A few years back, Zika caught a wave of attention during the epidemic in Central and South Americas. Although usually causing relatively mild disease in most patients, the virus is linked with devastating microcephaly in babies born to mothers actively infected by the virus. In the last two years, the disease dropped out of the limelight after experiencing a relatively short-lived focus. So, what happened to Zika? Should we still be concerned?
In short
- Zika is a virus in the flavivirus family, most commonly spread by the Aedes aegypti mosquito
- Infection can be asymptomatic or cause a flu-like illness, but is associated with birth defects including microcephaly in babies born to mothers with active infection
- 95% of Zika virus (ZIKV) cases in the continental US were in travelers returning from affected areas
- In the U.S. territories, the vast majority (99.6%) of ZIKV illness cases occurred in Puerto Rico by local mosquito-borne transmission
- Zika virus illness cases peaked in 2016 and decreased dramatically the following year (2017) by 91% in the U.S. states and 98% in the U.S. territories
- U.S. case trends continue to decline thus far in 2018
- The striking decline of infections is likely due to herd immunity as previously infected individuals became immune in endemic areas
Background
Zika virus (ZIKV) is a virus in the flavivirus family, typically transmitted by Aedes aegypti mosquitoes (1). The Aedes aegypti mosquito is endemic to many regions of particular interest to surfers as in the map below. Other forms of Zika transmission include mosquito-borne from the Aedes albopictus species, sexual, iatrogenic (blood transfusion), and vertical transmission (from mother to fetus) (2). Common symptoms are low grade fever, rash, conjunctivitis, and muscle pain; though most patients have no symptoms whatsoever (3). A major concern with Zika outbreaks is the incidence of microcephaly and other birth defects in babies born to mothers with active infection (4). Back in 2016, Surfers Medical Association board member, Andrew Nathanson, MD, wrote a great article in SM outlining the disease and other mosquito-borne diseases to watch out for in many of our beloved surf destinations. For more information regarding the disease geography, symptoms, and treatment, read the full article here. For a quick hit: Top 5 Things to know about Zika from the CDC.
Map available from the Center for Disease Control and Prevention. A searchable map with zoom functions is available here.
Has Zika been a major concern in the United States?
The concern for Zika outbreak in the Americas began after the first confirmed case of the virus in Brazil in May of 2015 (5). ZIKV became a nationally reportable disease in 2016; however, cases have been collected in the ArboNET database since 2015 and are available from the CDC’s cumulative provisional data (6). The data for reported Zika virus disease cases is collected for the continental U.S. and Hawaii and separately for the U.S. territories due to different exposure and transmission patterns.
In the 50 U.S. states, the vast majority of Zika cases occurred in travelers to endemic areas. By contrast, nearly all Zika cases in the territorial U.S. were contracted locally by route of mosquito bites. Below is the most current cumulative data of all Zika cases since 2015 (7).
Cumulative Zika Cases 2015-2018: Continental U.S. and Hawaii
Between January 1, 2015 and May 2, 2018, 5,700 symptomatic Zika virus disease cases were reported in the U.S. states. These excluded congenital disease cases and cases in the U.S. territories. Of the U.S. states cases, 95% were in patients who travelled to affected areas. Four percent of cases were presumed to be contracted through mosquito-borne transmission within the continental U.S. Only 55 cases were acquired through other vectors. These included: 52 cases of sexual transmission, 2 cases of laboratory transmission, and a single case of transmission from person-to-person through by an unknown means.
“Between January 1, 2015 and May 2, 2018, 5,700 symptomatic Zika virus disease cases were reported in the U.S. states … 95% were in patients who travelled to affected areas”.
Cumulative Zika Cases 2015-2018: U.S. Territories
The data for U.S. territories paint a very different picture. The US territories include Puerto Rico, Guam, Northern Mariana Islands, the U.S. Virgin Islands, and American Samoa, though the vast majority of Zika cases were in Puerto Rico. In the same time period between January 1, 2015 and May 2, 2018, a cumulative 37,229 symptomatic Zika virus disease cases were reported in the US territories, nearly seven times more than in the U.S. states. A striking 37,082 cases (99.6%) were transmitted locally by mosquito bite exposure and only 147 cases occurred in patients who returned from travel to affected areas. None of the cases were reported as ‘acquired through other routes’, though it’s worth noting that sexually transmitted cases are not reported for these regions since the local transmission make it impossible to discern mosquito-borne from sexually transmitted disease. The combined U.S. territory population only makes up about 1% of the total U.S. population but nearly 87% of the cumulative Zika virus disease cases were reported in the U.S. territories, most of which were acquired locally.
“A cumulative 37,229 symptomatic Zika virus disease cases were reported in the US territories, nearly seven times more than in the U.S. states…(99.6%) were transmitted locally by mosquito bite exposure”.
Is the problem getting better or worse?
The trend of ZIKV illness cases has been dramatic since the disease became a nationally reportable illness in 2016 (8). In short, 2016 saw a massive peak in both the U.S. states and territories followed by a precipitous drop the following year.
Continental U.S. and Hawaii
In 2015, before widespread reporting, there were 62 cases in the U.S. states, and all were in travelers that returned from endemic areas. 2016 saw 5,168 cases with about 95% occuring in travelers. There were 224 cases acquired through presumed local mosquito-borne transmission; 218 in Florida and 6 in Texas. In 2017, only 449 cases were reported, a 91% decrease from the previous year. 97% were in returning travelers and 7 cases were acquired through presumed local mosquito-borne transmission (2 in Florida and 5 in Texas). From the beginning of January 2018 through May 2, 2018 there were 21 reported cases, all from travelers returning from endemic regions (7).
