No evidence of chikungunya virus and antibodies shortly before the outbreak on Sri Lanka

A massive outbreak of chikungunya disease occurred on Sri Lanka in 2006. Reasons for the explosive nature of the epidemic are being intensively discussed. According to recognised and anecdotal concepts, absence of human population immunity against chikungunya virus (CHIKV) might have supported virus amplification. However, formal proof of concept is lacking. This study determined the prevalence of anti-CHIKV IgG antibodies as well as CHIKV RNA shortly before the outbreak. Two hundred and six human sera were collected from patients with acute febrile illness in 2004/2005. Validated indirect immunofluorescence and real-time RT-PCR assays for dengue as well as CHIKV were employed. Laboratory evidence of dengue virus infection was seen in 67% of patients, indicating virus activity and exposure to Aedes spp. vectors. These vectors are the same as for chikungunya. However, no evidence of acute or previous chikungunya infection could be demonstrated in the same cohort. This study gives formal evidence that the absence of human population immunity correlated with a large chikungunya epidemic.