Characteristics of the cases
Sixty-nine patients tested for LACE by MHS in 2010-2012 were classified as either confirmed (n=58) or probable (n=11). The majority of cases were white (93%), non-Hispanic (94%), male (54%), and under the age of 16 (80%) with a median age of 8 years (Table 2). The occurrence of LACE followed the typical arboviral seasonality with 97% of cases occurring from May to October with a peak in late
49
summer (Figure 7). The median time between symptom onset and hospitalization was 3 days (range: 0- 31). Upon hospital evaluation, the majority of LACE case visits were inpatient (96%), with a median length of stay of 4 days (range: 0-11) (Table 3).
The most common signs and symptoms among LACE cases were clinically measured fever >38⁰C (81%), headache (70%), vomiting (65%), altered mental status (42%), seizures (30%), and nuchal rigidity (9%). For arboviral testing, acute serum was collected for all but 2 cases (97%), and 86% of cases provided acute CSF, while convalescent serum (9%) and CSF (3%) were less frequently tested. Paired testing of acute and convalescent serum or CSF was recorded in only 6% of the cases. Additional diagnostic tests to rule out other pathologic causes of viral encephalitis included Herpes Simplex virus (65%), enterovirus (55%), and Rocky Mountain Spotted Fever (14%), all of which were negative with the exception of one enterovirus co-infection (Table 3).
Characteristics of the cohort and subcohort
The full cohort, which includes the 69 cases, consisted of 501 individuals. The majority were non-Hispanic (97%), white (91%), male (52%), and over the age of 20 (59%) with a median age of 28 years (Table 2). Individuals from the cohort were tested and enrolled in the cohort throughout the study period, with an increase in the months around the typical arboviral season (Figure 7). The subcohort consisted of 409 patients with successfully geocoded addresses, after exclusions. Excluded patients did not differ with respect to any of the measured demographics, with the exception of race, where 10 of 15 American Indians were excluded (results not shown).
Of the 409 individuals in the subcohort, 58 (14%) were LACE cases. The majority of the subcohort lived in single-family homes (61%) in urban (52%) areas. Developed areas (55%) were the most common habitat type around the residence, followed by forest (35%), and farmland (11%) (Table 4). The mean number of objects quantified within 50 meters of the home was 7.5, with a median of 6
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objects (range: 0-36) (results not shown). Sixty-one percent of residencies had 7 or fewer objects quantified around the home (Table 4).
Analysis of risk factors
There was no difference in LACE risk by gender, race, type of home, and the number of objects around the home in bivariate associations within the subcohort. The risk of LACE among patients under the age of 15 was more than 7 times the risk of LACE in patients at least 15 years of age (RR: 7.69; 95% CI 4.22, 14.04), and the risk of LACE was higher among Hispanic patients when compared to non- Hispanic patients (RR: 2.68; 95% CI: 1.18, 6.09). Individuals living in rural areas had a higher risk of LACE than those in urban areas (RR: 2.23; 95% CI: 1.34, 3.73), and patients residing in forested areas had a higher risk of LACE when compared to developed areas (RR: 2.69; 95%: 1.64, 4.42). The risk of LACE among those living in forested areas was 3.08 times the risk of those in non-forest areas (95% CI: 1.89, 5.02) (Table 4).
When adjusted for age, the only confounder identified, the RRs were attenuated but the associations between LACE and those living in forested areas (RR: 2.08; 95% CI: 1.31, 3.32) and those living in rural areas (RR: 1.76; 95% CI: 1.09, 2.84) remained. Due to the high correlation between these two variables, a jointly exposed forested and rural variable was created to avoid collinearity in the fully adjusted model. The risk of LACE for those living in rural forested areas was 2.57 times that of
individuals living in rural and/or non-forested areas after adjustment for age (RR: 2.57; 95% CI: 1.61, 4.08). When controlling for age, there remained no difference in LACE risk by any other measured peridomestic risk factors (Table 5). The estimations of adjusted RRs among the full cohort, in sensitivity analyses, were similar to those presented in the subcohort (results not shown).
