Dengue-Associated Hemophagocytic Lymphohistiocytosis: A Narrative Review of Its Identification and Treatment

Dengue’s lack of specific treatments beyond supportive care prompts a focus on uncovering additional pathophysiological factors. Dengue-associated hemophagocytic lymphohistiocytosis (HLH), characterized by dysregulated macrophage activation and cytokine storm, remains underexplored despite its potential to worsen disease severity and mortality. While rare, dengue-associated HLH disproportionately affects severe cases, significantly impacting mortality rates. To mitigate high mortality, early identification and familiarity with dengue-associated HLH are imperative for prompt treatment by clinicians. This narrative review therefore aims to examine the current clinical and therapeutic knowledge on dengue-associated HLH, and act as a resource for clinicians to improve their management of HLH associated with severe dengue. Dengue-associated HLH should be considered for all cases of severe dengue and may be suspected based on the presence of prolonged or recurrent fever for >7 days, or anemia without intravascular hemolysis or massive bleeding. Diagnosis relies on fulfilling at least five of the eight HLH-2004 criteria. Treatment predominantly involves short courses (3–4 days) of high-dose steroids (e.g., dexamethasone 10 mg/m2), with additional therapies considered in more severe presentations. Notably, outcomes can be favorable with steroid therapy alone.


Introduction
Dengue is one of the most important vector-borne diseases globally, affecting large geographical regions and contributing to symptomatic disease in 60-100 million new cases annually [1,2].Each year, the estimated global impact of dengue includes 10,000 deaths, half a million years of life lost to premature mortality, half of million years lived in disability, a million disability-adjusted life-years, and costs of USD 8.9 billion [1][2][3].
Mosquito control, the use of personal protection, and vaccination are means for preventing dengue infection.Three tetravalent vaccines have demonstrated efficacy against dengue infection in Phase III clinical trials: the three-dose CYD-TDV (Dengvaxia ® , Sanofi S.A. Paris, France) [4,5], the two-dose TAK-003 (Qdenga ® , Takeda Pharmaceutical Company Limited, Tokyo, Japan) [6,7], and the one-dose Butantan-DV (Instituto Butantan, São Paulo, Brazil) [8].Of note, CYD-TDV paradoxically increased the risk of severe disease in seronegative recipients through unknown immunopathologic mechanisms, and thus should only be given to persons with at least one previous dengue infection [9].
Once an individual is infected, the overall management of dengue is supportive [10].Capillary leak and hypotension are corrected with fluid therapy, bleeding with blood product support, cardiomyopathy with inotropic agents to increase cardiac function, and respiratory failure with mechanical ventilation.Although good supportive care may mitigate the poor outcomes of dengue, the lack of any specific treatment limits the potential for further improvement of health outcomes.Efforts directed at uncovering and treating the underlying pathophysiology of severe dengue are therefore urgently needed.
As severe dengue is marked by increased plasma leakage, much research has aimed to preserve capillary permeability [11].Nonetheless, another aspect of severe dengue-that of dengue-associated hemophagocytic lymphohistiocytosis (HLH)-may be overlooked, and it is not mentioned in the World Health Organization dengue management guidelines in 2009 [10].
HLH, in general, is a rare diagnosis that may be associated with a variety of infectious triggers (especially Epstein-Barr virus [12], dengue virus [13], and scrub typhus [14]), autoimmune disorders [15][16][17], malignancy [18][19][20], and familial genetic mutations [21].It is predominantly found in children, with an incidence of about 1 in 100,000 hospitalized children [22].Adult cases are increasingly recognized [23,24].As a subset of HLH, dengueassociated HLH is even more uncommon among all dengue patients, occurring with an incidence of 0.44 cases per 100,000 children per year [25].Nonetheless, dengue-associated HLH is found more commonly in severe dengue patients; it was observed in 15 (13.4%) of 112 children with severe dengue [26] and in 5 (38.5%) of 13 non-survivors in a study of 114 patients admitted to an adult intensive care unit [27].Unlike severe dengue, which has no known specific treatment [28], HLH in general may benefit from the prompt application of specific therapies to destroy immune cells and suppress inflammation (e.g., with dexamethasone and possibly HLH-specific chemotherapy [24,29]), which may then mitigate unfavorable outcomes like further organ dysfunction and mortality [26,[30][31][32].
Case fatality rates for dengue-associated HLH range from 4.5% when active surveillance was conducted [25], to 14.6% in a systematic review of published reports [32], and to 43% in an adult intensive care unit series [33].To mitigate high mortality, early identification and familiarity with dengue-associated HLH are imperative for prompt treatment by clinicians.This paper therefore aims to review the current clinical and therapeutic knowledge on dengue-associated HLH, and act as a resource to allow clinicians to improve their management of HLH associated with severe dengue.

