Sloth Fever

by Gertrud U. Rey

Recent news headlines have featured a mysterious and presumably new illness termed “sloth fever.” More aptly named Oropouche fever, the disease is caused by Oropouche virus (OROV), an arthropod-borne virus that is transmitted primarily through the biting midge Culicoides paraensis. OROV typically circulates in sloths, non-human primates, and birds, but it can also spill over into humans when they are bitten by infected midges. Some species of mosquitoes can also transmit the virus, but there is currently no evidence of any human-to-human transmission.

The virus is in fact not new – it was originally isolated in 1955 in Trinidad and Tobago from the blood of a patient with fever. Since its discovery, OROV has caused regular epidemics in Latin American countries, and it is one of the most common arthropod-borne viral infections in Brazil. Increased travel and climate change-associated geographic spread of arthropod vectors have led to a heightened awareness of OROV fever as an emerging infectious disease. Recent public health reports have documented multiple cases of OROV infection in US and European travelers who returned from Cuba and Brazil, and two young and otherwise healthy individuals in Brazil recently died from the infection, manifesting the first recorded OROV-associated fatalities since this virus was identified almost 70 years ago.

OROV is a member of the genus Orthobunyavirus, a group of about 170 different vector-borne viruses. The viral particle is 80 to 120 nm in diameter and houses a spherical, negative-sense, single-stranded RNA genome that is surrounded by a lipid envelope. OROV typically infects a variety of cells found in peripheral blood; however, it also infects neural cells, and in rare cases it crosses the blood-brain barrier, potentially leading to meningitis and encephalitis. Symptoms occur about three to ten days after a person is bitten by the infected insect and closely resemble those associated with dengue fever, Zika fever, chikungunya, and malaria (e.g., fever, headache, joint, muscle pain, etc.). Although the majority of infections are mild and most people recover with no lingering effects, some individuals experience a recurrence of symptoms, a feature that distinguishes OROV fever from other arthropod-borne infections.

Epidemiological reports from Brazil have also documented several cases of OROV-infected mothers who gave birth to infants with microcephaly, a condition in which the baby’s head is much smaller than expected, leading to severely impaired intellectual development. This observation suggests that the virus is transmitted from mother to child in utero, and that viral infection may cause microcephaly in the developing fetus. You may recall similar reports during the 2016 Zika virus epidemic, when a series of successive publications demonstrated that Zika virus infection in pregnant individuals could cause microcephaly in their infants. The CDC therefore suggests that pregnant persons consider postponing travel to outbreak-prone areas.

There are currently no vaccines to prevent OROV fever, and there are no specific treatments or antivirals to treat an infection. The best way to prevent infection with OROV or any other arthropod-borne pathogen is by preventing bug bites through the use of insect repellent, appropriate clothing, window screens, and mosquito netting. If you live in affected areas, you can reduce your chances of exposure to midges and mosquitoes by keeping your yard free of any containers that can hold standing water. Flower pots, outdoor pet dishes, and even tiny bottle caps with rainwater can serve as breeding puddles for mosquitoes and flies. Increased awareness by public health officials and researchers will hopefully soon inspire OROV-specific antiviral therapeutics and other treatment strategies.

[This outbreak was discussed on TWiV #1144, clinical update with Daniel Griffin.]