Dengue & Chikungunya

Two Aedes-borne arboviral fevers that dominate the Indian monsoon and post-monsoon season. Both are transmitted by the same day-biting, container-breeding Aedes aegypti, share an overlapping clinical picture, yet differ sharply in their feared complications — plasma leakage and shock in dengue, versus chronic crippling arthritis in chikungunya. This is high-yield Community Medicine territory: vector indices, WHO severity grading, NS1 window, and the dengue-versus-chikungunya distinction are perennial favourites.

Definition & classification

Dengue is an acute febrile illness caused by the dengue virus (DENV), a single-stranded RNA Flavivirus (family Flaviviridae) with four antigenically distinct serotypes — DENV-1, 2, 3 and 4. Infection with one serotype confers lifelong homologous immunity but only transient (months) heterologous cross-protection, after which a second infection with a different serotype increases the risk of severe disease.

Chikungunya ("that which bends up" in the Makonde language, describing the stooped posture from arthralgia) is caused by the chikungunya virus (CHIKV), an RNA Alphavirus (family Togaviridae). There is only one serotype, and infection gives long-lasting (probably lifelong) immunity, so a person rarely gets chikungunya twice.

WHO dengue case classification (2009, revised)

The older DF / DHF / DSS scheme is still examined, but the operational 2009 classification is now standard.

Category	Defining features
Dengue without warning signs	Fever + ≥2 of: nausea/vomiting, rash, aches/pains, leucopenia, positive tourniquet test
Dengue with warning signs	Above + abdominal pain/tenderness, persistent vomiting, clinical fluid accumulation (ascites/pleural effusion), mucosal bleeding, lethargy/restlessness, liver enlargement >2 cm, rising haematocrit with falling platelets
Severe dengue	Severe plasma leakage (shock/respiratory distress), severe bleeding, or severe organ involvement (AST/ALT ≥1000, impaired consciousness, myocarditis)

High-yield: The single most important pathophysiological event in severe dengue is plasma leakage, detected as a rising haematocrit with simultaneously falling platelet count. Bleeding is not the defining feature — leakage is.

Old DHF grading (still asked)

Grade	Features
I	Fever + non-specific symptoms; only positive tourniquet test (Hess test)
II	Grade I + spontaneous bleeding (skin/mucosa)
III	Circulatory failure — weak rapid pulse, narrow pulse pressure ≤20 mmHg, hypotension, cold clammy skin (DSS)
IV	Profound shock — undetectable BP and pulse (DSS)

High-yield: Grades III and IV constitute Dengue Shock Syndrome (DSS). Narrow pulse pressure (≤20 mmHg) is the cardinal sign of impending shock.

The vector — Aedes aegypti

Both viruses are transmitted chiefly by Aedes aegypti (and secondarily Aedes albopictus, the "Asian tiger mosquito"). Vector biology is the most heavily tested portion of this topic in Community Medicine.

• Day biter — peak biting in early morning and late afternoon (so bed-nets at night are relatively ineffective; this is the basis for daytime protection advice).
• Container breeder — breeds in clean, stored water in artificial containers: discarded tyres, coolers, flower pots, tanks, tins, coconut shells, tree holes.
• Domestic and peridomestic, highly anthropophilic (prefers human blood).
• Flight range is short — about 100 metres, hence cases cluster around the breeding focus.
• Eggs survive desiccation for months on container walls — explaining sudden post-monsoon surges.
• Black-and-white striped legs; rests with body parallel to surface.

High-yield: Aedes is a clean-water, container breeder and a day biter with a flight range of only ~100 m. Contrast with Anopheles (malaria, clean water, night biter) and Culex (filaria/JE, dirty/polluted water, night biter).

Extrinsic & intrinsic incubation

• Extrinsic incubation period (in the mosquito): ~8–10 days after a blood meal, after which the mosquito is infective for life.
• Intrinsic incubation period (in humans): dengue 4–10 days (typically 4–7); chikungunya 2–4 days (range 1–12).
• Viraemia in humans appears ~1 day before fever and lasts ~5 days — the infective period during which a feeding Aedes picks up the virus.

