Zika virus, belonging to the family Flaviviridae and genus Flavivirus, is transmitted to humans by mosquito bites but can also be contracted through sexual and vertical transmission (1). Zika virus was first detected in Brazil in 2015 and has since spread with the same speed as chikungunya virus through South and Central America and the Caribbean Islands, despite a shorter and lower viremia in humans and a longer replication period in the vector (2,3). As with other arboviruses, Zika virus viremia is commonly measured in venous blood, even though mosquitoes introduce virus into cutaneous capillary blood. We conducted a prospective descriptive study with Zika virus patients during the Zika virus epidemic in French Guiana to evaluate the kinetics of Zika virus RNA load in serum samples collected sequentially from venous and skin capillary blood.

The study population comprised 21 symptomatic and consenting Zika virus patients infected during March‒September 2016. We confirmed Zika virus infection by real-time reverse transcription PCR (RT-PCR) of serum and urine samples provided by patients during the first few days after symptom onset. We obtained serum samples from the venous and cutaneous capillary blood (collected from the fingertip) sequentially 1‒18 days after the onset of symptoms. The median age of the population was 40 (range 28–63) years and the sex ratio (male:female) was 1.6. We observed no co-morbidities, and all participants were found to be free of dengue and chikungunya virus infections by methods previously described (4,5).

We extracted RNA from 150-µL samples using the QIAamp Viral RNA kit (QIAGEN, Hilden, Germany) and performed Zika virus RNA amplification using the RealStar Zika Virus RT-PCR Kit 1.0 (Altona Diagnostics GmbH, Hamburg, Germany) according to the manufacturer’s instructions. We performed Zika virus RNA quantification using a reference strain provided by the European Zika Virus Archive (SKU no. 001N-01648) and estimated the Zika virus RNA load as log₁₀ copies per milliliter Technical Appendix Table).

Zika virus RNA loads in capillary blood correlated with those in venous blood (Spearman correlation test, r = 0.54, p<0.0001) but were significantly higher in the capillary samples (Wilcoxon signed rank test, p = 0.0003), except for 3 patients (nos. 2, 13, and 21; Technical Appendix Table). The median duration of Zika virus detection after symptom onset was significantly greater in capillary blood than in venous blood (p = 0.005 by log rank test; hazard ratio = 2.99, 95% CI 1.39–6.43), even though the duration of detection in capillary blood was underestimated; RNA was still detectable in the last capillary blood samples taken from 8 patients (nos. 1, 5, 6, 8, 11, 15, 16, and 19; Technical Appendix Table). The duration of Zika virus RNA detection was greater in capillary than in venous blood for 12 (57%, 95% CI 34%–78%) of the 21 patients (nos. 1, 3, 5, 6, 7, 8, 11, 14, 15, 16, 19, and 20). The maximum duration of RNA detection in capillary blood samples was 18 days after the onset of symptoms (seen with patient no. 19) with a load of 1.9 log₁₀ copies/mL. The duration of detection was equal between the 2 compartments for 7 (33%, 95% CI 15%–57%) of 21 patients (nos. 4, 9, 10, 12, 13, 17, and 18) and longer in venous blood for 2 (10%, 95% CI 1%–30%) of 21 patients (nos. 2 and 21) (Technical Appendix Table).

These data raise questions about the consequences of longer persistence and higher loads of Zika virus RNA in the cutaneous capillary blood compartment, although we did not test Zika virus replication capacity. The higher load and longer detection of Zika virus RNA in this compartment might be attributable to Zika virus replication in permissive cells of the skin (e.g., human dermal fibroblasts and epidermal keratinocytes); capillaries; or both (6). If the Zika virus RNA observed in the serum samples taken from the capillary compartment reflects the presence of infectious virus particles, symptomatic Zika virus‒infected patients would need to be shielded from mosquitoes for a longer period than is currently practiced to limit potential vectorborne transmission.

This study comparing the kinetics of Zika virus RNA load between the venous and capillary compartments highlights the need to further investigate the infectivity and pathophysiology of the virus located in the often neglected capillary compartment. These findings provide new information on this biologic compartment, which plays a key role in vectorborne transmission and transmission dynamics. Moreover, these observations, if validated with more patients and extended to other vectorborne infections, will be vital for preventing and controlling the transmission of Zika virus and other arboviruses.

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