# Dengue and Flavivirus Co-Infections: Challenges in Diagnosis, Treatment, and Disease Management
## 3 Co-Infection: Occurrence and Clinical Relevance  
In regions where multiple DENV serotypes are concurrently transmitted, clinical cases caused by more than one serotype may be more prevalent than previously recognized [10]. In endemic or hyperendemic areas, it offers greater opportunities for mosquitoes to acquire two or more DENV serotypes simultaneously [11]. This is partly explained by vector competence, which is shaped by a complex interplay of biological and ecological factors, including viral interactions within the vector, mosquito immune responses, and feeding behavior [12]. In a co-infection, the viruses may interact synergistically or competitively in the mosquito's midgut, affecting the virus's capacity to replicate, disseminate, and eventually spread [13]. Simultaneous infections by multiple serotypes are expected to severely impact the clinical manifestations of dengue. When two DENV serotypes coexist within an individual, they might elevate the overall DENV viremia level, infect high numbers of leukocytes, and potentially induce substantial cytotoxicity due to the culminating cytokine storm [14], ultimately leading to severe symptoms and disease emergence [15]. This was evident in previously described dengue co-infections, where a strong correlation with severe clinical disease, including dengue hemorrhagic fever (DHF) and dengue shock syndrome (DSS), was documented, possibly due to markedly heightened immune response and  
Figure 1: PRISMA flowchart of literature search strategy. increased viremia [16; 17]. Although some studies reported no significant differences in disease severity between mono- and co-infections, there is still a lack of empirical evidence delineating DENV-DENV interactions and immune dysregulation in co-infections, therefore, warranting further rigorous investigations [18; 19]. Despite the menacing health effects of dengue co-infections, the exact underlying mechanism remains uncertain. The complexity of flaviviral co-infections becomes more intricate when it involves different flaviviruses, such as DENV, (ZIKV), WNV, YFV, Saint Louis encephalitis virus (SLEV), and Japanese encephalitis virus (JEV). Among these, co-infections of DENV and ZIKV are particularly noteworthy and frequently reported in prevalent regions, including New Caledonia in 2014 [20], Brazil in 2015, and Colombia in 2016 [21]. The occurrence is primarily due to the overlapping dengue and Zika endemicities. [PERSON] et al. [22] deduced that regions experiencing DENV epidemics are at high risk for ZIKV outbreaks, suggesting a close correlation between the two flaviviruses. The correlation is highly attributable to their shared vectors and similar environmental factors that support their spread and transmission. For example, _Aedes aegypti_, the main mosquito species responsible for transmitting both DENV and ZIKV, thrives in urban environments with high population density and poor sanitation in tropical regions [23]. A more pressing health concern is the high degree of nucleotide similarity (approximately 60%) between ZIKV and DENV. Increasing clinical and experimental evidence points to a substantial level of antibody cross-reactivity between the two viruses. Specifically, the cross-reactivity can lead to antibody-dependent enhancement (ADE), where antibodies developed against dengue can potentially enhance the adversity of Zika. As a result, it further complicates the disease management and public health response [24; 25; 26]. Apart from becoming a public health burden, ADE is a significant concern in dengue and Zika vaccine development [27]. ADE arises when antibodies produced from a previous infection or vaccination, characterized as either non-neutralizing or suboptimal, aid in the entry of the virus into host cells, leading to increased viral replication and worsening the disease severity [28]. Therefore, a feasible dengue or Zika vaccine must elicit protective immunity to individuals without compromising the risk of severe diseases upon subsequent infections [29].