Etiology & pathophysiology

Dengue — secondary infection theory of DHF/DSS

The classical explanation for why severe dengue clusters in secondary infections is Antibody-Dependent Enhancement (ADE):

1. First infection with one serotype generates antibodies. →
2. On secondary infection with a different serotype, the pre-existing, non-neutralising (sub-neutralising) antibodies bind the new virus. →
3. These antibody–virus complexes are taken up more efficiently into Fc-receptor-bearing monocytes/macrophages, enhancing viral replication. →
4. Massive immune activation releases cytokines (TNF-α, IL-6, etc.), causing capillary endothelial dysfunction → plasma leakage, haemoconcentration, thrombocytopenia and shock.

High-yield: The secondary infection theory (Halstead) via antibody-dependent enhancement explains why DHF/DSS is commoner on a second heterologous infection. This is also why the CYD-TDV (Dengvaxia) vaccine is recommended only for sero-positive individuals — in sero-negatives it can prime ADE and worsen a future natural infection.

Other contributors: complement activation, transient bone-marrow suppression (thrombocytopenia, leucopenia), and increased vascular permeability that is reversible within 24–48 hours — fluid management must respect this short critical window.

Chikungunya

CHIKV directly infects fibroblasts, muscle and joint tissue, provoking an intense innate immune and cytokine response that targets synovium and periarticular structures — explaining the disproportionate, sometimes chronic, arthralgia. It does not typically cause plasma leakage or shock.

Clinical features

Dengue — the triphasic illness

Classic "break-bone fever" runs in three phases:

1. Febrile phase (days 1–3): abrupt high fever, severe retro-orbital pain, frontal headache, myalgia, arthralgia, flushing, and a transient macular rash. →
2. Critical phase (days 4–6, around defervescence): this is when plasma leakage occurs. Paradoxically, danger rises as fever falls. Warning signs appear here. →
3. Recovery phase (days 7–10): reabsorption of leaked fluid; a characteristic "isles of white in a sea of red" confluent petechial rash with islands of sparing; bradycardia; pruritus of palms and soles.

High-yield: Beware the patient who is defervescing on day 4–6 — that is precisely when shock develops. Falling fever in dengue is not reassuring.

Chikungunya

• Sudden high fever + severe, symmetrical, polyarticular arthralgia/arthritis involving small joints (hands, wrists, ankles) — the dominant and distinguishing feature.
• Maculopapular rash, often more pruritic and prominent than dengue.
• Chronic arthralgia persisting weeks to months (occasionally years) in up to a third of patients is the hallmark sequel.
• Mortality is low; deaths mainly in neonates (vertical transmission) and the elderly with comorbidities.

Diagnosis & investigation of choice

Test selection is timing-dependent, and this is repeatedly examined.

Test	Window of positivity	Comment
NS1 antigen	Day 1–5 of fever (peaks early)	Investigation of choice in the first 5 days; can be positive even before antibodies appear
RT-PCR / viral isolation	Day 1–5	Most specific; allows serotyping; used in surveillance/research
IgM (MAC-ELISA)	From ~day 5, peaks ~2 weeks	Investigation of choice after day 5; indicates recent/current infection
IgG	Late, rises in secondary infection	A 4-fold rise in paired sera confirms; high early IgG suggests secondary infection

High-yield: NS1 antigen is the test of choice in the first 5 days; switch to IgM ELISA after day 5. There is a diagnostic "window" around day 5–6 where NS1 is waning and IgM not yet reliably up — combined NS1+IgM kits cover this gap.

Supportive labs in dengue: thrombocytopenia (platelets <100,000 in DHF), leucopenia, rising haematocrit (≥20% rise = significant leak), raised transaminases, hypoproteinaemia, and pleural effusion/ascites on USG (a sensitive early sign of leakage).

Tourniquet (Hess capillary fragility) test: inflate BP cuff to midway between systolic and diastolic for 5 minutes; ≥10–20 petechiae per square inch (2.5 × 2.5 cm) is positive — a marker of capillary fragility used in resource-limited settings.

