Emerging and re-emerging arbovirus infections pose a serious public health threat in tropical and subtropical regions worldwide [2, 3]. In this study, we observed that half of the study population was seropositive for dengue virus, however, a major proportion of our study population was susceptible to endemic and emerging viruses. The presence of a large and susceptible population, and the abundant presence of mosquito vector populations are important factors that contributed to the recent large outbreaks of ZIKV and CHIKV reported in Brazil and other Latin American countries, upon the introduction of the viral diseases in the continent. Also, a lack of adequate molecular diagnostic tools, and the antibody cross-reactivity between these endemic viruses, contributed to their spread and delayed diagnosis.
All four dengue serotypes are endemic in Brazil, and at least two of the four serotypes circulate in most of the Brazilian states [7, 9, 26, 27]. For example, in Manaus, Amazonas state, all four DENV serotypes have been detected . Likewise, seroprevalence have been shown to be in the range of 15 to 80%, depending on the population and region; in addition, the prevalence of DENV infection has been shown to increase with age [5, 7, 9, 19, 29]. We observed a maximum seroprevalence of 58.4% for DENV-2 among our cohort. Also, HIA titres for DENV-1 and 2 antibodies were higher compared to DENV-3 and 4, which may be partially explained by the late introduction of these viruses in Brazil [7, 26]. However, we did not observe any correlation between age and seroprevalence frequencies as observed in other studies. Military personnel are dynamic populations, and they move constantly from one region to another for duty; this factor could explain the discrepancy observed between the lack of correlation between age and seroprevalence in our cohort as compared with the general population that resides in the same location over a period of time. Our data suggest that an important factor in the development of the overwhelming epidemic of ZIKV and CHIKV in Brazil could be attributed to the low level or absence of pre-existing antibody responses, combined with the ample presence of Aedes aegypti in the urban setting [11, 29, 30, 31].
More individuals had antibodies reactive against the Yellow fever vaccine strain 17D compared to the wildtype strain. Moreover, the decreasing reactivity toward wild type strain may be an indicator of gradual decrease of antibody titres overtime after vaccination. Recently, demonstrated a need for booster doses at 10 years after primary vaccination in travellers to endemic region, endemic population, and high-risk individuals like military personnel to heighten the 17DD-YF specific immune response and to achieve efficient immunity.
The presence of heterologous neutralizing antibody titres are inversely correlated with the severity of patients with a second DENV infection [8, 32, 33, 34]. Additionally, over 30% of the individuals had antibodies against major endemic and emerging Flaviviruses tested in this study. We cannot rule out multiple flavivirus infections in the same individual. These findings, along with other reports, indicate that SLEV, WNV, ILHV and ROCV circulation in Brazil is largely unknown, and there may be epidemiological implications of the co-circulation of these arboviruses [16, 19]. Overall, in vivo or cohort studies are needed to ascertain role of multiple Flavivirus infection in cross-protection or induction of a severe disease [8, 13, 15, 35]. Regarding their role in disease and protection, low avidity antibodies against DENV have been shown to participate in severe disease, also poorly-neutralizing antibodies can participate in antibody-dependent enhancement (ADE) in DENV infections [36, 37, 38]. Recent studies have demonstrated a role of DENV antibodies in causing ADE during ZIKV infection [39, 40]. On the other hand, several studies have also demonstrated a lack of ADE like cytokine storm and partial protective role of these flavivirus cross-reactive antibodies upon ZIKV infection [41, 42]. Nevertheless, comprehensive in vivo studies are necessary to ascertain role of these cross-reactive antibodies in ADE during ZIKV and other Flavivirus infections.
MAYV is endemic in the Amazon region and there have been imported cases in other regions of Brazil [21, 43]. A prevalence of more than 40% for MAYV has been described in some Amazonian communities . However, we did not observe any positive samples for the Alphavirus tested in our study. Similarly, we observed very low prevalence to OROV belonging to Peribunyaviridae. OROV is still localized in the Amazon region and is responsible for causing neurological disease in urban and rural areas .
One of the limitations of the present study is that it was not feasible to perform a neutralization assay to distinguish between dengue serotypes and confirm or rule out multiple infections or cross-reactivity. Although, most study participants were young male adults, these results are in accordance with previous studies on DENV prevalence [9, 29]. Furthermore, given the size and geographical differences in Brazil, the estimates from one region or state cannot be used to understand epidemiology from the whole of Brazil. On the other hand, soldiers are a high-risk group because of the activities they are involved and their contact with endemic regions such as the Amazon rainforest. A very small proportion of the study population reported previous malaria infection, which suggests that most of the individuals lived in urban regions or have spent little time in the rural Amazon region, where 99.9% of the malaria cases are described . This might explain the low level or absence of antibodies against arboviruses described in the Amazonian region, such as OROPV and MAYV (Sakkas et al. 2018; Esposito & Fonseca 2017). Moreover, only 17% of the individuals tested in this study self-reported dengue infection, which was three times lower than that observed by serology. These unreported dengue infections could be asymptomatic subclinical infections or self-limiting fever without diagnosis or clinically not diagnosed as dengue. Our cluster analysis suggests that ~ 60% of our cohort comprehended a low Flavivirus-positive population and therefore susceptible to infection. Theoretically, susceptible military personnel could act as disease-spreading agents when returning to civil life when the combination of a susceptible population and specific vectors are present. More detailed serological surveys together with vector population assessment and viral detection strategies are needed to further characterize the extent of favourable factors that can contribute to future outbreaks, and to forecast potential public health needs.
Overall, mosquito control measures and integrated vector management are essential for control of all arboviruses, and were effective in controlling ZIKV and CHIKV outbreaks in Brazil and worldwide [46, 47, 48]. However, vector control precedes the decrease in herd immunity and the increase in the availability of susceptible populations [11, 32]. Adaptions of these emerging viruses to urban vectors like Aedes aegypti or Culex quinquefasciatus and the decreasing herd immunity to them might facilitate further epidemics of endemic and emerging arboviruses [2, 3]. Currently, differential clinical diagnosis is a major challenge when multiple viruses that cause similar clinical symptoms co-circulate [13, 14]; moreover, the lack of adequate diagnostics tools can limit early identification and efforts to block outbreaks [11, 49]. Warmer weather conditions brought on by the El Niño phenomenon and the destruction of the Amazon native forest can encourage a faster breeding and maturation cycles for Aedes and Anopheles mosquito populations . Hence, an improved vector and febrile syndrome surveillance, along with basic understanding of arboviral-disease pathogenesis mechanisms are essential to prevent future outbreaks and can contribute to better control strategies.