Dengue - A Mosquito Borne Viral Disease
A Topic For Curious Patients
Dengue is part of a series on mosquito borne diseases.
No other animal infects and causes disease as effectively as mosquitos, which are the deadliest animals in the world to humans. Infections caused by mosquitos kill over 700,000 people a year.
Structure of a dengue virus Dengue viruses in tissue
Dengue viruses are Flaviviruses, transmitted through the bite of infected Aedes aegypti or Aedes albopictus mosquitos. Dengue is caused by one of four related viruses: Dengue virus 1, 2, 3, and 4. For this reason, a person can be infected as many as four times with the different dengue viruses.
The history of dengue is not well known, but a dengue-like outbreak in humans was recorded in a Chinese medical encyclopedia in 992. In the 1700s dengue was known as breakbone fever. Queen Luisa of Spain used the word dengue while writing about her recovery from it in 1801. No one is sure about dengue’s etymology, but the word dengue in Spanish means affectation, or careful, and may have described the stiff, painful movements of people with dengue fever. Another theory is that the name came from a Swahili phrase “Ka dinga pepo”, or “disease caused by an evil spirit”.
Dengue is common in more than 100 countries around the world. About three billion people live in endemic areas, and every year up to 400 million people get infected with dengue, approximately 100 million people get symptomatic illness, and 22,000 die from severe dengue.
Dengue is common in the U.S. territories of Puerto Rico, the U.S. Virgin Islands, and American Samoa. Nearly all dengue cases reported in the U.S. mainland are from travelers infected elsewhere. In 2019, there were 1,203 cases of dengue reported in the U.S., and 56 cases reported in U.S. territories. Dengue is a frequent cause of infection in Central and South America, East Africa, Southeast Asia and the Pacific Islands.
Global Dengue Risk Map
World Health Organization(WHO) Clinical Dengue Definitions 
Dengue is defined by a combination of ≥2 clinical findings in a febrile person who traveled to, or lives in a dengue-endemic area. Clinical findings include nausea, vomiting, rash, aches and pains, a positive tourniquet test (see below), low white blood cell count, or the following warning signs which may predict severe dengue: abdominal pain or tenderness, persistent vomiting, clinical fluid accumulation, bleeding from the lining of the mouth or gastrointestinal tract, lethargy, restlessness, and liver enlargement.
Severe dengue is defined as dengue with any of the following symptoms: severe plasma leakage leading to shock or fluid accumulation with respiratory distress, severe bleeding, or severe organ impairment such as liver inflammation, impaired consciousness, or heart impairment.
Dengue Clinical Disease and Treatment 
Most dengue infections are asymptomatic, with only 25% of dengue virus infections estimated to be symptomatic. It commonly presents as a mild to moderate, nonspecific, acute febrile illness. Approximately 5% of patients with dengue progress to severe dengue, a life-threatening disease. The early clinical findings of severe dengue are nonspecific, but recognizing early signs of shock, and promptly initiating intensive care unit therapy can reduce the risk of death from severe dengue to less than one person out of 200.
A study of antibody dependent enhancement of dengue in children found that the level of serum antibodies from a first infection determined the risk of severe dengue disease on reinfection. The presence of high dengue antibody titers in an individual are protective, but people with intermediate antibody titers had a much higher likelihood of developing severe dengue, even greater than those with low antibody titers. In that study the chances of getting severe dengue during a second infection was 1.6% in the high antibody titer group, 11.4% in the intermediate antibody titer group, and 6.6% in low antibody titer group. It is thought that particularly in people with intermediate dengue antibody titers, some binding of antibodies to the dengue virus occurs, but the antibody does not necessarily neutralize the virus. This creates a virus-antibody complex that appears to facilitate viral entry into host cells, and can trigger an immune cascade that leads to severe dengue. This is felt to be the reason why a dengue re-infection with a different strain, or first infection after dengue vaccination, may increase the risk of contracting severe dengue.
Clinical Course of Dengue
Dengue begins abruptly after a typical incubation period of five to seven days, and has three phases: febrile, critical, and convalescent or recovery.
The febrile phase lasts two to seven days. Other signs and symptoms may include severe headache, pain behind the eyes, muscle, joint, or bone pain, and/or a rash. Some patients have an injected oropharynx (inflamed, red throat) and facial redness in the first 24–48 hours after onset. Bleeding may occur from thrombocytopenia (low platelet count), or in more ill patients from a coagulopathy (bleeding disorder from the infection). Thrombocytopenia results from transient bone marrow suppression, and increased peripheral destruction of platelets in dengue. In this phase, patients may have minor hemorrhagic manifestations such as bruising, nose bleeds, bleeding gums, or blood in the urine. Petechiae (pinpoint spots of bleeding in the skin) may occur, and a positive tourniquet test may aid in the diagnosis of dengue. A positive tourniquet test result is ten or more petechiae per one square inch in the arm, after inflating a blood pressure cuff halfway between the systolic and diastolic pressures for two minutes.
Positive Tourniquet Test With Petechiae 
The critical phase of dengue begins when the fever ends, and typically lasts 24–48 hours. Most patients clinically improve during this phase, but there can be substantial plasma leakage (a development in which the fluid portion of blood leaks from blood vessels into the surrounding tissue) due to a marked increase in vascular permeability. Patients with severe plasma leakage may have pleural effusions (fluid around the lungs), ascites (fluid in the abdominal cavity), edema(excess fluid and swelling of the skin), and low serum protein. Liver enlargement may occur. Once hypotension(low blood pressure) develops, irreversible shock and death may ensue despite resuscitation. Patients can also develop severe hemorrhagic manifestations such as vomiting blood , bloody stools, or vaginal bleeding. Increased bleeding time may be seen in severe dengue. Uncommon complications include myocarditis (heart inflammation or infection), pancreatitis, and encephalitis (brain inflammation or infection).