Identification and Treatment of Dengue-Associated HLH in Adults and Children >12 Years of Age
Table 1 contains case reports or series of 39 non-pregnant patients with dengue-associated HLH.Persistent or recurrent fever over a duration exceeding 7 days was the main presenting feature for most patients.Few patients had comorbid medical conditions; these included diabetes mellitus [44][45][46], hypertension [47], and obesity [48].Patients were diagnosed using the criteria outlined by the HLH-2004 study [41], with 33 cases (84.6%) reporting hemophagocytosis on bone marrow aspirate (1 case was determined postmortem [49]), 1 case reported a false negative finding [50]), and no cases mentioned any procedural complications or difficulties.

Identification and Treatment of Dengue-Associated HLH in Adults and Children > 12 Years of Age
Table 1 contains case reports or series of 39 non-pregnant patients with dengueassociated HLH.Persistent or recurrent fever over a duration exceeding 7 days was the main presenting feature for most patients.Few patients had comorbid medical conditions; these included diabetes mellitus [44][45][46], hypertension [47], and obesity [48].Patients were diagnosed using the criteria outlined by the HLH-2004 study [41], with 33 cases (84.6%) reporting hemophagocytosis on bone marrow aspirate (1 case was determined postmortem [49]), 1 case reported a false negative finding [50]), and no cases mentioned any procedural complications or difficulties.
Treatment of dengue-associated HLH was usually with short courses (about 3-4 days) of high-dose intravenous steroids, e.g., dexamethasone 10 mg/m 2 daily [51], with some patients tapering steroids over 2-8 weeks [40,50,[52][53][54][55].One case initiated and continued dexamethasone orally for a total of 26 days [56].Other agents like intravenous immunoglobulin [57][58][59] or etoposide [60,61] were only occasionally used, which departs from the HLH-2004 study treatment protocol.In some reports, good clinical outcomes were achieved by using IV immunoglobulin alone, without steroids [57].In addition, no other chemotherapy or hematopoietic stem cell transplantation was reported.With steroids alone, no study reported an increased risk of secondary bacterial or fungal infection.
One case involving a pregnant patient with dengue-associated HLH has been reported [62].The patient was a 28-year-old woman at 38 weeks gestation and had gestational diabetes, controlled by the diet [62].Dengue-associated HLH was identified based on fever for three days, persistent anemia despite blood transfusion, thrombocytopenia, hepatosplenomegaly from day six with hepatitis, high lactate dehydrogenase (759 IU/L), high ferritin (6930 mcg/L), high triglyceride (4.3 mmol/L), and hemophagocytosis on bone marrow aspirate.The main treatment administered was intravenous methylprednisolone 1 g for three days, followed by oral prednisolone 1 mg/kg for two weeks, and steroid taper over six weeks.Despite severe postpartum hemorrhage requiring massive transfusion, the patient made a complete recovery.Based on this case, pregnancy status does not appear to confer any substantial differences in diagnosis or treatment of dengue-associated HLH, although there may be an increased bleeding risk.

Identification and Treatment of Dengue-Associated HLH in Young Children up to 12 Years of Age
Table 2 contains case reports or series of 97 young children with dengue-associated HLH.The clinical presentation of young children resembled that of adults.Persistent or recurrent fever over a duration exceeding 7 days was the main presenting feature for most patients.Comorbid conditions were few, and included diabetes, obesity, asthma, and congenital heart disease [25,48].Like adults, young children were diagnosed using the criteria outlined by the HLH-2004 study [41].However, unlike adults, about two-thirds of these cases (63 patients) did not report the performance of bone marrow aspiration.
Treatment of dengue-associated HLH in young children appeared to involve either steroids alone [48,70,71], immunoglobulin alone [72], or a combination of steroids and immunoglobulin [67].Other agents like etoposide [73] were only occasionally used.No other chemotherapy or hematopoietic stem cell transplantation was reported.With steroids alone, or with combination steroid and immunoglobulin therapy, only one study suggested an increased risk of secondary bacterial infection [74].

