RIO DE JANEIRO — The governor of the Minas Gerais State in southeastern Brazil declared a public health emergency on Friday over an outbreak of yellow fever that appears to have killed at least 10 people so far and led to reports of more than 100 suspected cases of the disease.
The state authorities said Friday they were investigating 133 suspected cases of yellow fever, of which 20 were considered probable, pending further testing. They said they were also looking into reports of 38 deaths, 10 of them suspected of being caused by yellow fever, according to the State Health Secretariat’s website.
The state health authorities said the number of suspected cases had more than doubled in recent days: 48 suspected cases had been reported as of Wednesday, and that figure rose to 110 on Thursday.
According to the World Health Organization, yellow fever is an acute viral hemorrhagic disease transmitted by mosquitoes. Symptoms include fever, headache, muscle pain, nausea and vomiting. A small number of patients develop severe symptoms, and about half of those die within seven to 10 days.
A spokeswoman for the Minas Gerais State Health Secretariat said all the cases reported so far were sylvatic, meaning that the infection has been passed by mosquitoes in the wild, in rural areas where monkeys are known to carry the disease.
Small outbreaks are occasionally set off in rural towns when loggers, miners or other forest workers catch the disease and transmit it to local mosquitoes. Those outbreaks may die out spontaneously with the arrival of cold or windy weather, or they may be quelled by vaccination campaigns.
But if yellow fever reaches cities infested with Aedes aegypti mosquitoes — the same species that carries the Zika virus — it can set off outbreaks that spread so fast that vaccination teams and mosquito-control teams cannot contain them.
Health specialists said the sudden increase in cases was cause for alarm.
“It is a significant increase,” said Dr. Jesse Alves, an infectious diseases specialist at the Emilio Ribas Hospital in São Paulo, an institution run by the state government there.
Dr. Alves noted that Brazil suffered cyclical outbreaks of sylvatic yellow fever in rural areas like those in Minas Gerais. The last one was in 2009 and led to a number of deaths.
“This is not the first time this has happened,” Dr. Alves said. “We know the virus has a cyclical behavior, and from time to time we see activity in areas of transmission. But each time this happens it is of concern because we could see the return of yellow fever to Aedes aegypti mosquitoes in urban areas.”
The government of Minas Gerais said Friday that the declaration of a public health emergency expanded its ability to deal with the growing crisis, allowing it to assign contracts for more workers and services without having to go through the rounds of bidding normally required under Brazilian law.
“We are taking all the preventive measures, especially in affected areas in the rural zone,” Gov. Fernando Pimentel said in comments published on the State Health Secretariat’s website on Friday. “Everything that is necessary is being done, and with the help of everyone and the awareness of the population, we will overcome this moment.”
The state has received 735,000 vaccines from Brazil’s Ministry of Health, the state spokeswoman said. People in rural areas and towns affected by the outbreak have been lining up to receive vaccinations, according to local news reports.
Although many of the areas where the Minas Gerais state government has reported cases of yellow fever are rural, some cases are in small towns. One death likely to have been caused by yellow fever has been reported in Ubaporanga, a town of 12,000 people. In Ladainha, a town of 18,000 people, the disease is suspected in the deaths of three people.
“The question is why people weren’t vaccinated in these areas,” Dr. Alves said. “Minas Gerais has long been known as a risk area for yellow fever. That is what most calls my attention.”