami-Dade County mosquito control inspector, uses a Golden Eagle blower to spray pesticide to kill mosquitos as the county fights to control the Zika virus outbreak Aug. 24, 2016 in Miami Beach, Fla.

The Zika virus has been responsible for birth defects in thousands of infants and the deaths of nearly two dozen adults since it erupted in the Americas two years ago.

It has since been detected in more than 50 countries in South, Central and North America and in the Caribbean. The number of cases has dropped dramatically in many of those countries, but the virus hasn’t disappeared in any of them. There also are several countries, most notably Argentina and Peru, where the number of cases may still be on the rise.

“CDC expects Zika virus will continue to circulate at low levels in most regions where it has been introduced,” said Benjamin Haynes, a spokesman for the U.S. Centers for Disease Control and Prevention. "Anyone living in or traveling to areas with risk of Zika may be at risk for infection."

For that reason, Graman said people should continue to be cautious while traveling and protect themselves from mosquitoes in areas where Zika is known to lurk.

"I’d think the admonitions still apply, particularly if you’re pregnant or going to be pregnant. The risk still exists in those cases, even if it’s less," he said. "I think those admonitions need to apply until people are given the all-clear."

Decline in cases 'not unexpected'

When Zika hit public consciousness in the United States early last year, there was little discussion of the likelihood that the virus would burn its way through countries so quickly. But public health officials now say they knew all along that’s what would happen.

“A decline in the numbers of Zika cases from 2016 to 2017 is not unexpected,” the New York state Department of Health said in a written statement to the Democrat and Chronicle.

Good news and bad about Zika in New York

Just how big that decline has been is not completely clear.

Florida, which had the most cases of any state in 2016, measured a 71-percent drop over the first half of this year compared to the same period last year. New York, which had the second-most cases of any state, shows a 56 percent decline year-to-year. 

Other states that had significant numbers of Zika cases last year — Texas, California, New Jersey and Pennsylvania among them — don't release month-to-month totals but appear to be on track to end 2017 with significant reductions compared to last year's case numbers.

The CDC maintains a nationwide Zika database that shows even larger reductions in every single state. But its 2017 case numbers are much lower than those released by the states, raising questions about the timeliness of the agency's data. CDC officials failed to respond to repeated inquiries about the data. 

Nearly 5,400 Zika cases have been reported in the U.S. so far, according to the CDC, and 95 percent of those infections were acquired when U.S. residents traveled offshore to locations where the virus was raging through the local population. (The other cases were acquired through sexual contact or blood transfusions.)

Zika spreads when a mosquito picks up the virus from an infected person’s blood and then bites someone else. At the height of the epidemic in places like Brazil and Venezuela, mosquitoes swarming in heavily populated areas passed the virus to thousands of people a week.

But once infected, people develop immunity. If they’re bitten again, the virus is destroyed in their blood and cannot be passed on to another mosquito. As immunity spreads through a populace, the number of potential victims and the number of mosquitoes carrying the virus both decline. The epidemic dies back.

For that reason, the number of new cases in one-time Zika strongholds such as Puerto Rico, Colombia, Brazil, Venezuela and Honduras is very small so far this year compared to what it had been, according to data published by the Pan American Health Organization.

And that means visitors from other places, such as the United States, are much less likely to encounter mosquitoes carrying Zika.

“The word out of those areas is they are on the other side of what we call the epi curve. There’s an explosion of the disease, there’s some kind of saturation and then the numbers start to go down,” said Chris Van Deusen, a spokesman for the Texas Department of State Health Services. “There’s a smaller reservoir for people who travel to get infection.”

Local transmission takes a break

Most people who contract the Zika virus show no symptoms at all. Others may suffer from mild flu-like symptoms such as fever, headaches, conjunctivitis and muscle aches. The symptoms usually pass in a week and people often don’t seek medical care.

But in some cases, the infection proves very serious. Infants can be born with head deformities, development problems and brain damage if their mothers contract the virus while pregnant. Some infected adults have developed Guillain-Barré syndrome, which can result in paralysis and death.

The virus, which is similar to those that cause yellow fever and dengue fever, was initially observed in 1947 and spread slowly across the globe, first causing mass illness in several Pacific islands in the 2000’s and then exploding in Brazil in 2015.

Unsuspecting travelers brought the virus back to the U.S. and were bitten by mosquitoes that then pass Zika onto other victims. The first locally transmitted case was found near downtown Miami in July 2016, with more than 200 additional cases eventually arising in three South Florida counties.

A half-dozen locally acquired Zika cases were found in Brownsville, Texas, near the Mexican border, late last year.

Officials in both states responded with aggressive mosquito-control campaigns and public education, and there has been no recurrence so far this year.

“The strategies we used last year — draining standing water, covering screens and doors with well-fitting screens and using repellent — still work,” said Mara Gambineri, spokeswoman for the Florida Department of Health.

Barrington Sanders and Joseph Blackman with the Miami-Dade County mosquito control department inspect a neighborhood for any mosquitos or areas where they breed as the county works to eradicate mosquitos carrying the Zika virus on April 5, 2017 in Miami Beach, Fla. .

Whether this pattern will continue the rest of the year isn’t clear.

“CDC is not able to predict how much Zika virus will spread in the continental United States during 2017,” said Haynes, the agency spokesman.

The worst-case scenario would be one in which Zika arose somewhere in the United States and defied local efforts to control it, allowing it to spread more widely than was the case last year.

“Many areas in the United States have the types of mosquitoes that can become infected with and spread Zika virus," Haynes noted.

Two species of mosquito that are well-established in the U.S., Aedes Albopictus and Aedes Aegypti, are known to be capable of carrying the virus.

According to the latest CDC mapping, the latter species' permanent range in this country is in border regions , primarily in southern Florida, Texas, Arizona and California.

A. Albopictus, also known as the Asian tiger mosquito, is established over a much wider swath of the U.S. — the South, the lower Midwest and the Mid-Atlantic states.

The northern-most point in its range is metropolitan New York City and the lower Hudson Valley, which presents the possibility that local transmission of the virus could occur in this state.

The newly released maps also show A. Albopictus with a foothold in Seneca, Madison and Oneida counties in upstate New York. State health officials say, however, that those were one-time discoveries and that A. Albopictus does not permanently inhabit any place in upstate.

Local transmission of the virus in upstate New York is theoretically possible, but only under just the right circumstances. 

Zika an unlikely threat in upstate New York

Local transmission in the New York City region could certainly happen, but isn't considered terribly likely.

"Transmission within the United States, even in areas where Aedes aegypti is present, is expected to be minimal and sporadic," the state health department said in its statement. "Good mosquito control, coupled with good sanitation, should make outbreaks like what was seen in Central and South America unlikely to occur here in the United States."

SORR@Gannett.com