Challenges in the Diagnosis and Treatment of Dengue-Associated HLH
Dengue-associated HLH is often associated with severe dengue, which is dengue infection with any of the following: severe plasma leakage (leading to shock, fluid accumulation with respiratory distress), severe bleeding, severe organ involvement (aspartate aminotransferase or alanine aminotransferase ≥1000 U/L, impaired consciousness, or other organ failure) [10].The clinical features of severe dengue can mask the identification of HLH, which means that clinicians need to maintain a high index of suspicion and actively look for the presence of any of the eight HLH-2004 criteria.Fortunately, assessment of most of these criteria should be available in routine clinical practice.Some other clinical features can help distinguish severe dengue from dengue-associated HLH, as these are more commonly found in the latter: (1) prolonged or recurrent fever of >7 days [32,50] (consider also co-infection, which occurred in 10.4% of 122 cases [32]), and (2) anemia without intravascular hemolysis or massive bleeding [32,75].
Bone marrow examination is not necessary when enough criteria have been fulfilled [32], although bone marrow aspiration seems to be safe in many reports involving adults and older children with dengue-associated HLH, consistent with the low 0.08% adverse event rate found in an audit performed by the British Society for Haematology across 63 hospitals [70].Additionally, when it is available, specialized testing for elevated interleukin-2 receptor (i.e., CD25) can help distinguish dengue-associated HLH from severe dengue alone [71].
For the treatment of dengue-associated HLH, no randomized trials are available, which makes treatment selection both challenging and uncertain.Nonetheless, published experience from contemporary cases mainly relies on supportive care with or without the use of short courses (3-4 days) of high-dose steroids [51].Although some patients had a favorable outcome without steroids [45,64], the high risk of mortality and the relatively low risk of short courses of steroids argue for using steroids to treat dengue-associated HLH.Another reason to support the use of short courses of high-dose steroids is that there is no apparent way of predicting the self-limiting trajectory of HLH with supportive management alone.
Specific steroid treatment could be IV methylprednisolone or dexamethasone alone, without the need for IV immunoglobulin or chemotherapy.Dexamethasone may be preferred when HLH involves the central nervous system, as it has superior penetration across the blood-brain barrier compared to prednisolone [78].Dexamethasone may even be initiated orally [56], given its good bioavailability of 81% [79].Similarly, prednisone and prednisolone have good oral bioavailability of >80% [59], although their upfront use in dengue-associated HLH has not been reported.Some authors have suggested combined therapy of steroids with immunoglobulin [59], an example being methylprednisolone 1 g/day for 3-5 days and immunoglobulin 1 g/kg/day for 2 days [80].In addition, clinicians can consider IV immunoglobulin [58] and etoposide as rescue therapies, and use IV immunoglobulin as an alternative in case steroids cannot be given [57,81].Dengue-associated HLH should not require chemotherapy or hematopoietic stem cell transplantation.Finally, for persistently or recurrently febrile patients, it remains important to consider and promptly treat bacterial co-infection, which occurred in 33% of dengue-associated HLH cases [26].

