Public Health Perspectives is pleased to welcome Dr Jessica Taaffe to the blog to discuss dengue fever. For more about Dr Taaffe, see the end of this post.
Dengue is no joke – it causes a “bone-breaking” illness and fever that basically wipes you out. Several members of my family in Paraguay can attest to that (this country is currently experiencing the “worst epidemic in history,” with over 4,500 cases from July to November 2013, and the season isn’t over yet). The disease starts with one dengue virus-infected mosquito bite. Four to ten days later, sickness may start with a mild headache, but this quickly progresses into high fever (up to 106F!) and an incredibly painful headache with muscle, bone, and joint pain. The associated limb pain is so agonizing that the disease became known as ‘breakbone fever,’ even though no bones are actually broken. Most people recover within a week or so, but it can leave them weakened for weeks afterward. And, if you’ve been infected once, this doesn’t mean that you are immunologically protected from infection again. In fact, if the second infection originates from a different serotype than the first infection, you’re at risk for severe dengue. That’s right, severe dengue. As if the symptoms from uncomplicated dengue weren’t severe enough, a second infection could result in hemorrhagic fever or shock. Severe dengue requires hospitalization and can be life-threatening, among whom a great many affected are children.
Dengue is mostly found in tropical and sub-tropical climates, and the mosquitos carrying it are able to survive in urban habitats in those climates. Although most of dengue cases exists in Southeast Asia and Latin America, incidence has been increasing over the past few decades, including spreading to new areas, which now alarmingly includes autochthonous (locally acquired) dengue outbreaks in Florida, Texas, and Hawaii since 2001. Even more so, local transmission of dengue was reported in France and Croatia in 2010. And, climate change isn’t helping – rising temperatures may allow dengue virus transmitting mosquitos to persist longer and geographically expand their domain. What’s even scarier is that Aedes aegypti, one mosquito species capable of transmitting dengue, has recently been found in California. This means that local transmission of dengue could also become established on the West Coast.
The WHO reports that over 2.5 billion people (that’s over 40% of the world’s population) are at risk for dengue (meaning, they live in an areas with dengue transmission), and an estimated 50-100 million infections occur worldwide each year. This, though, is likely an underestimate, as much of dengue illness is underreported, especially since up to one half of infections are asymptomatic (ref: CDC). Indeed, a study published in Nature earlier this year, estimated the total global burden to be 390 million, with only 96 million being apparent through clinical illness. Dr. Donald Shepard of Brandeis University further expanded on this topic at the past American Society of Tropical Medicine and Hygiene (ASTMH) Annual Meeting (I attended this wonderful science and global health conference in November, and much of what I’ll mention here was discussed at this meeting). His analyses using expansion factors (a value multiplied to reported cases to give a more accurate estimate of infection incidence) in Southeast Asia suggested that, on average, only 13.2% of clinical dengue episodes are reported to surveillance systems. There is also great variance among regions in the expansion factor value (7 for South East Asia, 25 for the Americas, and 130 for South Asia), demonstrating how factors contributing to underreporting (misdiagnosis, poor surveillance systems, availability of rapid diagnostic tests, unhospitalized cases missed, etc) differ across the world.
If you like to travel internationally, you should know that travelers are not only at risk for the disease, but also help spread it. Dengue is the second most common cause of fever in people returning from Australia, and it is found in travelers from South East Asia almost three times more than malaria, said Dr. Annelies Wilder-Smith of Singapore’s Nanyang Technological University at the ASTMH conference, referring to GeoSentinel surveillance data. Smith also discussed how international travel has been responsible for the spread of dengue globally, through both introduction of mosquito vectors and the virus itself. One of dengue’s mosquito vectors, Aedes albopictus, was first introduced into the Americas in 1985 through ship travel (tires containing stagnant water served as mosquito breeding ground), and the 2012 dengue outbreak in the European island of Madeira could be traced back to airline travel from Venezuela. The link to travel was so strong in the latter case that it was airline travel data that first identified the outbreak origin. This was later confirmed by molecular epidemiological data that showed sequence similarity between the Madeira dengue viruses to those circulating in Latin America. Travel has also promoted the introduction of new dengue serotypes into regions of the world in which they previously did not exist. Remember my earlier point of the increased risk of severe dengue through cross-serotype infection? Certainly, more than one dengue serotype in any one region is just bad news.
We have no vaccine, we cannot predict who will get severe dengue, nor do we know how to ameliorate the clinical severity of the disease. If you get dengue, there is no treatment to stop the disease. Basically, you have to let it take its (painful) course and hope it doesn’t progress into severe dengue. Oxford University’s Dr. Bridget Willis explained at the ASTMH conference that efforts to suppress the immune response (responsible for clinical symptoms) with corticosteroids or inhibit the viral life cycle with anti-virals have not panned out. And while many dengue vaccine candidates are in the pipeline, we are just not there yet. The most developed dengue vaccine candidate from Sanofi Pasteur disappointingly protects only against three out of four of the predominant serotypes, leaving out the most pathogenic serotype, DENV 2.
But, I don’t want to be completely depressing about dengue – let’s put a positive and hopeful spin on this. Dengue is becoming “An Increasing Global Health Problem” (the aptly named title of a two part symposium series at the ASTMH meeting this year) and people are starting to take notice. The dengue sessions at ASTMH were very well attended, if not packed, and a number of media sources have recently taken interest. More attention given to the growing problem of dengue can hopefully promote our efforts against it.
Biomedically, we need to better understand the disease, in order to develop appropriate vaccines and therapeutics against it, including clinical management of disease; increased investment in dengue research and development is imperative here. From a public health perspective, more accurate estimates of the burden of disease, both clinically and economically, may convince policy makers to ramp up national and international efforts to control spread of disease. At the local level, measures to improve vector control, through pesticide use or community outreach campaigns to reduce stagnant water sources, may help address local dengue epidemics. Additionally, biomedically-driven solutions, such as the introduction of genetically-modified mosquitos, whose offspring cannot survive, or Wolbachia-infected mosquitos that resist dengue infection, into natural mosquito populations could aid public and environmental health programs in the future.
My purpose with this post is to build awareness of dengue, including where we stand globally in our efforts against it. I don’t expect this post to mobilize communities, governments, and international organizations to address the global dengue epidemic; that’s already being carried out by other excellent groups, initiatives, and organizations (Eliminate Dengue and Dengue Vaccine Initiative are two examples). However, I sincerely hope that whatever your background (scientist, clinician, global/public health profession, policy maker, member of general public, etc), you give dengue a second (or first) thought. This is a global disease that is not going away anytime soon, and awareness is the first step in addressing it.
Dr. Jessica Taaffe is a biomedical scientist committed to advancing global health through scientific practice, communication, and advocacy. An expert in non-human primate immunology, she currently researches severe malaria anemia in monkeys at the National Institutes of Health and focused her PhD on HIV/SIV immunopathogenesis at the University of Pennsylvania. Dr. Taaffe is also a scientific writer/consultant for The World Bank. An avid fan of social media, she can be found on Twitter (@JessicaTaaffe), Tumblr (http://www.tumblr.com/blog/signoradavinci), and LinkedIn (http://www.linkedin.com/in/jtaaffe), sharing her wide variety of passions, from science and global health to music and fashion. Views here and on social media are her own and do not reflect policies or opinions of her employers.