Virology, ecology, epidemiology, pathology, and treatment of eastern equine encephalitis.

Eastern equine encephalitis (EEE) was one of the first-recognized neuroinvasive arboviral diseases in North America, and it remains the most lethal. Although EEE is known to have periodic spikes in infection rates, there is increasing evidence that it may be undergoing a change in its prevalence and its public health burden. Numerous factors shape the scope of EEE in humans, and there are important similarities with other emergent viral diseases that have surfaced or strengthened in recent years. Because environmental and ecological conditions that broadly influence the epidemiology of arboviral diseases also are changing, and the frequency, severity, and scope of outbreaks are expected to worsen, an expanded understanding of EEE will have untold importance in coming years. Here we review the factors shaping EEE transmission cycles and the conditions leading to outbreaks in humans from an updated, multidomain perspective. We also provide special consideration of factors shaping the virology, host-vector-environment relationships, and mechanisms of pathology and treatment as a reference for broadening audiences.

Real-time RT-PCR for Venezuelan equine encephalitis complex, Madariaga, and Eastern equine encephalitis viruses: application in human and mosquito public health surveillance in Panama.

Eastern equine encephalitis virus (EEEV), Madariaga virus (MADV), and Venezuelan equine encephalitis virus complex (VEEV) are New World alphaviruses transmitted by mosquitoes. They cause febrile and sometimes severe neurological diseases in human and equine hosts. Detecting them during the acute phase is hindered by non-specific symptoms and limited diagnostic tools. We designed and clinically assessed real-time reverse transcription polymerase chain reaction assays (rRT-PCRs) for VEEV complex, MADV, and EEEV using whole-genome sequences. Validation involved 15 retrospective serum samples from 2015 to 2017 outbreaks, 150 mosquito pools from 2015, and 118 prospective samples from 2021 to 2022 surveillance in Panama. The rRT-PCRs detected VEEV complex RNA in 10 samples (66.7%) from outbreaks, with one having both VEEV complex and MADV RNAs. VEEV complex RNA was found in five suspected dengue cases from disease surveillance. The rRT-PCR assays identified VEEV complex RNA in three Culex (Melanoconion) vomerifer pools, leading to VEEV isolates in two. Phylogenetic analysis revealed the VEEV ID subtype in positive samples. Notably, 11.9% of dengue-like disease patients showed VEEV infections. Together, our rRT-PCR validation in human and mosquito samples suggests that this method can be incorporated into mosquito and human encephalitic alphavirus surveillance programs in endemic regions.

Efficacy of a brain-penetrant antiviral in lethal Venezuelan and eastern equine encephalitis mouse models.

Venezuelan and eastern equine encephalitis viruses (VEEV and EEEV, respectively) are mosquito-borne, neuroinvasive human pathogens for which no FDA-approved therapeutic exists. Besides the biothreat posed by these viruses when aerosolized, arthropod transmission presents serious health risks to humans, as demonstrated by the 2019 outbreak of EEE disease in the United States that resulted in 38 confirmed cases, 19 deaths, and neurological effects in survivors. Here, we describe the discovery of a 2-pyrrolidinoquinazolinone scaffold, efficiently synthesized in two to five steps, whose structural optimization resulted in profound antiviral activity. The lead quinazolinone, BDGR-49, potently reduced cellular VEEV and EEEV titers by >7 log at 1 µM and exhibited suitable intravenous and oral pharmacokinetic profiles in BALB/c mice to achieve excellent brain exposure. Outstanding in vivo efficacy was observed in several lethal, subcutaneous infection mouse models using an 8-day dosing regimen. Prophylactically administered BDGR-49 at 25 mg kg-1 per day fully protected against a 10× LD50 VEEV Trinidad donkey (TrD) challenge in BALB/c mice. Similarly, we observed 70% protection when 10× LD50 EEEV FL93-939-infected C57BL/6 mice were treated prophylactically with BDGR-49 at 50 mg kg-1 per day. Last, we observed 100% therapeutic efficacy when mice, challenged with 10× LD50 VEEV TrD, were dosed at 48 hours after infection with BDGR-49 at 25 mg kg-1 per day. Mouse brain viral titers at 96 hours after infection were reduced to values near the limit of detection. Collectively, these results underscore the substantial development potential of a well-tolerated, brain-penetrant lead compound that shows promise in preventing and treating encephalitic alphavirus disease.

