Clearing forests in the Amazon helps mosquitoes thrive and can send malaria rates soaring, U.S. researchers reported on Wednesday.

They found a 48 percent increase in malaria cases in one county in Brazil after 4.2 percent of its tree cover was cleared.

Their findings, published in the journal Emerging Infectious Diseases, shows links between cutting down trees, a rise in the number of mosquitoes and infections of humans.

“It appears that deforestation is one of the initial ecological factors that can trigger a malaria epidemic,” said Sarah Olson of the University of Wisconsin, who worked on the study.