Since I published Part I and Part II of this series in July, the ongoing chikungunya epidemic in the Americas has mostly fallen out of the headlines. I suspect that the 400,000+ people who have been infected with Chikungunya since then would still find the epidemic to be newsworthy. So Common Science® is here to fill in that information void.

If you don’t have the time to read Parts I and II, here is the nickel summary: Chikungunya is a virus that is spread from person to mosquito to person. While it causes severe joint pain and high fever, it usually resolves in 7-10 days and is rarely fatal. From the 1950s, when it was first identified, until the 1990s, it was confined to Africa. In the 1990s, it began to spread to Asia, with noteworthy outbreaks in India and Reunion Island. Given that the species of mosquitoes that can transmit Chikungunya are present in the Western Hemisphere, it was only a matter of time until the virus arrived here as well. Therefore, to those who have been paying attention, it was no surprise when the first locally-acquired case of Chikungunya in this hemisphere was reported on the island of St. Martin in December of 2013. Thus began an epidemic which has infected nearly three quarters of a million people and is still going strong.

Before I proceed to an update on the Chikungunya epidemic in the Americas, I need to address some inaccurate information that has been circulating in the media. Some of which, I am sad to say, I included in my earlier columns.

Nearly all of the reports I have seen on Chikungunya have included the statement that it can be spread by two species of mosquitoes, aedes aegypti (yellow fever mosquito) and aedes aldopictus (Asian tiger mosquito). While this statement is true for some strains of Chikungunya, it is not true of the current strain spreading in the Americas. This strain can only be spread by aedes aegypti, which has a much smaller geographic range in the United States compared to aedes aldopictus. My earlier columns included the incorrect implication that both species created a risk in the U.S and in North Carolina. To make matters worse, the map I used previously for the geographic range for aedes aegypti in the United States was based on data collected in 1978, and thus out of date. Here is the old map:

 

aedes_aegypti03

I have since located this map based on 2013 data, which shows a far more limited range, shown in blue, for aedes aegypti in the U.S.

aedes aegypti us

I do not know the reasons for the reduction in range. Please note that since North Carolina is not within the current range of aedes agypti, we are not at risk of an epidemic here.

In the earlier columns, I estimated that at least 80 million people in the U.S. lived within areas which were at risk of a Chikungunya epidemic. Based on this more updated information, I now estimate the number to be 20 million.

Now that I have finished falling on my sword, let me give you an update on what has been happening since I published Chikungunya Part I. The data table below shows the cumulative number of cases in 2014. (1)

Dominican Republic Americas(Canada to Argentina) United States (imported) United States (locally acquired)
July 13 136,000 260,000 138 0
September 5 430,000 660,000 700 8

The Dominican Republic has accounted for approximately two thirds of the 660,000 cases that have occurred in the Americas. The United States has now had 700 imported cases, primarily from tourists returning from the Caribbean, and eight locally-transmitted cases, all in southern Florida.

In order to fully appreciate the momentum of this epidemic, I find it helpful to consider this graph of the cumulative number of cases of Chikungunya in the Americas during 2014:

Americas sep 2014

Sort of breathtaking, isn’t it? The shape of the graph is quite typical for epidemic diseases, beginning with a period of slow growth, called the induction phase, followed by rapid exponential growth. Eventually the number of new infections will level off and the epidemic will subside.

So what does the future hold for the Western Hemisphere’s first ever Chikungunya epidemic? To discuss this, we need to dip our toes into the mathematics of epidemics. The key parameters in determining the dynamics of an epidemic are the average number of additional people infected by each victim, called the R value, and the lag time between when a person becomes infected and when they begin to infect others. As long as R remains greater than 1, the epidemic will continue to spread. In a population with no immunity, which until December of 2013 included effectively everyone in the Western Hemisphere, the initial R value for chikungunya is about 4. The onset of symptoms occurs approximately 7 days from infection and the victim is infectious to others, with a mosquito providing the mechanism of transfer, during the period of 2-6 days after symptoms present. Therefore, in a naïve population, each victim infects an average of 4 people in the period 9 to 13 days after he or she contracts the disease. As you can see from the graph above, these parameters for Chikungunya have resulted in the dramatic spread of the virus.

In order for an epidemic to end, the R value must drop to below one. There are several ways in which the R value for Chikungunya can decline with time. For example, all of the impacted countries have been attempting to lower the transmission rate by advising people on how to avoid mosquito bites and by spraying pesticides to kill mosquitoes. As I will explain below, at least for this epidemic, it appears that these public health interventions are serving primarily as “we-are-doing-something” theater rather than having any real impact. By a significant margin, the most important factor in reducing the R valve over time is the growth in the population of people who have become immune to Chikungunya by having caught and survived the disease. An infected mosquito only sustains a sufficient amount of virus to infect a new human for a few days. Therefore, as more and more people become immune, the mosquito runs out of time to encounter a susceptible human.

We don’t yet know what percentage of a regional population will need to become immune before this particular strain of Chikungunya burns itself out, but we can draw some inferences from the data. The graph below shows the percentage of the populations of both Guadeloupe and the Dominican Republic who have been infected by Chikungunya as a function of time after the disease reached those countries:

new Guad DR graph

Guadeloupe was impacted a couple of months before the Dominican Republic, and you can see that after 35 weeks of the epidemic a little over 16% of the population has contracted Chikungunya. Despite having extra time to educate the public and to initiate mosquito control measures, we can see from the graph the behavior of the epidemic in the Dominican Republic has been nearly identical to what has occurred in Guadeloupe. This similarity between these two trends suggests that the public health interventions initiated by the Dominican Republic were not effective, and that the spread of the disease has been governed primarily by the innate characteristics of the virus.

Since every strain of Chikungunya has different characteristics, we don’t yet know what percentage of the population will need to suffer through the disease until R drops below 1. In past epidemics, it is common for 25 to 45 percent of a local populace to catch Chikungunya in the first wave of the epidemic. Looking at the data from Guadeloupe, 25 to 35 percent does not seem unreasonable. If this is correct, and also applies to the Dominican Republic, then they would experience 2.1 million more cases before reaching 25% of their population.

Based on the new information I have included in this column, let me update my estimate of the potential impact of Chikungunya in the U.S. Given that there are 20 million people living in the geographic range of aedes aegypti, the worst case scenario is likely in the range of 5 million cases. However, our air-conditioned, closed-doors-and-windows lifestyle is likely to keep the number lower than this. I’ll be keeping an eye on Florida to see how things develop.

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(1) Given the pace of this epidemic is it hard for me to stay current.  As I am editing this on September 18th, the number of cases in the Americas is now over 700,000, cases imported to the U.S. are over 1,000, Florida has reported its 9th locally acquired case, and both Brazil and Guatemala have reported their first locally acquired cases.