Chikungunya Is Coming: Part I
I have some troubling news to share with you. If not this year, then next, the United States is almost certain to experience an epidemic from a disease of which you may not have heard, chikungunya virus. Before I explain why I feel so confident in this prediction, let me give you some background on chikungunya.
Chikungunya was first identified in Tanzania in the early 1950s; however, historical records as well as a review of virus’ genetic markers suggest that it first evolved around the year 1700. The virus circulates among humans, monkeys, rodents, bats and some birds. Chikungunya is transferred from one animal to another via the bite of two species of mosquitoes, aedes agypti and aedes albopictus. The incubation period in humans is typically 2-3 days and the symptoms include fever of up to 104 ºF, rashes, swelling of the legs and, in particular, severe joint pain. While the fatality rate for chikungunya is low, at 0.4%, this is not a disease you want to get.
Like many other viruses, you can only get chikungunya once, since surviving the disease gives you immunity to future infections. When the virus is introduced to an area where no one has immunity and the mosquitoes required to spread it are present, an epidemic can spread with frightening speed. When an epidemic strikes, the rate of infection grows rapidly until approximately 30-50% of the population develops immunity. And while the news media in the U.S. has been focused on whether Sarah Palin will join The View, chikungunya has been on the move in the Caribbean.
The first ever recorded case of locally-transmitted chikungunya in North or Central America occurred on the island of St. Martin in December of 2013. Since then, an epidemic has spread like wild fire, particularly in the Dominican Republic. The Graph below shows the number of infections reported during the first ninety days of the epidemic in the Dominican Republic, with day zero being April 1 of 2014.
In the first 90 days of the epidemic, this nation of 10 million people had 135,000 cases! If the behavior of this epidemic follows the pattern of the outbreak on Reunion Island in the India Ocean from 2005-2006, the Dominican Republic can expect approximately two to four million cases of chikungunya to occur within a year. To put it lightly, an epidemic of this proportion will be very challenging and disruptive to the citizens and economy of the Dominican Republic. Overall, there have been over a quarter of a million cases in the Caribbean, including Puerto Rico and the U.S. Virgin Islands, in the first six months of 2014.
Now let’s consider the probability of a chikungunya outbreak in the continental United States. For starters, essentially zero percent of the population of the United States is immune to chikungunya. Next consider the maps below showing the current ranges of aedes agypti and aedes albopictus in the U. S.
As you can clearly see, both species of mosquito are resident in large portions of the country, including North Carolina. For the purposes of the calculations below I estimated that approximately 25% of the U.S. population lives within the current range of one or both of these mosquitoes. All that is needed for an epidemic to get started is for one or more infected people to arrive here on boats or planes and the mosquitoes to bite them at the right time. The data on this front is not encouraging. According to the Centers for Disease Control (CDC), so far this year there have been 138 confirmed cases of people arriving in the continental US, primarily returning vacationers, with a chikungunya infection acquired while abroad. This number is unprecedented. So far we have been extremely lucky that there have been no reported cases of local transmission.
While my position is that current conditions make a near-term chikungunya epidemic in the U.S. almost certain, the statement made by the CDC on their website is more measured. Their position is, “There is a risk that the virus will be imported to new areas by infected travelers.” Therefore, either I am being alarmist or they are too concerned about creating a panic to be more forthright about the risk.
If I am correct and the Southeastern United States is about to have an epidemic similar to the one in the Dominican Republic, we’d have one million cases in the first ninety days! That would make for a few headlines.
So is there anything we can do to prevent this? Should we have been more prepared that we are? For my thoughts on that, you need to come back next week. In the meantime, I’d think about stocking up on insect repellent.
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