It was only a matter of time. From the moment the first cases of mosquito-borne chikungunya virus were detected spreading through the Caribbean, authorities were expressing concern about the possible introduction into North America.
Chikungunya virus is a mosquito-borne virus that causes potentially severe illness. Outbreaks have occurred in many parts of the world but, until recently, it hadn’t been reported from the Western Hemisphere.
Since the first reports of chikungunya virus detection among residents of the French side of St. Martin in the Caribbean surfaced in December 2013, there have been over 350,000 cases in Caribbean, Central America and South America. [update 4 September 2014: The total estimated cases now is over 650,000] [update 6 November 2014: Total estimated cases now 750,000] [update 12 December 2014: now over 1 million suspected cases of disease!]
While recent concern about mosquito-borne disease has focused on the risk of dengue associated with travellers attending the FIFA World Cup in Brazil, the number of imported cases of chikungunya virus into the U.S. has steadily increased with approximately 250 cases being reported this year. There have been imported cases in the past but now a locally acquired case has been reported in Florida.
The most recent update provided by the CDC [6 Novemebr 2014] reports that the total number of returning travellers to the U.S. is now 1,600, a staggering figure when compared to the pre-outbreak average of 28 cases per year. With about 9 million people travelling between the United States and Caribbean each year, the risks of the virus being introduced to the U.S. and triggering additional local outbreaks are not insignificant.
So what does this mean if I live in the U.S. or I’m about to visit?
There have been mosquito-borne pathogens introduced into North America in the past. Most famously was West Nile virus. Introduced in 1999, the virus has spread across the country and in the past 15 years or so infected almost 40,000 people and causing approximately 1,600 deaths.
Will Chikungunya virus so the same? No.
West Nile virus spread so widely because mosquitoes found all through the U.S. (over 40 different mosquito species) can carry the virus and birds, particularly those associated with urban areas act as reservoirs for the virus. What this means is that the virus could more easily spread amongst local mosquitoes and wildlife. This won’t be the case with chikungunya virus.
There are only two mosquito species associated with the spread of chikungunya virus in the U.S.. Aedes aegypti and Aedes albopictus. These two species are not as widespread or abundant as those that transmit West Nile virus.
Aedes aegypti, the Yellow Fever mosquito, is already a concern. This species is the most important mosquito internationally in the transmission of dengue viruses. It has been responsible for locally acquired cases of dengue in some southern regions of U.S. However, this mosquito is not found in many regions of North America.
Aedes albopictus, the Asian Tiger Mosquito, has a slightly greater distribution. This mosquito is more tolerant of temperate climates and so is found across many more regions within North America than Aedes aegypti. As Aedes albopictus has been responsible for some explosive outbreaks of chikungunya virus elsewhere in the world, concern was that this species may drive a similar outbreak in North America. Fortunately, some genetic studies have indicated that the current strain of chikungunya virus circulating in the Western Hemisphere is less prone to explosive outbreaks of disease driven by Aedes albopictus. However, these two mosquito species still hold the potential to spread chikungunya virus and should be considered a risk to be managed.
When considering the risk posed by Aedes aegypti and Aedes albopictus, other factors need to be considered. As opposed to West Nile virus, there are no local wildlife hosts of chikungunya virus that will assist the spread of the virus. Chikungunya virus, like dengue viruses, are typically spread by mosquitoes from person to person (as opposed to the spread of West Nile virus by mosquitoes between birds and people). Problem is people are the most likely culprits in spreading the pathogen anyway. They move around a lot too.
While there may be some uncertainty associated with predictions of what will happen regarding chikungunya and Aedes albopictus in the U.S., perhaps for the best indication of what may happen with chikungunya virus is to look at dengue. There have been significant increases in the activity of dengue in South America in recent decades. Notwithstanding the burden of disease on those countries, this increasing activity also increases the risk of infected travellers sparking local outbreaks in North America and has prompted warnings from local authorities. International travel has been identified as a critical issue in the global spread of dengue. However, while travellers are returning to North America infected with dengue, and there have been some locally acquired cases of dengue, there hasn’t been any major outbreaks recently (despite the history of major outbreaks in U.S. prior to the 1940s).
There is little doubt that there will continue to be some more locally acquired cases in Florida or other states where Aedes aegypti or Aedes albopictus are present. The risk will be determined by travellers bringing with them the virus from elsewhere and any reduction in the activity of chikungunya within the Americas will reduce the risk of introduction, and subsequent risk of local cases. A recent study confirmed the importance of Aedes albopictus as a vector with field collected specimens showing a strong preference for human hosts (as opposed to other mammals or birds).
The two mosquitoes primarily associated with chikungunya virus transmission are associated with water holding containers. These are typically found around the home. Ensuring that opportunities for these mosquitoes are minimised is critical. These mosquitoes can be found in any container that holds water, from pot plant saucers to rainwater tanks and from pet water bowls to water collecting on tarpaulins covering boats or trailers. If you see wrigglers about, tip the water out!
There is no vaccine but the use of mosquito repellents can be an effective prevention strategy. If used correctly, these will provide protection. However, keep in mind that the mosquitoes that transmit chikungunya virus primarily bite during the day (as opposed to the evening like other species). You’ll need to get that repellent on first thing in the morning. You can also read this recent post of mine on how best to combine sunscreens and mosquito repellents.
A good summary and analysis of chikungunya activity in the Americas is provided here (thanks to Ramon Martinez). You can also follow some of the coverage at Examiner by Charles Simmins. A perspective on the current (and possibly historic) activity of chikungunya virus in the Western hemisphere is provided in this New England Journal of Medicine piece.
**UPDATE 7 August 2014** There have now been 4 locally acquired cases of chikungunya virus in U.S. (all in Florida) but more importantly, mosquitoes collected locally in Texas have now tested positive to chikungunya virus. Local authorities are responding with extra control activities.
The photograph of Aedes albopictus at the top of this piece is by Stephen Doggett, Medical Entomology, Pathology West ICPMR Westmead.
Why not join the conversation on Twitter