Via the blog BugBitten, Krisztian Magori has a highly informative post: Happy Birthday, Chikungunya (in the Americas)! Excerpt:
The virus was first found in 1953 in Tanzania, and until recently was considered one of many minor mosquito-borne diseases in Asia and Africa. However, a single mutation from alanine to valine at position 226 of the envelope glycoprotein of the virus led to increased adaptation in terms of infectivity to Aedes albopictus mosquitoes. This adaptation led to a large outbreak in the French Island of Reunion in 2005-2006, which then spread further through the Indian Ocean to the India and SE Asia in general. In 2007, a returning traveller from India initiated an outbreak in Ravenna, Italy, which resulted in 205 local cases. In 2014, 11 cases of locally acquired chikungunya were confirmed in SE France.
During its 1st year, the chikungunya outbreak in the Americas has become a really big boy! As of December 12, 2014, there have been 1,031,757 suspected and confirmed locally transmitted cases documented across 43 countries. The only countries without documented local transmission at this point in time are Argentina, Bolivia, Canada, Chile, Cuba, Ecuador and Peru. As vectors are present in some of these countries the lack of documented cases might be due to a lack of surveillance or transparency rather than lack of transmission.
These huge numbers still reflect only a portion of the true number of cases. For instance, in Puerto Rico only 20% of laboratory confirmed cases had been diagnosed with chikungunya and only 8% of cases were reported to the Puerto Rico Department of Health. This may be because a portion of cases are mild or asymptomatic, as have been 40% of all cases on St. Martin. This is significantly higher than during previous outbreaks, such as on Reunion.
In addition, in many of the countries involved patients might self-medicate outside of the public health system during a chikungunya outbreak and therefore not be detected. I have heard from @DokteCoffee that this occurred on Haiti.
Project Tycho at the University of Pittsburgh has created the best animation I have seen so far showing the spread of chikungunya across the Americas, but this only went to the end of August 2014.
While chikungunya is generally regarded to have a low mortality rate, 115 deaths have been attributed to the disease so far, mostly in very young or very old patients with other co-morbidities. One consequence of large localized chikungunya outbreaks is the collapse of local health systems due to the large number of patients seeking help, such as happened in Colombia. Another issue is the potential economic effects, both due to lost productivity, and the reduction in tourism in affected countries.
So what can we expect in the 2nd year of chikungunya in the Americas? We can be sure that chikungunya won’t just disappear, and it hasn’t yet finished spreading and infecting people in new locations. While many countries such as the Dominican Republic had hundreds of thousands of cases, local transmission is only starting in other countries such as in Paraguay. On St. Martin, seroprevalence studies found that 12-20% of the population has been infected, which is still lower than the seroprevalence at the end of the outbreak in Reunion, so transmission might still continue for many months.
If 20% of the population in all of 43 countries with local transmission becomes infected, that would mean many millions more cases by the end of next year. Eleven locally transmitted cases were reported in Florida, as well as 1900 imported cases. In addition to Florida, Aedes aegypti is also present in southern Texas, in several counties of California, and even in isolated locations in Georgia, providing conditions for local transmission in these areas as well.