51 E. Discussion
This is the largest patient-level epidemiologic study assembled to assess LACE risk factors, with 69 cases among our cohort of 501 individuals who underwent arboviral testing. The overall volume of arboviral tests conducted indicated a keen awareness of LACE among MHS physicians in their active surveillance efforts. The cases identified from MHS represented almost one quarter of all LACE cases in the US reported to CDC’s ArboNET from 2010-2012, and represented 95% of the western NC cases reported by NCDPH.74
The demographics of the LACE cases were similar to previous findings, with the exception of gender. Historically, white males make up between 60-80% of reported cases.14,39-41 However, in this study, the gender distribution was more balanced, potentially indicating similar mosquito exposure among males and females. Clinically, symptoms of the LACE cases were comparable to prior studies, with febrile illness reported in conjunction with neuroinvasive symptoms. Laboratory diagnostics reinforce that few patients undergo follow-up testing for arboviral disease; only 6% (n=4) had paired acute and convalescent tests. This could be attributable to the ability to confirm tests using a single specimen, but does not include individuals who may have had follow-up visits outside MHS.
The quantification of LACE risk factors from our study provides practical, easily communicated information on the risk of LACE in peridomestic areas of western NC. By design, the risk of LACE in our study population was higher than that of the general population, but the cohort is generally
representative of the demographics of western NC, reported as 89% white, 5% Hispanic, and 49% male.76
This study identified a clear association between LACE risk and residence in rural forested areas; the risk of LACE for those living in rural forested areas was more than 2.5 times the risk for those living in non-forested and urban areas. This association is ecologically well founded and commonly understood for LACE, but the only forest metric to be identified as a significant risk factor in prior individual level
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epidemiologic analyses is the number of treeholes around a residence. In studies by Woodruff et al. (1992) and Erwin et al. (2002), proximity to the forest edge had an elevated but non-significant
association with LACE risk.39,47 Perhaps our broader approach of assessing the predominant habitat type around the home, rather than specifically quantifying the distance to the forest or the potentially unseen number of treeholes, is a more practical means of assessing risk and translating this risk for the general public.
Rural residence is another commonly cited risk factor for LACE, and was identified as a risk factor in our bivariate and age-adjusted analyses. However, this variable was highly correlated with presence of forest and had insufficient statistical independence for proper assessment — those living in rural areas were much more likely to live in a forested area than those in urban areas. Furthermore, no cases lived in urban forested areas and very few lived in rural non-forested areas. For these reasons, the joint exposure of rural forested areas was used with the confidence there were no mixing of effects within this study.
Investigations into additional peridomestic risk factors yielded no clear association, reinforcing the finding that location of the residence is the most important risk factor for LACE. Residence in mobile homes was a hypothesized risk factor, as it may increase mosquito exposure due to potentially poorer quality construction and lack of window screens or air conditioning, but no associations were found in this study. These variables were assessed by Woodruff et al. (1992) and Erwin et al. (2002) with similarly null associations with LACE. These studies also quantified the number of tires or other containers around the yard and the general tidiness of the yard, finding no associations with LACE risk.39,47 Our
quantification of objects around the home was a general approximation of potential containers and yard tidiness, but also found no association with LACE risk.
It must be noted that this study assumed that the most likely place of exposure was around the home and assumes the peridomestic exposures were constant between the time of measurement and
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the likely exposure period. The former is a common assumption of peridomestic risk factor studies, and the latter is justifiable given that land use change often occurs slowly and all exposures were assessed from data derived during the study period. Our subcohort analysis was also limited by the exclusion of non-geocodable addresses and residence outside of the western NC study area. There was no method to identify missing residence or areas visited without violating study protocols, so data were considered missing completely at random. Non-geocodable addresses are commonly the result of post office boxes or indeterminate addresses in rural areas and American Indian Reservations, potentially introducing differential misclassification of exposure.64 However, any bias introduced by these missing data was minimal, based on results of our sensitivity analyses. Lastly, all patients included in this cohort were tested for arboviral disease, thus implying clinical suspicion of LACE and potential bias towards patients with known risk factors such as age or residence in forested areas. This potential bias may have masked additional risk factors. However, aside from selection bias by age, which was controlled for in the analyses, the distribution of peridomestic risk factors in our study suggests little, if any, preferential selection.
