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Table.  Human EEE Cases and 12-Month SPI From July of Each Year in Massachusetts
Human EEE Cases and 12-Month SPI From July of Each Year in Massachusetts
1.
National Centers for Environmental Information.  Climate at a glance: statewide rankings. https://www.ncdc.noaa.gov/cag/statewide/rankings/19/pcp/200910. Accessed December 17, 2019.
2.
Letson  GW, Bailey  RE, Pearson  J, Tsai  TF.  Eastern equine encephalitis (EEE): a description of the 1989 outbreak, recent epidemiologic trends, and the association of rainfall with EEE occurrence.  Am J Trop Med Hyg. 1993;49(6):677-685. doi:10.4269/ajtmh.1993.49.677PubMedGoogle ScholarCrossref
3.
Mahmood  F, Crans  WJ.  Effect of temperature on the development of Culiseta melanura (Diptera: Culicidae) and its impact on the amplification of eastern equine encephalomyelitis virus in birds.  J Med Entomol. 1998;35(6):1007-1012. doi:10.1093/jmedent/35.6.1007PubMedGoogle ScholarCrossref
4.
Armstrong  PM, Andreadis  TG.  Eastern equine encephalitis virus—old enemy, new threat.  N Engl J Med. 2013;368(18):1670-1673. doi:10.1056/NEJMp1213696PubMedGoogle ScholarCrossref
5.
Rocheleau  JP, Arsenault  J, Ogden  NH, Lindsay  LR, Drebot  M, Michel  P.  Characterizing areas of potential human exposure to eastern equine encephalitis virus using serological and clinical data from horses.  Epidemiol Infect. 2017;145(4):667-677. doi:10.1017/S0950268816002661PubMedGoogle ScholarCrossref
6.
Downs  J, Vaziri  M, Jenkins  A, Unnasch  T.  Validation of a risk index model for predicting eastern equine encephalitis virus transmission to horses in Florida.  J Med Entomol. 2018;55(5):1143-1149. doi:10.1093/jme/tjy067PubMedGoogle Scholar
7.
National Centers for Environmental Information.  National Climate Report–annual 2012 warmest seasons and calendar year periods on record. https://www.ncdc.noaa.gov/sotc/national/201213/supplemental/page-3#MA. Accessed December 17, 2019.
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    Research Letter
    Infectious Diseases
    January 31, 2020

    Association of Human Eastern Equine Encephalitis With Precipitation Levels in Massachusetts

    Author Affiliations
    • 1Department of Medicine, Warren Alpert Medical School, Brown University, Providence, Rhode Island
    • 2Division of Infectious Diseases, Rhode Island Hospital, Providence
    • 3Department of Epidemiology and Infection Control, Rhode Island Hospital, Providence
    JAMA Netw Open. 2020;3(1):e1920261. doi:10.1001/jamanetworkopen.2019.20261
    Introduction

    From 2009 through 2018, only 3 to 15 cases of Eastern equine encephalitis (EEE) virus neuroinvasive disease in the United States were reported to the Centers for Disease Control and Prevention annually.1 The enzootic cycle of the EEE virus involves Culiseta melanura mosquitos and passerine birds inhabiting freshwater swamps. The likelihood of human EEE infection is associated with ecosystems where C melanura thrive. The spread of the EEE virus from infected birds to humans requires the presence of other mosquitos that feed on both birds and humans (ie, bridge vectors), such as Coquillettidia pertubans, Aedes sollicitans, or Ochlerotatus canadensis. Increased precipitation throughout the year maintains C melanura larval habitat.2 Additionally, global warming may amplify the EEE virus life cycle3 and expand EEE virus geographic distribution.4 I hypothesized that higher than usual precipitation during the preceding year from a midsummer month would be associated with increased human EEE infections.

    Methods

    This cross-sectional study used the Standard Precipitation Index (SPI) in the 6 regions of Massachusetts. The SPI measures the probability of precipitation over a given period of time based on long-term precipitation records. The lower the SPI, the dryer the conditions, and the higher the SPI, the wetter the conditions compared with long-term historical data. These data were obtained from the Massachusetts Department of Conservation and Recreation. The number of human EEE infections each year from January 1, 1999, until December 1, 2019, was obtained from the Massachusetts Department of Public Health. The exposure variable was the SPI in the 6 regions of Massachusetts during 12 months, calculated from July of each year (eg, the SPI of 2019 includes August 1, 2018, through July 31, 2019), and the outcome variable was the number of human EEE cases in Massachusetts. Two-sided Fisher exact test was used to determine significance at P < .05. An a priori study size calculation was not performed. Statistical analysis was performed using VassarStats (Richard Lowry). Data were analyzed from September 18 to 20, 2019.

    Per the Common Rule, this study did not require ethics board review because no protected health data of any kind were accessed or reviewed. The Strengthening the Reporting of Observational Studies in Epidemiology (STROBE) reporting guideline for cross-sectional studies was followed.

