Unwelcome mosquito-borne disease Chikungunya spreads in the Americas

On December 6, 2013, local authorities on the Caribbean Island of Saint Martin reported 2 confirmed cases of locally acquired chikungunya. By December 20, 2013, local authorities in St. Martin have announced that a total of 26 cases have been confirmed, with onset of symptoms between October 15 and December 19. In addition, they have 12 probable and up to 145 more suspected cases. The Dutch part of the island also reported 3 confirmed cases. While these 29 cases might seem small, they mark the arrival and spread of a novel, nasty mosquito-borne disease in the Americas, with potentially catastrophic consequences.

Chikungunya fever (spelled as “chik-en-gun-ye”, here you can hear the correct pronounciation) is a disease caused by the chikungunya virus, a member of the Alphavirus family, a relative of e.g. Ross River virus in Australia. It is transmitted by mosquitoes in the Aedes genus, predominantly the yellow fever mosquito (Aedes aegypti) and the Asian Tiger Mosquito (Aedes albopictus). Chikungunya fever is characterized by fever, headache, fatigue, nausea, vomiting, muscle pain, rash, and joint pain (http://www.cdc.gov/chikungunya/symptoms/). Fatigue and incapacitating joint pain can last for weeks or months. The pain is apparently so bad that the term “chikungunya” means “that which bends up” in the Kimakonde language of Mozambique. There is no licensed vaccine (although there is ongoing vaccine development), and there is no specific antiviral therapy available, only supportive treatment (i.e. painkillers). The only good thing about this pathogen is that the number of deaths attributed to infection is low.

The virus was first found in 1953 in Tanzania, and was considered one of many minor mosquito-borne diseases in Asia and Africa until recently. However, a single mutation at position 226 of the envelope glycoprotein of the virus from alanine to valine 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. Then in 2010, 2 cases of locally acquired chikungunya were confirmed in SE France.

Since these events, many researchers, including myself, worried about the expansion of chikungunya into the Americas, and its potential consequences. In the Americas, the conditions are sufficient for the local transmission of chikungunya, given the presence of efficient vectors. All areas that currently support dengue transmission in the Americas, including parts of the US, would definitely support chikungunya transmission. However, while Aedes albopictus is generally believed to be a less competent vector for dengue relative to Aedes aegypti, the same is not true for chikungunya, especially the novel strains adapted to this mosquito species. Therefore, it has been suggested that areas that lack Aedes aegypti but have endemic Aedes albopictus populations (notably the Eastern US) would be susceptible to chikungunya outbreaks. Extensive outbreaks over such large geographic scales, even with a very low mortality rate, could quickly overwhelm the existing health-care infrastructure, given that chikungunya outbreaks typically have a high attack rate (proportion of susceptible population infected), and could potentially lead to a Category 4 or 5 event on the Infectious Disease Impact Scale. The economic costs of such an outbreak in terms of vector control response, healthcare, and lost productivity are potentially enormous both in dollars and in Disability-adjusted life-years (DALYs). A simulation model by a group of researchers at Cornell has found that large outbreaks could occur in locations such as Miami, FL year-round. Accordingly, the CDC, WHO and PAHO have prepared plans and guidelines for the eventual introduction of chikungunya into the Americas, and set up a network of laboratories capable of testing samples for chikungunya infection. Now, those plans are mobilized, implemented, and put to the test.

Following the report of the initial 2 cases, the WHO Global Alert and Response system has issued an advisory on the outbreak, as well as the CDC. Major media outlets such as NPR , CBS, The Huffington Post, and the New York Times have featured reports of the outbreak, but many others are yet to cover it. One of the best media reports are by Robert Harriman (@bactiman63) on the Global Dispatch, featuring an interview with the legendary Robert Nasci , Chief of the Arboviral Diseases Branch in the Division of Vector-Borne diseases at the CDC. However, several major media outlets, notably CNN, are yet to report on the outbreak. It hasn’t been featured on the popular Nature and Science websites either.

Prevention campaigns have been launched on both the French and the Dutch part of the island, including emergency mosquit0 control and education, but given that they have been battling a dengue outbreak at least since December 2012, the outlook is not good for control. In fact, the original article says that the “authorities concede the spread of the virus will be inevitable”. Unfortunately, on December 19, 2013, local authorities on the island of Martinique have reported 2 confirmed, locally transmitted cases, indicating that the virus has already spread from Saint Martin. In addition, a single imported case from Martinique was detected in French Guyane. The translocation of the virus to the mainland of South America would ensure its spread over the continent. At this moment, the true spatial extent of the outbreak is unclear. One can argue, that in today’s interconnected world, having the disease in the Caribbean does not increase the risk of importation into other parts of the Americas significantly, as there are already substantial numbers of imported cases from endemic countries. However, Sint Maarten received more than 350,000 tourists from Jan-Sep 2013, 187,550 from the United States, constituting a large pool of susceptible hosts that could initiate local outbreaks in the US upon their return. Another question is whether Aedes albopictus in the US would even be capable of initiating local outbreaks, even if a substantial number of imported cases would occur. Despite hundreds of imported cases in the last decade, no local outbreaks occurred in the mainland US. While this could be only sheer luck, some argue that the fact that Aedes albopictus, unlike Aedes aegypti, bites animals as well as people, is sufficient to break the transmission chain before local outbreaks would occur. However, the recent case of locally acquired dengue on Long Island, transmitted probably Aedes albopictus, is a vivid counterexample to such claims. Whatever the case, the emergence and spread of chikungunya transmission in the Americas is a game-changer, and we have to do everything possible to contain it from spreading further. As one member of the Vector Control group (Richard Pollack) stated on LinkedIn, let’s “hope transmission will simply die out and that this incident will be an interesting footnote to history.” That would be an almost magical Christmas present at this point.

 

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