The big news on the Zika virus front is welcome: the infection rate among U.S. residents is down dramatically from a year ago.
In Florida, Zika cases tumbled 71% during the first half of 2017 compared with the same period in 2016, according to a recent USA Today article. In New York, cases dropped 56%. Texas, California, New Jersey and Pennsylvania are also expected to end the year with much lower numbers compared with 2016.
While the public is breathing a collective sigh of relief, the U.S. Centers for Disease Control and Prevention (CDC) is advising people to still use caution (not to mention plenty of mosquito repellant and, when pregnancy is a possibility, condoms) in any areas where Zika is a known risk.
“CDC expects Zika virus will continue to circulate at low levels in most regions where it has been introduced,” said Benjamin Haynes, CDC spokesman, in the USA Today report. "Anyone living in or traveling to areas with risk of Zika may be at risk for infection."
Although favorable, the drop in Zika incidence hasn’t exactly come as a surprise to public health officials.
As the article explained, when Zika cases are at epidemic proportions, like they were in Brazil and Venezuela last year, mosquitoes spread the virus rapidly by biting infected people and passing the infection on to others they bite. However, after a person becomes infected, he or she develops immunity. As the number of people with Zika immunity grows, the number of mosquitoes carrying the virus shrinks — as does the number of people likely to pick it up.
Subsequently, the epidemic begins to wane.
“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,” Chris Van Deusen, Texas Department of State Health Services, said in the USA Today report. “There’s a smaller reservoir for people who travel to get infection.”
The significant drop in Zika incidence, combined with newly gained insight into the limitations of the most widely used blood test for Zika, has prompted the CDC to step back from a previous recommendation that advised routine Zika testing for pregnant women without symptoms but who may have been exposed to the virus.
On July 24, updated guidance recommended pregnant women with recent possible Zika exposure — as well as Zika symptoms, which can include fever, rash, joint pain, red eyes, and headache — be tested to diagnose the cause of their symptoms and the subsequent likelihood of Zika-related fetal abnormalities.
“As the prevalence of Zika virus disease declines, the likelihood of false-positive test results increases,” the CDC explained in the report. “In addition, emerging epidemiologic and laboratory data indicate that, as is the case with other flaviviruses, Zika virus IgM antibodies can persist beyond 12 weeks after infection. Therefore, IgM test results cannot always reliably distinguish between an infection that occurred during the current pregnancy and one that occurred before the current pregnancy, particularly for women with possible Zika virus exposure before the current pregnancy.”
Although representatives from the American College of Obstetricians and Gynecologists and the Society for Maternal Fetal Medicine reportedly worked with the CDC in revising the guidance, some in the obstetrics community expressed concern over the change.
“By not testing an asymptomatic pregnant patient, we are keeping her from getting the increased surveillance she would get if test results are positive and taking away the mother’s options if fetal abnormalities are found, since they won’t be found until after birth,” Rita Driggers, MD, medical director of maternal and fetal medicine division, Sibley Memorial Hospital in Washington, DC, told the Washington Post.
Others noted the test’s lack of specificity combined with the increased risk of false-positives isn’t exactly helpful to women, either.
“When the disease incidence decreases, the test performs even worse than it did when there was lots of disease,” Laura Riley, MD, vice chair of the obstetrics department at Massachusetts General Hospital, said in the Washington Post article. “What the patient then gets is a potentially false positive, and that creates nothing but hysteria.”
“It’s not perfect,” conceded Henry Walke, MD, MPH, in the article. “We’re trying to negotiate between false positives … vs. missing perhaps one true positive.”