Residents are strongly advised to curtail outdoor activities between dusk and dawn because mosquitoes infected with eastern equine encephalitis have been found in the area. On Aug. 24, after an alpaca close to the Plympton and Halifax town line was killed by the virus, the communities were designated “critical risk,” which means the state Department of Public Health considers the area “highly likely” to see multiple cases of human disease. EEE causes brain swelling that results in death in a third of all cases, according to the federal Centers for Disease Control and Prevention. To date, there have been three confirmed human cases of EEE in Massachusetts residents this year, including one case that resulted in the death of a Worcester man in his 70s. Last year, there were two cases of EEE acquired in Massachusetts, including a fatal case in a Bristol County resident.