Co-occurrence probabilities between mosquito vectors of West Nile and Eastern equine encephalitis viruses using Markov Random Fields (MRFcov).

Mosquito vectors of eastern equine encephalitis virus (EEEV) and West Nile virus (WNV) in the USA reside within broad multi-species assemblages that vary in spatial and temporal composition, relative abundances and vector competence. These variations impact the risk of pathogen transmission and the operational management of these species by local public health vector control districts. However, most models of mosquito vector dynamics focus on single species and do not account for co-occurrence probabilities between mosquito species pairs across environmental gradients. In this investigation, we use for the first time conditional Markov Random Fields (CRF) to evaluate spatial co-occurrence patterns between host-seeking mosquito vectors of EEEV and WNV around sampling sites in Manatee County, Florida. Specifically, we aimed to: (i) quantify correlations between mosquito vector species and other mosquito species; (ii) quantify correlations between mosquito vectors and landscape and climate variables; and (iii) investigate whether the strength of correlations between species pairs are conditional on landscape or climate variables. We hypothesized that either mosquito species pairs co-occur in patterns driven by the landscape and/or climate variables, or these vector species pairs are unconditionally dependent on each other regardless of the environmental variables. Our results indicated that landscape and bioclimatic covariates did not substantially improve the overall model performance and that the log abundances of the majority of WNV and EEEV vector species were positively dependent on other vector and non-vector mosquito species, unconditionally. Only five individual mosquito vectors were weakly dependent on environmental variables with one exception, Culiseta melanura, the primary vector for EEEV, which showed a strong correlation with woody wetland, precipitation seasonality and average temperature of driest quarter. Our analyses showed that majority of the studied mosquito species' abundance and distribution are insignificantly better predicted by the biotic correlations than by environmental variables. Additionally, these mosquito vector species may be habitat generalists, as indicated by the unconditional correlation matrices between species pairs, which could have confounded our analysis, but also indicated that the approach could be operationalized to leverage species co-occurrences as indicators of vector abundances in unsampled areas, or under scenarios where environmental variables are not informative.

Pearls & Oy-sters: Seronegative Eastern Equine Encephalitis in an Immunocompromised Stem Cell Transplant Recipient.

Altered mental status in immunosuppressed patients has a wide differential diagnosis. In this case, a 27-year-old man presented with encephalopathy, nausea, vomiting, and fevers. His medical history was significant for acute myeloid leukemia in remission after allogenic hematopoietic stem cell transplantation 17 months prior complicated by graft vs host disease affecting his skin treated with sirolimus. A lumbar puncture was performed with a lymphocytic pleocytosis, mildly elevated protein, and negative Gram-stain and bacterial and fungal cultures. His examination deteriorated, and he became comatose with loss of pupillary and corneal reflexes. An MRI of his brain demonstrated T2/fluid-attenuated inversion recovery signal abnormality involving the bilateral basal ganglia, mesial temporal lobes, and entire brainstem along with bilateral temporal parenchymal and leptomeningeal enhancement. Ultimately, diagnosis was made through metagenomic PCR sequencing from his CSF. This case highlights diagnostic challenges in immunosuppressed patients because antibodies against the causative antigen were negative (potentially related to decreased antibody production in the setting of immunosuppression).

Multi-season transmission model of Eastern Equine Encephalitis.

Eastern Equine Encephalitis (EEE) is an arbovirus that, while it has been known to exist since the 1930's, recently had a spike in cases. This increased prevalence is particularly concerning due to the severity of the disease with 1 in 3 symptomatic patients dying. The cause of this peak is currently unknown but could be due to changes in climate, the virus itself, or host behavior. In this paper we propose a novel multi-season deterministic model of EEE spread and its stochastic counterpart. Models were parameterized using a dataset from the Florida Department of Health with sixteen years of sentinel chicken seroconversion rates. The different roles of the enzootic and bridge mosquito vectors were explored. As expected, enzootic mosquitoes like Culiseta melanura were more important for EEE persistence, while bridge vectors were implicated in the disease burden in humans. These models were used to explore hypothetical viral mutations and host behavior changes, including increased infectivity, vertical transmission, and host feeding preferences. Results showed that changes in the enzootic vector transmission increased cases among birds more drastically than equivalent changes in the bridge vector. Additionally, a 5% difference in the bridge vector's bird feeding preference can increase cumulative dead-end host infections more than 20-fold. Taken together, this suggests changes in many parts of the transmission cycle can augment cases in birds, but the bridge vectors feeding preference acts as a valve limiting the enzootic circulation from its impact on dead-end hosts, such as humans. Our what-if scenario analysis reveals and measures possible threats regarding EEE and relevant environmental changes and hypothetically suggests how to prevent potential damage to public health and the equine economy.

