Dengue fever
Dengue is transmitted by several species of mosquito within the genus Aedes, principally A. aegypti. The virus has four different types; infection with one type usually gives lifelong immunity to that type, but only short-term immunity to the others. Subsequent infection with a different type increases the risk of severe complications. As there is no vaccine, prevention is sought by reducing the habitat and the number of mosquitoes and limiting exposure to bites.
Treatment of acute dengue is supportive, using either oral or intravenous rehydration for mild or moderate disease, and intravenous fluids and blood transfusion for more severe cases. The incidence of dengue fever has increased dramatically since the 1960s, with around 50–100 million people infected yearly. Early descriptions of the condition date from 1779, and its viral cause and the transmission were elucidated in the early 20th century. Dengue has become a global problem since the Second World War and is endemic in more than 110 countries. Apart from eliminating the mosquitoes, work is ongoing on a vaccine, as well as medication targeted directly at the virus.
Signs and symptoms
Clinical course
The characteristic symptoms of dengue are sudden-onset fever, headache (typically located behind the eyes), muscle and joint pains, and a rash. The alternative name for dengue, "break-bone fever", comes from the associated muscle and joint pains.[1][8] The course of infection is divided into three phases: febrile, critical, and recovery.[9]The febrile phase involves high fever, often over 40 °C (104 °F), and is associated with generalized pain and a headache; this usually lasts two to seven days.[8][9] At this stage, a rash occurs in 50–80% of those with symptoms.[8][10] It occurs in the first or second day of symptoms as flushed skin, or later in the course of illness (days 4–7), as a measles-like rash.[10][11] Some petechiae (small red spots that do not disappear when the skin is pressed, which are caused by broken capillaries) can appear at this point,[9] as may some mild bleeding from the mucous membranes of the mouth and nose.[5][8] The fever itself is classically biphasic in nature, breaking and then returning for one or two days, although there is wide variation in how often this pattern actually happens.[11][12]
In some people, the disease proceeds to a critical phase, which follows the resolution of the high fever and typically lasts one to two days.[9] During this phase there may be significant fluid accumulation in the chest and abdominal cavity due to increased capillary permeability and leakage. This leads to depletion of fluid from the circulation and decreased blood supply to vital organs.[9] During this phase, organ dysfunction and severe bleeding, typically from the gastrointestinal tract, may occur.[5][9] Shock (dengue shock syndrome) and hemorrhage (dengue hemorrhagic fever) occur in less than 5% of all cases of dengue,[5] however those who have previously been infected with other serotypes of dengue virus ("secondary infection") are at an increased risk.[5][13]
The recovery phase occurs next, with resorption of the leaked fluid into the bloodstream.[9] This usually lasts two to three days.[5] The improvement is often striking, but there may be severe itching and a slow heart rate.[5][9] Another rash may occur with either a maculopapular or a vasculitic appearance, which is followed by peeling of the skin.[14] During this stage, a fluid overload state may occur; if it affects the brain, it may cause a reduced level of consciousness or seizures.[5] A feeling of fatigue may last for weeks afterwards.[14]
Associated problems
Dengue can occasionally affect several other body systems,[9] either in isolation or along with the classic dengue symptoms.[6] A decreased level of consciousness occurs in 0.5–6% of severe cases, which is attributable either to infection of the brain by the virus or indirectly as a result of impairment of vital organs, for example, the liver.[6][12]Other neurological disorders have been reported in the context of dengue, such as transverse myelitis and Guillain-Barré syndrome.[6] Infection of the heart and acute liver failure are among the rarer complications.[5][9]
Cause
Virology
Main article: Dengue virus
The dengue virus genome (genetic material) contains about 11,000 nucleotide bases, which code for the three different types of protein molecules (C, prM and E) that form the virus particle and seven other types of protein molecules (NS1, NS2a, NS2b, NS3, NS4a, NS4b, NS5) that are only found in infected host cells and are required for replication of the virus.[13][15] There are four strains of the virus, which are called serotypes, and these are referred to as DENV-1, DENV-2, DENV-3 and DENV-4.[2] All four serotypes can cause the full spectrum of disease.[13] Infection with one serotype is believed to produce lifelong immunity to that serotype but only short term protection against the others.[2][8]
The severe complications on secondary infection occurs particularly if someone previously exposed to serotype DENV-1 then contracts serotype DENV-2 or serotype DENV-3, or if someone previously exposed to type DENV-3 then acquires DENV-2.[15]
