Erin Capina, 127 YinD
History and Epidemiology
Dengue is an old mosquito borne disease that originated in monkeys before crossing over to humans, a phenomenon known as cross species transmission (other examples include Bird flu, MERS, and HIV-AIDS). Dengue’s exact age and region of origin is up for debate. There are different schools of thought about dengue’s timeline and region of origin, going from as little as 100 years ago to as much as 800 years ago and the four dengue viruses evolved in either Africa or Southeast Asia (1). Dengue entered the world beyond its original region in either the 18th or 19th century with the expansion of shipping and trading (2).
The mosquito vector for dengue is the Aedes mosquito, primarily the Aedes aegypti mosquito, which lives in tropical and sub-tropical regions of the world, primarily between the latitudes 35°N and 35°S where the winter temperatures do not go below 10°C, and normally do not live at altitudes above 1000m (3). Dengue virus is actually four distinct but similar serotypes (serotypes are groups within a single species of microorganisms that have distinct surface features in common): DENV-1, DENV-2, DENV-3, and DENV-4 (1).When multiple dengue serotypes co-circulate in a community the community is said to hyper-endemic (4). All four dengue serotypes can now be found in parts of North America, South America, Africa, Asia, and Australia (5). Dengue viruses exist in two environments: urban or endemic, where humans and mosquitoes are the only known hosts, settings and forested settings, where mosquito borne viruses are transmitted between nonhuman primates and sometimes make the jump from nonhuman primates to humans (6).
The consequences of World War II in Asia, the war effort and troop movements, helped expand the geographical distribution and density of Aedes aegypti mosquitoes thereby helping accelerate the spread of dengue throughout the Southeast Asian region (4). The first recorded epidemic of dengue hemorrhagic fever occurred in Manila, Philippines in 1953-1954 followed closely by Bangkok, Thailand, Malaysia, Singapore, and Vietnam. By the 1970s epidemic dengue fever and dengue hemorrhagic fever had spread throughout the region thanks to the rapid urbanization that Southeast Asia experienced post-World War II (4). However, during the same post-World War II period, dengue remained localized to the Southeast Asia region because of the geographic isolation of the Pacific islands and the eradication program to control urban yellow fever in the Americas, Aedes aegypti is also a vector for yellow fever, zika, and chikungunya4. In the 1970s, however, the American yellow fever eradication program was dismantled allowing Aedes aegypti and dengue to expand their geographical reach in the Americas (4). The increased ease of modern transportation not only allowed people and commodities to move around the world relatively easily but also allowed for the mosquitoes and dengue to move on around just as easily (4). These factors and the complacency that followed the yellow fever eradication allowed dengue to regain its foothold in the world (4).
The World Health Organization and a recent study by Bhatt et al. estimate the global burden of dengue to be 390 million dengue infections per year (95% confidence interval 284-528 million) of which 96 million infections are apparent infections, manifest clinical disease,. Much of the global burden of dengue lies in Asia, 70% of all apparent infections, with India alone accounting for 34% of the global total (8). A surprising aspect from Bhatt et al.’s study is the hidden burden of dengue in Africa, 16% of the global total, is nearly equal to the total in the Americas 14%. This shows that Africa’s burden is often hidden by dengue being mistaken for other similar diseases, under reporting, and highly variable treatment seeking behaviors (8). Climate change also has the potential to stretch the habitat of Aedes aegypti past its current geographical limits and put more areas of the world and more people at risk for dengue.
Patients who show a clinical manifestation of dengue are classified as having dengue or severe dengue but the majority of dengue infections are asymptomatic, meaning patients do not show symptoms despite having the disease (6). Severe dengue happens when the following conditions occur: “Plasma leakage resulting in shock, accumulation of serosal fluid sufficient to cause respiratory distress, or both; severe bleeding; and severe organ impairment” (6). The illness is divided into three phases: a febrile phase, a critical phase around defervescence—when a fever breaks, and a spontaneous recovery phase (6).
The febrile phase—the initial phase—is characterized by: high fever, vomiting, headache, muscle pain, possibly a transient rash, and joint pain. The potentially severe muscle and joint pain give dengue its infamous nickname, breakbone fever. Children can have higher fevers than adults but are usually less symptomatic. Mild bleeding manifestations (bleeding from the gums or nose, petechiae—small red or purple spots caused by bleeding into the skin—, or easy bruising) and a palpable liver are also common during this phase. Laboratory results will show an elevated immune response. This phase lasts for three to seven days, after which most people recover complication free (6).