U.S. Territories
Similar trends were seen in the U.S. territories. In 2015, 10 Zika cases were reported, all occurring in Puerto Rico, 9 of which were by presumed local transmission and one by travel. In a dramatic spike in 2016, 36,512 cases were reported, 99.6% were by presumed local mosquito transmission, 97% occurring in Puerto Rico. Cases in 2017 dropped to 662, a striking 98% decrease. Finally, only 45 cases were reported thus far in 2018, with virtually all contracted locally (7).
From 2016 to 2017, Zika virus illness cases decreased by 91% in the U.S. states and 98% in the U.S. territories.
Note: Data is presented for 2018 is provisional through May 2, 2018.
To see a month-by-month breakdown of laboratory-confirmed symptomatic cases with illness onset in 2016-2018 reported to ArboNET in the United states here.
What is caused such a massive decline in Zika cases?
The striking decline of infections is likely due to herd immunity in endemic areas (9). Individuals previously infected by Zika virus develop immunity against future infection. Once the number of immune individuals reaches a critical proportion, the likelihood of transmission plummets. A new analysis of Zika virus trends in Latin America and the Caribbean (LAC) suggests that the epidemic is “by and large over, with incidence projected to be low in most cities in LAC in 2018”. Even with substantial declines in incidence, travelers to endemic areas should take preventive precautions so as to avoid other mosquito-borne diseases such as Dengue and Chikungunya.
Zika is endemic to all my favorite surf spots, what can I do to protect myself from infection?
Unfortunately, the Ayedes aegypti mosquito tends to hang around many of our favorite surf spots. Although under investigation, there are no vaccines to protect against Zika at this time (8). Your best bet is protection from mosquito bites. Mosquito repellents with the following ingredients have been studied and shown to be effective: DEET, picaridin, IR3535 oil of lemon eucalyptus (OLE), para-menthane-diol (PMD), or 2-undecanone (10). The U.S. Environmental Protection Agency has a useful registry you can search to find the best repellent for your needs. When possible, cover up with long-sleeved shirts and long pants. Clothing impregnated with permethrin is available or you can treat your clothing, shoes, tents, and other items yourself. Note that there is high resistance to permethrin in areas that have long histories of its use for mosquito control, especially in Puerto Rico (10). At night, use a mosquito net or window screens.
More great tips on how to protect yourself from mosquito bites can be found at
CDC Guide for Zika Prevention.
Other Zika-related resources:
CDC’s Guidelines for Healthcare Providers
CDC’s Guide to Reporting Zika
References
(1) Petersen, L. R., Jamieson, D. J., Powers, A. M., & Honein, M. A. (2016). Zika Virus. The New England Journal of Medicine, 374(16), 1552–1563. http://doi.org/10.1056/NEJMra1602113
(2) Silva, G. S., Richards, G. A., Baker, T., Hidalgo, J., Jimenez, J. I. S., & Amin, P. (2018). Zika virus: Report from the task force on tropical diseases by the world Federation of Societies of intensive and critical care medicine. Journal of Critical Care. http://doi.org/10.1016/j.jcrc.2018.03.030
(3) Nathanson, A., MD. (2016, August 29). Should you Frika about Zika? Surfing Medicine Journal. Retrieved May 28, 2018, from http://journal.surfersmedicalassociation.org/should-you-frika-about-zika/
(4) Centers for Disease Control and Prevention (CDC) (2017, October 6). Key Messages – Zika Disease. Retrieved May 28, 2018 from https://www.cdc.gov/zika/pdfs/zika-key-messages.pdf
(5) Oduyebo T, Polen KD, Walke HT, et al. Update: Interim guidance of health care providers caring for pregnant women with possible Zika virus exposure—United States (including U.S. territories), July 2017. MMWR Morb Mortal Wkly Rep 2017;66:781–93. https://www.ncbi.nlm.nih.gov/pubmed/28749921
(6) Lockwood CJ, Ros ST, Nielsen-Saines K. (2018, April 5). Zika virus infection: Evaluation and management of pregnant women. Retrieved May 28, 2018 from https://www.uptodate.com/contents/zika-virus-infection-evaluation-and-management-of-pregnant-women?sectionName=PREVENTION&topicRef=106169&anchor=H1053429363&source=see_link#H1053429363
(7) Centers for Disease Control and Prevention (CDC) (2018, May 3). Zika Cases in the United States. Retrieved May 28, 2018 from https://www.cdc.gov/zika/reporting/case-counts.html
(8) Hall V, Walker WL, Lindsey NP, et al. Update: Noncongenital Zika Virus Disease Cases — 50 U.S. States and the District of Columbia, 2016. MMWR Morb Mortal Wkly Rep 2018;67:265–269. DOI: http://dx.doi.org/10.15585/mmwr.mm6709a1
(9) O’Reilly K, Lowe R, Edmunds J, et al. Projecting the end of the Zika virus epidemic in Latin America: a modelling analysis, 2018. bioRxiv 323915; DOI: https://doi.org/10.1101/323915
(10) Centers for Disease Control and Prevention (CDC) (2018, February 5). Prevent Mosquito Bites. Retrieved May 28, 2018 from https://www.cdc.gov/zika/prevention/prevent-mosquito-bites.html