Eastern Equine Encephalomyelitis in Michigan: Historical Review of Equine, Human, and Wildlife Involvement, Epidemiology, Vector Associations, and Factors Contributing to Endemicity.

Eastern equine encephalomyelitis (EEE) is a mosquito-borne viral disease that is an emerging public health concern in the state of Michigan. Although Michigan has one of the highest incidence rates of EEE in the United States, much of the information known about cases in humans, equines, and other animals residing in Michigan is unpublished. This article summarizes such information and explores spatial trends in the historic distribution of EEE in Michigan. Outbreaks in Michigan have occurred over an 80-yr interval, involving only horses in 1942-1943 and 1973-1976, and then episodically from 1980 to 2020, and involving horses, humans, and wild and domestic animals. An estimated 1,036 equine cases (confirmed and suspected) and 36 confirmed human cases have occurred, including 10 in 2019 (6 deaths) and 4 in 2020 (2 deaths). Human cases ranged in age from 1 to 81 yr; 70% were male, and fatality rate of 34.3%. Equine and human cases occurred from July to October, peaked in August, and cluster in space in southwestern and southeastern lower Michigan. Cases occurred in glacial outwash and ice-contact landscapes in glacial interlobate zones. EEE virus (EEEV) was recovered from Culiseta melanura, Coquillettidia perturbans, five species of Aedes, and other mosquito species near horse and human case sites. Virus isolations or presence of neutralizing antibodies in several passerine species of birds suggest broad EEEV-bird associations. White-tailed deer and other wildlife were also affected. Geographic spread to northern areas of the state suggests expansion of this disease system into new and unsuspected foci.

Magnetic resonance imaging, clinicopathologic findings, and clinical progression of a puppy with confirmed Eastern equine encephalitis virus.

A 5-month-old puppy was evaluated for rapidly progressive neurologic signs and pyrexia. Magnetic resonance imaging showed multifocal meningoencephalitis with transtentorial and foramen magnum herniation. A cerebrospinal fluid tap revealed highly cellular fluid, and the puppy was euthanized. Histopathology showed lymphoplasmacytic and neutrophilic meningoencephalitis. Viral polymerase chain reaction testing for Eastern equine encephalitis was positive. Rapid progression of neurologic signs and respiratory arrest necessitated mechanical ventilation. Severe hypernatremia, most consistent with central diabetes insipidus, developed. Key clinical message: Transtentorial and foramen magnum herniation and high cerebrospinal fluid cell counts may be indicators of poor prognosis. Brain death, respiratory arrest, and central diabetes insipidus may also ensue with Eastern equine encephalitis infection.

Imagerie par résonance magnétique, résultats clinicopathologiques et progression clinique d'un cas confirmé d'infection par le virus de l'encéphalite équine de l'Est chez un chiot. Un chiot de 5 mois a été évalué pour des signes neurologiques progressant rapidement et une pyrexie. L'imagerie par résonance magnétique a montré une méningo-encéphalite multifocale avec hernies transtentorielle et au foramen magnum. Un prélèvement de liquide céphalo-rachidien a révélé un liquide hautement cellulaire et le chiot a été euthanasié. L'histopathologie a montré une méningo-encéphalite lymphoplasmocytaire et neutrophilique. Un test de réaction en chaîne par la polymérase pour l'encéphalite équine de l'Est était positif. La progression rapide des signes neurologiques et l'arrêt respiratoire ont nécessité une ventilation mécanique. Une hypernatrémie sévère, plus compatible avec un diabète insipide central, s'est développée.Message clinique clé:Une hernie transtentorielle et au foramen magnum de même qu'un nombre élevé de cellules dans le liquide céphalorachidien peuvent être des indicateurs de mauvais pronostic. La mort cérébrale, l'arrêt respiratoire et le diabète insipide central peuvent également s'ensuivre avec une infection par l'encéphalite équine de l'Est.(Traduit par Dr Serge Messier).

Deadly Neuroinvasive Mosquito-Borne Virus: A Case of Eastern Equine Encephalitis.

