Dengue, chikungunya, zika: tell them apart

A twenty-eight-year-old man returns to the health unit on the fifth day of an illness that began with high fever, body aches and pain behind the eyes. He says he is better, because the fever has been gone since the day before. He reports, however, that he vomited three times that morning and that he has belly pain. The region is in an arbovirus season, and the room is full of patients with fever and skin spots. The scene is fictional and was written for teaching. This text uses that picture as a setting for clinical reasoning: how to tell dengue, chikungunya and zika apart when they circulate together, why the moment the fever subsides calls for more attention, not less, and how to recognize the signs that change management.

NEXO is a daily medical diagnosis game: one clinical case a day, up to six progressive clues, and ICD-10 proximity feedback across five levels.

The text avoids being a protocol. Management of these infections follows national and international guidelines and depends on the structure of each service. The interest here is reasoning: how to look at three similar diseases without confusing what matters, how to follow a patient over the days and how to keep an apparent improvement from ending surveillance. If you play the case of the day in NEXO, you will recognize the move: the clue that changes the case sometimes arrives when everything seemed settled.

Key points

• Dengue, chikungunya and zika are transmitted by the same mosquitoes, have similar pictures and can circulate in the same region and even in the same patient.
• In dengue, warning signs usually appear around the moment the fever subsides, which marks the start of the critical phase.
• Fluid management in dengue requires balance between giving too little and giving too much, with frequent reassessment.
• Joint pain in chikungunya can persist for months after the acute phase.
• Zika is usually mild in adults, but infection during pregnancy can affect the fetus in any trimester.

Why three similar viruses are a setting for reasoning

Guidance from the United States Centers for Disease Control and Prevention notes that dengue, chikungunya and zika are transmitted by the same mosquitoes, have similar clinical features and can circulate at the same time in the same region, including with coinfection (CDC, chikungunya, clinical signs). Guzman and colleagues describe dengue as endemic in parts of the Eastern Mediterranean, the Americas, Southeast Asia, the Western Pacific and Africa, with sporadic local transmission in Europe and the United States (Guzman and colleagues, 2016).

The reasoning challenge is twofold. On one hand, telling apart three similar diseases that carry different risks. On the other, following over time a disease that can change behavior after a few days. A broad clinical review of dengue was published by Simmons and colleagues in NEJM.

What the scene teaches

In the opening scene, the patient says he is better because the fever is gone. At the same time, he reports vomiting and abdominal pain. Both pieces of information arrive together, and the first tends to reassure. Reasoning needs to give weight to the second. The post on signs, symptoms and history shows how to interpret what the patient brings, including when the clues point in different directions.

A disease that changes over the days

Many febrile diseases follow a simple course: the fever rises, the patient is ill and then improves. Dengue has a critical moment in the middle of that path, and that is where reasoning by phases makes a difference. Knowing which day of the illness the patient is on is as important as knowing what he feels.

Telling them apart at the bedside

The Centers for Disease Control and Prevention guidance describes useful differences among the three diseases (CDC, chikungunya, clinical signs). Chikungunya is more likely to cause high fever, intense joint pain, skin spots and a drop in lymphocytes. Dengue is more likely to cause a fall in neutrophils and platelets, bleeding, shock and death. Zika is usually mild, but it can cause congenital infection.

The table below summarizes these differences in a didactic way. It simplifies a complex field: the diseases overlap, and the diagnosis depends on the context and the tests available.

Disease	Clues that weigh in favor	What is most concerning
Dengue	Drop in platelets and neutrophils, bleeding	Critical phase, shock and bleeding
Chikungunya	High fever, intense joint pain, skin spots	Joint pain that persists for months
Zika	Usually mild picture, itchy skin spots	Infection during pregnancy and effect on the fetus

Thinking of dengue first

The same guidance recommends considering and ruling out dengue in a patient with suspected chikungunya, because dengue carries a higher risk of severe complications (CDC, chikungunya, clinical signs). It is the "cannot miss" reasoning: when two similar hypotheses carry different risks, the more dangerous one guides surveillance until it is ruled out. The post on differential diagnosis with clues shows how to order hypotheses by severity.

When there are two infections at the same time

The guidance cited above notes that coinfection is possible where the viruses circulate together (CDC, chikungunya, clinical signs). For reasoning, this means that finding an explanation for part of the picture does not end the search. A patient with intense joint pain who seems to have chikungunya still deserves the same surveillance for dengue warning signs. Finding one diagnosis and stopping the search is a form of premature closure, and the context of cocirculation makes that risk more concrete.

