Annie Kao
Kimberly Poggemeyer

Descriptive Epidemiology of Coccidioidomycosis Using GIS, San Diego County, 1990-2000.

Coccidioidomycosis, a fungal infection caused by inhalation of spores found in the soil, is endemic in the southwest United States and parts of Mexico. Surveillance data show the incidence of coccidioidomycosis to be significantly higher in certain border regions of San Diego County and affects specific minority populations. Possible risk factors associated may include increased excavation and construction activities. This paper integrates reported cases during 1990-2000 with geographic and land development data. Geographically defined risk factors and temporal trends are also assessed. GIS facilitates the identification of high-risk areas in San Diego County for targeted intervention and prevention strategies.

Background

Coccidioidomycosis is an infection caused by the inhalation of fungal spores from Coccidioides immitis, a dimorphic fungus that grows primarily in desert soil. Soil ecology studies have found the organism to be present in hot, dry, alkaline soil. During moist seasons, C. immitis forms arthroconidia or arthrospores, which lie dormant in the soil until some disruption of the hyphae occurs, usually by wind (dust storms), earthquake, recreational activity, archeological excavation, construction, or land cultivation. Infections occur when arthrospores are inhaled by the host. Regions identified with highest endemicity in North America include the South Western portion of the United States and the desert areas of northern Mexico.

Approximately 60% of the infected population are asymptomatic or present with minor symptoms similar to influenzae-like illness. Disseminated or extrapulmonary disease develop in approximately 1 in 200 infected cases, and are more frequently found in men, pregnant women, immunocompromised individuals, and minority populations such as Hispanics, African Americans, and Filipinos.

This study aims to describe the epidemiology of coccidioidomycosis in San Diego County from 1990-2000, and to assess possible spatial clusters of cases. Geographically defined risk factors and temporal trends are also assessed.

Methods

Between 1990 and 2000, over 400 cases of coccidioidomycosis were reported to the San Diego County Health and Human Services Agency. Analysis of the data was conducted using EpiInfo version 6.04c and SPSS version 10.0. Maps illustrating average annual rates of disease and year-specific rates and maps of geographic characteristic within the county were created using ArcView 3.2. Case reports included demographic information, date of diagnosis, date of report and onset, and zip codes. Rates for zip code areas were calculated using disease frequency or counts in the numerator and the total populations within each zip code as the denominator. Besag-Newell corrections for areas with variable populations were used to normalize the crude rates. Chi-square test for linear trend was used to test the yearly rates of coccidioidomycosis over time.

Descriptive Analysis

The geographic location of San Diego County, low seasonal rainfall, mild winters, and warm summers, result in dry soil and dusty conditions that allow for C. immitis to be viable in the soil micro-flora. Previous reports of residents and their activities in association with Coccidioidomycosis indicate that widespread areas of the county may harbor C. immitis in the soil. Onset of illness was frequently reported during winter months, though monthly yields were not specific to any one year. In 2000, county incidence rates for coccidioidomycosis rose from 1.22 to 2.13 per 100,000 population, a 43% increase from the previous year. Trends analysis of the crude rates over the last few years show a significant linear increase in disease rates from 1996 to 2000 (Chi-square=4.6, p<.05).

Analysis of the data from 1990 to 2000 showed that the number of cases and respective rates were higher in the South Region compared to other Health Services Regions, and remained consistently high for each subsequent year. African-Americans showed higher rates in all regions with the exception of the South Region, where Asians exhibited higher rates. Possible risk factors may include increased excavation and construction activities in these areas and a higher proportion of minority populations.

Discussion

This study utilizes the combined tools of surveillance, biostatistics and spatial analysis to examine and assess the epidemiology of coccidioidomycosis in San Diego County. It provides an exploratory approach to assessing public health data where many variables are dynamic. Other risk factors which contributes to the transmission of C. immitis include influences by wind, climatic changes, soil characteristics and their ability for dispersal, and seismic events such as earthquakes. Additional risk factor information about the individual and his environment may provide further explanations related to increased incidence and may help epidemiology and public health researchers maximize the ability to predict and target areas with potentially high environmental influences.

The information conveyed is more useful when decisions for targeted prevention, resource allocation, and issues of cost-effectiveness are considered. In addition, areas of highest risk may be identified for further studies and preventive efforts and monitored for reporting efficiency. This study also emphasized the need to promote better data ascertainment regarding the risk factors and geo-referenced data of coccidioidomycosis to reflect the location of exposure. Finally, the collaboration of geography and epidemiology offer more creative methods to study the distribution and dissemination of disease.

References or Acknowledgments

Layers of San Diego County were provided by SANGIS.

Population data was provided by SANDAG.

Annie S. Kao, MPH, MS
Epidemiologist, San Diego County Health and Human Services Agency
1700 Pacific Hwy, MS:P511C-A
San Diego, CA 92186
Tel: (619) 515-6620
Fax: (619) 515-6644

Kimberly Poggemeyer, MPH
Senior Epidemiologist
County of San Diego