Dengue Fever

INTRODUCTION

Dengue fever virus is considered the most important arbovirus in terms of morbidity, mortality and economic cost with an estimated 100 million cases of dengue fever occurring throughout the world annually. Dengue is transmitted by mosquito and occurs in epidemic and endemic proportions throughout tropical and subtropical regions of the world. Infection with dengue virus causes a wide number of clinical symptoms which range in severity. These include fever, a maculopapular rash and headache. Primary infection with dengue usually results in a febrile, self limiting disease, however, secondary infection may result in severe complications such as dengue shock syndrome (DSS) or dengue haemorrhagic fever (DHF). Patients diagnosed with dengue in endemic areas such as South East Asia generally have secondary infection, whereas patients in non endemic areas are usually diagnosed with primary infection. Characteristic antibody responses to the disease enable serological diagnosis and differentiation between primary and secondary dengue.

MORPHOLOGY

•  RNA viruses
•  belong to family Flaviviridae
•  four serotypes (1, 2, 3 and 4)
•  different strains within each serotype

PATHOGENESIS

•  transmitted by mosquito, principally Aedes aegypti
•  incubation time ranges from 3 to 10 days

CLINICAL ASPECTS

Primary Infection

• acute febrile illness of sudden onset
• fever lasting 3 to 5 days
• headache, myalgia, arthralgia or muscular pain, retro-orbital pain, anorexia
• fine mculopapular rash on extremities
• recovery may be associated with fatigue and depression
• chidren usually have milder disease than adults

Secondary Infection

• over 90% of cases of DHF and DSS occur in patients previously infected with the virus
• symptoms are similar to those seen in primary infection, although after a period of 3 to 7 days the patient goes on to display haemorrhagic symptoms
• bleeding, particularly in skin (petichiae), occaisionally in gunms and nose
• increased vascular permeability, resulting in leakage of plasma into extravascular spaces and which leads to hypovolaemia
• haemorrhagic symptoms
• reduced blood pressure
• vascular changes and coagulopathy
• circulatory shock
• vomiting and abdominal pain
• lymphadenopathy and hepatomegaly may occur
• presence of blood in stools, vomitus, urine

ANTIBODY RESPONSE

Infection will result in lifelong immunity to that serotype, but only temporary immunity to other serotypes

Primary Infection

• IgM antibodies appear approximately 5 days after onset of    symptoms and rise for the next 1-3 weeks
• IgM antibodies detectable for up to 6 months
• IgG are detectable at approximately 14 days after onset of     symptoms and are maintained for life

Secondary Infection

• pproximately 5% patients do not produce detectable levels of specific IgM
• IgM titre can be slower to rise in secondary infection
• IgG appears approximately 2 days after symptoms appear
• IgG titre significantly higher in secondary infection

DIAGNOSIS

• may not be diagnosed correctly in endemic areas due to generalised and non specific clinical manifestations
• based mainly on serological methods, as this method is useful in     distinguishing primary from secondary infection

Haemagglutination Inhibition Assays (HAI)

• traditional method of diagnosis
• sera must be acetone or kaolin treated before testing
• requires paired sera collected at least 7 days apart
• variance in potency of haemagglutinins made in different     laboratories has lead to doubts regarding general applicability

ELISA

• pre-treatment of sera is not required
• serial dilution not required - diagnosis can be made from a single serum specimen
• diagnosis can be from a single serum sample

TREATMENT

• No Specific treatment for primary dengue

Secondary Infection

•  intravenous fluid replacement and use of plasma expanders
•  oxygen therapy
•  blood transfusions in cases of severe bleeding
•  heparin for severe haemorrhage

REVENTION

• Presently no vaccine for prevention of disease
• interruption of breeding cycles of mosquitoes, particularly in stagnant water around the home
• use of insect repellent and insecticidal treatment and spraying