Transmission
Dengue can also be transmitted via infected blood products and through organ donation.[20][21] In countries such as Singapore, where dengue is endemic, the risk is estimated to be between 1.6 and 6 per 10,000 transfusions.[22] Vertical transmission (from mother to child) during pregnancy or at birth has been reported.[23] Other person-to-person modes of transmission have also been reported, but are very unusual.[8]
Predisposition
Severe disease is more common in babies and young children, and in contrast to many other infections it is more common in children that are relatively well nourished.[5] Women are more at risk than men.[15] Dengue can be life-threatening in people with chronic diseases such as diabetes and asthma.[15]Polymorphisms (normal variations) in particular genes have been linked with an increased risk of severe dengue complications. Examples include the genes coding for the proteins known as TNFα, mannan-binding lectin,[1] CTLA4, TGFβ,[13] DC-SIGN, and particular forms of human leukocyte antigen.[15] A common genetic abnormality in Africans, known as glucose-6-phosphate dehydrogenase deficiency, appears to increase the risk.[24] Polymorphisms in the genes for the vitamin D receptor and FcγR seem to offer protection against severe disease in secondary dengue infection.[15]
Mechanism
When a mosquito carrying dengue virus bites a person, the virus enters the skin together with the mosquito's saliva. It binds to and enters white blood cells, and reproduces inside the cells while they move throughout the body. The white blood cells respond by producing a number of signaling proteins, such as interferon, which are responsible for many of the symptoms, such as the fever, the flu-like symptoms and the severe pains. In severe infection, the virus production inside the body is greatly increased, and many more organs (such as the liver and the bone marrow) can be affected, and fluid from the bloodstream leaks through the wall of small blood vessels into body cavities. As a result, less blood circulates in the blood vessels, and the blood pressure becomes so low that it cannot supply sufficient blood to vital organs. Furthermore, dysfunction of the bone marrow leads to reduced numbers of platelets, which are necessary for effective blood clotting; this increases the risk of bleeding, the other major complication of dengue fever.[24]Viral replication
Once inside the skin, dengue virus binds to Langerhans cells (a population of dendritic cells in the skin that identifies pathogens).[24] The virus enters the cells through binding between viral proteins and membrane proteins on the Langerhans cell, specifically the C-type lectins called DC-SIGN, mannose receptor and CLEC5A.[13] DC-SIGN, a non-specific receptor for foreign material on dendritic cells, seems to be the main point of entry.[15] The dendritic cell moves to the nearest lymph node. Meanwhile, the virus genome is replicated in membrane-bound vesicles on the cell's endoplasmic reticulum, where the cell's protein synthesis apparatus produces new viral proteins, and the viral RNA is copied. Immature virus particles are transported to the Golgi apparatus, the part of the cell where some of the proteins receive necessary sugar chains (glycoproteins). The now mature new viruses bud on the surface of the infected cell and are released by exocytosis. They are then able to enter other white blood cells, such as monocytes and macrophages.[13]The initial reaction of infected cells is to produce interferon, a cytokine that raises a number of defenses against viral infection through the innate immune system by augmenting the production of a large group of proteins mediated by the JAK-STAT pathway. Some serotypes of dengue virus appear to have mechanisms to slow down this process. Interferon also activates the adaptive immune system, which leads to the generation of antibodies against the virus as well as T cells that directly attack any cell infected with the virus.[13] Various antibodies are generated; some bind closely to the viral proteins and target them for phagocytosis (ingestion by specialized cells and destruction), but some bind the virus less well and appear instead to deliver the virus into a part of the phagocytes where it is not destroyed but is able to replicate further.[13]
Severe disease
Further information: Antibody-dependent enhancement
It is not entirely clear why secondary infection with a different
strain of dengue virus places people at risk of dengue hemorrhagic fever
and dengue shock syndrome. The most widely accepted hypothesis is that
of antibody-dependent enhancement
(ADE). The exact mechanism behind ADE is unclear. It may be caused by
poor binding of non-neutralizing antibodies and delivery into the wrong
compartment of white blood cells that have ingested the virus for
