State Senator Marc Pacheco, a Democrat from Taunton, will host a briefing on Eastern equine encephalitis Thursday with John Auerbach, the Massachusetts health commissioner, who will outline the state’s changes to its surveillance and response plan for mosquito-borne diseases. The session is set for 7:30 to 9:30 p.m. at Raynham Middle School on Titicut Road. Last year, the state had two cases of Eastern equine encephalitis. One was a Raynham resident who died from the illness, and the other was an out-of-state residents believed to have been infected while visiting the southeastern section of the state. Last winter, Raynham’s health agent, Alan Perry, with colleagues from other area communities, lobbied state officials for more aggressive treatment to control mosquito populations.