Incubation and onset

The incubation period of dengue is usually 5 to 7 days (CDC, dengue, clinical picture), and that of chikungunya is 3 to 7 days (CDC, chikungunya, clinical signs). These intervals overlap, so the time since exposure rarely separates the two diseases. The post on base rate in diagnosis notes that what is circulating in the region at that moment weighs heavily on the probability of each hypothesis.

Dengue in three phases

The Centers for Disease Control and Prevention guidance describes dengue in three phases: febrile, critical and recovery (CDC, dengue, clinical picture). The febrile phase usually lasts 2 to 7 days. The critical phase begins around defervescence, when the fever subsides, and lasts 24 to 48 hours.

The paradox of improvement

The moment the fever subsides is, for the patient, a sign of improvement. For clinical reasoning, it is the moment of greatest attention. The same guidance describes that warning signs usually appear around defervescence and that the patient may look well despite early signs of shock, with narrowing of the difference between systolic and diastolic pressure (CDC, dengue, clinical picture).

How many do poorly

According to the same guidance, about 1 in 20 patients with dengue progresses to severe dengue (CDC, dengue, clinical picture). Most recover without complications, and that is precisely why recognizing the few who will worsen requires method. The post on prognostic reasoning discusses how to think about what comes after the diagnosis.

Counting the days

Asking which day of the illness the patient is on helps place the phase. The clinical management described by the Centers for Disease Control and Prevention places the critical phase between the third and eighth day after symptom onset (CDC, dengue, clinical management). A patient on the fifth day, without fever and with vomiting, is exactly in the window that calls for careful reassessment.

The warning signs

The Centers for Disease Control and Prevention guidance lists the warning signs described by the World Health Organization in 2009: abdominal pain, persistent vomiting, fluid accumulation, mucosal bleeding, lethargy or restlessness and an enlarged liver (CDC, dengue, clinical picture). The World Health Organization dengue guidelines contain the full classification.

What systematic reviews show

Tsheten and colleagues gathered 143 studies on predictors of severe dengue. Being a child, having a secondary infection, diabetes or kidney disease were risk factors, and signs such as a rise in hematocrit with a drop in platelets, abdominal pain, lethargy, vomiting, an enlarged liver, ascites, pleural effusion and melena were associated with severe progression (Tsheten and colleagues, 2021). The authors concluded that the results support the warning signs of the World Health Organization.

The first days count too

Sangkaew and colleagues analyzed 150 studies on the febrile phase. Diabetes, hypertension, kidney disease and cardiovascular disease, as well as vomiting, abdominal pain, bleeding and fluid accumulation in the febrile phase, were associated with progression to severe disease. Lower platelets and albumin and higher liver enzymes in the first four days were also associated with this risk, as was secondary infection (Sangkaew and colleagues, 2021).

The sign as a question

Each warning sign works as a direct question: is this patient entering the critical phase? For reasoning, a single sign changes management, and their absence today does not guarantee their absence tomorrow. The post on pretest probability shows how a new clue changes the probability of an outcome.

Fluids and analgesia: neither too little nor too much

The clinical management described by the Centers for Disease Control and Prevention groups patients according to warning signs and severity (CDC, dengue, clinical management). Without warning signs, oral hydration is usually enough. With warning signs or shock, intravenous isotonic crystalloids come in, adjusted by clinical status and hematocrit.

The two sides of error

The same guidance highlights the importance of avoiding both insufficient hydration and fluid overload (CDC, dengue, clinical management). Too little fluid exposes the patient to shock. Too much fluid, especially when leakage decreases in the recovery phase, exposes the patient to accumulation in the lungs and abdomen. The post on diagnostic uncertainty discusses how to decide when both sides carry risks.

Reassess often

Adjusting fluids by clinical status and hematocrit, as described in the guidance, assumes repeated reassessments (CDC, dengue, clinical management). In practice, this means returning to the patient at short intervals, comparing the current exam with the previous one and recording what changed. The reasoning here is dynamic: the decision made a few hours ago was correct for that moment and needs to be confirmed or corrected at each new assessment. A simple spreadsheet with time, vital signs, urine output and test results helps the team see the trend, which usually says more than a single value.

What the trials show

Wills and colleagues compared three solutions in the resuscitation of children with dengue shock syndrome, with 383 children with moderate shock and 129 with severe shock, and there was one death in the study (Wills and colleagues, 2005). The authors concluded that lactated Ringer's is indicated in the initial resuscitation of moderate shock, and the need for rescue colloid was similar among the solutions.

Platelets and analgesia

The guidance from the Centers for Disease Control and Prevention recommends paracetamol for fever, advises avoiding aspirin and nonsteroidal anti-inflammatory drugs in dengue and advises against prophylactic platelet transfusion (CDC, dengue, clinical management). For reasoning, this has a practical consequence: faced with fever and joint pain in an area of cocirculation, the possibility of dengue must be considered before any anti-inflammatory drug.

