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HLH in dengue in detail

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PMID: 38668287

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Hemophagocytic Lymphohistiocytosis (HLH) in Dengue

1. What HLH Is

HLH (also called hemophagocytic syndrome, or macrophage activation syndrome when secondary to rheumatologic disease) is a hyperinflammatory syndrome in which macrophages/histiocytes become pathologically activated and phagocytose blood cells and their precursors in the bone marrow, spleen, and lymph nodes. It is driven by a cytokine storm - impaired cytotoxic clearance of infected/activated cells (by NK cells and CD8+ T cells) leads to sustained antigen exposure, excessive interferon-gamma release, and downstream macrophage activation with toxic release of TNF, IL-6, and other proinflammatory cytokines, producing a picture resembling sepsis or SIRS - Robbins & Kumar Basic Pathology, p. 401.
HLH is classified as:
  • Primary/familial HLH - inherited defects in genes required for cytotoxic granule function (perforin/PRF1, Munc13-4, syntaxin-11, Munc18-2), X-linked lymphoproliferative disease (SH2D1A/XIAP), or HLH with partial albinism (Chediak-Higashi, Griscelli, Hermansky-Pudlak type 2) - Harrison's Principles of Internal Medicine, 22E.
  • Secondary HLH - triggered by infection (viral, most classically EBV, but also dengue and other arboviruses), malignancy (especially T/NK-cell lymphomas), or autoimmune/rheumatologic disease (macrophage activation syndrome).
Dengue-associated HLH falls into the infection-associated secondary category, though an underlying subclinical predisposition (heterozygous cytotoxicity-gene defects) may lower the threshold in some patients.

2. Why Dengue Triggers HLH

Dengue virus infects monocytes/macrophages and dendritic cells directly, and the intense cytokine response seen in severe dengue (the same cytokine storm implicated in plasma leakage and dengue shock syndrome) creates fertile ground for secondary HLH. The proposed mechanism mirrors other viral triggers: dengue-infected monocytes/macrophages are not efficiently cleared by cytotoxic lymphocytes, sustained IFN-gamma drives macrophage hyperactivation, and this results in hemophagocytosis of erythrocytes, platelets, and leukocyte precursors in the marrow. Textbook cross-references list dengue alongside EBV and CMV as recognized infectious triggers of secondary/acquired HLH (Dermatology, 2-Vol Set 5e; Red Book 2021 Report of the Committee on Infectious Diseases, p. 425/487).

3. Epidemiology and Clinical Suspicion

Dengue-associated HLH is rare but is disproportionately seen in severe dengue and carries a meaningfully higher mortality than uncomplicated dengue if unrecognized. Per the most current dedicated review (See KC, Pathogens 2024, PMID 38668287):
Clinicians should suspect dengue-associated HLH when a dengue patient has:
  • Fever persisting or recurring beyond 7 days (dengue fever typically resolves by day 5-7)
  • Anemia that is disproportionate and not explained by intravascular hemolysis or overt massive bleeding
  • Worsening cytopenias and worsening liver function despite the usual defervescence period
  • Progressive hepatosplenomegaly
This overlaps heavily with warning signs of severe dengue, which is why HLH is frequently missed or diagnosed late, it "hides" behind a background of expected dengue-related cytopenia and transaminitis.

4. Diagnostic Criteria - HLH-2004

Diagnosis of HLH (including dengue-associated cases) requires at least 5 of the following 8 HLH-2004 criteria:
  1. Fever
  2. Splenomegaly
  3. Bicytopenia (affecting at least 2 of: hemoglobin <9 g/dL, platelets <100,000/microL, neutrophils <1000/microL)
  4. Hypertriglyceridemia (fasting triglycerides ≥3 mmol/L, ~265 mg/dL) and/or hypofibrinogenemia (≤1.5 g/L)
  5. Hemophagocytosis demonstrated on bone marrow, spleen, or lymph node biopsy
  6. Low or absent NK-cell activity
  7. Ferritin ≥500 mcg/L (in florid HLH, often exceeds 10,000 mcg/L, which is highly specific)
  8. Elevated soluble CD25 (soluble IL-2 receptor) ≥2400 U/mL
The major practical challenge in dengue is that fever, splenomegaly, bicytopenia, and elevated ferritin already occur to some degree in severe dengue itself, so the diagnosis rests on the degree and persistence of these abnormalities plus bone marrow confirmation of hemophagocytosis when clinical suspicion is high (Robbins & Kumar Basic Pathology; Henry's Clinical Diagnosis and Management by Laboratory Methods).