Chikungunya: RT-PCR in the first ~5–7 days (viraemic), then IgM ELISA thereafter. Routine bloods may show lymphopenia; thrombocytopenia is mild and shock is absent.

Management / drug of choice

There is no specific antiviral; management is supportive, and fluid therapy is the cornerstone of dengue care.

• Antipyretic of choice: paracetamol. Avoid aspirin, NSAIDs and steroids — they aggravate bleeding/gastritis and platelet dysfunction.
• Fluids: isotonic crystalloids (Ringer lactate / normal saline) titrated against haematocrit and urine output during the critical phase; colloids for refractory shock. Avoid over-hydration once the recovery phase begins (risk of fluid overload as leaked plasma is reabsorbed).
• Platelet transfusion is not given for a numerical count alone; reserve for active significant bleeding or counts typically <10,000–20,000 with bleeding risk.
• Chikungunya: paracetamol for fever; analgesics and physiotherapy for arthralgia; NSAIDs may be used for joint pain only after dengue has been excluded (because co-circulation is common and NSAIDs are dangerous in dengue).

High-yield: In any acute febrile illness during a dengue-endemic season, give paracetamol and withhold NSAIDs/aspirin until dengue is excluded.

Stepwise outpatient-to-shock approach

Fever + warning signs? → if none and tolerating orally, home care with oral fluids + paracetamol + daily platelet/Hct review → if warning signs appear → admit, IV crystalloids, monitor Hct → if narrow pulse pressure / shock → aggressive fluid resuscitation (DSS protocol), ICU.

Prevention & vaccine

• CYD-TDV (Dengvaxia): live attenuated tetravalent vaccine; WHO advises use only in sero-positive individuals (pre-vaccination screening) because of ADE risk in sero-negatives.
• TAK-003 (Qdenga): newer tetravalent vaccine with broader approval.
• No licensed chikungunya vaccine in routine Indian use (newer candidates emerging).

Vector control & key indices

Source reduction is the mainstay; the larval/entomological indices are extremely high-yield.

Index	Formula	Use
House (premise) Index	(Houses positive for larvae / houses inspected) × 100	% of houses infested
Container Index	(Containers positive / containers inspected) × 100	% of water containers infested
Breteau Index (BI)	(Number of positive containers / 100 houses inspected)	Best single index; relates containers to houses

High-yield (cut-offs): A Breteau Index < 5 and a House Index < 1% indicate low transmission risk. A Breteau Index ≥ 50 signals high risk of an outbreak. The Breteau Index is considered the best/most informative single larval index because it links containers to houses.

Control measures:

• Source reduction — eliminate/empty/cover water containers; weekly "dry day"; manage coolers and overhead tanks (the single most effective, sustainable measure).
• Larvicides — temephos (Abate) in potable water; biological control with larvivorous fish (Gambusia, Poecilia).
• Space spraying of adulticides (pyrethroids/malathion fogging) during outbreaks to kill adult mosquitoes quickly.
• Personal protection — repellents, full-sleeved clothing, mesh screens; effective by day because the vector is a day-biter.
• Health education & community participation — the backbone of sustainable Aedes control.

Complications

Dengue: Dengue Shock Syndrome, severe haemorrhage (GI bleed, menorrhagia), dengue myocarditis, hepatitis/fulminant hepatic failure, acute kidney injury, dengue encephalopathy/encephalitis, fluid-overload pulmonary oedema (iatrogenic), and expanded dengue syndrome (unusual organ involvement). Maculopapular rash desquamation in recovery.

Chikungunya: chronic destructive arthritis/arthralgia (months–years), tenosynovitis, rare neurological (encephalitis, Guillain-Barré) and ocular involvement, vertical transmission causing severe neonatal disease.

