Transmission of Chikungunya Virus in an Urban Slum, Brazil

I n the 21st century, chikungunya virus (CHIKV) has emerged as a mosquitoborne disease of global relevance, causing large epidemics because of its widespread dissemination in tropical and subtropical areas (1).Infected persons usually develop an acute febrile illness associated with joint pains, myalgia, headache, and other signs and symptoms that can lead to misdiagnosis with other arboviral illnesses, such as dengue virus (DENV) and Zika virus (ZIKV) infections.Noteworthy with chikungunya, the arthralgia is often severely debilitating and may last for months to years (1,2).After the introduction of CHIKV into the Caribbean region in 2013, the virus spread rapidly, causing large outbreaks (3,4).In certain Caribbean islands, such as Puerto Rico and the US Virgin Islands, the rate at which CHIKV infection was symptomatic was estimated at >70% (5,6).However, it remains unclear whether the attack rates in outbreaks in large population centers in the Americas created sufficiently high levels of herd immunity to preclude subsequent epidemics.Furthermore, 2 CHIKV strains were introduced and are cocirculating in the Americas; it is unclear whether the proportions of symptomatic infections differ on the basis of strain type, population, or region.In Brazil, CHIKV was first detected in September 2014, almost simultaneously in the cities of Oiapoque, in the northern state of Amapá, where the Asian genotype was implicated (7), and in Feira de Santana, in the northeast state of Bahia, where the East/Central/ South African (ECSA) genotype was detected (7,8).The virus spread rapidly throughout the country, reaching all states by 2015 (9), and peaked in 2016, when ≈280,000 probable cases were recorded (10).The northeast region was the most affected by CHIKV (9,10,11); this same area was also the most affected by ZIKV in 2015-2016 (12,13).In Salvador (population 2.9 million [14]), the capital of Bahia state, which is located ≈100 km from Feira de Santana, we retrospectively identified that CHIKV had been circulating since September 2014 (15), but outbreaks first occurred between June and November 2015 (12,15).In this study, we used the prevalence of CHIKV IgG as an indicator of all (i.e., symptomatic and asymptomatic) previous infection in a slum