The Oropouche virus, which is linked to a disease known as “sloth fever” due to its potential to infect sloths, has reported its first cases in the United States. Historically confined to the Amazon region, this insect-borne virus has been spreading since late 2023, prompting growing international concern. More than 8,000 confirmed human infections have been recorded across the Americas this year, with Brazil seeing the majority, alongside cases in Peru, Bolivia, Colombia, and Cuba.
In July, Brazilian authorities confirmed the deaths of two adults from the virus, marking the first fatalities since the virus’s discovery nearly 70 years ago. Investigations are ongoing into possible fetal deaths and malformations linked to the virus, which has shown the capability to transmit from pregnant individuals to their fetuses. Currently, there are no available vaccines or treatments for Oropouche fever.
This month, the Pan American Health Organization raised the Oropouche virus’s risk level from moderate to high, acknowledging both its geographical spread and the emergence of fatal cases, which is significant for a virus previously known to result in mild to moderate symptoms. On August 23, the World Health Organization deemed the public health risk at the regional level as high and low on a global scale. The Centers for Disease Control and Prevention (CDC) in the U.S. has called for heightened surveillance of individuals returning from affected areas. Cases have been detected in travelers returning to the U.S., Spain, Italy, and Germany from Brazil and Cuba, including 20 travelers from Cuba to the U.S. as reported by the CDC on August 27.
The virus, part of the Orthobunyavirus genus, is primarily transmitted to humans through a midge, Culicoides paraensis, rather than mosquitoes, although other vectors cannot be ruled out. Discovered in 1955 in Trinidad and Tobago, it was first identified in Brazil in 1960. Outbreaks have historically occurred in the Amazon region but are now noted outside this area for the first time, raising concerns of potential local transmissions, especially given the widespread presence of the Culicoides paraensis midge across the Americas.
Symptoms of Oropouche fever are similar to those of other arboviruses, including fever, headache, muscle and joint pain, pain behind the eyes, nausea, and vomiting, making diagnosis based solely on symptoms challenging. Severe cases may arise, but most typically resolve within a week. Recent findings suggest a potential association between the virus and microcephaly in newborns, as antibodies against Oropouche have been detected in affected infants. Additionally, confirmed cases of mother-to-child transmission have been documented. Two previously healthy young women with Oropouche symptoms also died, marking them as the first reported fatalities linked to the virus.
While it is too soon to determine the frequency of these fatal cases among adults and infants, the overall symptoms remain consistent with past outbreaks.