In the critical phase a systemic vascular leak—“leaky” blood vessels throughout the whole body—occurs around the time that the fever breaks, during this time a patient may seem deceptively well until abnormally low blood pressure sets in and then irreversible shock and death might follow even with aggressive treatment. During the transition from febrile to critical, between days four and seven of illness, it is important that clinicians be aware of the warning signs that a patient is developing clinically significant vascular leakage. The warning signs include: persistent vomiting, increased severe abdominal pain, a tender and enlarged liver, a high or increasing hematocrit level (the volume percentage of red blood cells in the bloodstream) that is concurrent with a rapid decrease in the platelet count, fluid in body cavities, mucosal bleeding (bleeding from the mucosal linings, membranes that line various body cavities), and lethargy or restlessness. During the critical phase hemorrhagic manifestations (major skin bleeding, mucosal bleeding) are common. In children clinically significant bleeding occurs rarely but in adults clinically significant bleeding can occur without obvious precipitating factors and only minor plasma leakage (6).
The recovery phase is marked by significant improvement. The changed vascular permeability from the critical phase is short-lived, within 48-72 hours it reverts back to a normal level spontaneously, during this time there is also a rapid improvement in other symptoms. During this period another rash may appear, this rash may be a mild rash or a severe itchy lesion. Adults may have profound fatigue for weeks after recovery (6).
Currently there are no antivirals available to treat a dengue infection; the only things available are supportive care measures such as fluid management (6).
Original Antigenic Sin and Antibody Dependent Enhancement
A unique feature to dengue is known as antibody dependent enhancement (ADE). After a first infection with any of the four dengue serotypes the immune system develops an immune response to that specific serotype, this also gives a person lifelong immunity to that serotype as well as limited cross-protective immunity to the remaining three serotypes for a short period of time, but a second infection with a different dengue serotype will cause a more severe disease the second time around. The reason for this phenomenon is ADE. In ADE, the antibodies from the primary dengue infection will actually help spread the new dengue serotype. If this was an infection of the same dengue serotype then the antibodies would bind with the dengue particles and neutralize the virus but in this case the infection is of a novel serotype so the existing dengue antibodies are unable to neutralize the virus when they bind with the dengue particles. This antibody-virus complex then attaches to Fcy receptors on macrophages, phagocytic—cells that ingest harmful foreign particles, bacteria or dead or dying cells—white blood cells, helping the virus infect the macrophages and effectively turning them into Trojan horses. The end result is that the dengue virus can replicate undetected and eventually generate high enough virus titers to cause severe disease (10, 11). ADE has important effects on dengue vaccine development because the potential for a more severe response with subsequent infections means that creating four different serotype specific vaccines is practically out of the question.
The concept of original antigenic sin is that the immune system will prioritize an immunological memory response based on a previous infection when encountering a new, slightly different version of the same entity (12). In dengue, a primary infection with a specific serotype creates memory B and T cells, long term immune memory (B cells produce antibodies and “tag” invaders to be engulfed by phagocytes while T cells are involved in actively fighting an infection), and these cells are capable of responding rapidly compared to naïve cells during a second infection. However, because the second infection is caused by a different dengue serotype- the responding memory B and T cells may not have optimal affinity to this new infection because the immune response is skewed by the memory of the previous infection (12). Understanding and accounting for original antigenic sin is important in the development and application of vaccines. In dengue understanding and accounting for original antigenic sin has important implications for vaccine development because once an immune response against one dengue serotype has been established it is unlikely that repeated boosting will change that response. Therefore, any vaccine will have to create balanced responses to all four serotypes in one dose (13).
Prevention remains the best defense against dengue; this is done via vector control. Controlling Ades mosquitoes, particularly the Aedes aegypti, is a complex problem given Aedes aegypti’s close relationship with humans. They feed mainly on humans and can live inside human homes effectively allowing them to feed 24 hours a day. Chemical treatment for water stored in containers for domestic use is one way to control mosquito larvae. However, increasing levels of resistance and household rejections of adding the chemicals to their water along with the difficulty in achieving regular and high levels of coverage make this method difficult (5). The use of biological control agents such as mesocyclops, a copepod, in community programs has had success but is also susceptible to failing without community support and vigilance once funding stops and re-infestation can occur (14). Reducing mosquito breeding grounds in and around the home requires vigilance, commitment and sustained behavior change.