Future Directions
Future research is required to uncover the prognostic factors in dengue-associated HLH per se, and to identify individuals who may continue to benefit from supportive treatment alone (i.e., without HLH-specific treatment like steroids).In addition, while there are numerous case reports and series, the epidemiology of dengue-associated HLH remains unclear.Given the relatively rarity of dengue-associated HLH, especially for important special populations like pregnant women, studying its epidemiology would likely require a global registry.
Additionally, the treatment of dengue-associated HLH appears to be less intensive than for other forms of HLH (e.g., familial HLH), but optimal specific treatment remains unknown.While conventional randomized clinical trials of multiple treatment options would be difficult to achieve for a rare condition, an adaptive trial design may be used to efficiently study the effects of treatment with fewer patients and shorter clinical investigation times [82].In the case of dengue-associated HLH, such an adaptive trial design may be applied in the comparison of supportive treatment alone, steroids, intravenous immunoglobulin, and chemotherapy, with regards to improving organ dysfunction and mortality.
Novel agents that have been used in non-dengue-associated HLH could be further studied in the context of dengue-associated HLH.Possible medications include emapalumab, a human anti-interferon-gamma antibody [83], tocilizumab, a monoclonal antibody targeting the interleukin-6 receptor [84], and anakinra, a recombinant interleukin-1 receptor antagonist [85], although these may increase the risk of secondary infection.Another promising agent is ruxolitinib [86], an orally administered Janus kinase inhibitor that has been shown to selectively dampen the cytokine storm in primary HLH while preserving the immune response against infection.

Conclusions
Dengue, a significant global health concern, lacks specific treatments beyond supportive care, leaving room for improved outcomes.While efforts have primarily focused on managing capillary permeability and bleeding, the presence of dengue-associated HLH demands attention.Characterized by dysregulated macrophage activation and cytokine storm akin to sepsis, dengue-associated HLH presents challenges in diagnosis and management.Despite its rarity, dengue-associated HLH significantly elevates mortality rates in severe dengue cases.Diagnosis relies on fulfilling HLH-2004 criteria, while treatment typically involves short courses of high-dose intravenous steroids.Notably, outcomes can be favorable with steroid therapy alone.Future research should focus on prognostic factors and treatment optimization, potentially through adaptive trial designs.Enhanced understanding and management of dengue-associated HLH could significantly improve clinical outcomes and inform global dengue management strategies.

Pathogens 2024 , 15 Figure 1 .
Figure 1.Pathophysiology of dengue-associated hemophagocytic lymphohistiocytosis.Dengue virus infection triggers the activation of cytotoxic T-cells, leading to cytokine release, macrophage activation, and subsequent dysfunction of vital organs.Images from Science Figures (open license) (accessed from https://sciencefigures.org on 30 March 2024).Figure constructed based on information from Hines et al. [35] and Keenan et al. [36].

Figure 1 .
Figure 1.Pathophysiology of dengue-associated hemophagocytic lymphohistiocytosis.Dengue virus infection triggers the activation of cytotoxic T-cells, leading to cytokine release, macrophage activation, and subsequent dysfunction of vital organs.Images from Science Figures (open license) (accessed from https://sciencefigures.org on 30 March 2024).Figure constructed based on information from Hines et al. [35] and Keenan et al. [36].

Figure 1 .
Figure 1.Pathophysiology of dengue-associated hemophagocytic lymphohistiocytosis.Dengue virus infection triggers the activation of cytotoxic T-cells, leading to cytokine release, macrophage activation, and subsequent dysfunction of vital organs.Images from Science Figures (open license) (accessed from https://sciencefigures.org on 30 March 2024).Figure constructed based on information from Hines et al. [35] and Keenan et al. [36].

Figure 2 .
Figure 2. Bone marrow smear with hemophagocytosis.The large cell in the center of the image is a macrophage that has phagocytosed several red blood cells and platelets.Image by Petter Quist-Paulsen (license CC BY SA 3.0) (accessed from https://sml.snl.no/hemofagocytisk_lymfohistio-cytose_-_HLH on 30 March 2024).

Figure 2 .
Figure 2. Bone marrow smear with hemophagocytosis.The large cell in the center of the image is a macrophage that has phagocytosed several red blood cells and platelets.Image by Petter Quist-Paulsen (license CC BY SA 3.0) (accessed from https://sml.snl.no/hemofagocytisk_lymfohistiocytose_-_HLH on 30 March 2024).

Table 1 .
Selected reports of dengue-associated HLH in non-pregnant adults and children > 12 years old.

Table 2 .
Selected reports of dengue-associated HLH in young children up to 12 years of age.

Table 3 .
Clinical management of dengue-associated HLH.
* Diagnosis of the HLH requires 5 of the following 8 criteria as outlined by the HLH-2004 study: (1) fever,