Discussion
This systematic review summarises the existing literature on ZIKV coinfections. Specifically, it describes the estimated frequencies of reported ZIKV coinfections and their clinical spectrum. The search identified 34 studies conducted between 2014 and 2019, which reported 213 cases of ZIKV coinfection with 10 different pathogens. ZIKV coinfections were detected across 10 countries, primarily in Latin America. CHIKV and DENV were the predominantly reported ZIKV coinfecting agents and the only ZIKV coinfections for which population frequencies were described. ZIKV coinfection frequencies among ZIKV-infected cases varied significantly between location and population type. The vast majority of ZIKV-coinfected cases were reported to present with the signs and symptoms described for uncomplicated ZIKV monoinfections and defined by the WHO.1 However, complications were reported to arise in 9% of ZIKV-coinfected cases in cohort and cross-sectional studies.
This is the first systematic review to study how frequently individuals with ZIKV infection have a coexisting infection of any kind. The variation in frequencies reported for ZIKV/arbovirus coinfections among the ZIKV-infected individuals reported in this study was likely influenced by differences in study design and the selected study population. Factors, such as study location, season and study period in relation to the ZIKV outbreak, will have additionally influenced ZIKV coinfection frequency estimates. As expected, ZIKV coinfections were relatively more common in studies conducted during concurrent arbovirus outbreaks.14 44 These differences in study design, timing and location make it difficult to generalise ZIKV coinfection frequency estimates, but provide important knowledge that arbovirus coinfections can occur in up to half of ZIKV-infected cases in certain contexts. Our findings are consistent with a systematic review of CHIKV/DENV coinfections, which found the frequency of CHIKV/DENV coinfections reported in 28 studies ranged from 1% to 36%.45 The heterogeneity across studies also reflects the difficulty in estimating the background level of ZIKV infections (ie, the denominator for assessing coinfection frequencies), given the diagnostic challenges in identifying acute ZIKV infections.46
Overall, the evidence identified in this review suggests that ZIKV coinfections appear to present with a mild clinical presentation similar to that previously described for ZIKV monoinfections. Of note, GI and URT symptoms, which are considered uncharacteristic for ZIKV, were reported to occur not infrequently in ZIKV/DENV, ZIKV/CHIKV and ZIKV/CHIKV/DENV-coinfected cases. While the evidence base from animal model studies of ZIKV coinfection is limited to date, two studies have compared ZIKV infection among rhesus macaque models with and without simian immunodeficiency virus or chimeric simian HIV.47 48 Whereas coinfected macaques were observed to have lower peak Zika viral loads with a longer clearance time in both investigations, the area under the viral load curves did not appear to differ substantively by coinfection status, potentially suggesting an overall limited impact of coinfection on disease progression but raising questions about the role of lentiviral coinfection in onward transmission.47 48
Although the existing reports suggest that coinfections do not appear to markedly alter the clinical presentation of uncomplicated ZIKV disease in humans, the findings from this review highlight a need for additional high quality research investigating whether coinfections may influence complication risks. Based on the limited available evidence, the complications described for ZIKV coinfections appear to be broadly similar to those reported for ZIKV monoinfections.49 However, 33% of the coinfection-related complications appeared to be atypical for ZIKV monoinfections, but were consistent with complications previously documented for the coinfecting pathogens (eg, bleeding in 10% of ZIKV/DENV cases and persistent arthralgia in 6% of ZIKV/CHIKV cases).50 51 In addition, among deaths of ZIKV-coinfected cases, three of the nine cases had immune deficiencies and one ZIKV/Leptospira spp-coinfected case died from complications established for Leptospira spp infections.40 The remaining five deaths reported from ZIKV coinfections were three fetal deaths, one case following multiorgan failure and one case following encephalitis.22 28 Additionally, some complications may have been missed, especially those that occurred after the acute infections, as the follow-up period of the individual studies may have not been adequate to detect late-onset complications. Further research (eg, an ongoing cohort study of ZIKV/HIV coinfections in pregnant women52) will be valuable for discerning the relative risk of complications of ZIKV coinfection versus monoinfections.
This review had strengths and limitations. ZIKV is an emerging infectious disease of significant public health concern, and this is the first systematic review of the frequency, types and clinical presentation of ZIKV coinfections. The study employed a broad search strategy including search terms for all potential coinfecting pathogens and using multiple languages to identify all available evidence. Most importantly, the review included only qRT-PCR-confirmed ZIKV coinfections, which is the most accurate way to diagnose acute coinfections (ie, due to the very short time window of qRT-PCR testing (<7 days)) and limits misdiagnosis, which is of particular importance with the high cross-reactivity reported from arbovirus serology testing. On the other hand, by focusing on concurrent infections, the current review was unable to appraise the potential impact of recent infections; for example, it has been previously reported that pre-existing immunity to DENV, which shares a common vector and circulates in most of the countries reporting ZIKV coinfection, may influence the clinical presentation of ZIKV infection.53 The additional limitations of this review mainly stem from the lack of available high-quality evidence on ZIKV coinfections. Notably, the majority of included studies were rated level 4 or 5 according to the OCEBM Levels of Evidence. Only seven studies were rated level 2 or above. Additionally, the reported ZIKV coinfection types may have been influenced by the underlying prevalence of coinfecting pathogens in the population and the applied diagnostic practices (ie, multiplex testing vs testing on clinician's suspicion). The use of specific case definitions in included cross-sectional and case series studies (eg, fever and rash15) may have also introduced a selection bias that potentially led to an over-representation of specific symptoms associated with ZIKV coinfection reported for a given study (eg, reporting 100% of cases as presenting with fever and rash).15 Finally, the studies selected for this systematic review only included symptomatic ZIKV-infected cases, which represent only approximately 40% of all ZIKV cases.2 It is likely that the actual frequency of ZIKV coinfections may be higher as many cases will be asymptomatic and therefore never seek medical attention. However, the recently implemented multiplex PCR assay, which tests for CHIKV, DENV and ZIKV simultaneously, will likely improve the detection of ZIKV/arbovirus coinfections and facilitate future assessment of the frequency of ZIKV coinfections.54
In conclusion, the findings of this review suggest that the cocirculating arboviruses, CHIKV and DENV, are the most common ZIKV coinfection types and may, in specific populations and epidemiological contexts, occur in up to half of ZIKV infections. The evidence collated in this systematic review suggests coinfections do not markedly alter the generally mild clinical presentation of uncomplicated ZIKV disease. However, additional and better quality evidence should be prioritised in future outbreaks to corroborate the estimates of the frequency of ZIKV coinfections and to interrogate the importance of ZIKV coinfections in the development of ZIKV-related complications, especially for ZIKV coinfections with CHIKV and DENV.