Epidemiologic and spatiotemporal trends of Zika Virus disease during the 2016 epidemic in Puerto Rico

Abstract

Background: After Zika virus (ZIKV) emerged in the Americas, laboratory-based surveillance for arboviral diseases in Puerto Rico was adapted to include ZIKV disease.

Methods and findings: Suspected cases of arboviral disease reported to Puerto Rico Department of Health were tested for evidence of infection with Zika, dengue, and chikungunya viruses by RT-PCR and IgM ELISA. To describe spatiotemporal trends among confirmed ZIKV disease cases, we analyzed the relationship between municipality-level socio-demographic, climatic, and spatial factors, and both time to detection of the first ZIKV disease case and the midpoint of the outbreak. During November 2015-December 2016, a total of 71,618 suspected arboviral disease cases were reported, of which 39,717 (55.5%; 1.1 cases per 100 residents) tested positive for ZIKV infection. The epidemic peaked in August 2016, when 71.5% of arboviral disease cases reported weekly tested positive for ZIKV infection. Incidence of ZIKV disease was highest among 20-29-year-olds (1.6 cases per 100 residents), and most (62.3%) cases were female. The most frequently reported symptoms were rash (83.0%), headache (64.6%), and myalgia (63.3%). Few patients were hospitalized (1.2%), and 13 (<0.1%) died. Early detection of ZIKV disease cases was associated with increased population size (log hazard ratio [HR]: -0.22 [95% confidence interval -0.29, -0.14]), eastern longitude (log HR: -1.04 [-1.17, -0.91]), and proximity to a city (spline estimated degrees of freedom [edf] = 2.0). Earlier midpoints of the outbreak were associated with northern latitude (log HR: -0.30 [-0.32, -0.29]), eastern longitude (spline edf = 6.5), and higher mean monthly temperature (log HR: -0.04 [-0.05, -0.03]). Higher incidence of ZIKV disease was associated with lower mean precipitation, but not socioeconomic factors.

Conclusions: During the ZIKV epidemic in Puerto Rico, 1% of residents were reported to public health authorities and had laboratory evidence of ZIKV disease. Transmission was first detected in urban areas of eastern Puerto Rico, where transmission also peaked earlier. These trends suggest that ZIKV was first introduced to Puerto Rico in the east before disseminating throughout the island.

Fig 1. Reported arboviral disease cases by diagnostic test result and reported week of illness onset, November 1, 2015–December 31, 2016 (N = 71,618).

Top: Confirmed and probable Zika virus (ZIKV) disease cases (n = 39,717), and cases negative for ZIKV infection (gray bars; n = 31,681); Middle: Confirmed and probable dengue cases (n = 198); Bottom: Confirmed and probable chikungunya cases (n = 149).

Fig 2. Zika virus disease cases by sex and number per 1,000 population by age group (N = 39,637*), Puerto Rico, November 1, 2015–December 31, 2016.

Percentages in black indicate the proportion of female cases by age group. *Neither age nor date of birth were available for 80 cases.

Fig 3. Incidence of laboratory-positive Zika virus disease cases (N = 39,578^†) per 1,000 residents by municipality of residence, Puerto Rico, November 1, 2015–December 31, 2016.

*Indicates the location of sites for enhanced surveillance for arboviral diseases ^†Municipality of residence was not available for 139 cases.

Fig 4. Observed and model estimated week of first confirmed Zika virus disease case, outbreak midpoint, and total confirmed cases, Puerto Rico.

A: the color of the points in each municipality refers to the observed and model estimated week of first confirmed Zika virus disease case. B: the color of the points in each municipality refers to the observed and model week of the midpoint of confirmed Zika virus disease cases. C: the color of the points in each municipality refers to the observed and model estimated total confirmed cases. Grey diagonal line refers to perfect linear relationship of the observed and estimated weeks and cases (i.e. R² = 1).

Fig 5. Weekly number of confirmed Zika virus disease cases by municipality, Puerto Rico, November 1, 2015–December 31, 2016 (ordered from largest to smallest municipal population, top to bottom).

The points indicate the week of the first confirmed Zika virus disease case (yellow) and the week in which the midpoint of confirmed Zika virus disease cases occurred (red).