None: Eastern equine encephalitis (EEE) is a rare and potentially fatal neuroinvasive disease with a high mortality rate of > 30%. It is an uncommon vector-borne illness, with an average of 8 cases reported in the United States annually. Alarmingly, in 2019 alone, the Centers for Disease Control and Prevention (CDC) confirmed 38 cases of EEE virus in the United States, reported from 10 states. In this report, we describe a 42-year-old man who worked primarily in wooded areas and presented to a hospital in southern New Jersey with an intractable headache and global facial paraesthesia. He reported multiple tick bites in the weeks prior to his presentation. Based on high clinical suspicion, cerebrospinal samples were sent to the CDC, which confirmed the diagnosis of EEE. The patient was treated with supportive care, and, after spending 9 days on mechanical ventilation in the intensive care unit, he was extubated and subsequently had some improvement of his symptoms with the implementation of an extensive physical therapy program. We hope this report will contribute to increasing awareness among the public health and medical communities regarding the increasing number of EEE cases and the importance of following prevention measures, especially in areas with high prevalence and early recognition of the disease for treatment.

Designing multivalent immunogens for alphavirus vaccine optimization.

There is a pressing need for vaccines against mosquito-borne alphaviruses such as Venezualen and eastern equine encephalitis viruses (VEEV, EEEV). We demonstrate an approach to vaccine development based on physicochemical properties (PCP) of amino acids to design a PCP-consensus sequence of the epitope-rich B domain of the VEEV major antigenic E2 protein. The consensus "spike" domain was incorporated into a live-attenuated VEEV vaccine candidate (ZPC/IRESv1). Mice inoculated with either ZPC/IRESv1 or the same virus containing the consensus E2 protein fragment (VEEVconE2) were protected against lethal challenge with VEEV strains ZPC-738 and 3908, and Mucambo virus (MUCV, related to VEEV), and had comparable neutralizing antibody titers against each virus. Both vaccines induced partial protection against Madariaga virus (MADV), a close relative of EEEV, lowering mortality from 60% to 20%. Thus PCP-consensus sequences can be integrated into a replicating virus that could, with further optimization, provide a broad-spectrum vaccine against encephalitic alphaviruses.

Eastern Equine Encephalitis Virus in Mexican Wolf Pups at Zoo, Michigan, USA.

During the 2019 Eastern equine encephalitis virus (EEEV) outbreak in Michigan, two 2-month old Mexican wolf pups experienced neurologic signs, lymphohistiocytic neutrophilic meningoencephalitis with neuronal necrosis and neuronophagia, and acute death. We identified EEEV by reverse transcription real-time PCR and in situ hybridization. Vector mosquitoes were trapped at the zoo.

A Review of the Control of Aedes aegypti (Diptera: Culicidae) in the Continental United States.

Aedes aegypti (L) is an anthropophilic mosquito involved in the transmission of a variety of viral pathogens worldwide including dengue, chikungunya, yellow fever, and Zika viruses. This species, native to Africa, is well established in the continental U.S. (CONUS) and occasionally contributes to localized outbreaks of viral diseases. In the last seven decades, mosquito control programs in the CONUS have been focused on vectors of eastern equine encephalitis, St. Louis encephalitis, and West Nile viruses, as well as nuisance species. Aedes aegypti receives little control focus except during outbreak periods, which has led to a lack of information on appropriate and effective control options targeting Ae. aegypti in the CONUS. As such, in the event of an Ae. aegypti-borne arboviral outbreak in the CONUS, there are limited evidence-based control recommendations or protocols in place. Autochthonous outbreaks of Ae. aegypti-borne pathogens have occurred recently in the CONUS, including dengue outbreaks in 2010 and 2013, a chikungunya outbreak in 2014, and the 2016 outbreak of Zika virus. The increasing frequency of Ae. aegypti-borne outbreaks necessitates increased attention and research on control of this species to prevent and mitigate future outbreaks. This review consolidates and synthesizes the available literature on control of Ae. aegypti, specifically within the CONUS, focusing on data generated through operational applications as well as field and semifield experiments. The purpose of this review is to identify and highlight areas where additional research is needed. The review covers chemical control and insecticide resistance, biological control, source reduction, trapping, and alternative techniques.

Eastern Equine Encephalitis Virus: A Scoping Review of the Global Evidence.

