Dengue fever, Dengue Hemorrhagic Fever, Dengue Shock Syndrome, Severe Dengue
During the 19th century, dengue was considered a sporadic disease, causing epidemics at long intervals. However, dramatic changes in this pattern have occurred and currently, dengue ranks as the most important mosquito borne viral disease in the world.
Epidemiological trends in South East Asia;the first epidemic of DHF in South East Asia occurred in 1954 in Manila, Philippines. Following this, epidemics have occurred in nearly all countries in this region, and currently are a major public health problem in worldwide.
1. Causative agents:
Dengue viruses (DENV) are flavi-viruses and include 4 types (serotypes; DENV-1,-2,-3,-4). All 4 DENV serotypes can cause dengue and have been associated with severe dengue, including DHF/DSS with a fatal outcome. It is important to note, that the majority about 75% of DENV infections are asymptomatic in both children and adults. Long-term, serotype-specific protective immunity is produced by infection with each serotype, but there is no long-term, cross-protective immunity following infection.
An increase risk of severe dengue is associated with the presence of heterotypic antibody and with viral strains that have greater virulence and/or epidemic potential. DENV serotypes can be further defined into genotypes based on differences in the envelope gene sequence; molecular epidemiologic studies may assist in tracking disease transmission patterns.
2. Occurrence:
Dengue virus transmission has now become endemic in most countries located in the tropics and subtropics.
In dengue endemic areas, transmission occurs year-round with peak disease incidence usually occurring during the rainy season and in areas of high Aedes aegypti prevalence. Epidemic cycles at 2-5 years interval, with disease incidence exceeding the expected annual increase in incidence. In most endemic areas more than one DENV serotype will circulate over time, and usually 2 or more serotypes circulate simultaneously.
In some island nations, DENV is reintroduced periodically and produces large epidemics. All tropical and subtropical regions of Asia, the Americas, the Caribbean, Oceania, and Africa should be considered at risk for dengue. Areas that border dengue endemic regions may experience DENV introductions and epidemics. Epidemics may occur wherever vectors are present and DENV is introduced, whether in urban or rural areas.
Good risk maps for dengue are available, as well as mapping programs with real-time reporting of reported dengue activity.
3. Reservoir:
Where endemic, DENV is maintained in a human/ Ae. Aegypti mosquito cycle, in which human infections are either with or without symptoms. There is a sylvatic monkey/ mosquito cycle, which may spill over into human populations of southeastern Asia and western Africa.
4. Incubation period : from 3-14 days, commonly 4-7 days
5. Transmission :
Bite of infective mosquitoes, principally Ae. Aegypti. This is a day-biting species, with increased biting activity for 2 hours after sunrise and several hours before dusk. Dengue outbreaks have been attributed to Ae. Aegypti and to a lesser extent, Aedes albopictus. The latter is a periurban species abundant in Asia that has spread to the USA, the Caribbean, Central and South America, the Pacific, parts of southern Europe and Africa. Other Aedes spp. mosquitoes associated with DENV transmission include Aedes polynesiensis and several species of the Aedes scutellaris complex.
Each of these species has a particular ecology, behavior, and geographical distribution. Patients are infective for mosquitoes during their period of viremia, from shortly before, until the end of the febrile period. The mosquito becomes infective 8-12 days after the viremic blood-meal and remains so for life.
Because of the approximately 7-day viremia in infected persons, blood borne transmission is possible through exposure to infected blood, organs, or other tissues. In addition, perinatal DENV transmission occurs with the highest risk among infants born to mothers acutely ill around the time of delivery.
6. Risk groups
People of all ages living in dengue endemic areas should be considered at risk of oninfection. In most areas, disease incidence is highest in children, although increasing numbers of adult cases are being reported from both rural and urban areas. Perinatal DENV transmission can occur with being febrile. In addition, infants infected with DENV at 6-12 months of age, and born to mothers previously infected with DENV, are at increased risk for severe dengue. Because of warning levels of transplacentally transferred maternal IgG anti DENV and immune enhanced DENV infection.
7. Clinical features
Mild to moderately severe acute febrile illness that usually follows three phases: febrile, critical and convalescent. Patients with dengue often have sudden onset of fever, which lasts for 2-7 days and may be biphasic.
Other signs and symptoms include intense headache, myalgia, arthralgia, bone pain, retro-orbital pain, anorexia, vomiting, macular or maculopapular rash, and minor hemorrhagic manifestations, including petechiae, ecchymosis, purpura, epistaxis, bleeding gums, hematuria, or a positive tourniquet test. Some patients have injected oropharynx and facial erythema in the first 24-48 hours after onset.
Warning signs of progression to severe dengue occur in the late febrile phase, around the time of defervescence, and include persistent vomiting, severe abdominal pain, mucosal bleeding, difficulty breathing, signs of hypovolemic shock, and rapid decline in platelet count with an increase in hemotocrit (hemoconcentration).
Dengue hemorrhagic fever (DHF) and dengue shock syndrome (DSS) are now classified as subsets of severe dengue according to the 2009 WHO Guidelines. Timely identification of dengue patients with warning signs is critical to ensure proper anticipatory guidance and initiation of supportive treatment. Severe dengue can occur in both children and adults.
8. Diagnosis :
Laboratory confirmation for the clinical diagnosis of dengue can be made using a single serum specimen obtained during the febrile phase of the illness (days 0-7 after onset of fever) with diagnostic testing to detect DENV and immunoglobulin class M (IgM) anti-DENV. DENV viremia occurs for 5-6 days before and after fever onset.
Testing for immunoglobulin class G [IgG] anti-DENV is not useful for dengue diagnosis since a high proportion of persons in endemic areas have preexisting, cross-reactive IgG antibody from previous DENV infections (secondary DENV infection).
9. Prevention :
(1) Presently no vaccine, chemoprophylaxis, antiviral agent is available to prevent or treat dengue; prevention of bites from the vector mosquito is the only means of prevention.
- Vector control includes public education and community programs to eliminate mosquito vector larval habitats, which for Ae.aegypti include water-holding containers close to or inside human habitation (e.g., old tries, flowerpots, trash, food, or water storage containers).
- Community surveys to determine the density of vector mosquitoes and identify productive larval habitats should complement plans for the elimination, management, or treatment of mosquito production sites with appropriate larvicides.
- Communities should be educated about personal protection against day-biting mosquitoes by using repellents, bed-nets, screening, and protective clothing.
- Prevention measures are required year-round; once increased dengue activity is identified it is usually too late for reactive vector control activities to be effective.
(2) Because vector control and personal protection have generally been ineffective in preventing seasonal and epidemic increases in dengue or mitigating an epidemic once it has begun, timely identification of cases and good clinical management of dengue patients to prevent mortality and morbidity is essential.
(3) This should include ongoing education of the public about dengue and warning signs, ongoing education of health care professionals in best clinical practices, evaluation of health care practices related to clinical outcomes, and planning by health care facilities to meet demands placed upon them by the annual seasonal increase in cases or by periodic epidemics.
Credit ; Heymann L. David.(ed). (2015), Control of Communicable Diseases, Manual,20thed, Washington. DC, American Public Health Association.pp. 144-148
Khay Mar Mya
MBBS (Ygn), M.Med.Sc (Public Health), PhD (Public Health), Dip.Med.Edu,
Rector (Retd), University of Public Health, Yangon
