As much of the United States settles into peak mosquito season, health officials across the country face a considerable challenge in conducting Zika surveillance. Operating under the unfortunate reality of limited resources, states can offer only limited Zika testing. In most states, testing algorithms prioritize pregnant women due to the additional risk posed to the fetus (eg, microcephaly), and testing typically excludes asymptomatic individuals. A testing strategy that focuses on those most vulnerable (eg, pregnant women) makes sense in the context of limited resource availability; however, incomplete surveillance makes it difficult to detect local transmission and determine the extent of the affected population.

On July 26, Hidalgo County, Texas announced what is believed to be the first local transmission of Zika in Texas for 2017. The case was identified through prenatal Zika testing, and the Hidalgo County Department of Health and Human Services praised the clinician who ordered the test, citing his/her “forward thinking” as a key factor in identifying the infection. The Hidalgo County report notes that more than 6,000 Zika tests have been conducted since the state Department of Health and Human Services recommended that Zika testing be part of routine prenatal care for six Texas counties at elevated risk for Zika transmission. This program is in line with updated CDC guidance, which recommends testing for asymptomatic pregnant women with “ongoing possible Zika exposure,” among other subpopulations of pregnant women at elevated risk.

Given that 80% of Zika infections are thought to be asymptomatic, targeted testing strategies (eg, those focusing on pregnant women) likely miss cases, which may lead to delays in detecting local transmission. Because surveillance is being conducted for only a small portion of the potentially exposed population, there is a considerable chance that local Zika transmission could be ongoing without detection by local surveillance programs. Additionally, surveillance reports based almost solely on testing pregnant women are likely underreporting Zika cases, and the unidentified cases could be sustaining the Zika infection in local mosquito populations or spreading the infection directly via sexual activity.

In most of the country, the reintroduction of Zika virus each year will be required to establish infection in a mosquito population capable of transmitting Zika to humans. In some parts of the southern United States, however, particularly southern Florida and Texas, winter temperatures are not typically cold enough to kill Aedes aegypti mosquitoes or their eggs/larvae, potentially allowing Zika infection in local mosquito populations to persist through the winter months. The documented introduction of Zika—and local transmission—in southern Florida and Texas in 2016 increases the likelihood that Zika could become endemic in these parts of the United States.

Based on the best information available to them, Hidalgo health officials have identified no evidence of ongoing local Zika transmission. That being said, we—as health officials and as a country—must be aware that the current testing and surveillance system is, by design, going to result in an underreporting of Zika cases. The current testing guidance does not attempt to ascertain the total incidence of Zika virus infection or the total burden of disease; it is an admirable attempt to optimize the use of limited resources by focusing them on the highest-risk populations. If we do desire to obtain a more complete picture of Zika transmission, considerable investment—beyond the small, one-time emergency allotment from Congress—is required to develop better diagnostic capabilities, build testing capacity, and integrate surveillance networks across jurisdictions.

Without additional resources to conduct more complete Zika surveillance, health officials are continually operating under uncertainty regarding the presence and extent of local transmission. Such uncertainty makes it critically important that health departments implement effective public education campaigns that encourage people to limit their exposure to mosquitoes and underscore the importance of testing of high-risk individuals as well as proactive vector control efforts to limit Zika risk in at-risk communities.