The Dengue Virus and Possibilities of Antiviral Treatment
By Nicholas Vitale
The dengue virus is a mosquito borne RNA positive virus of the Flavivirus genus that causes dengue fever (Figure 1). This illness affects about 100 million individuals a year across the globe, is extremely prevalent in many underdeveloped tropical areas, and there is no known antiviral treatment for the virus. Due to the lack of treatment, the affected areas are forced to try to control the populations of mosquitoes that transmit the virus, which can be challenging. Due to the vast amount of people that this potentially fatal virus affects there has been a large amount of research in recent years focusing on the mode of the viral action and the different ways to fight it. Some research groups have had some success in stopping the viral action of dengue. For example, the Talarico et al. group had success in fighting the dengue virus using the antiviral activity of sulfated polysaccharides, but the effectiveness of the treatment depends on the serotype of the virus (of which there are 5) . Clearly the viral activity of dengue is complex and not completely understood, but there are still many other possibilities for treatment of this illness.
History of Dengue Virus
The first isolation of the Dengue virus (DENV1) was found in 1943 in areas of the Southern Pacific region like the French Polynesian Islands and other land masses like Japan, but to date there have been four different isolates of the dengue virus found . After the discovery of DENV1 it was constantly rising in prevalence mainly in the Southern Asia region, which sparked the interest of researchers to learn more of the possibly fatal disease. With the increased interest in the virus three different types of isolates were found across the globe: in 1944 DENV2 was discovered in Indonesia, in 1953 DENV3 was discovered in Thailand, and DENV4 was also first discovered in Thailand . Much of the research into the newly prominent disease was sparked by the epidemics of dengue fever in the Philippines that occurred at various times throughout the 1950’s .
Despite the rather quick discovery of all different versions of the virus, it seemed that there was a relative drop off in the rise of reported cases of dengue fever, which lead to decreased interest in the research of the virus. Due to many different reasons such as increased air travel and lack of knowledge of the disease in the 1980’s there was a large increase in prevalence of the virus globally . Rather than being confined to the Southern Asia and Pacific ocean regions, the virus was seen to spread into Southern and Central America as well as more prominently in Asia affecting highly populated areas like India and China (Figure 2). This exponential growth seen of the Dengue viruses across the globe has not stopped and does not seem as if it is going to in the nearby future. The new reports of the disease are now being collected in a virus type specific manner, which is very important in order to understand and regulate the disease (more on this in “Epidemiology, Signs, and Symptoms of Dengue Fever”).
Although today the disease is seen to be very prevalent in many warm highly populated regions of the globe it still may also be very present in similar but underdeveloped areas of the globe like central Africa . Researchers believe that being able to more fully obtain up-to-date reports from all areas of the globe affected by the Dengue viruses will increase the ability of the scientific community to understand and regulate the prevalence of the virus . The tracking of the disease has proven to show that all four types of Dengue virus have no trouble integrating into new lands where the disease has not yet been. This finding is rather foreboding as in our modern world of constant transportation microbes from foreign lands are constantly being introduced into new areas and the Dengue viruses have shown that they can thrive under these circumstances. Further, the tracking of the spread of this mosquito borne disease will have important implications for ongoing research. With this knowledge it is very important to continue to track the spread of Dengue viruses in order to understand, detect, and react to the growing burden of the viruses.
Most recently, a new serotype of the Dengue virus has been found making it so that there are now five phylogenetically different strains of the virus . The discovery occurred in 2013 by researchers in Bangkok . This throws a very large hook into all of the ongoing research of the disease and possibilities of immunizations. This news is especially devastating considering that an immunization for the four known serotypes of the Dengue virus was about to undergo clinical trial late in 2012 . The discovery of this new serotype puts a large burden on the researchers of the virus. Since there is no anti-viral or drug remedy for the disease the discovery of a new serotype pushes back all the researchers that have been attempting to create immunizations and other remedies many years.
Epidemiology, Signs, and Symptoms of Dengue Fever
The four studied serotypes of the dengue virus are phylogenetically and antigenically different from one another, which adds even more difficulty in trying to understand this virus and its epidemiology . This phylogenetic difference between the four virus types makes it so that after a host is infected by one type and successfully fights it off, the host is now immune to that dengue virus type, but is still very susceptible to the three other virus types . This characteristic of the dengue viruses makes studying its epidemiology that much more challenging. Also, the sequential infection of different serotypes of Dengue can put people at higher risk of obtaining other diseases like dengue hemorrhagic fever (DHF) and dengue shock syndrome (DSS) . DHF is a more severe version of dengue fever that involves bleeding that occurs underneath the patient’s skin . DSS is very similar to dengue fever but is a syndrome that causes a collapse of the circulatory system.
