Oscar Ugarte, the health minister, said authorities were screening sugar and fish meal exports from Ascope province, located about 325 miles north-west of Lima.
Chicama beach, a popular draw for tourists to Peru, is not far away.
Mr Ugarte said the boy, who had Down syndrome, died of bubonic plague on July 26.
He said on Monday that most of the infections were bubonic plague, with four cases of pneumonic plague.
The former is transmitted by flea bites, the latter by airborne contagion. The disease is curable if treated early with antibiotics.
The first recorded plague outbreak in Peru was in 1903. The last, in 1994, killed 35 people.
The bubonic plague is believed to have been the cause of the Black Death, which swept through Europe in the 14th century, killing one third of the population, or more than 25 million people.
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