Disease detectives are on the lookout for obscure viruses that can be spread among people by traveling insects, and quickly become a widespread problem. Scientific papers are filled with illnesses to watch. Four particular viruses now stand out to virologists and epidemiologists, although it is not certain any of the ailments will become the next Zika or West Nile virus. But researchers give several reasons to keep a close eye on this quartet:
"For 10 years now I've been thinking that Mayaro is right on the cusp of being able to amplify in humans and being transmitted efficiently by Aedes aegypti mosquitoes," says Scott Weaver, a virologist at The University of Texas Medical Branch at Galveston. Mayaro causes a disease that is clinically indistinguishable from the mosquito-borne chikungunya virus: fever, chills, rash and the characteristic joint pain that can last longer than a year. It does matter, though, which of these viruses circulates in your body. Once there are vaccines against Mayaro and chikungunya, and drugs to treat them (so far there are none), they will likely be virus-specific.
Similarities with chikungunya are also why Mayaro might become a widespread problem. Both were originally transmitted by forest mosquitoes, infecting people in places like the Amazonia. Yet chikungunya has already adapted to be transmitted by urban mosquitoes, such as A. aegypti or Aedes albopictus—as of September 2016 in the Americas alone there were over 100,000 confirmed cases of the disease. Such adaptations may be happening with Mayaro, too. In lab experiments A. aegypti and A. albopictus have been shown as potential vectors for Mayaro. What's more, in recent years there were cases in large Brazilian cities—far from forests. In September 2016 the virus was detected in Haiti, further suggesting possible adaptations to an urban cycle. "It could just be that the right strain was introduced into that area or it could be that the virus has mutated to facilitate urban transmission," Weaver says.
RIFT VALLEY FEVER
For most people, a Rift Valley infection means a simple illness of fever and chills. Yet in some it can progress to a hemorrhagic disease, accompanied by abnormal bleeding, or to inflammation of the brain. Half of Rift Valley hemorrhagic cases are fatal, and there is no treatment.
Since its discovery in Kenya in the early 20th century, the Rift Valley virus has been limited to Africa. But in 2000 it left the continent, reaching the Arabian Peninsula. By 2014 there have been numerous outbreaks causing tens of thousands of human cases and millions of livestock deaths. In recent years more people have ended up with the hemorrhagic disease (10 percent versus 1 percent from early observations). "The jury is still out whether the virus is mutating to be more virulent or, more likely, we are just much better at detecting severe infections in Africa," says Brian Bird, a virologist at the University of California, Davis School of Veterinary Medicine One Health Institute.
The problem with Rift Valley fever is that it can be spread by over 30 species of mosquitoes, 19 of them native to North America. Both wild and domestic animals can serve as a reservoir of the disease. "The availability of immunologically naive livestock to perpetuate the disease in North America or in Europe is concerning. If it were to come here, the effects on animal health and economy would be profound," Bird says. A likely way for Rift Valley fever to arrive into the U.S. would be via virus-carrying mosquitoes on airplanes or ships. Luckily, the work on a vaccine is quite advanced.
CRIMEAN-CONGO HEMORRHAGIC FEVER
Crimean-Congo hemorrhagic fever is transmitted by ticks—which is good news because tick-borne infections are likely to spread more slowly than mosquito-carried ones. The bad news is that Crimean-Congo has fatality rates up to 40 percent. Identified in 1944, the virus has already managed to travel from Africa all the way to China. In recent years the virus's spread seems to have accelerated: In 2002 it arrived in Turkey, infecting 10,000 people by 2015; by 2011 the virus reached India; and in September 2016 the first two cases were registered in Spain. One person died. "It's very difficult to say if that was a one-off introduction of the virus or an indicator of changes in the environment allowing the virus to spread to this area of Europe," Bird says.
Besides bleeding from almost anywhere in the body, the disease can cause rash and fever. The antiviral drug ribavirin has been used to treat Crimean-Congo, but it is not very effective.
The virus is transmitted by ticks, mostly from the Hyalomma genus, that with climate change are creeping into new territories. They have already been found as far north as Germany (in 2015) and even in the U.K., hitchhiking on migratory birds. Whether there are ticks that could transmit Crimean-Congo in North America remains unknown.
"After an unusually warm summer in Europe we are facing an unprecedented outbreak of Usutu among birds in Germany, a massive die-off, and colleagues from France, Belgium, Hungary, the Netherlands are also reporting similar phenomena. That's new—earlier Usutu outbreaks were always limited geographically," says Daniel Cadar, virologist at Bernhard-Nocht Institute for Tropical Medicine in Hamburg.
Usutu, an avian virus transmitted by commonplace Culex mosquitoes, causes a disease that is a West Nile virus fever lookalike: headaches, fever, neurological problems. But so far, most Usutu infections in humans end without symptoms. According to a study published this September in ClinicalMicrobiology and Infection, over 6 percent of Italians around Modena have been recently and unknowingly infected with the virus. Serious symptoms are most likely to occur in people with compromised immunity, as it was reported in 2013 in Croatia—on just three cases. Yet Weaver says we shouldn't underestimate this infection. "Usutu is not far from where West Nile fever was 20 years ago. There were some small outbreaks in eastern Europe, but for the most part people thought it was not a serious pathogen. Because Usutu is an avian virus, it could easily circulate in many different parts of the world, just like West Nile virus," he cautions. And climate change could mean more infected birds, and more potential for human-virus contact, too.