The Dengue Virus is transmitted between people and mainly by Aedes aegypti and Aedes albopictus mosquitoes, which are present around the Earth . These vectors are by far the most common way someone will become infected by the virus. Mosquitoes become vectors for the disease by biting an infected host and allow the virus to incubate in their blood for 8-10 days, once this occurs the mosquito is infected for the rest of its lifespan  (Figure 3). Symptoms of the disease usually occur 4-13 days after infection and last for around 3-10 days  (Figure 3). The symptoms that occur in a host include very high fever (the main symptom), severe joint pain, migraines, rash, mild bleeding manifestation, and low white blood cell count . Some of the warning signs of Dengue fever are severe abdominal pain, tarry stool, persistent vomiting, drowsiness, difficulty breathing, and bleeding from nose or gums . There is no known treatment for the disease, but physicians recommend the use of acetaminophen and to drink plenty of fluids .
In the subtropical and tropical regions affected, the Dengue virus is an endemic occurring every year usually coinciding with peak mosquito population levels . These endemic areas are also at risk of full-blown epidemics as they are highly populated areas where transmission of the disease can occur rapidly. The Dengue virus is an emerging disease where about 40% of the entire world’s population lives in an at risk area. The World Health Organization (WHO) estimates that nearly 50 to 100 million people are infected per year and approximately 20,000 of the cases result in death . Dengue fever is a significant disease for a huge amount of the world’s population, yet there is still no approved drug or immunization. This is a large problem that many different research groups are currently working on to eliminate the damage that the Dengue virus can do .
Possibilities of Remedies against the Dengue virus
There is no known cure, anti-viral drug, or immunization for the dengue virus and all five of its serotypes . Earlier we classified the Dengue virus as an emerging pathogen and this has been one of the reasons for the extensive amount of research into creating a remedy that blocks the viral activity of all five serotypes of the Dengue virus . With the very recent discovery of the fifth serotype of Dengue virus some of the following research is outdated, but still valid for the DENV1, DENV2, DENV3, and DENV4. Much of the theoretical remedies look to stop the viral activity of Dengue that occurs in a host, but this is already after infection and most likely after the onset of some of the terrible signs and symptoms that come with the disease. Due to these facts, a lot of research has been focused on creating an immunization for all serotypes of the virus, which is particularly beneficial as all serotypes of the virus are prevalent across the world, and one type of drug may only end of working on one type of Dengue virus. Further, Dengue virus is an RNA positive virus, meaning that it uses host molecular machinery for replication . This knowledge of how the virus acts within a host is how researchers start to figure how to stop its pathogenic activity . Now these researchers are in the very early stages of creating a drug to combat the Dengue virus, but their work in finding weaknesses in the virus’ activity in the host is key information for vaccine and drug developers.
One action that has been deemed necessary for the Dengue virus’ success within a host is its ability to cause vascular leakage, which allows for the virus to induce shock and travel more freely within the body of the host  . The Beatty et al. group has done extensive research on trying to link a Dengue protein to the virus’ ability to incite vascular endothelium permeability (or vascular leakage), and they were very successful. The group found a Dengue associated protein called NS1, which induces vascular leak and has been found in four serotypes of the virus . They not only found a key protein involved in the virus’ pathogenic action, but they also created a serum that blocks the negative effects that NS1 induces. They found that when a mouse was given this NS1-immunity serum and also was infected with a lethal dose of DENV2 that mouse was able to live, but when mice were not given the serum they died from the DENV2 dose . These results confirm NS1 as an instigator of necessary steps of the virus’ success within a host and therefore a possible target for anti-dengue drugs . Beatty and colleagues’ findings are a huge step forward in creating a vaccine and possible drugs to eliminate Dengue’s pathogenic behavior in a host.