F. Conclusions
Western NC continues to identify a substantial proportion of the nationally reported LACE cases, and the continued support of active surveillance efforts is critical in this endemic area. The results of this study identify residence in rural forested areas as a peridomestic risk factor for LACE, and emphasize that location of residence is the most important risk factor for LACE. For those living in rural forested areas, the risk of LACE was over 2.5 times the risk of LACE for those living in non-forested and urban areas. Other potential peridomestic risk factors measured had no association with LACE risk. Practically, the risk factors identified in this study can be easily communicated and assessed by the general public, with the aim of increasing awareness and knowledge of LACE risk factors in endemic areas.
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Table 2. Demographic characteristics of confirmed and probable LACE cases (n=69) and full cohort (n=501), Mission Health System, 2010-2012.
LACE Cases Full Cohort
Demographicsa n (%) n (%) Number of Patients 69 (100) 501 (100) Gender Female 32 (46) 239 (48) Male 37 (54) 261 (52) Race White 62 (93) 443 (91) American Indian 4 (6) 15 (3) Black 1 (1) 29 (6) Ethnicity Non-Hispanic 65 (94) 485 (97) Hispanic 4 (6) 16 (3)
Age Group (Years)
0-5 21 (30) 67 (13) 6-10 25 (36) 61 (12) 11-15 9 (13) 47 (9) 16-20 1 (1) 32 (6) 21-30 4 (6) 63 (13) 31-40 0 (0) 53 (11) 41-50 3 (4) 45 (9) 51-60 1 (1) 42 (8) 61-70 3 (4) 47 (9) >70 2 (3) 43 (9)
Median Age (Range) 8 (1-74) 28 (0-91) a
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Table 3. Clinical and laboratory characteristics of confirmed and probable LACE cases (n=69), Mission Health System, 2010-2012.
Clinical & Laboratory Characteristics of Cases
LACE Cases n (%)
Time to Hospitalization (Days)
Mean (St Dev) 4 (3.9)
Median (Range) 3 (0-31)
Type of Hospital Visit
Inpatient 66 (96)
Emergency Department 2 (3)
Observation Unit 1 (1)
Length of Stay (Days)
Mean (St Dev) 4.5 (2.3)
Median (Range) 4.0 (0.3-10.8)
Signs and Symptoms
Fever (clinically measured > 38⁰C) 56 (81)
Headache 48 (70)
Vomiting 45 (65)
Altered Mental Status 29 (42)
Seizure 21 (30)
Nausea 16 (23)
Nuchal Rigidity 6 (9)
Arboviral Panel Tests
Acute Serum 67 (97)
Acute CSF 59 (86)
Convalescent Serum 6 (9)
Convalescent CSF 2 (3)
Paired Acute & Convalescent (Serum or CSF) 4 (6)
Related Diagnostic Tests
HSV 45 (65)
Enterovirus 38 (55)
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Table 4. Potential risk factors for LACE measured among the Mission Health System subcohort (n=409), 2010-2012.