Many communities opt for insecticide spraying as a way to control adult mosquitoes and prevent epidemics. This method is highly visible and lets community members see that their officials are doing something; unfortunately this method is also ineffective and quite expensive (4). In reality, spraying in response to a dengue epidemic is akin to shutting the barn door after the horse has bolted if no other measures are taken. A new approach to vector control involves genetic modification. One approach is to release genetically modified males that will mate with wild females and sterilize them in the process thereby reducing the next generation of mosquitoes (6). Another approach involves introducing strains of the obligate intracellular bacteria wolbachia into Aedes aegypti embryos; wolbachia infected Aedes aegypti mosquitoes are partially resistant to dengue virus infection and these infected mosquitoes can invade wild populations, which suggests the possibility of widespread biological resistance in Aedes aegypti populations (6).
There are currently two vaccine candidates in phase III clinical trials. Dengvaxia, CYD-TDV, is manufactured by Sanofi Pasteur and is the vaccine farthest along in development. It is a live attenuated tetravalent vaccine—containing a weakened form of the virus and effective against all four serotypes. CYD-TDV is a chimeric (composed of more than one organism, in this case, cloning parts of one virus into another virus) three dose vaccine, yellow fever is the replicative backbone but it carries the structural proteins of dengue,(15,16). The results from clinical trials indicate that CYD-TDV showed strong protection against hospitalization and severe dengue but recent data from a long term safety assessment demonstrate that three years after vaccination the risk of hospitalization for CYD-TDV recipients compared to placebo recipients in participants less than nine years old and that the risk was highest for subjects ages two to five years old (16,17). The clinical trials also indicate a lower efficacy for dengue naïve individuals compared to those with prior dengue exposure (16). Because of the safety concern regarding children younger than nine, the vaccine has been licensed for individuals aged 9-45 living in highly endemic areas and is currently licensed in three countries: Mexico, the Philippines, and Brazil (16, 17).
TV003 is another dengue vaccine in production. TV003 is also a live attenuated tetravalent vaccine. However, unlike CYD-TDV it is not a chimeric vaccine, rather each serotype has a small sequence deleted from its genetic code (15, 17). TV003 was developed by scientists at the United States’ National Institute of Health (15). TV003 used a human challenge model to evaluate its efficacy before further evaluation in a dengue endemic area. The human challenge model included 41 participants, six months after receiving or not receiving the intervention all participants were challenged with the selected dengue challenge virus. The results demonstrated that completed TV003 protected the 21 participants who received the vaccine when challenged with the dengue challenge virus. In contrast, the dengue challenge virus strain was recovered from the blood of all 20 placebo participants (17). In January of this year TV003 entered a large scale phase III trial in Brazil (18).
The journey towards creating effective and safe dengue vaccine has been one marked by decades of research, new insights, new roadblocks, more questions, and funding issues. However, CYD-TDV and TV-003 represent new hope that an effective and safe dengue vaccine—possibly more than one—is within our reach. If global climate change does in fact expand Aedes aegypti’s range to new, previously untouched places then a dengue vaccine would certainly be welcomed with open arms.
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(15) Live attenuated vaccines. Dengue Vaccine Initiative website. http://www.denguevaccines.org/live-attenuated-vaccines. Accessed August 19, 2016.
(16) Questions and answers on dengue vaccines. World Health Organization website. http://www.who.int/immunization/research/development/dengue_q_and_a/en/. Accessed August 19, 2016.
(17) Kirkpatrick BD, Whitehead SS, Pierce KK, et al. The live attenuated dengue vaccine TV003 elicits complete protection against dengue in a human challenge model. Science Translational Medicine. 2016; 8(330): 1-8
(18) Dengue vaccine enters phase 3 trial in Brazil. National Institutes of Health website. https://www.nih.gov/news-events/news-releases/dengue-vaccine-enters-phase-3-trial-brazil . January 14, 2016. Accessed August 19, 2016.