Chikungunya: when the pain stays

Guidance from the Centers for Disease Control and Prevention describes the joint pain of chikungunya as usually bilateral and symmetric. Acute symptoms often clear in 7 to 10 days, but joint pain can persist for months or years (CDC, chikungunya, clinical signs). In the acute phase, where dengue circulates or in people who have traveled to those regions, paracetamol is the first choice for fever and joint pain until dengue is ruled out, to reduce the risk of bleeding; after that, nonsteroidal anti-inflammatory drugs, corticosteroids, including topical ones, and physical therapy can help with persistent joint pain (CDC, chikungunya, treatment).

How many still have pain

Paixão and colleagues gathered data from 6532 patients and found that about 43 in 100 had not recovered at three months and about 21 in 100 still had symptoms at twelve months. The proportion varied according to the virus lineage (Paixão and colleagues, 2018).

The reasoning in follow-up

For clinical reasoning, chikungunya is a disease that does not end when the fever passes. The follow-up visit needs to ask about pain, stiffness, limitation for activities of daily living and impact on work. A patient who returns weeks later with pain in the hands and feet brings a clue that is explained by the recent history, as long as someone asks about it.

When the pain has another cause

Joint pain after a fever can have more than one explanation. Other causes of joint pain remain possible, and reassessment over the months helps separate the expected course from a different picture. The post on clinical decision making helps you think about when to order a new test and when to follow up.

Zika in pregnancy

Brasil and colleagues followed pregnant women with skin rash in Rio de Janeiro: among 345 participants, 182 had confirmed Zika virus infection (Brasil and colleagues, 2016). Adverse pregnancy outcomes occurred in 46 in 100 pregnant women with Zika, versus 11.5 in 100 in those without the infection (Brasil and colleagues, 2016). The authors observed adverse outcomes in all trimesters, despite mild maternal symptoms.

The risk by trimester

Hoen and colleagues analyzed 555 fetuses and infants of pregnant women with symptomatic Zika infection in the French territories of the Americas. Neurological and ocular defects consistent with congenital syndrome occurred in 7 in 100 (Hoen and colleagues, 2018). The frequency was 24 in 189 when the infection occurred in the first trimester, 9 in 252 in the second and 6 in 114 in the third.

A mild disease with a serious consequence

The mild picture in a pregnant woman can lead to underestimating the disease. The reasoning needs to separate the severity for the woman from the severity for the fetus. Asking about pregnancy, or the possibility of pregnancy, is part of the assessment of every woman of childbearing age with fever and skin rash in a region where Zika circulates.

What to ask and record

In a woman of childbearing age with fever and skin rash, the visit gains its own questions: date of the last menstrual period, possibility of pregnancy, recent travel by the patient and her partner, and similar symptoms in people in the same household. Recording these answers in the medical record helps whoever will follow the pregnancy later. Clinical reasoning here extends beyond the acute episode, because today's decision can guide tests and visits in the following months.

The bias of the patient who looks well

Croskerry described how cognitive errors lead to diagnostic errors and proposed strategies to reduce them (Croskerry, 2003). In arboviral diseases, one bias stands out: trusting the appearance of the moment.

The fever that passed

In the opening scene, the patient feels better because the fever has subsided. If the reasoning stops there, the vomiting and abdominal pain remain unexplained. The post on heuristics in clinical reasoning shows how an overall impression can hide specific clues.

Guidance for the return visit

A patient with probable dengue going home does not end the reasoning, it only changes who keeps watch. From that point on, it is the patient, or the family, who will notice if something has changed at home. Explaining in simple words which signs call for an immediate return, such as abdominal pain, vomiting that will not stop, bleeding or drowsiness, and agreeing on a date for reassessment shifts part of the vigilance safely. Checking whether the person understood, by asking them to repeat the instructions in their own words, is a short step that prevents misunderstandings.

Pause before discharge

Before releasing a patient with probable dengue, a short pause helps: which day of the illness are they on, is there any warning sign, did they understand when to come back? The post on diagnostic pause shows how a short moment of checking improves the decision.

Questions that protect

Some questions help keep the focus:

1. Which day of the illness is the patient on?
2. Did the fever subside recently?
3. Is there any warning sign today?
4. Was dengue considered before any anti-inflammatory?
5. Is the patient pregnant or could she be?

How NEXO helps train this reasoning

In NEXO, the case of the day is fictional, with a real clinical presentation and up to six progressive clues. You propose a diagnosis by searching for the ICD-10 code and receive feedback on proximity in five levels, which shows whether the guess landed on the exact code, the same category, the same group, the same chapter or a different chapter. This format speaks to the theme of this text: similar diseases tend to sit in the same chapter, and the feedback shows whether you reached the right category.

