The Centers for Disease Control and Prevention keeps a frequently updated tally of the number of pregnant women diagnosed with the Zika virus in the United States and U.S. territories.
Most weeks, the number ticks slowly higher. But last month, it surged.
On May 12, the CDC reported that 157 pregnant women in the U.S. and 122 pregnant women in U.S. territories had tested positive for the Zika virus, up from 48 and 65 respectively just one day earlier. The numbers jumped most dramatically in states where the virus has been particularly worrisome; in Florida, the number of reported cases in pregnant women went up four-fold: from 9 to 36.
But the apparent upward leap in cases isn’t as scary a sign for those watching the Zika epidemic as it might sound. In reality, it reflects the CDC’s use of a new system for tracking the vector-borne virus: Now the agency is including all pregnant women who test positive for Zika, not just those who show visible symptoms of the disease. That means that rising numbers reflect the fact that the CDC is using two different definitions for what constitutes a case of Zika. For the general population, a person must test positive for the disease and show symptoms, while a pregnant woman is counted if she tests positive, even if she is asymptomatic.
Zika is not always a highly visible infection. Roughly 20 percent of Zika infectees have symptoms that include rashes, fever, pink eye, and joint pain. But recent research suggests that pregnant women — even when they demonstrate no such signs of illness — may still be at risk for passing on a birth defect to their child.
For epidemiologists, this presents a difficult accounting challenge. “The Zika virus in non-pregnant persons is still a relatively mild illness,” said Dr. Margaret A. Honein, an epidemiologist and chief of the CDC’s birth defects branch. “The Zika virus in pregnant persons is dire.”
Because of the risk issue, the CDC had already set up special methods of counting the number of people with Zika. They use ArboNET, the CDC’s traditional registry for tracking infectious disease, along with separate registries for pregnant women.
The CDC’s current Zika case definition is based on an interim position statement approved by the Council of State and Territorial Epidemiologists in February, but an updated version will be voted on later this month.
As it subtly shifts the definition of what constitutes a case of Zika, the CDC is walking a delicate line — especially as it’s still studying the virus and figuring out exactly who is at risk. Public health officials very much want to be accurate, without inciting a panic. Asked how the CDC is balancing these ethics, Honein replied, “this is really an unusual situation.”
“This is the first time ever that a mosquito-borne infection has been shown to cause a major birth defect,” she said. “This is quite unusual, our response is specifically tailored to protecting pregnant women and their fetuses.”