To the Editor: Zika virus (ZIKV) has been recognized as an emerging mosquito-borne
flavivirus since outbreaks were reported from Yap Island in 2007 (
), French Polynesia in 2013 (
), and Cook Island and New Caledonia in 2014 (
). It has joined dengue virus (DENV) and chikungunya virus (CHIKV) as global public
health threats (
). ZIKV infection typically causes a self-limited dengue-like illness characterized
by exanthema, low-grade fever, conjunctivitis, and arthralgia, and an increase in
rates of Guillain-Barré syndrome have been observed during ZIKV outbreaks (
In Brazil, clusters of cases of acute exanthematous illness have been reported from
various regions since late 2014, and in April 2015, ZIKV was identified as the etiologic
). In May 2015, the Brazilian Ministry of Health recognized circulation of ZIKV in
Brazil. We report epidemiologic findings for an ongoing outbreak of acute exanthematous
illness in the population of Salvador, the third largest city in Brazil.
The Salvador Epidemiologic Surveillance Office (ESO) was first alerted to cases of
an acute exanthematous illness early in 2015. Reporting of cases increased during
March, and in April the ESO established 10 public emergency health centers in Salvador
as sentinel units for systematic surveillance of patients with acute exanthematous
illness of unknown cause. The units searched retrospectively for suspected cases by
review of medical charts of patients treated since February 15, continued with prospective
case detection, and submitted weekly reports of identified cases to the ESO.
During February 15−June 25, a total of 14,835 cases of an indeterminate acute exanthematous
illness were reported from the 12 sanitary districts in Salvador. The overall attack
rate was 5.5 cases/1,000 persons (4.6 cases/1,000 men and 6.3 cases/1,000 women, 8.2
cases/1,000 children <15 years of age, 5.4 cases/1,000 persons 15–39 years of age,
and 3.8 cases/1,000 adults >40 years of age).
The epidemic curve peaked in the first week of May, which was 1 week after molecular
diagnosis of ZIKV in 8 patients residing ≈50 km from Salvador and during a period
of intense media coverage of the outbreak (Figure) (
). Reporting of suspected dengue cases in Salvador did not vary substantially from
that in other years and was >5 times lower: 2,630 cases, of which 165/366 (45.1%)
were positive for dengue IgM, 20/590 (3.4%) positive for dengue virus nonstructural
protein 1, and 1/11 (9.1%) positive for dengue virus by reverse transcription PCR
(Figure). During the same period, 58 cases of suspected chikungunya were reported
and 24 patients with suspected Guillain-Barré syndrome were hospitalized.
Reported cases of indeterminate acute exanthematous illness and suspected dengue fever
in Salvador, Brazil, by date of medical care, February 15−June 25, 2015. Letters indicate
specific events. A) February 15: systematic reporting of cases of acute exanthematous
illness of unknown cause begins in Salvador. B) April 13: Salvador Epidemiologic Surveillance
Office releases its first epidemiologic alert about the outbreak in Salvador. C) April
29: Zika virus is confirmed in 8 samples from patients residing ≈50 km from Salvador
and media coverage of the outbreak intensifies (http://www.correio24horas.com.br/detalhe/noticia/doenca-misteriosa-que-atinge-cidades-baianas-e-identificada-como-zika-virus/?cHash = 74792c41f3128395ba0ffa5e1ed9dbbe).
D) May 14: Brazilian Ministry of Health announces circulation of Zika virus in Brazil
E) June 11: Brazilian press announces that cases of Zika virus infection have been
confirmed in 8 states in Brazil (http://www1.folha.uol.com.br/cotidiano/2015/06/1640752-virus-primo-da-dengue-zika-ja-tem-casos-confirmados-em-oito-estados.shtml).
The median age of case-patients was 26 years (interquartile range 11–39 years), but
all age groups were affected, which is a pattern typical of spread of new microorganisms
(or subtypes) in a susceptible population. Median duration of symptoms at time of
medical attention was 1 day (interquartile range 0–3 days). All patients had exanthema
and most (12,711/14,093 [90.2%]) had pruritus. Fever (4,841/13,786, 35.1%), arthralgia
(278/1,048 [26.5%]), headache (3,446/13,503 [25.6%]), and myalgia (223/1,033 [21.6%])
were less common.
Serum samples from some patients were examined for rubella IgM (2/200, 1.0% positive),
rubella IgG (15/18, 83.3% positive), measles IgM (0/11, 0% positive), dengue nonstructural
protein 1 (3/185, 1.6% positive), dengue IgM (17/80, 21.3% positive), parvovirus B19
IgM (0/1, 0% positive), and parvovirus B19 IgG (1/1, 100% positive). Reverse transcription
PCR was performed on 58 serum samples stored at −20°C and confirmed ZIKV in 3 (5.2%)
samples, CHIKV in 3 (5.2%) samples, DENV type 3 in 1 (1.7%) sample, and DENV type
4 in 1 (1.7%) sample.
Identification of ZIKV, CHIKV and DENV as etiologic agents of acute exanthematous
illness suggests that these 3 Aedes spp. mosquito−transmitted viruses were co-circulating
in Salvador and highlights the challenge in clinically differentiating these infections
during outbreaks. Although we were not able to determine the specific incidence of
each virus, the low frequency of fever and arthralgia, which are indicators of dengue
and chikungunya, point to ZIKV as the probable cause of several of the reported cases.
Furthermore, laboratory-confirmed cases of infection with ZIKV were simultaneously
identified in other cities within metropolitan Salvador (
) and in other states in Brazil (
). Low diagnosis of ZIKV infection is likely because viremia levels among infected
patients appear to be low (
The spread of ZIKV represents an additional challenge for public health systems, particularly
because of the risk for concurrent transmission of DENV and CHIKV by the same vectors,
Ae. aegypti and Ae. albopictus mosquitoes, which are abundant throughout tropical
and subtropical regions. To date, the largest outbreak of chikungunya in Brazil occurred
in 2014 in Feira de Santana, Bahia, ≈100 km from Salvador, where dengue is also prevalent
This report illustrates the potential for explosive simultaneous outbreaks of ZIKV,
CHIKV, and DENV in the Western Hemisphere and the increasing public health effects
of Aedes spp. mosquitoes as vectors. The apparent increase in reports of Guillain-Barré
syndrome during the outbreak deserves further investigation to elucidate whether this
syndrome is associated with ZIKV infection. Public health authorities in Brazil and
neighboring countries should plan accordingly.