Although limited, we had access to sufficient patients to perform data analysis in relation to the severity of the disease (severe illness was defined as fever .38.5uC, or maximum pulse rate .100 beats/minute, or nadir platelet count ,100610 9 /L). Using this definition, we observed that higher disease severity was associated with increased plasma levels of IL-1b and IL-6 and a decreased level in RANTES (Figure 3b ). IL-1b and IL-6, whose levels are already high in the CHIKV infected patients, are potent endogenous pyrogens [45] [46] [47] [48] . Therefore, elevations of IL-1b and IL-6 might account for the high fever in the severe cases. The increase production of IL-1b might also mediate the development of abrupt and persistent arthralgia since this cytokine is involved in the immunopathogenesis of many arthritic pathologies such as rheumatoid arthritis [49] . On the contrary, T cell chemokine RANTES levels were significantly suppressed in severe CHIKF patients. Platelets are a major reservoir of RANTES in the peripheral circulation [50] , and severe CHIKF was characterized by thrombocytopenia. Thus, as mentioned above for EGF, thrombocytopenia can also reduce levels of circulating RANTES. Low levels of RANTES correlate with disease severity and mortality in individuals with severe malaria, who were also correspondingly thrombocytopenic [51] . Interestingly, it was observed in other studies that RANTES levels were up-regulated in dengue [48] , except for one single report from Cuba [52] . Since the symptoms of CHIKF mimic those of dengue fever, results obtained from this study strongly suggest that RANTES could be a potential biomarker that differentiates between these 2 clinically very similar diseases.