LACE Cases (n=58)
Subcohort
(n=409) Crude Risk Ratio
n (%a) n (%b) (95% Confidence Interval) Number of Patients 58 (14) 409 (100) -- Age > 15 years 12 (4) 273 (67) REF < 15 years 46 (34) 136 (33) 7.69 (4.22, 14.04) Gender Female 26 (13) 200 (49) REF Male 32 (15) 209 (51) 1.18 (0.73, 1.90) Race White 54 (15) 367 (92) REF American Indian 1 (20) 5 (1) 1.36 (0.23, 7.98) Black 1 (4) 27 (7) 0.25 (0.04, 1.75) Ethnicity Non-Hispanic 54 (14) 398 (97) REF Hispanic 4 (36) 11 (3) 2.68 (1.18, 6.09)
Predominant Habitat Type
Developed 21 (9) 223 (55) REF
Forest 36 (25) 142 (35) 2.69 (1.64, 4.42)
Farmland 1 (2) 44 (11) 0.24 (0.03, 1.75)
Predominant Habitat Typec
Non-forest 22 (8) 267 (65) REF Forest 36 (25) 142 (35) 3.08 (1.89, 5.02) Rural/Urban Urban 19 (9) 213 (52) REF Rural 39 (20) 196 (48) 2.23 (1.34, 3.73) Home Type Single-family 41 (16) 249 (61) REF Multi-unit 5 (9) 56 (14) 0.54 (0.22, 1.31) Mobile Home 12 (12) 98 (24) 0.74 (0.41, 1.35) Other 0 (0) 6 (2) --
Objects With 50m of Residence
< 7 Objects 41 (16) 251 (61) REF > 7 Objects 17 (11) 158 (39) 0.66 (0.39, 1.12) a Row percentage b Column percentage c
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Table 5. Crude and adjusted risk ratios for peridomestic risk factors for LACE among the Mission Health System subcohort (n=409), 2010-2012.
Peridomestic Risk Factors
Crude RRa Age-Adjusted RR (95% CIb) (95% CI) Forest vs Non-forest habitat 3.08 (1.89, 5.02) 2.08 (1.31, 3.32) Rural vs Urban residence 2.23 (1.34, 3.73) 1.76 (1.09, 2.84) Mobile home vs Other home type 0.83 (0.46, 1.50) 0.74 (0.43, 1.28) Many vs < Few objects around homec 0.66 (0.39, 1.12) 0.78 (0.48, 1.27) Rural forested vs Urban and/or non-forest 3.72 (2.28, 6.05) 2.57 (1.61, 4.08)
a
Risk Ratio (RR) b
Confidence Interval (CI) c
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60 CHAPTER 5
MANUSCRIPT 2: SPATIAL CLUSTERING OF LA CROSSE ENCEPHALITIS IN WESTERN NORTH CAROLINA, 2000-2012
A. Overview
La Crosse Encephalitis (LACE) is the most prevalent arbovirus in North Carolina, and over 85% of statewide cases are reported from western NC. Due to the transmission dynamics of LAC virus, LACE is a highly focal disease and typical county-level analyses do not identify the areas of highest disease risk within these endemic counties. Using statewide reportable disease surveillance data from 2000-2012, confirmed and probable cases of LACE in NC were retrospectively characterized and aggregated at the census tract-level to determine the extent and location of spatial clusters of LACE cases. Two hundred twenty-eight cases were reported to the NC Division of Public Health at the NC Department of Health and Human Services during the 13-year study period, and the majority of cases where white, non- Hispanic, male, and under the age of 18. The highest incidence occurred in western NC, and tract-level analyses revealed significant heterogeneity across the study area. Spatial scan statistics were generated in unadjusted and covariate-adjusted models revealing several clusters of high LACE incidence
throughout western NC. Three high incidence clusters spanning multiple counties were identified in unadjusted analyses. Upon adjustment for children <18 years of age and residence in rural forested areas, the 3 high incidence clusters were more compact, and a previously unrecognized secondary cluster was identified. The clusters identified in this study occur at the sub-county level, and may not have been identified using traditional county-level analyses. The clusters identified in this study indicate areas of high LACE risk, and their detection will allow for targeted public health interventions within these areas to reduce the future occurrence of LACE in western NC.