Background: Eastern equine encephalitis virus (EEEV) is a mosquito-borne virus that is primarily found in North America and the Caribbean. Over the past decade there has been an increase in virus activity, including large outbreaks in human and horse populations. Predicted climate change is expected to affect the range of mosquitoes including vectors of EEEV, which may alter disease risk posing a public health concern. Methods: A scoping review (ScR) was conducted to identify and characterize the global evidence on EEEV. A thorough search was conducted in relevant bibliographic databases and government websites. Two reviewers screened titles and abstracts for relevance and the characteristics of relevant articles were extracted using a uniformly implemented data collection form. The study protocol was developed a priori and described the methods and tools used and this article follows the PRISMA-ScR guidelines for reporting ScRs. Results: The ScR included 718 relevant research articles. The majority of the articles originated from North America (97%) between 1933 and 2019. EEEV has been identified in 35 species of mosquitoes, over 200 species of birds, various domestic animals, wild mammals, reptiles, and amphibians. Articles identified in this ScR primarily covered three topic areas: epidemiology of hosts and vectors (344 articles) including surveillance results (138), pathogenesis of EEEV in hosts (193), and in vitro studies characterizing EEEV (111). Fewer articles evaluated the accuracy of diagnostic tests (63), the efficacy of mitigation strategies (62), transmission dynamics (56), treatment of EEEV in hosts (10), societal knowledge, attitudes, and perceptions (4), and economic burden (2). Conclusion: With the projected impact of climate change on mosquito populations, it is expected that the risk of EEEV could change resulting in higher disease burden or spread into previously unaffected areas. Future research efforts should focus on closing some of the important knowledge gaps identified in this ScR.

Eastern equine encephalitis and use of IV immunoglobulin therapy and high-dose steroids.

OBJECTIVE: To determine the clinical presentation and patient outcomes after treatment with IV immunoglobulin (IVIG), high-dose steroids, or standard of care alone in Eastern equine encephalitis (EEE), a mosquito-borne viral infection with significant neurologic morbidity and mortality. METHODS: A retrospective observational study of patients admitted to 2 tertiary academic medical centers in Boston, Massachusetts, with EEE from 2005 to 2019. RESULTS: Of 17 patients (median [IQR] age, 63 [36-70] years; 10 (59%) male, and 16 (94%) White race), 17 patients had fever (100%), 15 had encephalopathy (88%), and 12 had headache (71%). Eleven of 14 patients with CSF cell count differential had a neutrophil predominance (mean = 60.6% of white blood cells) with an elevated protein level (median [IQR], 100 mg/dL [75-145]). Affected neuroanatomic regions included the basal ganglia (n = 9/17), thalamus (n = 7/17), and mesial temporal lobe (n = 7/17). A total of 11 patients (65%) received IVIG; 8 (47%) received steroids. Of the patients who received IVIG, increased time from hospital admission to IVIG administration correlated with worse long-term disability as assessed by the modified Rankin Scale (mRS) (r = 0.72, p = 0.02); steroid use was not associated with the mRS score. The mortality was 12%. CONCLUSIONS: Clinicians should suspect EEE in immunocompetent patients with early subcortical neuroimaging abnormalities and CSF neutrophilic predominance. This study suggests a lower mortality than previously reported, but a high morbidity rate in EEE. IVIG as an adjunctive to standard of care may be considered early during hospitalization.

Eastern Equine Encephalitis Virus Seroprevalence in Maine Cervids, 2012-2017.

Eastern equine encephalitis virus (EEEV) first emerged in Maine in the early 2000s and resulted in an epizootic outbreak in 2009. Since 2009, serum samples from cervids throughout Maine have been collected and assessed for the presence of neutralizing antibodies to EEEV to assess EEEV activity throughout the state. We tested 1,119 Odocoileus virginianus (white-tailed deer) and 982 Alces americanus (moose) serum samples collected at tagging stations during the hunting seasons from 2012 to 2017 throughout the state of Maine. Odocoileus virginianus from all 16 counties were EEEV seropositive, whereas A. americanus were seropositive in the northwestern counties of Aroostook, Somerset, Piscataquis, and Franklin counties. Seroprevalence in O. virginianus ranged from 6.6% to 21.2% and in A. americanus from 6.6% to 10.1%. Data from this report in conjunction with findings previously reported from 2009 to 2011 indicate that EEEV is endemic throughout Maine.

Endemic and Epidemic Human Alphavirus Infections in Eastern Panama: An Analysis of Population-Based Cross-Sectional Surveys.

