If your wife is a researcher in medical entomology, you’ll often hear odd tidbits related to mosquito-borne diseases. For instance, did you know how cute malaria parasites can look under a microscope? I didn’t either, until I met Cassandra Urquhart. (Some other things I’ve heard described as “cute” since then include, but are not limited to: cockroaches, nematodes, spiders, earwigs, and male mosquitoes.) She’s fascinated by her own work with La Crosse virus, excited by new papers on dengue fever, and interested in how many of the mosquitoes she’s collected at sites around Knox County, Tennessee will test positive for West Nile virus. In her spare time, she reads books on the history of yellow fever and Chagas disease for fun. Don’t get me wrong—she cares about the human toll of such diseases. But as a scientist, she’s usually more curious than alarmed about them. However, when it comes to chikungunya virus, my cheerfully bug-obsessed wife gets far more serious—and so do many entomologists. So why is chikungunya different?
Chikungunya virus seems, at first, to have a lot in common with dengue virus, another mosquito-borne pathogen. Both cause extremely painful diseases—chikungunya’s name comes from a Makonde word meaning “that which bends up,” referring to the contortions sufferers put themselves through due to intense joint pain. Dengue’s nickname is breakbone fever. Both viruses are primarily transmitted by the Aedes aegypti mosquito, and both have been moving slowly closer to the United States over the past decades, with local cases of dengue fever already found in Florida and Texas.
Last week the Centers for Disease Control and Prevention announced the first locally acquired cases of chikungunya in the United States. A woman in Miami-Dade County and a man in Palm Beach County, neither of whom had left the country recently, both came down with the dreaded disease.
To understand why it’s chikungunya, not dengue, that makes entomologists so nervous, you’ll need to know a bit about another mosquito; Aedes albopictus (more pronouncably known as the Asian tiger mosquito). The Asian tiger mosquito is an invasive species that has spread over much of the eastern half of the United States since its introduction in 1985. These black-and-white-striped jerks are capable of spreading all sorts of diseases, including West Nile, dengue, and yellow fever. However, they often do so pretty inefficiently, with viruses found in only the tiniest minority of the mosquitoes tested. In the case of chikungunya, however, at least one strain has been shown to spread as easily in tiger mosquitoes as in Aedes aegypti. Ae. aegypti (which is also, confusingly, tiger-striped) is mostly confined to the Gulf Coast. It’s common in parts of Florida, Texas, and Louisiana. In most other places, however, Asian tiger mosquitoes have driven down their numbers through competition—good news for anyone concerned about dengue or yellow fever, but very bad news when it comes to chikungunya.
Adding to the reasons for alarm is the fact that chikungunya doesn’t need a reservoir—it can be spread directly from one human host to another. This is in contrast with several other mosquito-borne pathogens, including West Nile virus, which needs to replicate inside a bird before it can pass from a mosquito to a human. The special characteristics of tiger mosquitoes once again exacerbate the problem—these particular mosquitoes prefer feeding off of, and living close to, humans. (Many mosquitoes, in contrast, feed opportunistically on humans, while primarily targeting other animals.) Tiger mosquitoes are also daytime feeders, which means that while other species are taking a break, preferring to feed at dawn or twilight, the tigers keep chomping during the times of day when humans are most active.
Until last week, the only cases of chikungunya found in the United States had been in travelers, generally those returning from the Caribbean, where several outbreaks are currently raging. Before we got confirmation last week, I asked my wife’s adviser, Rebecca Trout Fryxell of the University of Tennessee Institute of Agriculture, just how likely she thought it was that chikungunya would eventually make it to the United States. “I think it’s inevitable” was her reply. “Entomologists have been watching it since the introduction of Aedes albopictus.”
Of several strains of the virus, only one has been identified that is easily transmitted by the Asian tiger mosquito. This strain, the one with the most potential to spread far and wide in the United States, has been found in only one place so far, the French island of Réunion in the Indian Ocean. This means the virus (in the form of its other strains) may remain confined for now to pockets of the southernmost United States where Aedes aegypti is common. But there’s no reason to think that the situation will stay that way. Chikungunya has shown its potential to spread more widely, and with no vaccine and no treatment, that risk is likely to grow.
Of course, that doesn’t mean that there is nothing we can do. According to Trout Fryxell, personal awareness remains the most effective tool we have for combating this disease. Travelers who think they may have contracted chikungunya should not delay in seeking medical attention. Adults should take measures to protect themselves and their families from mosquito bites by checking window screens for holes, wearing effective bug spray when outside, making sure kids apply and reapply bug spray regularly, and checking for and eliminating any sources of standing water on their property to prevent mosquitoes from finding human-adjacent breeding sites. (And be thorough—my wife informs me that the larvae of Aedes mosquitoes can survive in less than a tablespoon of water.) Visit the Control page of the American Mosquito Control Association to learn more about the most effective methods of prevention.
Above all, if you’ve recently returned from the Caribbean or are living in the Miami-Dade area and you’re feeling a bit feverish (or if you’re in so much pain that you feel compelled to contort yourself into bizarre positions), do not get bitten by any tiger-striped mosquitoes! Seriously, I know you’re feeling terrible, I know you’re wondering if the pain will ever stop, and if it’s worth even continuing to live, but spare a thought for the rest of us and avoid mosquitoes altogether, will you?