Convalescent/Recovery Phase 
In severe dengue, as plasma leakage subsides, the patient enters the convalescent or recovery phase, and begins to reabsorb the fluid that has leaked from the blood vessels into the tissues, such as edema, pleural effusions and ascites. As a patient’s hemodynamic status stabilizes there is a significant diuresis (more than normal urination) of the excess extracellular fluid. The patient’s blood count stabilizes, or may fall because of the dilutional effect of the reabsorbed fluid, and the white blood cell count usually starts to rise, followed by a recovery of platelet count. In the convalescent-phase rashes may start flaking off and become itchy.
Dengue and Pregnancy 
There is limited data about dengue during pregnancy. Perinatal transmission can occur, and maternal infection may increase the likelihood of symptomatic infection in the newborn. Of 41 perinatal transmission cases to fetuses described in the literature, all developed thrombocytopenia, most had evidence of plasma leakage typically with ascites or pleural effusions, and 39 were febrile. Nearly 40% had a hemorrhagic manifestation, and 25% were hypotensive at some point. Perinatally infected neonates typically become ill during the first week of life. Placental transfer of maternal dengue IgG antibodies (Immunoglobulin G- long lasting antibodies produced late in the course of an infection that survive for some time after an infection) from a previous infection does occur, but when the protective effect of these antibodies wanes, infants 6–12 months of age are at risk for severe dengue.
Laboratory Findings and Testing 
Course of a Dengue Infection
Laboratory findings commonly include low white blood cell count, low platelets, low serum sodium, elevated liver enzymes, and in the majority of cases, a normal erythrocyte sedimentation rate (A measure of systemic inflammation that typically would be expected to be elevated in an infection such a dengue.).
During days one to seven after symptom onset, dengue virus RNA can be detected with molecular tests, such as an RT-PCR(reverse transcriptase polymerase chain reaction such as is used for COVID-19 testing). NS1 is a dengue virus protein that also can be detected by some commercial tests. An IgM antibody should also be drawn. (Immunoglobulin M is an early phase antibody produced before IgG antibodies to help fight an infection.) A negative result from a RT-PCR, or IgM antibody test is not conclusive. After seven days post symptom onset, patients with initially negative RT-PCR, and IgM antibody tests from the first seven days of illness should have another sample tested for IgM antibodies, if dengue is suspected. During the convalescent phase, IgM antibodies are usually present and can be reliably detected. IgM antibodies against dengue virus can remain detectable for 3 months or longer after infection.
If a RT-PCR or NS1 test is positive for dengue, a current dengue diagnosis is confirmed. If the RT-PCR result is negative and the IgM antibody test is positive, the laboratory diagnosis is presumptive dengue virus infection.
Cross reactivity with other flaviviruses such West Nile, yellow fever, and Zika, is a limitation of dengue IgM antibody tests. Therefore, a patient with past flavivirus infection(s) may be falsely test positive for dengue virus IgM antibodies. To determine if dengue is causing the infection, IgM positive specimens should be tested for specific neutralizing antibodies by a more specific plaque reduction neutralization test (PRNT).
Whenever a pregnant woman is tested for dengue, it is recommended Zika also be tested for using an RT-PCR test, as they may present similarly.
No specific antiviral agents or treatments exist for dengue. Supportive care is recommended and patients should try to stay well hydrated, and avoid aspirin and other NSAIDS, such as ibuprofen, because of their anticoagulant properties. Fever can be controlled with acetaminophen and possibly tepid sponge baths. Febrile patients should avoid mosquito bites to reduce risk of further transmission to other people.
In severe dengue, ICU care may be required. Prophylactic platelet transfusions in dengue patients are not beneficial, and may contribute to fluid overload. Administration of corticosteroids has not demonstrated any benefit, except in the case of autoimmune-related complication such as immune thrombocytopenia purpura (bleeding from low platelet counts)
A vaccine to prevent dengue, Dengvaxia, administered in three doses six months apart, is licensed and available in some countries for people aged 9-45 years old. The World Health Organization recommends that the vaccine only be given to people with confirmed prior dengue virus infection, as previously uninfected people who are vaccinated and then get dengue have a higher chance of developing severe dengue. In 2019, Dengvaxia was FDA approved for use in children 9-16 years old, with laboratory confirmed prior dengue virus infection, living in an endemic area such as the U.S. territories of American Samoa, Guam, Puerto Rico or the U.S. Virgin Islands.
Both of these mosquito-borne viral diseases can present initially as a non-specific illness, making diagnosis difficult. Laboratory testing may help, but initially can be negative, requiring convalescent testing to make a firm diagnosis. Dengue is the deadliest, and in endemic areas initially treating for dengue is recommended until an alternate diagnosis is confirmed. West Nile virus (see Part 2) is the only one of the four infections discussed in these two articles which is currently endemic in the continental U.S., the others are typically seen in travelers who are either from, or who have visited endemic areas. Fortunately, with its threat of fetal abnormalities, Zika virus spread has significantly declined, but around the world dengue and chikungunya infections remain significant health issues.
The infections discussed in these two articles are just a small portion of the infections that mosquitos can transmit, which include both viruses and parasites, and are listed below.
Eastern equine encephalitis
La Crosse encephalitis
Rift Valley fever
Ross River virus disease
St. Louis encephalitis
Dirofilariasis (dog heartworm)
Zika, Dengue, West Nile, and Chikungunya read about all four mosquito borne diseases as a member of www.FibonacciMD.app or download PDF for $1.
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