Chennai Monsoon Fever: How Doctors Tell Dengue, Typhoid, Chikungunya and Leptospirosis Apart

Fever alone does not separate these four. The day of illness does. India's national treatment guidelines (NCDC and ICMR, MoHFW, November 2025) say to defer tests on day 1 or 2 of fever, start blood counts and dengue NS1 on day 3 or 4, and add blood cultures and IgM serology only after day 5.

Dengue, typhoid, chikungunya and leptospirosis all open the same way in Chennai between June and December: fever, headache, body ache, no rash yet, nothing that points anywhere. What tells them apart is a small number of clinical separators a physician looks for on examination, and a test sent on the correct day of fever. This article, written for patients at AJSMC in Egmore, Chennai, sets out both.

One thing before anything else. AJSMC is an outpatient multi-specialty centre in Egmore, Chennai, with no casualty unit, no trauma unit and no 24-hour emergency department. If someone with fever has breathlessness, bleeding, confusion or drowsiness, cold clammy skin, blood in vomit or stool, severe abdominal pain, or has stopped passing urine, do not come to an outpatient clinic. Call 108 or go straight to the nearest hospital with a 24-hour emergency department.

All four start as fever — what actually tells them apart?

In the first 48 hours, usually nothing does, and that is the honest answer. The separators appear as the illness declares itself: where the pain is, when the rash comes, what the eyes look like, and what the person was exposed to. The table below is the reasoning a physician works through at the examination couch.

Sources for this table: National Guidelines for Clinical Management of Dengue Fever (NCVBDC, MoHFW, 2023); National Guidelines for Clinical Management of Chikungunya Fever (NCVBDC, MoHFW, 2023); National Guidelines on Leptospirosis (NCDC, MoHFW, 2015); WHO Dengue Guidelines 2009; Indian Journal of Critical Care Medicine, 2021.

These four are not the only causes of monsoon fever in Chennai. Scrub typhus and malaria belong in the same list, and the NCDC and ICMR national guidelines of November 2025 name scrub typhus as one of the serologies to send after day 5 of fever. Co-infection also happens. That is why a doctor examines rather than picks from a menu.

Which test should be done on which day of fever?

This is the single most useful thing a patient in Egmore, Chennai can know, because a correct test sent on the wrong day reads negative and the illness gets missed. Each test detects a different thing, the virus or bacterium itself early, the antibody response later, and each has a window.

The chikungunya IgM window printed above is the one in the disease-specific NCVBDC 2023 chikungunya guideline — after day 7, and "not useful in first 7 days of illness" — while the NCDC and ICMR national treatment guidelines of November 2025 group chikungunya IgM with the serologies sent once fever has lasted more than 5 days; the two national documents differ, and the practical consequence is the same either way, that a negative chikungunya IgM taken before day 7 does not exclude the illness.

Two practical consequences follow. First, write the date the fever started on the laboratory request form, the NCVBDC 2023 dengue guideline instructs laboratories that the day of onset and the day of sample collection must both be recorded, so the lab knows whether to run NS1 or IgM. Second, if typhoid is suspected, the blood culture must be drawn before the first antibiotic dose, not after. Bacteria are present in very low numbers in typhoid — roughly half of culture-positive patients carry under 1 colony-forming unit per millilitre of blood, so the volume drawn matters too: sensitivity runs about 51% at 2 mL, 56% at 5 mL, 60% at 7 mL and 65% at 10 mL (Journal of Infectious Diseases, 2018).

What tests should be done on day 2 of fever, and what should wait?

Usually nothing on day 2. The National Treatment Guidelines for Antimicrobial Use in Infectious Disease Syndromes (NCDC and ICMR, MoHFW, Version 2.0, November 2025) stage the workup by day of fever, and the first stage is deliberately empty.

Read the first and last rows together. "Defer on day 1 or 2" is not permission to wait indefinitely. The NCVBDC 2023 dengue guideline is explicit that a complete blood count with haematocrit and platelet count is indicated in anyone whose fever has lasted more than three days, and immediately in anyone with a warning sign, whatever day of illness it is.

Can a dengue test be negative when you still have dengue?

Yes, and the commonest reason is the day it was sent. In a study of 94 confirmed dengue cases at JIPMER, Puducherry (Journal of Clinical and Diagnostic Research, 2016), NS1 was positive in 76.6% of samples taken on days 1–7 of fever but in only 4.3% of samples taken on days 8–14. Peak NS1 detection was days 2–5. WHO's own kinetics (Dengue Guidelines, 2009) show the mirror image for antibody: IgM is detectable in about 50% of patients by days 3–5, 80% by day 5 and 99% by day 10.