    Results

    There were 38 human EEE cases reported to the Massachusetts Department of Public Health from 1999 to 2019 (Table). At least 1 human EEE case occurred in Massachusetts in 10 of 14 years when the SPI was higher than 1.0 in 1 or more regions of Massachusetts. However, a human EEE case occurred in only 1 of 7 years when the SPI was lower than 1.0 in all 6 regions of Massachusetts (relative risk, 3.0; 95% CI, 1.2-7.2; P = .02). Of note, 2019 was the first year in the last 20 years that the 12-month SPI measured in July was higher than 2.0 for all Massachusetts regions except 1 region (the Cape and Islands), and there were 12 human EEE cases in 2019.

    Discussion

    This cross-sectional study found that a 12-month SPI below 1.0 across Massachusetts was associated with lower incidence of human EEE compared with a 12-month SPI higher than 1.0. A high 12-month SPI measured in July may suggest an increased risk of human EEE cases. Despite these data, estimating the risk of EEE infection is complex and not easily explained simply by precipitation data, and large-scale epidemiologic investigations of EEE are limited by the low number of cases diagnosed each year.5,6 Temperature also plays a role in risk of EEE cases, as exemplified by the 2012 data that included 7 human EEE cases. Notably, 2012 was the warmest year in Massachusetts from 1895 through 2012.7

    This study was limited by the absence of monthly SPI data and monthly data regarding human EEE cases. It is unclear if these results would be found in other states where EEE infections occur, as this study only involved data in Massachusetts.

    Conclusions

    The level of precipitation during the life cycle of the enzootic mosquito vector of EEE is an important variable determining the risk of human infection. In the 21st century, as in millennia before, human health cannot be separated from conditions in our environment.

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    Article Information

    Accepted for Publication: December 4, 2019.

    Published: January 31, 2020. doi:10.1001/jamanetworkopen.2019.20261

    Open Access: This is an open access article distributed under the terms of the CC-BY License. © 2020 Mermel LA. JAMA Network Open.

    Corresponding Author: Leonard A. Mermel, DO, ScM, Division of Infectious Diseases, Rhode Island Hospital, 593 Eddy St, Providence, RI 02903 (lmermel@lifespan.org).

    Author Contributions: Dr Mermel had full access to all of the data in the study and takes responsibility for the integrity of the data and the accuracy of the data analysis.

    Concept and design: Mermel.

    Acquisition, analysis, or interpretation of data: Mermel.

    Drafting of the manuscript: Mermel.

    Critical revision of the manuscript for important intellectual content: Mermel.

    Statistical analysis: Mermel.

    Conflict of Interest Disclosures: None reported.

    References
    1.
    National Centers for Environmental Information.  Climate at a glance: statewide rankings. https://www.ncdc.noaa.gov/cag/statewide/rankings/19/pcp/200910. Accessed December 17, 2019.
    2.
    Letson  GW, Bailey  RE, Pearson  J, Tsai  TF.  Eastern equine encephalitis (EEE): a description of the 1989 outbreak, recent epidemiologic trends, and the association of rainfall with EEE occurrence.  Am J Trop Med Hyg. 1993;49(6):677-685. doi:10.4269/ajtmh.1993.49.677PubMedGoogle ScholarCrossref
    3.
    Mahmood  F, Crans  WJ.  Effect of temperature on the development of Culiseta melanura (Diptera: Culicidae) and its impact on the amplification of eastern equine encephalomyelitis virus in birds.  J Med Entomol. 1998;35(6):1007-1012. doi:10.1093/jmedent/35.6.1007PubMedGoogle ScholarCrossref
    4.
    Armstrong  PM, Andreadis  TG.  Eastern equine encephalitis virus—old enemy, new threat.  N Engl J Med. 2013;368(18):1670-1673. doi:10.1056/NEJMp1213696PubMedGoogle ScholarCrossref
    5.
    Rocheleau  JP, Arsenault  J, Ogden  NH, Lindsay  LR, Drebot  M, Michel  P.  Characterizing areas of potential human exposure to eastern equine encephalitis virus using serological and clinical data from horses.  Epidemiol Infect. 2017;145(4):667-677. doi:10.1017/S0950268816002661PubMedGoogle ScholarCrossref
    6.
    Downs  J, Vaziri  M, Jenkins  A, Unnasch  T.  Validation of a risk index model for predicting eastern equine encephalitis virus transmission to horses in Florida.  J Med Entomol. 2018;55(5):1143-1149. doi:10.1093/jme/tjy067PubMedGoogle Scholar
    7.
    National Centers for Environmental Information.  National Climate Report–annual 2012 warmest seasons and calendar year periods on record. https://www.ncdc.noaa.gov/sotc/national/201213/supplemental/page-3#MA. Accessed December 17, 2019.
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