The small, northeastern state of Massachusetts has confirmed a fourth case of the rare and deadly Eastern equine encephalitis virus (EEEV), which appears to have killed the latest patient.

The US Centers for Disease Control and Prevention (CDC) says the mosquito-borne virus is pretty rare - the entire country usually averages only seven cases annually - because the "transmission cycle takes place in and around swampy areas where human populations tend to be limited".  But the four cases in Massachusetts, the first in the state in 13 years, were all detected since the beginning of the month.  EEEV is fatal in roughly 30 percent of infections.

The latest patient was identified on Sunday, a woman over the age of 50 from southern Bristol County.  Within hours, family members confirmed the wide of a local Labor Union officials had died in hospital from EEEV.

In addition to the human infections, EEEV was detected in birds and farm animals.

The Massachusetts Department of Public Health is working with other agencies to coordinate a spraying campaign in areas deemed a high risk for EEEV to eradicate as many mosquitoes as possible in these late summer days.