NS1 and IgM together reached 97.8% in that one Puducherry cohort of 94 patients. That is a single study, not a guarantee, and no combination of tests is definitive. The point a patient should take from it is the opposite of reassuring: a negative test sent on the wrong day of fever has not excluded the illness, and the fever still needs reviewing.

Two other points on test choice are worth knowing because they contradict what is still widely sold. The NCVBDC 2023 dengue guideline states that rapid diagnostic test accuracy is largely unvalidated, that sensitivity and specificity vary batch to batch, and that the national programme does not recommend rapid tests for diagnosing or managing dengue, a positive rapid test makes a case only "probable". And the NCDC and ICMR National Treatment Guidelines of November 2025 say of enteric fever, in one line: blood culture remains the gold standard, and avoid using the Widal test for diagnosis. None of this means overriding your own doctor. It means asking a reasonable question, which test is being sent, and on which day of fever.

Which symptoms point to dengue rather than typhoid?

Speed and shape. Dengue in Chennai arrives suddenly, with high fever of 38.5 °C or more, pain behind the eyes, generalised body ache, and a rash that appears after the third or fourth day of fever over the face, neck, chest and abdomen. NCVBDC 2023 defines it as an acute febrile illness of 2–7 days with two or more of: headache, retro-orbital pain, myalgia, arthralgia, rash, bleeding, low platelets or low white cells.

Typhoid climbs instead of spiking. Incubation is 7–21 days from swallowing contaminated food or water; week 1 brings a step-ladder fever with chills and a pulse slower than the temperature would predict; week 2 brings abdominal pain, an enlarged liver and spleen, and rose spots; week 3 carries the risk of intestinal perforation (Indian Journal of Critical Care Medicine, 2021). If a fever has been climbing for eight days rather than raging for four, dengue is the less likely of the two.

One typhoid fact prevents a common mistake. The NCDC and ICMR guidelines of November 2025 note that fever may take four to five days to settle even on the correct antibiotic, so persistent fever on day 3 of treatment is not by itself a reason to switch drugs.

Why do my calves hurt after walking through flood water?

Because that combination is the classic presentation of leptospirosis, and Chennai carries a large share of it. The NCDC 2015 national guideline calls calf, abdominal and lumbosacral muscle pain "a characteristic finding" that is very useful in separating leptospirosis from other causes of fever, usually with a raised serum CPK. It goes further: in an endemic area, all cases of fever with muscle pain and conjunctival suffusion should be treated as suspected leptospirosis. Conjunctival suffusion is redness of both eyes without discharge, most marked on the inner lids.

The bacterium enters through skin cuts and mucous membranes after contact with water contaminated by animal urine — flood water, stagnant water, sewage, rodent-infested surroundings. Incubation is usually 5–14 days, with a range of 2–30 days.

There is a trap in the course of the illness that patients in Egmore, Chennai fall into every monsoon. The NCDC 2015 guideline describes leptospirosis as biphasic: the symptoms decrease or disappear within two to three days, then reappear and may progress to severe disease. Feeling better on day 3 is not proof of recovery. Jaundice, when it occurs, starts after 4 to 7 days of illness. About 90% of patients have the milder anicteric form and 5–10% the icteric form.

Leptospirosis is also where waiting turns dangerous fastest. The NCDC 2015 guideline's referral criteria are severe muscle pain, passing little or no urine, jaundice, blood in the cough, breathlessness, confusion, cold extremities and an irregular pulse. Over 90% of deaths from leptospirosis are from bleeding into the lungs and from kidney failure, and death can follow within hours to two days of severe lung involvement. Any of those signs means calling 108 or going directly to a hospital with a 24-hour emergency department and intensive care, not to an outpatient clinic, and not to AJSMC.

When does joint pain mean chikungunya?

When the joints hurt more than the muscles, the pain is symmetrical and mostly in the small joints of the hands and feet, and it is still there after the fever has gone. The NCVBDC 2023 chikungunya guideline describes severe debilitating polyarthralgia beginning soon after fever onset, typically polyarticular and symmetrical, involving peripheral and predominantly small joints, sometimes with knees and shoulders. In Indian series of 1,638 acute cases, joint pain or arthritis occurred in 96–100%, muscle pain in 80–99% and rash in 31–94%.