Key differentials

Feature	Dengue	Chikungunya
Virus / family	Flavivirus (RNA)	Alphavirus, Togaviridae (RNA)
Serotypes	4 (DENV 1–4)	1
Dominant symptom	Retro-orbital pain, myalgia	Severe, symmetric arthralgia/arthritis
Plasma leakage / shock	Yes (hallmark of severe disease)	No
Thrombocytopenia	Marked	Mild
Bleeding	Common (DHF)	Rare
Chronic sequel	Usually none	Chronic arthritis (weeks–years)
Mortality	Significant (DSS)	Low
Incubation	4–10 days	2–4 days

Other look-alikes: malaria (periodic fever, no rash, blood smear/RDT positive), leptospirosis (conjunctival suffusion, myalgia, jaundice, AKI — and may coexist with dengue post-monsoon), enteric fever (relative bradycardia, sustained fever), scrub typhus (eschar, post-monsoon), Zika (mild, congenital microcephaly, same Aedes vector), and acute viral hepatitis. Tip: when arthralgia dominates, think chikungunya; when bleeding/leakage dominates, think dengue.

High-yield: Aedes aegypti transmits all four — dengue, chikungunya, Zika and yellow fever — a single-best-answer favourite. (Yellow fever does not occur in India, but India maintains strict vaccination certificate requirements for travellers from endemic zones precisely because the competent vector is present.)

Mnemonics & eponyms

• Aedes is a "DAY" biter and breeds in "ABCD" — Artificial containers, Breeds in clean water, Container index, Day biting.
• "4 D's of severe dengue": Defervescence danger, Dropping platelets, Dropping pulse pressure, Drip (fluids) is treatment.
• Halstead's secondary infection theory = ADE eponym.
• Hess test = tourniquet/capillary fragility test.
• Breteau = the index named after the entomologist; remember it counts positive containers per 100 houses.

Recently asked / exam angle

• NS1 antigen positive in the first 5 days — single most asked diagnostic point; "best test on day 2 of fever" → NS1.
• Breteau Index definition and cut-off (<5 low risk; counts positive containers per 100 houses); and "which is the best larval index?" → Breteau.
• Narrow pulse pressure ≤20 mmHg = DSS (DHF grade III) — grading questions.
• Vector for dengue/chikungunya/Zika/yellow fever = Aedes aegypti; flight range ~100 m; day biter; clean-water container breeder.
• Antibody-dependent enhancement / secondary infection theory explaining DHF and the sero-status requirement for Dengvaxia.
• Paracetamol yes, NSAIDs/aspirin no in dengue.
• Defervescence (day 4–6) = critical phase when leakage and shock occur.
• Chikungunya = chronic arthralgia; differentiating it from dengue by the joint involvement and absence of leakage.
• Larvivorous fish — Gambusia/Poecilia; larvicide — temephos.
• Tourniquet/Hess test positive ≥10–20 petechiae per square inch.

Rapid revision

1. Aedes aegypti = day-biting, clean-water container breeder, flight range ~100 m; transmits dengue, chikungunya, Zika and yellow fever.
2. Dengue = Flavivirus, 4 serotypes; chikungunya = Alphavirus, 1 serotype.
3. NS1 antigen is the test of choice days 1–5; IgM ELISA after day 5.
4. Severe dengue is defined by plasma leakage → rising haematocrit + falling platelets.
5. DHF III–IV = DSS; pulse pressure ≤20 mmHg is the cardinal warning sign.
6. Secondary heterologous infection + antibody-dependent enhancement → DHF/DSS (Halstead).
7. Critical phase = around defervescence, day 4–6 — danger rises as fever falls.
8. Paracetamol for fever; avoid aspirin/NSAIDs/steroids in dengue.
9. Platelet transfusion only for active bleeding, not for a number alone.
10. Breteau Index = positive containers per 100 houses; BI <5 / House Index <1% = low risk; best single larval index.
11. Vector control: source reduction, temephos larvicide, larvivorous fish (Gambusia), fogging in outbreaks.
12. Chikungunya hallmark = severe symmetric arthralgia with chronic joint pain; no shock, mild thrombocytopenia. Dengvaxia only in sero-positives.