SÃO PAULO—Brazil’s biggest city has called in the army to help combat a deadly outbreak of dengue fever that has sickened hundreds of thousands of people nationwide.

Soldiers will next week begin going door-to-door in some of São Paulo’s hardest-hit neighborhoods to educate residents on fighting mosquitoes, Mayor Fernando Haddad said on Friday.

A severe drought in southeastern Brazil has spurred residents to hoard water, often in makeshift containers, providing breeding grounds for mosquitoes that spread the disease whose symptoms can include intense muscle pain, convulsions and high fever.

Dengue has killed 132 people in Brazil in just the first 12 weeks of the year, a nearly 30% jump from the same period in 2014. Three-quarters of those deaths have been in São Paulo state, which has registered more than half of the 460,502 cases reported in Brazil in the first quarter. The state had more cases in the first 12 weeks of this year than it did in all of 2014.

In São Paulo city, where confirmed cases have soared more than fourfold to nearly 32,000 over the period, Mr. Haddad said that 50 army soldiers are being trained to advise residents on how to store water and most effectively use repellents and insecticides.