The weather phenomenon El Niño and global warming fueled the spread of the Zika virus in South America, scientists at the University of Liverpool say.

The study comes after The World Health Organization declared last month that Zika, which has been linked to birth defects and neurological complications, like microcephaly, will no longer be treated as an international emergency, but as a “significant enduring public health challenge.”

As of Dec. 14, there have been a total of 4,617 Zika cases in U.S. states and 34,268 in U.S. territories, including Puerto Rico, according to the Centers for Disease Control and Prevention. As of Dec. 15, 48 countries and territories in the Americas have confirmed the transmission of Zika virus disease since 2015, according to the Pan American Health Organization.

Researchers used a new epidemiological model that analyzed how climate impacts the spread of Zika by both of its major vectors, the yellow fever mosquito (Aedes aegypti) and the Asian tiger mosquito (Aedes albopictus). That model can also be used to predict the risk of future outbreaks, as well as helping public health officials with carrying out mosquito control measures and travel advice.

El Niño and Global Warming

“It’s thought that the Zika virus probably arrived in Brazil from Southeast Asia or the Pacific islands in 2013” said population and epidemiology researcher Dr. Cyril Caminade. “However, our model suggests that it was temperature conditions related to the 2015 El Niño that played a key role in igniting the outbreak – almost two years after the virus was believed to be introduced on the continent.”

When the Zika outbreak occurred in 2015, the risk of transmission was greatest in South America, researchers found. The spread of the Zika virus was likely caused by a combination of El Niño and climate change, which both created “conducive conditions for the mosquito vectors,” scientists said.

El Niño, which occurs every three to seven years, sees unusually warm temperatures in the Pacific Ocean, which causes extreme weather globally. However, 2015’s El Niño was one of the strongest on record, and was nicknamed “Godzilla.”

“In addition to El Niño, other critical factors might have played a role in the amplification of the outbreak, such as the non-exposed South American population, the risk posed by travel and trade, the virulence of the Zika virus strain and co-infections with other viruses such as dengue,” said Caminade.

Zika is “Here to Stay”

Scientists say although the WHO declared that Zika is no longer an emergency, the threat is “here to stay.”

“Zika is not going away, and so the development of tools that could help predict potential future outbreaks and spread are extremely important,” said Prof. Matthew Baylis, from the University’s Institute of Infection and Global Health. “Our model predicts a potential seasonal transmission risk for Zika virus, in the southeastern United States, southern China, and to a lesser extent over southern Europe during summer.”

The study was conducted in collaboration with the Liverpool School of Tropical Medicine and Public Health England.