Reuters: Prior exposure to dengue or West Nile could make Zika worse: report
“Prior infection with West Nile or dengue — two viruses closely related to Zika — can make Zika symptoms worse, U.S. researchers said on Thursday. The findings in mice, published in the journal Science, confirm studies in cells suggesting that prior infection with dengue could worsen the effects of Zika…” (Steenhuysen, 3/30).

Science: Dengue may bring out the worst in Zika
“…If such ‘antibody-dependent enhancement’ (ADE) also takes place in people, it could have helped fuel Zika’s recent explosion in Brazil, where more than 90 percent of people in some communities have been infected with dengue…” (Cohen, 3/30).

VOA News: Study: Prior Dengue Infection May Make Zika Worse
“…In the study of mice, researchers at the Icahn School of Medicine at Mount Sinai in New York gave the rodents human antibodies to dengue virus from 141 infected individuals. Another group of animals received antibodies from 146 individuals with West Nile virus. The mice were then exposed to Zika virus. Only 21 percent of mice with the antibodies from dengue patients survived being infected with Zika. Similar results were reported in mice that received antibodies from West Nile patients…” (Berman, 3/30).