61 B. Introduction
La Crosse (LAC) virus is the most commonly reported cause of pediatric encephalitis in the United States, and the most prevalent endemic arbovirus in North Carolina.4,39,77 La Crosse Encephalitis (LACE) is the neuroinvasive presentation of LAC infection and is characterized by febrile illness
accompanied by neurologic dysfunction, and has been a nationally notifiable disease since 1996.4,12,18 Most LAC infections are subclinical, yet children are disproportionately affected by neuroinvasive disease—over 90% of LACE cases were under 15 years of age in reviews of reportable disease data.4,14,38 LACE is most prevalent in the upper Midwest and throughout the Appalachian regions of North Carolina, Tennessee, West Virginia, and Ohio, which account for almost 70% of all LACE cases reported from 1999-2007.12
The distribution of LACE in endemic areas is highly focal, as a result of the natural transmission cycle of LAC virus. LAC virus is transmitted by the eastern treehole mosquito (Aedes triseriatus), a woodland container-breeding mosquito, and the primary reservoirs are small mammals including the eastern chipmunk (Tamias striatus), eastern gray squirrel (Sciurus carolinensis), and red fox (Vulpes fulva). Ae. triseriatus prefers to breed in root cavities and treeholes in forested areas, but has adapted to the anthropogenic environment breeding in tires and other shaded artificial containers in suburban areas (cite). Peridomestic areas and fragmented forest also provide suitable habitat for LAC virus reservoir species, maintaining the disease transmission cycle both in forested and residential areas.
Given the ecology of the LAC vectors and reservoirs, risk factors for LACE are those that increase contact with mosquitoes in endemic areas—residence in forested areas, time spent outdoors, and treeholes around the home are the primary peridomestic risk factors.39,47 Male children under the age of 15 are the classically identified high risk group due to children’s affinity for more severe disease
presentation and the idea that boys are more likely to spend time outdoors.4,14 Recent research by the authors identified age and residence in rural forested areas as risk factors for LACE in North Carolina.
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Combined with the locations of endemic areas, knowledge of these risk factors can contribute to awareness and prevention of LACE where it exists.
The highly focal nature of LACE cases and the risk factors for LACE lend themselves to
geographic analyses to identify areas of high disease risk, yet few studies exist due to the relatively small number of annual LACE cases occurring sporadically throughout endemic areas. Cases are commonly aggregated at spatial and temporal scales that result in large-scale analyses for reporting purposes with little regard for the fine-scale geographic occurrence of cases. Studies by Kitron et al (1997), Haddow and Odoi (2009), and Haddow et al (2009) began to address these issues by conducting analyses of high incidence clustering of LACE at varying spatial scales and among higher risk subpopulations, confirming the existence of geographically clustered areas of high LACE incidence.10,54,78
Assessing of geographic clustering can identify areas of high and low disease incidence, but methods must account for variance in populations among locations being studied. Clustering of disease may also be influenced by the distribution of risk factors within the population at each location. For example, LACE may be endemic in many areas of western NC, but the incidence of LACE may be considerably higher in an area with a higher proportion of risk factors such as children or individuals living in rural, forested areas. Upon adjustment for the risk factors, any clusters that remain are areas of high incidence independent of the risk factors of the underlying population. Such analyses may identify additional areas of disease risk not readily detected by unadjusted cluster analyses.
Within North Carolina, LACE incidence is highest among the counties in western NC, but county- level analyses are not conducive to identifying fine-scale areas of LACE risk. Using data reported to the North Carolina Division of Public Health (NC DPH) at the NC Department of Health and Human Services (NC DHHS), this study aims to summarize the historical cases of LACE in North Carolina and determine the existence of spatial clustering of LACE within census tracts throughout western NC. The results of
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this study will identify areas of high LACE risk in order to target public health education and prevention to reduce the burden of LACE in western NC.
C. Methods
Surveillance for LACE in North Carolina is coordinated by NC DPH and relies upon local health departments, healthcare practitioners, and diagnostic laboratories to identify cases of LACE throughout the state. Cases are classified as confirmed or probable, as defined by the CDC case definition for neuroinvasive arboviral disease in use at the time of case report, by NC DPH and maintained in the North Carolina Electronic Disease Surveillance System (NC EDSS) database, with case reports beginning in 1997.35,36
LACE cases among NC residents with an onset date between January 1, 2000 and December 31, 2012 were queried from NC EDSS along with select demographics and case information including age, gender, race, Hispanic ethnicity, address, date of onset, county of report, case classification, and patient