Classify the arboviruses. Describe dengue, chikungunya and Japanese encephalitis with vector control.

Definition and Classification

Dengue

• A flavivirus with four serotypes (DENV-1 to 4), transmitted by AEDES aegypti, a day-biting mosquito that breeds in clean stored water around houses — coolers, tanks, tyres, pots and construction sites
• Infection gives lifelong immunity to that serotype but only brief cross-protection against the others
• Antibody-dependent enhancement is the central concept — on secondary infection with a different serotype, pre-existing non-neutralising antibody binds the new virus and carries it into macrophages through Fc receptors, increasing viral load and provoking a cytokine storm. This is why severe dengue is largely a disease of second infections
• Three phases — febrile (2 to 7 days: high fever, severe headache, retro-orbital pain, myalgia and arthralgia giving "breakbone fever", rash); critical (around defervescence, lasting 24 to 48 hours, when plasma leakage occurs); and recovery, with reabsorption of fluid
• The danger is at defervescence, not at the height of the fever — the patient appears to be improving as the temperature falls, and it is precisely then that shock develops
• Warning signs — severe abdominal pain, persistent vomiting, mucosal bleeding, lethargy or restlessness, tender hepatomegaly, clinical fluid accumulation, and a rising haematocrit with a falling platelet count
• Severe dengue — shock from plasma leakage, severe bleeding, or organ impairment
• Diagnosis — NS1 antigen from day 1 to 5; IgM from day 5 onward; RT-PCR early; and serial full blood counts and haematocrit, which guide management more than any single test
• Treatment — there is NO specific antiviral; management is careful fluid replacement guided by haematocrit and urine output. paracetamol only — aspirin and NSAIDs are contraindicated because of bleeding risk. Platelet transfusion is for significant bleeding, not for a low count alone

Chikungunya and Japanese Encephalitis

Vector Control

• AEDES breeds in clean stored water close TO houses — coolers, overhead tanks, flower pots, discarded tyres, coconut shells and construction sites. Control is therefore a household and community matter: emptying, covering, and removing containers weekly
• CULEX breeds in dirty stagnant water — drains, cesspits and rice fields — so the interventions are entirely different
• Aedes bites by day, so bed nets protect poorly; repellents, full clothing, screens and removal of breeding sites are what work. Culex bites at night, so insecticide-treated nets do protect against Japanese encephalitis
• Larval measures — source reduction, larvivorous fish (Gambusia), temephos; adult measures — space spraying during outbreaks, which controls an epidemic but does not prevent one
• Source reduction is the durable measure; fogging is visible and reassuring but has only a transient effect on adult mosquitoes

Comparing the Common Acute Febrile Illnesses of the Indian Monsoon

• These illnesses overlap and co-infection occurs, so a systematic approach with rapid tests for several is more useful than treating a single presumed diagnosis; and the treatable ones — malaria, scrub typhus, leptospirosis, enteric fever — must not be missed while attention is on dengue

Applied Aspects

• Watch the patient at defervescence, not at the peak of fever, and teach the warning signs explicitly to patients and families before discharge
• Never give aspirin or NSAIDs in a suspected dengue illness; in India, where undifferentiated fever is common and self-medication is widespread, this warning must be given actively
• Do not transfuse platelets for a number alone; the indication is significant bleeding, and unnecessary transfusion exposes the patient to risk without benefit
• Japanese encephalitis is preventable by vaccination, and its inclusion in the immunisation programme in endemic districts of Uttar Pradesh, Bihar, Assam and West Bengal has substantially reduced disease
• Consider co-infection — dengue, chikungunya, malaria, scrub typhus and leptospirosis circulate together in the Indian monsoon, and more than one may be present
• Kyasanur Forest disease is a tick-borne haemorrhagic flavivirus infection of the Western Ghats, and Crimean-Congo haemorrhagic fever is transmissible from patient to healthcare worker — both require specific awareness and, for the latter, strict barrier precautions
• A dengue vaccine exists but is complicated by antibody-dependent enhancement; vaccinating a seronegative person may prime them for severe disease on first natural infection, so pre-vaccination screening is required
• Fogging reassures more than it achieves; source reduction of larval breeding sites is what actually reduces Aedes density, and it requires household participation
• Zika shares the Aedes vector with dengue and chikungunya and causes congenital microcephaly, so it must be considered in a pregnant woman with a febrile rash in an affected area
• Chikungunya arthritis may persist for months and is frequently dismissed; it responds to analgesia and physiotherapy, and patients should be warned at the outset that recovery is slow
• Japanese encephalitis leaves neurological sequelae in a large proportion of survivors, so vaccination in endemic districts prevents lifelong disability rather than merely death
• Man is a dead-end host for Japanese encephalitis, so isolating a patient is unnecessary; control is directed at pigs, mosquitoes and vaccination, not at the case
• Take a travel history within the incubation period; arboviral infections are geographically defined, and knowing where the patient has been narrows the differential immediately
• Aedes aegypti is an urban, domestic, day-biting mosquito, so dengue is a disease of towns and of construction sites rather than of forests — which is why it has risen with urbanisation