To use the case of the day as training, try three habits. Before each guess, ask which point of the illness the patient is at. When two hypotheses seem equally likely, ask which one carries more risk. After the result, note down which clue separated the similar hypotheses.

Three exercises with cases

Count the days. In each case, place the patient on the timeline of the illness before proposing the diagnosis.

Separate the similar ones. List two close hypotheses and write the clue that weighs most in favor of each.

Think of the special group. Reread the case imagining a pregnant woman or a child and see what changes.

The post on calibration of confidence in diagnosis helps adjust the certainty of the guess to what the clues actually show.

The limits on using the game are described on the NEXO economy page. If you research the teaching of clinical reasoning, the page for researchers explains how to get in touch. Professionals who write or review cases find information under curation and under creators, and institutions can learn about the proposal on the partnerships page.

Frequently asked questions

How do you tell dengue, chikungunya and zika apart?

By the clues that weigh most in each one. High fever with intense joint pain favors chikungunya, a drop in platelets and bleeding favor dengue, and a mild picture with itchy spots favors zika, such as the descending itchy spots described in pregnant women with zika (Brasil and colleagues, 2016). Tests and the local context complete the reasoning.

Why does dengue call for attention when the fever passes?

Because the critical phase begins around defervescence. That is when warning signs usually appear, and the patient may look well even with early signs of shock.

What are the warning signs of dengue?

Abdominal pain, persistent vomiting, fluid accumulation, mucosal bleeding, lethargy or restlessness and an enlarged liver. Any one of them changes the approach.

Which painkiller should be used when dengue is suspected?

The guidelines recommend paracetamol for fever and advise avoiding aspirin and nonsteroidal anti-inflammatories while dengue is a possibility.

How long does chikungunya pain last?

Acute symptoms usually pass in about a week to ten days, and joint pain may persist for months or years. A review found about 43 in every 100 patients still with symptoms at three months.

Why is zika a concern in pregnancy?

Because infection in a pregnant woman, even with mild symptoms, can affect the fetus. Studies show adverse outcomes in all trimesters, with the highest risk in the first.

Final checklist

• I considered the three diseases and remembered that they can circulate together.
• I thought of dengue first, because it carries more risk of severe complications.
• I asked which day of the illness the patient is on.
• I looked for warning signs, especially when the fever subsided.
• I avoided anti-inflammatory drugs while dengue was possible.
• I asked about pregnancy and planned follow-up for the joint pain.

Want to practice your reasoning with progressive clues? Play today's case.

References

1. Brasil P, Pereira JP Jr, Moreira ME, et al. Zika virus infection in pregnant women in Rio de Janeiro. N Engl J Med. 2016. PMID 26943629.
2. Centers for Disease Control and Prevention. Clinical features of dengue. CDC page.
3. Centers for Disease Control and Prevention. Clinical signs and symptoms of chikungunya. CDC page.
4. Centers for Disease Control and Prevention. Treatment and clinical care for dengue. CDC page.
5. Centers for Disease Control and Prevention. Treatment and prevention of chikungunya virus disease. CDC page.
6. Croskerry P. The importance of cognitive errors in diagnosis and strategies to minimize them. Acad Med. 2003. PMID 12915363.
7. Guzman MG, Gubler DJ, Izquierdo A, et al. Dengue infection. Nat Rev Dis Primers. 2016. PMID 27534439.
8. Hoen B, Schaub B, Funk AL, et al. Pregnancy outcomes after ZIKV infection in French territories in the Americas. N Engl J Med. 2018. PMID 29539287.
9. Paixão ES, Rodrigues LC, Costa MDCN, et al. Chikungunya chronic disease: a systematic review and meta-analysis. Trans R Soc Trop Med Hyg. 2018. PMID 30007303.
10. Sangkaew S, Ming D, Boonyasiri A, et al. Risk predictors of progression to severe disease during the febrile phase of dengue: a systematic review and meta-analysis. Lancet Infect Dis. 2021. PMID 33640077.
11. Simmons CP, Farrar JJ, Nguyen vV, Wills B. Dengue. N Engl J Med. 2012. PMID 22494122.
12. Tsheten T, Clements ACA, Gray DJ, et al. Clinical predictors of severe dengue: a systematic review and meta-analysis. Infect Dis Poverty. 2021. PMID 34627388.
13. Wills BA, Nguyen MD, Ha TL, et al. Comparison of three fluid solutions for resuscitation in dengue shock syndrome. N Engl J Med. 2005. PMID 16135832.
14. World Health Organization. Dengue: guidelines for diagnosis, treatment, prevention and control. 2009. Publication page.