Have we dodged another Zika bullet?

“I am rather like a mosquito in a nudist camp; I know what I ought to do, but I don’t know where to begin.”        -Stephen Bayne

In 2015, a new epidemic in Brazil became apparent. Zika fever-caused by the RNA virus originally discovered in the 1940s in an African forest-exploded onto the scene. In February 2016, the World Health Organization declared the outbreak a Public Health Emergency of International Concern, and heart-rending photos of some of the thousands of severely affected infants appeared on the front pages of our newspapers and magazines.

Many of these babies suffered from chronic seizures, CNS malformations, profound hearing loss, and-of note to us ophthalmologists-major macular and optic nerve lesions sufficient to cause severe vision loss. Adults also were infected, with cases of uveitis, retinitis, and neurologic dysfunction, including Guillain-Barré syndrome appearing.

The epidemic spread rapidly to neighboring South/Central American nations, as well as to the Caribbean. The increased number of documented cases of infection grew exponentially.

Then the disease reached the United States, with almost all cases appearing in individuals who had traveled outside the continental states, but a few cases appeared as the result of sexual transmission or from mosquito transmission.

Experts pointed out that globalization and the frequent travel of potentially infected persons around the world make dissemination of infectious organisms a near certainty. It appeared that this past summer might result in this disease reaching epidemic proportions in our country, and a number of states and medical institutions prepared for the worst.

This year, however, the number of newly diagnosed cases of Zika infection has plummeted, almost as dramatically as the number had skyrocketed the year before. More good news came with the finding that no cases of mosquito-transmitted infection occurred in the continental United States.

Now with mosquito season past in most parts of North America, it seems we can be confident the predicted spread will not come to pass-at least in this year. Citing more important needs elsewhere, the U.S. government has cut back on funding for research related to Zika, and at least one large pharmaceutical company, Sanofi, has decided to discontinue its promising vaccine development program.

Supposedly, we can now relax.

What changed?

Do we know what caused this felicitous outcome? Was it due to a brilliant public health intervention or effective mosquito eradication program? Was it detection and appropriate intervention with infected airline passengers? Did the virus mutate and become less virulent? Did herd immunity in the Brazilian, Caribbean, and Latin American populace interrupt the cycle of transmission from vector to victims? Was it luck?

No one (at least no one I’ve been able to find) knows for sure why this epidemic has waned so emphatically.

To me at least, that is a cause for concern. If it was just good luck or some other unknowable circumstance that caused this threat to virtually evaporate, what is to prevent bad luck or a reversal of the unknown factor to cause this virus to rear its ugly head again before long in the southern United States and other regions that harbor the mosquito known to transmit Zika? Have we dodged this bullet for good, or is there another on its way?

I believe we’ll be justified in relaxing when we can satisfy ourselves that we know what caused this illness to abate and that the current circumstances will not support its return. Until that time, relaxing does not seem wise to me.