5. Bone Marrow and Laboratory Findings

Bone marrow examination is the key confirmatory test, showing activated macrophages phagocytosing erythrocytes, platelets, and nucleated marrow precursor cells. Supporting labs typically include:
  • Markedly elevated ferritin (often >10,000 mcg/L, near pathognomonic when this high)
  • Hypertriglyceridemia
  • Hypofibrinogenemia
  • Low/absent NK-cell numbers and function
  • Elevated soluble IL-2 receptor (CD25)
  • Elevated transaminases, often with worsening liver dysfunction beyond what is typical for dengue alone

6. Treatment

Reported management of dengue-associated HLH, drawn from case series and the 2024 narrative review, differs from the classical etoposide-based HLH-94/HLH-2004 protocols used for malignancy- or EBV-driven HLH:
  • First-line: short courses (3-4 days) of high-dose corticosteroids, e.g., dexamethasone ~10 mg/m2 or IV methylprednisolone pulses (1 g/day x 3 days), followed by an oral steroid taper over weeks. Multiple case reports describe rapid defervescence and hematologic recovery with steroids alone.
  • Supportive care for dengue itself remains essential (fluid management, monitoring for shock/plasma leakage, transfusion support for severe cytopenia or bleeding) since there is no specific antiviral for dengue.
  • More severe or refractory presentations may warrant escalation with agents used in secondary HLH more broadly, such as etoposide, IVIG, or cytokine-directed therapy (e.g., IL-1 inhibition with anakinra has been described anecdotally in secondary HLH, allowing etoposide to be avoided).
  • Because dengue-associated HLH tends to respond well to steroids alone (unlike malignancy-associated HLH), aggressive cytotoxic immunosuppression is generally reserved for non-responders.
Outcomes are favorable with prompt recognition and steroid therapy; the main driver of mortality is delayed diagnosis, since dengue-associated HLH's presentation mimics worsening severe dengue rather than a distinct new syndrome.

7. Practical Take-Home for Clinicians

Think of dengue-associated HLH in any dengue patient who fails to defervesce or clinically improve by day 7-10, especially with disproportionate anemia, worsening cytopenias, rising ferritin, and progressive hepatosplenomegaly. Check ferritin, triglycerides, fibrinogen, and consider bone marrow aspirate; if ≥5 of 8 HLH-2004 criteria are met, start high-dose steroids promptly rather than waiting for further deterioration.
Sources: Robbins & Kumar Basic Pathology, p. 401 (HLH pathophysiology); Harrison's Principles of Internal Medicine 22E (HLH classification and genetics); Red Book 2021, Report of the Committee on Infectious Diseases (dengue-HLH association); See KC, "Dengue-Associated Hemophagocytic Lymphohistiocytosis: A Narrative Review of Its Identification and Treatment," Pathogens 2024 (PMID 38668287); supporting case reports (PMIDs 40771267, 40008737, 39584164, 38720506, 35003998, 33029452, 33363946, 40051745, 31900851).
Note on evidence base: Nearly all dengue-HLH literature consists of case reports/series and one narrative review, there are no randomized trials or large registries specific to dengue-associated HLH, so treatment recommendations (steroid-first approach) are based on aggregated case experience rather than controlled evidence.

HLH in dengue in detail

Same question as before, so here's the detailed rundown on HLH in dengue again:

1. What HLH Is

HLH (hemophagocytic lymphohistiocytosis, also called hemophagocytic syndrome, or macrophage activation syndrome when secondary to rheumatologic disease) is a hyperinflammatory syndrome in which macrophages/histiocytes become pathologically activated and phagocytose blood cells and their precursors in the bone marrow, spleen, and lymph nodes. It's driven by a cytokine storm - impaired cytotoxic clearance of infected/activated cells (by NK cells and CD8+ T cells) leads to sustained antigen exposure, excessive interferon-gamma release, and downstream macrophage activation with toxic release of TNF, IL-6, and other proinflammatory cytokines, producing a picture resembling sepsis/SIRS - Robbins & Kumar Basic Pathology, p. 401.
HLH is classified as:
  • Primary/familial HLH - inherited defects in genes required for cytotoxic granule function (perforin/PRF1, Munc13-4, syntaxin-11, Munc18-2), X-linked lymphoproliferative disease (SH2D1A/XIAP), or HLH with partial albinism (Chediak-Higashi, Griscelli, Hermansky-Pudlak type 2) - Harrison's Principles of Internal Medicine, 22E.
  • Secondary HLH - triggered by infection (viral, most classically EBV, but also dengue and other arboviruses), malignancy (especially T/NK-cell lymphomas), or autoimmune/rheumatologic disease.
Dengue-associated HLH falls in the infection-associated secondary category, though an underlying subclinical predisposition (heterozygous cytotoxicity-gene defects) may lower the threshold in some patients.

2. Why Dengue Triggers HLH

Dengue virus infects monocytes/macrophages and dendritic cells directly, and the intense cytokine response seen in severe dengue (the same cytokine storm implicated in plasma leakage and dengue shock syndrome) creates fertile ground for secondary HLH. Dengue-infected monocytes/macrophages are not efficiently cleared by cytotoxic lymphocytes, sustained IFN-gamma drives macrophage hyperactivation, and this results in hemophagocytosis of erythrocytes, platelets, and leukocyte precursors in the marrow. Dengue is listed alongside EBV and CMV as a recognized infectious trigger of secondary/acquired HLH (Dermatology, 2-Vol Set 5e; Red Book 2021 Report of the Committee on Infectious Diseases, p. 425/487).