Madariaga virus (MADV) has recently been associated with severe human disease in Panama, where the closely related Venezuelan equine encephalitis virus (VEEV) also circulates. In June 2017, a fatal MADV infection was confirmed in a community of Darien Province. We conducted a cross-sectional outbreak investigation with human and mosquito collections in July 2017, where sera were tested for alphavirus antibodies and viral RNA. In addition, by applying a catalytic, force-of-infection (FOI) statistical model to two serosurveys from Darien Province in 2012 and 2017, we investigated whether endemic or epidemic alphavirus transmission occurred historically. In 2017, MADV and VEEV IgM seroprevalences were 1.6% and 4.4%, respectively; IgG antibody prevalences were MADV: 13.2%, VEEV: 16.8%, Una virus (UNAV): 16.0%, and Mayaro virus: 1.1%. Active viral circulation was not detected. Evidence of MADV and UNAV infection was found near households, raising questions about its vectors and enzootic transmission cycles. Insomnia was associated with MADV and VEEV infections, depression symptoms were associated with MADV, and dizziness with VEEV and UNAV. Force-of-infection analyses suggest endemic alphavirus transmission historically, with recent increased human exposure to MADV and VEEV in Aruza and Mercadeo, respectively. The lack of additional neurological cases suggests that severe MADV and VEEV infections occur only rarely. Our results indicate that over the past five decades, alphavirus infections have occurred at low levels in eastern Panama, but that MADV and VEEV infections have recently increased-potentially during the past decade. Endemic infections and outbreaks of MADV and VEEV appear to differ spatially in some locations of eastern Panama.

National Seroprevalence and Risk Factors for Eastern Equine Encephalitis and Venezuelan Equine Encephalitis in Costa Rica.

Eastern equine encephalitis and Venezuelan equine encephalitis are endemic neglected tropical diseases in the Americas, causing encephalitis in both horses and humans. In 2013, a cross-sectional study was performed in 243 horses located in the highlands and lowlands throughout Costa Rica. Serum samples were analyzed with an IgG ELISA and confirmed by the plaque-reduction neutralization test (PRNT80). Venezuelan equine encephalitis virus (VEEV) and Eastern equine encephalitis virus (EEEV) overall seroprevalences by the PRNT80 were 36% (95% confidence interval [CI]: 29.9-42.5; 78/217 horses) and 3% (95% CI: 1.3-5.9; 6/217 horses), respectively. Both the viruses occurred in the lowlands and highlands. Rainfall and altitude were associated with VEEV seropositivity in the univariate analysis, but only altitude <100 meters above sea level was considered a risk factor in the multivariate analysis. No risk factors could be identified for the EEEV in the multivariate analysis. This is the first study that estimates the seroprevalence of the EEEV and VEEV in Costa Rican horses. The VEEV is widely distributed, whereas the EEEV occurs at a much lower frequency and only in specific areas. Clinical cases and occasional outbreaks of both viruses are to be expected.

Impact of the Southern Oscillation Index, Temperature, and Precipitation on Eastern Equine Encephalitis Virus Activity in Florida.

Eastern equine encephalitis virus (EEEV), an Alphavirus from family Togaviridae, is a highly pathogenic arbovirus affecting the eastern United States, especially Florida. Effects of the Southern Oscillation Index (SOI), precipitation, and cooling degree days on EEEV horse case data in Florida from 2004 to 2018 were modeled using distributed lag nonlinear models (DLNMs). The analysis was conducted at statewide and regional scales. DLNMs were used to model potential delayed effects of the covariates on monthly counts of horse cases. Both models confirmed a seasonal trend in EEEV transmission and found that precipitation, cooling degree days, and the SOI were all predictors of monthly numbers of horse cases. EEEV activity in horses was associated with higher amounts of rainfall during the month of transmission at the statewide scale, as well as the prior 3 mo at the regional scale, fewer cooling degree days during the month of transmission and the preceding 3 mo and high SOI values during the month and the previous 2 mo, and SOI values in the prior 2 to 8 mo. Horse cases were lower during El Niño winters but higher during the following summer, while La Niña winters were associated with higher numbers of cases and fewer during the following summer. At the regional scale, extremely low levels of precipitation were associated with a suppression of EEEV cases for 3 mo. Given the periodicity and potential predictability of El Niño Southern Oscillation (ENSO) cycles, precipitation, and temperature, these results may provide a method for predicting EEEV risk potential in Florida.