In this edition of our On the Horizon series, we take a look at an obscure virus that may cause an important emerging disease in 2017, or may remain in obscurity for much of the world.
That is, except for South America, where the virus is estimated to have caused over 500,000 infections since its discovery in 1955. Its name? Oropouche, named after an area of Trinidad, where it was first isolated from a 24-year-old forest worker.
Like Zika, Oropouche is an arbovirus, spread by biting insects. However, while Zika is spread by mosquitoes, Oropouche is spread between humans by the bites of Culicoides paraensis midges (a day-biting insect). Those infected with the virus suffer symptoms similar to dengue fever – high temperatures, joint pain, nausea, and malaise – that begin about a week after being bitten and can be very severe. While no deaths have been reported, the virus has been identified in the cerebrospinal fluid of patients with meningitis, suggesting a link between the two in some cases.
The majority of Oropouche cases have been reported in Brazilian states within the Amazon rainforest, where the virus is characterised by short-lived but ‘explosive’ outbreaks, often involving thousands of people. The virus was first isolated in the country in 1960, from a dead sloth near the site of the then in-construction Belém-Brasília highway.
The following year, an estimated 11,000 people were infected in the city of Belém, in the first recorded mass outbreak of the disease. Then, in 1980, the city was the site of an outbreak affecting over 100,000 people – the largest number so far. The virus has also been isolated thousands of miles to the north and south of Belém, in Panama and Peru, and evidence suggests that it could be circulating in other South American countries too.
Oropouche may be a virus to keep an eye on due to the wide geography of its vector. C. paraensis midges can be found in a slice of the Americas that stretches from northern Argentina to the mid-US. Last year, a team in Brazil reported what they believe to be the first case of the disease acquired in the country outside of the Amazon, while the WHO reported the first cases of the virus in the Cusco Region of Peru.
There’s still so much we don’t know about this virus. As with most emerging diseases, its natural reservoir is unknown – evidence of it has been found in both non-human primates and birds – and the way it circulates in the wild has yet to be uncovered. Results in the laboratory show that Oropouche can infect other species of biting insects, including mosquitoes, but this has yet to be seen in a natural setting.
There’s also the question of how many suspected cases are actually Oropouche infection – it’s certainly feasible that many could have been caused by dengue, for example.
What does seem likely however, is that habitat destruction and climate change will have a role to play in the future of Oropouche – increasing human contact with infected animals, and expanding the range of the vector.
To give an idea of Oropouche’s current level of obscurity, search for it on PubMed. To date, you’ll only find less than a hundred papers that reference it. It was a similar story for Zika pre-2015 (148 papers then, 2,129 now), but whether Oropouche follows a similar path to become a global health issue, or whether it remains a disease few outside of South America have heard of, is difficult to answer.
Thanks to Dr Natasha Tilston-Lunel from Boston University, and to Dr Derek Gatherer from Lancaster University for their invaluable help on the background of Oropouche.