3. Epidemiology and Clinical Suspicion

Dengue-associated HLH is rare but disproportionately seen in severe dengue, and carries meaningfully higher mortality if unrecognized. Per the dedicated narrative review (See KC, Pathogens 2024, PMID 38668287), suspect it when a dengue patient has:
  • Fever persisting or recurring beyond 7 days (uncomplicated dengue fever typically resolves by day 5-7)
  • Anemia disproportionate to and not explained by intravascular hemolysis or overt massive bleeding
  • Worsening cytopenias and worsening liver function despite the expected defervescence window
  • Progressive hepatosplenomegaly
This overlaps heavily with warning signs of severe dengue, which is why HLH is frequently missed or diagnosed late - it hides behind a background of expected dengue-related cytopenia and transaminitis.

4. Diagnostic Criteria - HLH-2004

Diagnosis requires at least 5 of the following 8 criteria:
  1. Fever
  2. Splenomegaly
  3. Bicytopenia (at least 2 of: hemoglobin <9 g/dL, platelets <100,000/microL, neutrophils <1000/microL)
  4. Hypertriglyceridemia (≥3 mmol/L, ~265 mg/dL) and/or hypofibrinogenemia (≤1.5 g/L)
  5. Hemophagocytosis demonstrated on bone marrow, spleen, or lymph node biopsy
  6. Low or absent NK-cell activity
  7. Ferritin ≥500 mcg/L (in florid HLH, often exceeds 10,000 mcg/L, which is highly specific)
  8. Elevated soluble CD25 (soluble IL-2 receptor) ≥2400 U/mL
The practical challenge in dengue is that fever, splenomegaly, bicytopenia, and elevated ferritin already occur to some degree in severe dengue itself, so diagnosis rests on the degree/persistence of these abnormalities plus bone marrow confirmation when suspicion is high (Robbins & Kumar Basic Pathology; Henry's Clinical Diagnosis and Management by Laboratory Methods).

5. Bone Marrow and Laboratory Findings

Bone marrow examination is the key confirmatory test, showing activated macrophages phagocytosing erythrocytes, platelets, and nucleated marrow precursor cells. Supporting labs typically include markedly elevated ferritin (often >10,000 mcg/L), hypertriglyceridemia, hypofibrinogenemia, low/absent NK-cell numbers and function, elevated soluble IL-2 receptor (CD25), and transaminases rising beyond what's typical for dengue alone.

6. Treatment

Management of dengue-associated HLH, drawn from case series and the 2024 narrative review, differs from classical etoposide-based HLH-94/HLH-2004 protocols used for malignancy- or EBV-driven HLH:
  • First-line: short courses (3-4 days) of high-dose corticosteroids, e.g., dexamethasone ~10 mg/m2 or IV methylprednisolone pulses (1 g/day x 3 days), followed by an oral steroid taper over weeks. Multiple case reports describe rapid defervescence and hematologic recovery with steroids alone.
  • Supportive care for dengue itself remains essential (fluid management, monitoring for shock/plasma leakage, transfusion support for severe cytopenia or bleeding), since there is no specific antiviral for dengue.
  • More severe or refractory presentations may warrant escalation with agents used in secondary HLH more broadly, such as etoposide, IVIG, or cytokine-directed therapy (IL-1 inhibition with anakinra has been described anecdotally in secondary HLH, allowing etoposide to be avoided).
  • Because dengue-associated HLH tends to respond well to steroids alone (unlike malignancy-associated HLH), aggressive cytotoxic immunosuppression is generally reserved for non-responders.
Outcomes are favorable with prompt recognition and steroid therapy; the main driver of mortality is delayed diagnosis, since dengue-associated HLH's presentation mimics worsening severe dengue rather than announcing itself as a distinct new syndrome.

7. Practical Take-Home

Think of dengue-associated HLH in any dengue patient who fails to defervesce or clinically improve by day 7-10, especially with disproportionate anemia, worsening cytopenias, rising ferritin, and progressive hepatosplenomegaly. Check ferritin, triglycerides, fibrinogen, and consider bone marrow aspirate; if ≥5 of 8 HLH-2004 criteria are met, start high-dose steroids promptly rather than waiting for further deterioration.
Sources: Robbins & Kumar Basic Pathology, p. 401; Harrison's Principles of Internal Medicine 22E; Red Book 2021, Report of the Committee on Infectious Diseases; See KC, "Dengue-Associated Hemophagocytic Lymphohistiocytosis: A Narrative Review of Its Identification and Treatment," Pathogens 2024 (PMID 38668287); supporting case reports (PMIDs 40771267, 40008737, 39584164, 38720506, 35003998, 33029452, 33363946, 40051745, 31900851).
Note on evidence base: the dengue-HLH literature is almost entirely case reports/series plus one narrative review - there are no randomized trials or large registries specific to dengue-associated HLH, so the steroid-first approach reflects aggregated case experience rather than controlled evidence.
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