Doctors raise alarm about rise in “sloth fever” infections linked to birth defects

A surge of infections from a tropical virus is raising alarm among public health officials because it appears to be causing birth defects similar to those associated with Zika virus. Known as Oropouche virus, it can trigger a fever that may cause pregnant people to miscarry or their babies to have birth defects known as microcephaly, or smaller than usual heads.

Disease caused by this virus currently has no treatment or vaccine, although most cases are generally asymptomatic or mild and resolve on their own. Epidemics of Oropouche fever have occurred multiple times in the past, with approximately 380,000 infections between 1961 and 2007. Last year, more than 16,000 cases were reported, including four deaths, with 8,600 cases in Brazil alone, many in places where the virus hadn’t been reported before. Over 100 of these cases traveled to the U.S, but there has been no local infections or transmission. All of this indicates that the virus is becoming more prevalent, posing greater risks to the public.

“Case numbers have been steadily increasing this year. In areas with previous Oropouche transmission the number of cases have approximately doubled compared to last year and there have been many reports of cases in new areas,” Dr. Stephen Vaughan, an infectious disease specialist at the University of Calgary, told Salon by email. “Due to the nonspecific presentation and limitations in testing for this virus in most countries, the number of reported cases are an underestimate of the true burden of disease.”

Vaughan co-authored a recent guide published in the Canadian Medical Association Journal warning travelers, especially pregnant people, to take precautions and avoid insect bites that can spread the illness.

“Since late 2023, new outbreaks have been reported in Bolivia, Brazil, Peru and Cuba, including among travelers returning to Canada and the United States,” the report states. Notably, more Canadians are ditching vacation plans in the U.S. and traveling to South America instead, according to a recent Business Insider report, in response to tariffs imposed by the U.S. government. But Oropouche can be hard to detect, and is easily confused with other diseases.

“Malaria, Zika, dengue and chikungunya are also endemic in some regions affected by Oropouche virus,” the report notes. Indeed, Oropouche can mimic dengue and chikungunya with similar symptoms of fever, chills, headache and myalgias, which can often cause the infection to be misdiagnosed. It primarily spreads from biting midges such as Culicoides paraensis, but like Zika, it can be transmitted via certain mosquitoes (Culex quinquefasciatus and Aedes serratus.) Non-human primates, including howler monkeys and sloths, can also be infected with Oropouche, which is why the disease is sometimes nicknamed “sloth fever.”

Infection can be prevented by wearing long-sleeved clothing and using mosquito nets, as well as chemical insect repellents such as DEET.

“As this is a recent outbreak, there aren’t any treatments or vaccines available, and vaccine development pipelines are in their infancy (pre-clinical),” Vaughan explained. But “arbovirus vaccines“ meant to target viruses transmitted by arthropods and insects “have been recently approved in many countries,” he said, including vaccines for dengue and Chikungunya, which are similar but more severe. 

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“One of the dengue vaccines has been authorized for use in the U.S.,” he continued, but only for patients with evidence of previous infection. “If Oropouche continues to circulate for a few more years and there is confirmatory evidence of birth defects in pregnant travelers then it is likely a vaccine will be prioritized.” That, however, would take years to move through the approval process.  

As with other pathogens, from bird flu to malaria, the increase in Oropouche infections can be explained partially by global heating and the ongoing destruction of the environment.

“Climate change has allowed the expansion of mosquito vectors into new areas exposing more humans,” Vaughan said. “Also, degradation of natural habitats have driven mosquitos into surrounding areas. Ease of travel (airplanes) and the relative lack of symptoms with Oropouche allow infected patients to carry the virus over long distances to new areas without detection, then expose a new population.”

To further complicate matters, surveillance of the Oropouche virus is likely imperiled by recent cuts at the U.S. Centers for Disease Control and Prevention, as well as the Trump administration’s decision to pull out of the World Health Organization.

“Decreased funding and inability to collaborate with international organizations will lead to increased threats from emerging pathogens,” including Ebola, mpox and Zika virus, Vaughan said. “Without properly supported surveillance networks, infections (including Oropouche) will continue to enter and circulate in the population prior to being recognized with the possibly of severe consequences.” He said he hoped the CDC would “continue to be the most important organization to identify, monitor and advise the public of ongoing and emerging) outbreaks. It is better to know which infections are going to cause the next outbreak (through surveillance) to act quickly allowing health authorities to make educated decisions.”

One significant outstanding question about Oropouche is whether it can spread via semen, which would effectively make it a sexually transmittable disease. Zika virus can be spread this way, though the viruses are entirely different species. A 42-year-old Italian man who visited Cuba in July 2024 contracted Oropouche fever, with tests revealing he was shedding the virus in his semen, though it’s not yet clear if that means infection can spread from person to person this way. “Our findings raise concerns over the potential for person-to-person transmission of [Oropouche virus] via sexual encounters and may have implications for sperm banking and assisted reproductive technologies,” a report in Emerging Infectious Diseases states, adding that more research is needed.

In the meantime, there have yet to be any reports of Oropouche infections in the U.S. or Canada and it’s still not entirely clear how damaging the virus can be to the health of babies.

“Currently, the number of cases in the U.S. remains low,” Vaughan said. “However, the possibility of infection is greater with travel to the Caribbean and South America. The possible association with birth defects is still under investigation. I would recommend that pregnant travelers discuss their travel itinerary with a travel health professional to determine their individualized risk.”