Dengue Fever (DF)
The symptoms of dengue fever are similar to acute fevers of viral origin. These are sudden onset of fever, headache, bodyache, joint pains, and retro – orbital pain. Other common symptoms are anorexia, altered taste sensation, constipation, colicky pain, abdominal tenderness, dragging pains in the inguinal region, sore throat and general depression. Patient may or may not have rash. Some of the patients may also show signs of bleeding from the gum, nose, etc.

Dengue Haemorrhagic Fever (DHF)
DHF is a severe form of dengue fever. Typically, it begins abruptly with high fever accompanied by headache, anorexia, vomitting and abdominal pain. During the first few days, the illness resembles classical Dengue Fever (DF), but a maculopapular rash is less common.

A haemorrhagic diathesis is commonly demonstrated by scattered fine petechiae on the extremities, face and trunk and in the axilae. A positive tourniquet test and a tendency to bruise at venipuncture site are always present. Bleeding from nose, gums and gastrointestinal tract are less common. Haematuria is extremely rare.

The liver is usually enlarged, soft and tender. Approximately 50% of patients have generalized lymphadenopathy.

The critical stage is reached after 2–7 days, when the fever subsides. Accompanying or shortly after a rapid drop in body temperature, varying degree of circulatory disturbances occur. The patient is usually restless and has cold extremities. Sometimes, there may be sweating.

In less severe cases, the changes in vital signs are minimum and transient. The patient recovers spontaneously or recovers after a brief period of therapy.

Dengue Haemorrhagic Fever is clinically confirmed by the positive tourniquet test (a blood pressure cuff is used to impede venous flow. A test is considered positive if there are > 20 petechiae/square inch).

Thrombocytopenia and haemoconcentration are constant findings in DHF. Haemoconcentration – indicating plasma leakage is always present.

In more severe cases, shock ensues and the patient may die within 12–24 hours. Prolonged shock is often complicated by metabolic acidosis and severe bleeding, which indicate a poor prognosis. If the patient is appropriately treated before the irreversible shock has developed, rapid recovery is the rule.

A major cause of deaths due to Dengue Haemorrhagic Fever is leakage of plasma in the pleural and abdominal cavities leading to hypovolaemic shock. Determination of haematocrit and platelet is essential for diagnosis and case management. The time course relationship between the fall in the platelet count and a rise in haematocrit level appears to be unique to DHF. These changes occur before the subsidence of fever and before the onset of shock and are correlated with the disease severity.

Encephalitic signs associated with intracranial haemorrhage, metabolic and electrolyte disturbances, and hepatic failure (a form of Reye’s syndrome) may occur. They are uncommon but carry a grave prognosis.

Dengue Shock syndrome (DSS)
All the above criteria, plus evidence of circulatory failure menifested by rapid and weak pulse, and narrow pulse pressure (< 20 mm Hg) or hypo–tension for age cold, clammy skin and altered mental status.