Oropouche virus spreads in South America as scientists warn of potential outbreaks in the U.S.

As Oropouche virus cases rise across South America, experts call for urgent action to prevent its spread, with the U.S. potentially at risk due to travel-associated cases.

In a recent review published in the journal Emerging Infectious Diseases, researchers reviewed available information on the epidemiology, diagnosis, and transmission-risk associations of the Oropouche virus (OROV) (genus Orthobunyavirus). Reports suggest that the disease has been reemerging and expanding in its range, particularly since late 2023. In the span of just nine months (January – September 2024), more than 9,000 cases and two deaths were reported from six South American nations, with additional reports of travel-associated cases in North America and Europe, highlighting the need for a preparedness review before a potential large-scale disease outbreak.

Review findings suggest that most of the United States has a low Oropouche virus transmission risk. Still, some areas, including Puerto Rico and southern Florida, are considered at elevated disease risk due to the frequent influx of potential carriers from Oropouche virus hotspots. Cuba, which is experiencing an active outbreak, has also contributed to the increasing number of cases seen in travelers returning to the U.S. and Europe. Lessons from previous vector-borne outbreaks and current transmission ecology studies in Cuba and other affected regions may aid future preparedness against the disease. Notably, more effective and specific detection and testing methodologies must be developed given the limitations of current targeted small (S) segment genome assays.

What is an Oropouche virus infection, and what is its background?

The Oropouche virus is a pathogen belonging to the Simbu serogroup of the genus Orthobunyavirus (Peribunyaviridae family). The virus was first detected in a febrile forest worker in Trinidad and Tobago, near the Oropouche River. It is primarily transmitted to humans via the bite of Culicoides paraensis, a species of biting midges. Until recently, the virus has been considered endemic to the Amazon basin, with reports before 2000 documenting outbreaks in Brazil, Peru, and Panama. Between 2000 and 2023, more recent reports observed the disease's range expansion to Argentina, Bolivia, Colombia, Ecuador, French Guiana, and Haiti.

Alarmingly, despite previous reductions in Oropouche virus disease prevalence, the past year (January to September 2024) has witnessed numerous localized outbreaks across South America, accounting for more than 9,000 infections and at least two deaths from just six Latin American nations. Notably, in June 2024, Cuba, previously disconnected from the disease, reported its first confirmed case, sparking concern about its rapid transmission and outbreak potential.

Despite almost seven decades since its initial discovery, scientific knowledge about the Oropouche virus remains limited. Notably, no vaccinations, cures, or even routine diagnostic confirmatory tests currently exist for the disease, presumably due to its primarily mild clinical symptoms and limited long-term impacts. Recent disease-associated mortality and its unprecedented range expansion have raised concerns about potential viral reassortment, highlighting the need for additional epidemiological research on the disease.

Symptoms, transmission, and diagnosis

Oropouche virus disease typically presents as an abrupt onset of high fever, severe headache, chills, myalgia, and arthralgia. Unfortunately, these symptoms overlap with those of dengue, Zika virus, malaria, and chikungunya, often resulting in misdiagnosis and sometimes camouflaging Oropouche virus disease outbreaks as extensions of ongoing malaria or dengue episodes. In severe cases, Oropouche virus disease may cause neurological symptoms, including retroorbital pain, dizziness, nausea, vomiting, and retroorbital or eye pain. In rare instances, symptoms may include severe abdominal pain, conjunctival injection, and hemorrhage.

Pathogen transmission generally occurs via either urban or sylvatic transmission cycles. The urban cycle is better documented and involves the back-and-forth transfer of pathogens between infected Culicoides paraensis and humans. While Culex quinquefasciatus, the southern house mosquito, has been suggested as a transmission vector, previous vector competency evaluations challenge this notion with inconclusive findings. However, the lesser-understood sylvatic transmission cycle requires substantial research, given the potential for a wide range of mammalian and avian hosts, including sloths, non-human primates, wild rodents, and birds.

Over half a million cases of Oropouche fever have been recorded in South America, with Brazil being the most affected country.

Despite the disease's high attack rate (up to 30% of a population can be infected from a single incidence), Oropouche virus disease diagnosis remains challenging due to the lack of a specialized diagnostic test and its symptomatic similarity with other vector-transmitted viral diseases (dengue, malaria, Zika). Current reverse transcription polymerase chain reaction (RT-PCR) assays may detect viral load, but these assays remain sensitive only for the first 5-7 days following infection, highlighting the need for developing more sensitive and specific diagnostic tests against the pathogen.

Risk of spread and preparedness of the United States

Despite the startling surge in Oropouche virus disease prevalence, current epidemiological investigations observe that the risk of epidemic events in the United States is low. This is predominantly due to the low range overlap between urban U.S. citizens and Culicoides paraensis, the latter of which are generally restricted to rural tree holes in the Midwestern and Southeast U.S.

Increased public health preparedness is essential, especially in areas like southern Florida and Puerto Rico, where the introduction of the virus via travelers is more likely. The U.S. can improve its outbreak preparedness by requesting that clinicians report suspected patients to public health departments, prompting the routine screening of localized cases reporting similar symptoms and triggering community-wide mitigation measures, including pest management (to prevent sylvatic transmission). As rare cases of vertical mother-to-unborn transmission are being documented, pregnant women should be monitored, and information about the disease should be widely distributed.

Journal reference:

• Guagliardo SAJ, Connelly CR, Lyons S, Martin SW, Sutter R, Hughes HR, et al. Reemergence of Oropouche virus in the Americas and risk for spread in the United States and its territories, 2024. Emerg Infect Dis. 2024 Nov, DOI – 10.3201/eid3011.241220, https://wwwnc.cdc.gov/eid/article/30/11/24-1220_article