Travel-Associated Zika Cases and Threat of Local Transmission during Global Outbreak, California, USA

Abstract

Zika and associated microcephaly among newborns were reported in Brazil during 2015. Zika has since spread across the Americas, and travel-associated cases were reported throughout the United States. We reviewed travel-associated Zika cases in California to assess the potential threat of local Zika virus transmission, given the regional spread of Aedes aegypti and Ae. albopictus mosquitoes. During November 2015-September 2017, a total of 588 travel-associated Zika cases were reported in California, including 139 infections in pregnant women, 10 congenital infections, and 8 sexually transmitted infections. Most case-patients reported travel to Mexico and Central America, and many returned during a period when they could have been viremic. By September 2017, Ae. aegypti mosquitoes had spread to 124 locations in California, and Ae. albopictus mosquitoes had spread to 53 locations. Continued human and mosquito surveillance and public health education are valuable tools in preventing and detecting Zika virus infections and local transmission in California.

Keywords: Aedes aegypti mosquitoes; Aedes albopictus mosquitoes; California; Mexico; United States; Zika; Zika virus; arboviruses; global outbreak; local transmission; travel-associated illness; vector-borne infections; viruses; zoonoses.

Figure 1

Number of human Zika virus infections in residents, by month and year of onset and country of travel (top 10 countries shown), California, USA, October 1, 2015–September 1, 2017. Month was determined by date of symptom onset for symptomatic persons or specimen collection date for asymptomatic persons.

Figure 2

Confirmed and probable symptomatic Zika virus infections, by symptom onset month and year, California, USA, October 2015–September 2017.

Figure 3

Locations where Aedes spp. mosquitoes were detected and residences of possibly viremic case-patients infected with Zika virus, central (A) and southern (B) California, USA, October 2015–September 2017. Insets show larger views of corresponding region.