destruction.[13][15] There is a suspicion that ADE is not the only mechanism underlying severe dengue-related complications,[1] and various lines of research have implied a role for T cells and soluble factors such as cytokines and the complement system.[24]Severe disease is marked by two problems: dysfunction of endothelium (the cells that line blood vessels) and disordered blood clotting.[6] Endothelial dysfunction leads to the leakage of fluid from the blood vessels into the chest and abdominal cavities, while coagulation disorder is responsible for the bleeding complications. Higher viral load in the blood and involvement of other organs (such as the bone marrow and the liver) are associated with more severe disease. Cells in the affected organs die, leading to the release of cytokines and activation of both coagulation and fibrinolysis (the opposing systems of blood clotting and clot degradation). These alterations together lead to both endothelial dysfunction and coagulation disorder.[24]
Diagnosis
|
Warning signs[25]
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| Abdominal pain | ||||
| Ongoing vomiting | ||||
| Liver enlargement | ||||
| Mucosal bleeding | ||||
| High hematocrit with low platelets | ||||
| Lethargy | ||||
The diagnosis should be considered in anyone who develops a fever within two week of being in the tropics or subtropics.[14] It can be difficult to distinguish dengue fever and chikungunya, a similar viral infection that shares many symptoms and occurs in similar parts of the world to dengue.[8] Often, investigations are performed to exclude other conditions that cause similar symptoms, such as malaria, leptospirosis, typhoid fever, and meningococcal disease.[5]
The earliest change detectable on laboratory investigations is a low white blood cell count, which may then be followed by low platelets and metabolic acidosis.[5] In severe disease, plasma leakage results in hemoconcentration (as indicated by a rising hematocrit) and hypoalbuminemia.[5] Pleural effusions or ascites can be detected by physical examination when large,[5] but the demonstration of fluid on ultrasound may assist in the early identification of dengue shock syndrome.[1][5] The use of ultrasound is limited by lack of availability in many settings.[1]
Classification
The World Health Organization's 2009 classification divides dengue fever into two groups: uncomplicated and severe.[1][25] This replaces the 1997 WHO classification, which needed to be simplified as it had been found to be too restrictive, though the older classification is still widely used.[25] The 1997 classification divided dengue into undifferentiated fever, dengue fever, and dengue hemorrhagic fever.[5][26] Dengue hemorrhagic fever was subdivided further into grades I–IV. Grade I is the presence only of easy bruising or a positive tourniquet test in someone with fever, grade II is the presence of spontaneous bleeding into the skin and elsewhere, grade III is the clinical evidence of shock, and grade IV is shock so severe that blood pressure and pulse cannot be detected.[26] Grades III and IV are referred to as "dengue shock syndrome".[25][26]Laboratory tests
Dengue fever may be diagnosed by microbiological laboratory testing.[25] This can be done by virus isolation in cell cultures, nucleic acid detection by PCR, viral antigen detection or specific antibodies (serology).[15][27] Virus isolation and nucleic acid detection are more accurate than antigen detection, but these tests are not widely available due to their greater cost.[27] All tests may be negative in the early stages of the disease.[5][15] PCR and viral antigen detection are more accurate in the first seven days.[14] A PCR test that can run on equipment also used to diagnose influenza was introduced in 2012 and will make PCR testing more accessible.[28]These laboratory tests are only of diagnostic value during the acute phase of the illness with the exception of serology. Tests for dengue virus-specific antibodies, types IgG and IgM, can be useful in confirming a diagnosis in the later stages of the infection. Both IgG and IgM are produced after 5–7 days. The highest levels (titres) of IgM are detected following a primary infection, but IgM is also produced in secondary and tertiary infections. The IgM becomes undetectable 30–90 days after a primary infection, but earlier following re-infections. IgG, by contrast, remains detectable for over 60 years and, in the absence of symptoms, is a useful indicator of past infection. After a primary infection the IgG reaches peak levels in the blood after 14–21 days. In subsequent re-infections, levels peak earlier and the titres are usually higher. Both IgG and IgM provide protective immunity to the infecting serotype of the virus. In the laboratory test the IgG and the IgM antibodies can cross-react with other flaviviruses, such as yellow fever virus, which can make the interpretation of the serology difficult.[8][15][29] The detection of IgG alone is not considered diagnostic unless blood samples are collected 14 days apart and a greater than fourfold increase in levels of specific IgG is detected. In a person with symptoms, the detection of IgM is considered diagnostic.[29]
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