Against dengue, the same guideline's head-to-head comparison is useful: chikungunya is fever with joint pain, abrupt, lasting 3–5 days, rash on day 2 or 3, bleeding uncommon, low white cells and low platelets infrequent, haematocrit normal. Dengue is fever with headache, body ache and bleeding, lasting 5–7 days, bleeding common, low white cells and low platelets common, haematocrit high, with a rash appearing after the third or fourth day of fever. That is the timing given in the NCVBDC 2023 dengue guideline and used throughout this article, while the comparison table in the NCVBDC 2023 chikungunya guideline prints the dengue rash at days 5–7.

Source: National Guidelines for Clinical Management of Chikungunya Fever, NCVBDC, MoHFW, 2023. Joint pain that outlasts the fever by weeks is not a treatment failure and not a new illness. It is the recognised course of chikungunya, and it is worth reviewing with a physician rather than self-medicating through it.

Which signs mean this is no longer something to watch at home?

For dengue, the danger window is not the peak of the fever. It is the moment the fever comes down. WHO (2009) places the critical phase around defervescence, when the temperature drops to 37.5–38 °C or less, usually on days 3–7 of illness, with significant plasma leakage lasting 24–48 hours. NCVBDC 2023 says 5–10% of patients in the febrile phase progress to that critical phase, which usually begins after the third or fourth day of fever. The fever settling is the point to watch hardest, not to relax.

These are the dengue warning signs listed by NCVBDC (MoHFW, 2023) and WHO (2009). Any one of them means the person needs to be assessed in a hospital, not observed at home:

• Persistent vomiting
• Severe abdominal pain or tenderness
• Lethargy, restlessness, or a sudden change in behaviour
• Bleeding — from the nose or gums, blood in vomit, black tarry stools, blood in urine, unusually heavy menstrual bleeding
• Red spots or patches on the skin
• Fluid collecting in the abdomen or around the lungs, or difficulty in breathing
• Liver enlargement of more than 2 cm
• Fainting or giddiness
• Pale, cold or clammy skin
• A rising haematocrit together with a rapidly falling platelet count on blood tests

If any of these appear, call 108 or go to the nearest hospital with a 24-hour emergency department. Do not come to the AJSMC outpatient clinic in Egmore with them and do not wait for the next outpatient session. AJSMC has no casualty unit, no trauma unit and no 24-hour emergency department, and severe dengue — shock, plasma leakage, severe bleeding or organ impairment — needs a hospital that can admit and give intensive care immediately. The same applies to leptospirosis with breathlessness, blood in the cough, confusion, jaundice, reduced urine output or cold extremities, and to typhoid with severe abdominal pain suggesting perforation.

What can I do at home in the first two days of fever?

The NCVBDC 2023 guidance for mild dengue managed at home is the safest general advice for undifferentiated monsoon fever in Chennai while the diagnosis is still unclear:

• Keep the body temperature below 100 °F. If it goes above, the dengue-specific instruction in the NCVBDC 2023 national guideline is paracetamol 10 mg per kg body weight per dose, at intervals of not less than six hours, with a maximum of 4 g per day in adults. For an infant or a small child, confirm the dose and the interval with the treating doctor before giving it.
• Avoid aspirin and NSAIDs such as ibuprofen and diclofenac. They increase the risk of bleeding and of stomach and kidney injury in these illnesses.
• Adequate oral fluids, including ORS or tender coconut water. Avoid carbonated drinks. In infants and small children, watch that fluids are not overdone.
• Bed rest, and tepid sponging of the forehead, armpits and limbs.
• Do not take antibiotics on your own. Apart from being ineffective for the viral illnesses here, an antibiotic taken before a blood culture is drawn lowers typhoid culture sensitivity by 34% (Journal of Infectious Diseases, 2018) and can cost the diagnosis outright.

Two things deserve plain statements. There is no specific antiviral for dengue or for chikungunya; treatment is supportive. And platelet counts do not drive transfusion — NCVBDC 2023 states that platelet transfusion is not recommended for a low platelet count alone, and the narrow exceptions to that are decided in hospital. Papaya leaf extract and similar remedies sold to "raise platelets" are recommended by no national guideline. They appear nowhere in NCVBDC 2023, and they are not a substitute for being reviewed.

When is fever in Chennai most likely to be one of these four?