Some research has been shown to be successful in eliminating Dengue virus from a host, but it only occurs under certain circumstances. Talarico and colleagues have discovered that sulfonated polysaccharides derived from the red seaweeds Gymnogongrus griffithsiae and Cryptonemia crenulata have inhibitory effects on the Dengue virus’ replication within a host  (Figure 1). The success of these plant extracts was indeed in vitro and was only successful on specific host cell types, but this should not take away from the significance of the finding. The inhibitory effects were only seen in infected Vero cells, which are commonly used in experiments of this type. The way sulfonated polysaccharides acts against the Dengue virus in the early stages of the virus' action within the host cell, but mainly act as an inhibitor of virus absorption . The researchers found that the sulfonated polysaccharides' anti-viral action once the virus was already within the host cell is diminished. This would lead us to believe that these extracts would be good to take as a supplement in areas where the virus is extremely prevalent. Further, the anti-viral action of the extracts was tested on infected human cells and returned little success . Despite this profound finding, the inhibitory effects of the isolates only occur successfully with the DENV2 serotype. The other serotypes of dengue were either only slightly inhibited by the sulfonated polysaccharides or were completely unaffected by their action, however the antiviral action of sulfonated polysaccharides was not tested on DENV5 since it was discovered after this research was done . This result may be disappointing, but it has other significance as it shows that possibly a drug that cures all serotypes of dengue is not the most amicable thing to do. The outcomes of this study prove that the Dengue virus is much more complex than we have once thought, but it also confirms that a vaccine for all serotypes of the virus is very plausible as you can have an immunity for each serotype in one all-encompassing vaccine.
With the rising and rapid spreading of Dengue virus within the 21st century researchers stepped up and did their part to create a vaccine against the four known serotypes and this new vaccine began clinical trials in 2015 . The group found that a single dose of a tetravalent dengue vaccine (TV005) given to adults elicited immunity to all four dengue serotypes within three months. Not only was this vaccine shown to be extremely effective against the four main serotypes of dengue, but it was extremely safe as none of the patients reported any negative symptoms or reactions to the dose . Today these findings have great implications as recently a fifth serotype of dengue has been isolated, so this effective vaccine will not work against this novel dengue strain  . However, the implication of this vaccine to the increasingly affected areas of the world can come with some very harmful repercussions. For instance, if this vaccine is given to one of the subtropical communities where all serotypes of dengue are prevalent, the virus with no known cure (DENV5) will still infect members of the community. Also, there could be some evolutionary effects on the Dengue viruses were DENV5 is selected for, as it is the only serotype that can successfully infect and replicate the majority of hosts. Despite some of the shortcomings of this groups’ hard work, it is still a huge step in the right direction towards curing dengue fever and creating immunity in the masses.
It has been suggested by many researchers and sufferers of dengue fever that there are some anti-viral effects of certain medicinal plants on the Dengue viruses . The finding of low-cost, safe, and effective drugs that combat dengue fever is of high interest as there is no cure and the disease is extremely prevalent in underdeveloped regions of the world were the access to high quality medical care is not where it should be. The Frederico et al. group had success in compiling a vast amount of data were they found that methanolic extract of Vernonia cinerea leaves and ethanol extract of Tridax procumbers stems possessed high inhibitory activates against DENV . Vernonia cinerea is a species of flowering shrub that widespread in Asian and African ecosystems, and already is produced commercially for some of its other properties . Tridax procumbers is a flowering weed or pest plant that is widely spread amongst tropical and subtropical regions of the world. These two species of plants were found to have the most successful inhibitory effects on the dengue virus’ pathogenic activity. They were also successful in eliminating the replication of all dengue serotypes in hosts . The ubiquity of these plants is a great sign for the possible creation of an over-the-counter dengue remedy while a true clinical drug continues to be created. The creation of a anti-DENV agent from these specific plant extracts would be groundbreaking as it would be an extremely cost effective remedy for dengue victims and it would be very easy to obtain by those affected in regions of the world were medical care is hard to come by.
Clearly there is a vast amount of great research going on to effectively create a remedy to this to the harmful dengue virus, but there is much more that can still be done. Many potential remedies and viral inhibitors have been proposed, but still no clinically approved drugs or cures have been created. As more research occurs and we start to understand how each serotype works to infect its host the challenge to find a “cure-all” becomes even more puzzling. Until a true remedy for the virus is created organizations must continue to educate and warn people of the dangers, signs, and potential outbreaks of dengue fever in their regions. Vector control practices must continue to increase their range and effectiveness in order keep this rapidly spreading disease at bay. Those who do get infected by the Dengue virus must also do their part and comply with any research groups that ask them to aid their studies by giving data and possibly partaking in clinical trials of potential vaccines or anti-dengue drugs.
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Authored by Nicholas Vitale for BIOL 238 Microbiology, taught by Joan Slonczewski, 2018, Kenyon College.