Controlling diseases carried by mosquitoes is a bit like playing whack-a-mole: As soon as you stamp out a disease carrier in one area, they’ll pop up in another. Taking into account the larger ecological picture could save you the trouble.
That’s certainly the case for chikungunya—pronounced “chick-in-GOON-ya”—a virus that’s relatively new to the Americas. The pathogen spreads from the bite of a thirsty mosquito and renders victims sore and feverish, sometimes for months on end. Until 2006, the virus had been restricted to tropical regions of Africa, with travelers occasionally bringing the virus home. The actress Lindsay Lohan was recently reported to have contracted the virus over the holidays while vacationing in French Polynesia. Locally transmitted outbreaks in the Indian Ocean islands and, last year, in the Caribbean and Florida have made chikungunya a virus to watch.
Panama got its first native case last May, and this prompted researchers at the Smithsonian’s Tropical Research Institute (STRI) to see how Asian tiger mosquitoes (Aedes albopictus), one of the carriers of chikungunya, spread across the country and how quickly they are on the move. According to their results, published today in PLoS Neglected Tropical Diseases, Asian tiger mosquitoes have invaded Panama by hitting the pavement, travelling quickly along human-built roadways.
But Asian tiger mosquitoes aren’t the only ones to carry the virus: Aedes aegypti mosquitoes carry chikungunya, too, and both species can spread the dengue virus, which has a much more substantial foothold in the Americas. “In a lot of countries of Central America, including Panama, people have been looking at A. aegypti solely,” says Jose Loaiza, an entomologist at STRI and Panama’s Instituto de Investigaciones Cientificas y Servicios de Alta Tecnologia. “Both species have to be controlled in order to bring down the cases of dengue and chikungunya,” he says.
Genetically modified mosquitoes provide a promising, if controversial, way to control such diseases. The U.K. biotech firm Oxitec has engineered sterile A. aegypti mosquitoes, which can spread their sterility to wild populations and cut the number of disease-carrying mosquitoes buzzing around and biting humans. However, the STRI researchers suggest that the Asian tiger mosquito's agressive invasion tactics could help it colonize areas where GM mosquitoes have wiped out local populations—spreading chikungunya and dengue as they go.
Using Panama As a Lab
While A. aegypti has been in Central and South America since the 17th century, the Asian tiger mosquito didn't appear in Panama City until 2002, probably via cargo ship transportation from Asia. Loaiza and his STRI colleague Matt Miller wanted to see how the newcomer mosquitoes were spreading in the wild and how the two species might be interacting.
Fortunately, Panama’s ministry of health routinely collects samples from local mosquito populations across the country, and the researchers were able to use collection data on Asian tiger mosquitoes to track the spread of the species across Panama. They modeled the speed and spread of the bloodsuckers based on factors that could be driving the invasion: climate, human behavior, roads or some combination of the three.
By 2009, the insect's invasion had expanded to the coastal city of Colon, and by 2013, it had invaded most of Western Panama and east of Panama City. It turns out that local roads and highways best matched the path of the bug over time. “It seems to move really quickly, especially when you consider that there are all of these other competitive mosquitoes around,” says Miller. Asian tiger mosquitoes even invaded dry areas that might have been more hospitable to A. aegypti.
The bugs are likely hitching a ride with humans. Often, they lay their larva in old tires and then hatch. As the tires travel, the bugs travel. The finding matches up with previous data mapping Asian tiger mosquitoes’ invasion of the U.S. and Europe, and confirms that even at a local level in the tropics, these mosquitoes still migrate using the same routes. “There’s no question, they’re great hitchhikers,” says Phil Lounibos, an entomologist at the University of Florida.
Asian tiger mosquitoes haven’t invaded all of Panama yet, and the researchers recommend spraying vehicles with insecticide to stem their spread. Since Panama already has agricultural road checks in place to reduce the spread of a cattle parasite called screwworm, they suggest that spraying could be implemented at checkpoints. The researchers argue that this strategy could reduce cases of chikungunya and dengue.
