Oropouche Fever: 9 Years After Zika, a New Threat Rises

Nine years ago, Brazilian health authorities reported the country’s first cases of a rare but mild viral infection called Zika. At first, there was little cause for concern. Researchers had known about Zika since 1947 when it was first isolated from a rhesus macaque in Uganda. The virus was thought to be common in animals in Asia and Africa, but rarely caused severe symptoms in humans and had only caused one major outbreak up to that point. That outbreak had sickened thousands across French Polynesia from 2013 to 2014, but even then, the disease remained mild. The next one, however, would be worse by several orders of magnitude.

Like so many other South American cities, Rio De Janeiro would be slammed by Zika in 2015 and 2016 despite the virus's relative rarity before 2013.

It’s thought that an individual infected with Zika, likely from French Polynesia, introduced the virus to Brazil in 2014, possibly during the FIFA World Cup. From there, the virus got into the mosquito population, which spread the virus across the Americas. The ensuing epidemic would see millions of people contract the virus in almost 100 countries.

For most, the disease progressed as expected, but a wave of congenital birth defects appeared in Brazil by late 2015, sparking concern over the effects of Zika in pregnant women. As a result, the World Health Organization (WHO) declared a Public Health Emergency of International Concern (PHEIC) the following February. Travel advisories were issued, genetically modified mosquitoes used to limit mosquito populations, and more than 200,000 troops deployed to fight the outbreak.

Though Zika can occasionally cause serious conditions in adults, the most vulnerable are unborn fetuses. 80% of Zika cases have no symptoms, allowing the virus to silently spread from mothers to fetuses, resulting in a slew of insidious congenital conditions. The most well-known and dramatic of these is microcephaly, a sometimes fatal condition where babies are born with smaller heads and improperly developed brains. After almost 5000 suspected cases of microcephaly, thousands of other severe Zika cases, and up to 18 billion dollars in economic damage, the 2015-16 Zika outbreak subsided, though Zika remained endemic in the Americas and continues to spread to this day.

Despite its initially mild clinical manifestation, Zika quickly proved to be a global health threat due to its link with birth defects like microcephaly. Source

Oropouche: Zika 2.0?

Zika is an Arbovirus, meaning that it’s primarily spread through arthropod vectors like ticks or mosquitoes. Other Arboviruses include Dengue, Japanese encephalitis, and Rift Valley. A common theme among this ragtag collection is a rise in infections in recent years, likely accelerated by shifting climates that affect their insect hosts. Sometimes, they can take decades to emerge. The 2015-16 Zika epidemic, however, shows that an outbreak can explode onto the global stage in a matter of months.

Though the threat of Zika has diminished, another Arbovirus is rapidly emerging in South America: Oropouche fever. Like Zika, it was first discovered in the mid-20th century and remained rare for decades, but has recently seen a surge in cases and is spreading outside of its traditional habitat. One factor in this shift is deforestation, which displaces animals that carry the midge (C. paraensis), the primary insect carrier of Oropouche. As a result, the virus is increasingly coming into contact with people and transmitting successfully. In 2024 alone, cases have risen substantially in Brazil, Bolivia, Peru and Colombia, and Cuba reported its first major outbreak. Even Europe had its first case this year in a traveler from Cuba.

A map of current Oropouche fever outbreaks. Source: CDC.gov

With the virus expanding into new countries and megacities for the first time, it’s clear that it cannot be confined to traditionally-affected areas around the Amazon basin, worrying experts. At present, little is known about Oropouche and no treatments exist. Meanwhile, the virus itself may be mutating to better spread in humans. Like the flu virus (perhaps the single most consistent cause of new epidemics and pandemics thanks to its ability to mutate), different strains of Oropouche can swap traits because their genetic code is split into separate segments. The negative health impact of such mutations is already apparent: since 2022, one such “reassortant” strain of Oropouche has driven a sharp increase in infections. While all strains of the virus appear to cause mild illness, the first suspected deaths are under investigation, and governments would do well to remember that Zika too was initially thought to be mild.

The concern is that further mutations, combined with deforestation and climate change, could cause an explosive epidemic, especially since most people have no immunity to the virus. From there, the effects could be unpredictable. For one, unlike Zika, Oropouche is spread by tiny midges in addition to mosquitoes, which can pass through mosquito nets and resist insect repellent. Additionally, now is not a good time for healthcare systems in South America. Since 2020, both COVID-19 and Dengue fever have overrun hospitals. If Oropouche can cause an explosive outbreak like Zika did, the consequences may be severe.

Midges like C. paraensis, the primary vector for Oropouche, can escape control measures for mosquitoes, making the virus more problematic than many other Arboviruses.

In light of the growing threat posed by Oropouche fever, it is imperative to act proactively. Increased surveillance, which has already begun in Brazil, combined with investment in treatment and control measure research, could save lives. Additionally, addressing the broader issues behind the emergence of viruses like Oropouche, namely deforestation and climate change, is essential to mitigate future risks.

The fact is that even abrupt health crises don’t appear overnight; Zika certainly didn’t. Neither did HIV, which circulated for decades in Africa before exploding worldwide. Even COVID-19 was preceded by multiple other new Coronavirus outbreaks. This time, let’s not wait for the next big threat to rear its ugly head.