The northeast monsoon, and the weeks after it. The India Meteorological Department's report on the 2024 northeast monsoon (IMD Chennai, IMDC-SR/18, March 2025) records that October to December is the chief rainy season for the Tamil Nadu, Puducherry and Karaikal subdivision, delivering 48% of its annual rainfall — 442.8 mm of 921.4 mm — with a normal onset date of 20 October over coastal Tamil Nadu. The fever numbers track the rain.

Two honest gaps. There is no published Tamil Nadu case count for typhoid; national modelling estimates 4.9 million typhoid cases and 7,850 deaths in India in 2023, an incidence of 360 per 100,000 person-years, with Tamil Nadu among five states accounting for 49% of cases and deaths (Lancet Regional Health – Southeast Asia, 2026). And no current state-level chikungunya count was obtainable, the defensible statement is that a decade-long IDSP analysis covering 2014–2023 found India's highest district-level chikungunya burden in Tamil Nadu, in Thanjavur, Theni and Vellore, while national chikungunya outbreaks fell from 77 in 2017 to 19 in 2023 (Cureus, 2025).

Which of these tests are run at the AJSMC laboratory in Egmore?

AJSMC has an in-house laboratory in Egmore, Chennai that handles blood and urine samples. A complete blood count with haematocrit and platelet count, the test the national guidelines ask for in anyone febrile beyond three days — is a routine laboratory test, and the date the fever started is what decides whether NS1 or IgM is the right dengue test to send on the day you come.

Serial monitoring is a different thing altogether, and it is worth being plain about it. Repeating the count through the critical phase of dengue and reading the haematocrit and platelet trend as it moves is inpatient work. A patient who needs serial monitoring needs a hospital with inpatient beds and 24-hour emergency care. AJSMC is an outpatient centre with no casualty unit, no trauma unit and no 24-hour emergency department, and it is not the place to watch a febrile patient who is deteriorating — anyone with a warning sign, or with a rising haematocrit and a falling platelet count, should go to a hospital that can admit.

Before travelling for a test, call 044 2532 2021 and ask which of the tests named in this article can be run in-house and which are sent to a referral laboratory, and by what time in the day the sample has to be given. Sending a sample to the wrong place wastes the very day of illness this article is about.

How do you avoid all four during the Chennai monsoon?

The four split neatly into two routes, and the precautions follow the route rather than the illness.

• Dengue and chikungunya, the Aedes mosquito, which bites in daylight. Empty, scrub and dry any standing water around the house weekly: overhead and ground-level tanks, buckets, plant trays, discarded tyres, coolers, terrace containers. Cover stored water. Use repellent and full-sleeved clothing during the day, and mosquito nets for anyone sleeping during daytime, including infants and people already ill with fever.
• Leptospirosis — contact with contaminated water. Avoid wading through flood water and stagnant water where possible. If you must, wear boots and cover any cut or abrasion. Wash and dry skin as soon as you are out. Control rodents and store food and grain closed. This matters especially in Chennai, which accounted for 35.4% of Tamil Nadu's confirmed leptospirosis cases between April 2021 and March 2024 (TN Directorate of Public Health, IDSP-IHIP, 2024).
• Typhoid — contaminated food and water. Drink water you are sure of, particularly during and after flooding when supply lines are contaminated. Wash hands before eating and after using the toilet. Be careful with cut fruit, ice and roadside water. A typhoid vaccine exists and is worth discussing with a physician for household members at risk.

When should you see a doctor?

See a doctor if a fever has lasted more than three days, whatever else you feel. That is the threshold at which the NCVBDC 2023 national guideline asks for a complete blood count with haematocrit and platelet count. See a doctor sooner than that if there was flood water exposure and the calves are tender, if the eyes are red without discharge, if the joints are hurting more than the muscles, or if the fever has been climbing steadily for over a week rather than spiking. Take the date the fever started with you. It decides which test is worth sending, and it is the single most useful thing you can bring to the consultation.

Outpatient consultations at AJSMC, Egmore, Chennai run Monday to Saturday, 10am to 9pm. Call 044 2532 2021 to book.

And to repeat the one line that matters most: AJSMC has no casualty unit, no trauma unit and no 24-hour emergency department. For breathlessness, bleeding, confusion or drowsiness, cold clammy skin, persistent vomiting, severe abdominal pain, or reduced urine output in someone with fever, call 108 or go straight to the nearest hospital with a 24-hour emergency department.