Researchers Identify Zika Incubation Period and Optimal Testing Methods

With Zika virus in the United States, the big question has been: is it local?

Now, researchers may have a better idea of just how to determine that—and how to better test for it.

Zika, the mosquito-borne virus that rose to recently prominence after an outbreak in Brazil that began in 2014 and eventually spread to the Caribbean, in the United States has mostly been linked with travel to affected regions, with only clusters of cases transmitted via bites sustained from “local” mosquitos in Florida last summer and in southern Texas late last year. Although more than 5,000 Americans have been diagnosed with the virus, according to the latest figures from the Centers for Disease Control and Prevention (CDC), the majority of these individuals were infected through travel to places such as Brazil or Puerto Rico, where the virus is endemic, or through sexual contact with infected persons, many of whom became infected while traveling.

An analysis performed by researchers at the CDC may offer clues as to how so-called “local” and “travel” cases of Zika differ. Indeed, in a study published in the May 2017 issue of the CDC’s Emerging Infectious Diseases, the researchers found that among symptomatic Zika-infected travelers, 99% experience symptoms within 2 weeks of exposure, and 50% within 1 week. Based on these overwhelming numbers, they concluded that those who develop Zika symptoms more than 2 weeks after travel to an endemic area “should be evaluated for alternative modes of transmission.” This latter point is key, in that it may help epidemiologists identify “local” cases sooner, and thereby implement vector-control strategies more rapidly.

The authors arrived at these findings after assessing 197 people with Zika virus over an 18-month period from 2015 to 2016. In these cases, Zika diagnosis was confirmed via testing for 134 (68%) persons, molecular testing for 57 (29%), and molecular and serologic testing for 6 (3%). Among the study group, the authors determined the median incubation period for the virus was 6.2 days. Overall, they determined that 5% of those infected would develop symptoms within 2.1 days of exposure, and 99% would become symptomatic within 13.6 days. Of the 112 (57%) persons in the study population who had history of travel, cases were confirmed for 79 (71%) and the median length of virus incubation period in these subjects was 5.8 days. Ultimately, the findings suggest that the incubation period for Zika virus is between 3 to 14 days.

In a second study published in the same issue of Emerging Infectious Diseases, a group of French researchers reports on the findings of their longitudinal analysis of Zika virus RNA quantification in whole-blood and plasma samples taken from 5 immunocompetent patients who had returned from the Caribbean or South and Central America with a benign form of Zika virus infection.

According to the authors, the 18 whole-blood and 21 plasma samples showed that the median duration of Zika virus was 22 days in whole blood and 10 days in plasma (P = 0.058). Mean viral loads of positive samples were 3.39 log copies/mL in whole blood (n = 13) and 2.52 log copies/mL in plasma (n = 6; P = 0.001). Interestingly, a point-to-point comparison of the samples revealed that Zika virus RNA was quantifiable in 23 whole-blood specimens (mean viral load was 3.50 log copies/mL) versus only 10 plasma samples (mean viral load 3.01 log copies/mL; P ≤ 0.018).

“Overall, our data show that use of whole-blood specimens is much more sensitive than use of plasma samples to detect Zika virus RNA, they write. “These data could be useful in recommending the use of whole blood instead of plasma for the molecular diagnosis of acute symptomatic and asymptomatic Zika virus infections and for the safety of whole blood and blood components from donors, as well as for the safety of organs, tissues, and cells from deceased and living donors.”

Brian P. Duleavy is a medical writer and editor based in New York. His work has appeared in numerous healthcare-related publications. He is the former editor of Infectious Disease Special Edition.