However, the threat for local transmission of these viruses is increasing as more infected, potentially viremic travelers arrive from affected areas. The risk for local transmission of DENV and CHIKV is currently low because of small numbers of infected travelers arriving in California and limited distribution of invasive Aedes mosquitoes in the state. We document the invasion and persistence in California of Ae. Of these likely viremic patients, 44% (43/98) with dengue and 58% (54/93) with chikungunya fever arrived or returned to a county with an infestation of invasive Aedes mosquitoes and, thus, represented a potential risk for virus transmission if bitten by an Ae. Of the 141 chikungunya fever case-patients, 93 (66%) were likely viremic in California, 58 (41%) had illness onset after arrival, and 35 (25%) had illness onset within 7 days before arrival.

Of the 133 dengue case-patients, 98 (74%) were likely viremic while in California, 71 (54%) had illness onset after arrival, and 27 (20%) had illness onset within 5 days before arrival. In 2014, a total of 141 chikungunya fever and 133 dengue cases were reported, all in persons with recent travel to DENV- or CHIKV-endemic or outbreak areas. During 2009–2013, CDPH was notified of 3 confirmed chikungunya fever cases, and the reported annual number of dengue cases increased from 36 to 125, all among returned travelers or visitors. Nonetheless, CDPH is notified of confirmed and probable chikungunya fever cases.

Dengue, but not chikungunya fever, is reportable in California.

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