Yellow fever outbreak first reported in Angola kills 21 people in Congo

An Angolan military member gives a girl a yellow fever vaccine in Luanda, Angola, in February.

(CNN)A yellow fever outbreak that killed hundreds in Angola appears to be moving to neighboring Congo, where it has left at least 21 people dead.

The Democratic Republic of Congo reported the deaths this week, the World Health Organization said in a statement.

It said between January and March, at least 151 people were suspected of having the disease in the Congo.

Some of the cases were detected in areas bordering Angola and "were imported" from there, the organization said.

At least 225 deaths have been reported in Angola as of this week, the nation's worst yellow fever outbreak in three decades. Most of the cases have been in the capital, Luanda.

"The report of yellow fever infection in travelers returning from Angola ... highlights the risk of international spread," the WHO statement said.

People gather for shots at a vaccination post during the campaign against yellow fever in the Kilamba neighborhood in Luanda, Angola.

Link to Zika virus

Yellow fever is transmitted by two types of mosquitoes, one of which is responsible for the Zika virus that has ravaged the Americas.

The yellow fever virus is transmitted when a mosquito bites an infected monkey and then bites a human.

Symptoms include fever, muscle pain, nausea, vomiting and loss of appetite, and can be confused for malaria or other mosquito-borne illnesses.

No treatment

A small percentage of infected people experience a second phase within 24 hours of becoming ill. It comes with more advanced symptoms, including jaundice, hemorrhaging and bleeding from the eyes, nose and mouth.

At least half of the patients who get the second phase of the disease die within 10 to 14 days.

There is no treatment for yellow fever, but patients can get supportive care.