Introduction
Dengue infections are one of the most rapidly emerging mosquito-borne viral infections in the world. It was estimated that 18% of the 58.4 million symptomatic cases of dengue were admitted to hospital in 2013, which is a staggering 10.5 million patients(1). Approximately 26% to 33.6% of hospitalized patients have shown to develop dengue haemorrhagic fever (DHF), while 0.6% to 1.5% require admission to the intensive care units(2-4). As 70% of dengue infections occur in Asia, dengue causes a huge burden to resource-poor economies in developing countries(5). Although the recent case fatality rates in many countries have declined to <0.5% due to intense monitoring and meticulous fluid management, the case fatality rates are still around 2.6% in some countries such as India(6). Therefore, there is an urgent need to develop biomarkers which can identify patients who will develop severe disease and effective drugs to treat dengue.
Endothelial dysfunction leading to vascular leak in the hallmark of DHF(7). Plasma leakage leads to accumulation of fluid in pleural and peritoneal cavities, and when severe, it causes reduction of blood pressure leading in poor organ perfusion resulting in shock and organ dysfunction. Many inflammatory cytokines, lipid mediators and secretory virus protein NS1 has been shown to contribute to the vascular leakage(8-11). The dengue virus (DENV) has been shown to activate mast cells, and the vascular leak in dengue mouse models has been shown to be mast cell dependent(12). Mast cell stabilizing drugs such as cromolyn and ketotifen and the leukotriene inhibitor montelukast, significantly reduced the vascular leak in dengue mouse models, implying their role in dengue pathogenesis.
We have previously shown that many inflammatory lipid mediators such as platelet activating factor (PAF) and secretory phospholipase A2 levels were higher in patients with DHF, and that endothelial dysfunction induced by dengue sera was inhibited by PAF-R blockers, in vitro, which also showed a tend towards a reduction in vascular leakage in patients with acute dengue(2, 9, 13). Leukotrienes are another group of inflammatory lipid mediators that are produced by the action of lipoxygenase enzymes on arachidonic acid substrates, and include LTB4, LTC4, LTD4 and LTE4 (14). These mediators are potent chemoattractants to neutrophils and eosinophils, and some are also known to increase vascular permeability and induce vascular leak (14, 15). LTE4 specifically has been shown to induce vascular leak, specifically by contracting the endothelial cells in the post-capillary venules(16). Serum leukotriene B4 levels were shown to be elevated in a small cohort of patients with acute dengue infection, when compared to healthy individuals and other febrile illnesses and correlated with inflammatory markers such as highly sensitive CRP(17). In vitro experiments have also shown that the DENV induced LTB4 from neutrophils, suggesting that there could be multiple sources of leukotrienes in acute dengue infection. However, if leukotrienes correlate with clinical disease severity, viraemia and serostatus and the relationship with the onset of vascular leak in acute dengue has not been investigated.
Currently all individuals who have warning signs of possible severe disease such as persistent vomiting, abdominal pain, leucopenia along with thromobocytopenia are admitted to the hospital(18). However, only 25% to 34% of such patients would develop complications such as DHF and organ dysfunction(2, 4, 19). The onset of plasma leakage usually occurs during 3 to 6 days and lasts for 24 to 48 hours, which is known as the critical phase(7, 18). Those who do not develop vascular leak have a self-limiting illness known as dengue fever, and usually recover without any intervention, except if accompanied by bleeding manifestations. However, due to the absence of a reliable biomarker, all patients who are admitted should be closely monitored, several times a day, which is a huge burden in overcrowded hospital facilities with limited staff. Several potential biomarkers have been identified such as serum chymase, serotonin, VEGF and tryptase levels(20-23). However, it would be useful to evaluate a urinary biomarker to predict the development of DHF during early illness as it would user friendly.
Urinary LTE4 has been shown to be a stable metabolite in urine and spot urine LTE4 levels have been evaluated as a diagnostic marker of atopic asthma in pre-school children.(24) In this study we evaluated the levels of urinary LTE4 and histamine in a large cohort of patients with varying severity of acute infection during early illness, their kinetics throughout the illness and also diurnal variations. We investigated the relationship between the urinary LTE4 levels, with the onset of vascular leakage, viral loads and serostatus.