Zika virus testing and infection rates in the United States examined by the US Centers for Disease Control and Prevention (CDC) between January 3rd and March 5th, 2016 by symptoms and pregnancy status reveal that so far, the rate of infection is relatively low, according to a recent study published in the CDC’s Weekly Morbidity and Mortality Report.

Sharoda Dasgupta, PhD, of the CDC’s Epidemic Intelligence Service and the Division of HIV/AIDS Prevention, and colleagues tracked rates of infection of people who visited or moved away from locations where Zika is actively being transmitted by the Aedes species mosquitoes.


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During this time 4,534 people from the US, including Washington DC, were tested, with 76.3% (3,335) of these patients being pregnant. Of the 1,541 people who showed one or more symptoms of the virus, such as fever, rash, arthralgia or conjunctivitis, Zika was confirmed in 182 (11.8%); 28 of whom were pregnant women. Only 0.3% (7) asymptomatic pregnant women tested positive for the virus.

“Approximately 99% of asymptomatic pregnant women who received testing did not have Zika virus infection. In the current US setting, where most exposure is travel-associated, the likelihood of Zika virus infection among asymptomatic persons is low,” researchers wrote in the report.

Dr Dasgupta and colleagues noted 5 limitations to their study. The first is the possible failure of patients to remember symptoms, because Zika virus testing occurred weeks after possible exposure. Secondly, the analysis only used tests from the CDC’s Arboviral Diseases Branch – other state health departments were also testing for Zika, and so the number is reported cases is actually higher. Heterogeneous exposure risk was not accounted for, and infection from travel can vary based on area, amount of time spent, type of accommodation and activities while visiting. These findings cannot be used for people living in active, local vector-transmitted Zika, and finally, patients with previous unknown flavavirus or who have been vaccinated against flavavirus types may make infection test results hard to read.

As of April 20, 2016 the CDC reports that there have been no cases of local vector-borne Zika virus in the United States. There are 388 cases of travel-acquired Zika in the US; among those 33 are in pregnant women, 8 have been sexually transmitted and 1 resulted in Guillain-Barré syndrome. In US territories, there have been 500 locally transmitted cases reported and 3 travel-associated cases.

The CDC recommends that physicians offer Zika virus testing to asymptomatic pregnant women who may be at risk due to travel or who have a partner that has traveled to an area where Zika is being transmitted locally. Dr Dasgupta and colleagues stressed that pregnant women who test positive for Zika must be monitored for adverse pregnancy outcomes and congenital virus infection.

Reference

Dasgupta S, Reagan-Steiner S, Goodenough D, et al. Patterns in Zika Virus Testing and Infection, by Report of Symptoms and Pregnancy Status — United States, January 3–March 5, 2016. MMWR 2016;65:395–399.

Centers for Disease Control and Prevention. Zika virus disease in the United States, 2015–2016. CDC. http://www.cdc.gov/zika/geo/united-states.html. Updated April 21,2016. Accessed April 28, 2016.