In 2016, Zika virus outbreaks spread throughout Latin America and the Caribbean. Travel notices currently are posted for 60 countries and territories including in Asia and the Pacific. The large majority of Zika infections identified in the U.S. have occurred among travelers to affected areas but local outbreaks also have occurred in South Florida and Brownsville, Texas, resulting in 218 and 6 cases, respectively.1 Numbers of reported Zika cases, however, are certainly a substantial underestimate of the true burden of infection as 80% of infections are asymptomatic and persons with mild symptoms may not seek care or be diagnosed. In Puerto Rico, for example, over a period during which about 25,000 cases were reported, screening of blood donors suggested that 13% of the population had been infected, translating to over 475,000 people, consistent with only 1 in 19 cases being reported.2 Although no mosquito-borne Zika transmission has occurred in Los Angeles County or anywhere in California, conditions sufficient for a local outbreak (introduction of the virus by travelers and the presence of Aedes species mosquitoes that transmit infection) are present. The Centers for Disease Control and Prevention (CDC) have identified LA County as one of the U.S. jurisdictions at highest risk for an outbreak.
Zika infection during pregnancy is the greatest concern due to adverse pregnancy and birth outcomes. Microcephaly associated with disruption of brain development has been identified in 4% of U.S. infants whose mothers were infected during pregnancy, similar to estimates from Brazil and other earlier outbreaks.3-5 Recent data from a Brazilian cohort study, however, suggests that Zika infection can cause a much broader range of defects and affect a much higher proportion of infants. From a cohort of 134 Brazilian women with symptomatic Zika infection during pregnancy, defects were identified by clinical examination or imaging in 42% of infants. Defects occurred in 55% of infants whose mothers were infected during the first trimester, 51% in the second, and 29% in the third. Abnormalities on exam included hypertonicity, clonus, hyperreflexia, abnormal movements, spasticity, contractures, seizures, abnormal fundoscopic exams, and abnormal hearing tests. Imaging showed cerebral calcification, cerebral atrophy, ventricular enlargement, hypoplasia of cerebral structures, and parenchymal brain hemorrhages.5 While the complete spectrum of congenital Zika virus disease and its long-term consequences are not yet known, the impacts on infants and their families are likely to be far greater than previously recognized.
With the onset of summer in LA County, Zika-associated risks increase. Warmer weather will lead to increased density of the Aedes mosquito vectors in Mexico and Central America, where in 2016, 76% of LA County’s reported Zika cases acquired the infection. Increased summer travel to other areas with Zika virus will also increase Zika cases in Los Angeles. More than half of the 98 Zika cases reported in the county last year were acquired during travel in July and August. And during summer, the risk of a local outbreak increases as more infections among travelers coincides with an expanding indigenous Aedes mosquito population.