A Not-So-Perfect Solution
The team also suggests that competition between these mosquitoes could affect the success of more sophisticated strategies, like the introduction of GM mosquitoes. Oxitec has focused their efforts on dengue and had some success in small-scale fieldwork in Brazil and the Cayman Islands. And in April 2014, an Oxitec trial led by the Gorgas Institute in Panama released 60,000 genetically modified A. aegypti mosquitoes in the Arraiján district, west of Panama City. By October, they saw a reduction of more than 90 percent in the local mosquito population in Panama, according to unpublished results from Oxitec.
But Miller and Loaiza’s modeling points to the possibility that Asian tiger mosquitoes could replace A. aegypti in areas where GM programs have killed off the local population of other species. Evidence from the southeastern U.S. suggests that’s what happened when Asian tiger mosquitoes invaded the area. They also suggest that wild A. aegypti populations from elsewhere might be able to reinvade these areas themselves.
Either scenario could aid the spread of chikungunya in Panama and facilitate future outbreaks of dengue. “In this particular case, where you’ve got two mosquitoes that can do the same thing, but the GM approach is only going to affect one of them, it doesn’t make sense at all,” says Miller. Panama’s first case of chikungunya popped up around the same time as the GM trial, but the researchers chalk that up to pure coincidence.
GM mosquito proponents argue that reintroduction is a problem all around. “Any vector-control effort would need to guard against re-introduction. This is not a feature or disadvantage of any particular approach,” says Luke Alphey, who studies vector-borne viral diseases at the Pirbight Institute in the U.K. Plus, Oxitec has been working on a genetically modified Asian tiger mosquito, though it hasn’t been tested in the field yet.
The success of a GM mosquito disease control program may come down to which mosquito is better at transmitting the disease to humans, and that’s complicated. Asian tiger mosquitoes may be better at sustaining the viruses outside of outbreaks and at spreading specific strains. Those strains haven't made it to this side of the Atlantic yet. Instead, A. aegypti mosquitoes are best suited to carrying the strain that's floating around the Caribbean at the moment. “Based on what we currently know about disease transmission of chikungunya, aegypti is to be more feared than albopictus,” says Lounibos. Evidence suggests that the same is true for dengue.
So replacing it might not be the worst thing in the world, right? “The worst case would be that you take out aegypti, and albopictus moves in, but you’ve replaced a very efficient vector with an inefficient one, and in the best case, you take out the efficient one and there’s no impact on the inefficient one,” says Chris Creese, communications manager at Oxitec.
This strategy hinges on dengue being a bigger problem in Central and South America and A. aegypti being a better carrier, and that could change. If one patient with the right strain shows up in Panama and gets bitten by a local Asian tiger mosquito, the other strain could soon spread across the country. “It’s just a matter of time,” says Miller.
The strain in the Americas could also mutate and evolve in a way that makes Asian tiger mosquitoes a more efficient carrier for the virus. That’s actually happened in three separate outbreaks on Indian Ocean islands. In either of these scenarios, invading Asian tiger mosquitoes could make chikungunya a major problem in the Americas. "There's evolutionary potential in the mosquitoes and in the virus," says Lounibos.
Cost could be the most significant factor in GM mosquitoes' success. At the moment, Oxitec isn’t sure how much they’ll charge, though some outside estimates come in the hundreds of thousands of U.S. dollars range. A wide-scale disease prevention program would require a lot of GM mosquitoes. “If you’re going to use GM mosquitoes in Panama, you’re going to need to release them at a very frequent pace, which may be really costly for the country,” says Loaiza. At this point, it’s unclear what countries will be willing to pay.
Loaiza and Miller aren’t writing the technology off completely. Genetic engineering might prove more successful if the circumstances were different, perhaps in conjunction with other weapons in the mosquito-fighting arsenal. Whatever happens, it’s clear that both species deserve a watchful eye. If we’re not careful, the Asian tiger mosquito could help elevate chikungunya to a much more widespread public health threat.