The Oropouche virus, known for causing a disease colloquially termed “sloth fever,” has been reported for the first time in the United States. Historically limited to the Amazon region, this insect-borne virus has been spreading its reach since late 2023, prompting growing international concern. There have been over 8,000 confirmed human cases across the Americas this year, with Brazil being the most affected, followed by Peru, Bolivia, Colombia, and Cuba.
In July, Brazilian authorities reported the first fatalities linked to Oropouche, with two adult deaths marking a significant and concerning development since the virus’s discovery nearly 70 years ago. Officials are also probing possible connections between the virus and fetal deaths and malformations, as studies indicate potential transmission from pregnant individuals to their fetuses. Currently, there are no vaccines or treatments available for this disease.
The Pan American Health Organization recently elevated the risk level for Oropouche from moderate to high due to the virus’s expanding geographical presence and the emergence of fatal cases, a departure from its historical pattern of causing mild symptoms. The World Health Organization has assessed the public health risk from the virus as high within the region and low globally. The U.S. Centers for Disease Control and Prevention is closely monitoring travelers returning from areas where the virus is prevalent. Cases have been reported among individuals traveling from Brazil and Cuba to the United States, Spain, Italy, and Germany.
The Oropouche virus belongs to the Orthobunyavirus genus and is typically spread through a midge known as Culicoides paraensis, unlike other more recognized vector-borne viruses such as dengue or Zika, which are transmitted by mosquitoes. Discovered in 1955 in Trinidad and Tobago and first identified in Brazil in 1960, the virus has occasionally emerged in human outbreaks predominantly within the Amazon region. Despite the ongoing surveillance efforts, it’s uncertain if the current outbreak is the largest in history.
Recent findings indicate local transmission in Cuba for the first time and imported cases in Europe and the United States, raising further alarms. The presence of the midge responsible for transmission spans from the United States to Argentina, increasing the risk of local epidemics whenever people are infected.
Symptoms of Oropouche fever mirror those seen with other arboviruses, including fever, headaches, and joint pain, complicating diagnosis, which requires laboratory confirmation. While most cases are mild and resolve within a week, some may develop severe forms leading to neurological or hemorrhagic issues. Recent observations involving newborns with microcephaly suggest a potential link to Oropouche infection, although no conclusive causal relationship has been established. Swabs taken from deceased fetuses have detected the Oropouche genome, indicating mother-to-child transmission in several cases.
Deaths of young women displaying severe symptoms indicative of Oropouche have also been documented, marking the first fatalities associated with the infection in the literature. The full extent of the virus’s threat remains unclear, as current evidence does not suggest a significant change in its symptoms compared to past outbreaks.