This project sought to discover the basic relationship between climate conditions and valley fever incidence, the climate variables that were important determinants of incidence, and if incidence could be predicted by climate conditions.
Researchers compared medical records documenting valley fever incidence and climate data from Kern County, CA and Pima County, AZ from 1989-1998. The data analysis revealed a complex relationship in which winter and summer rains play a key factor. The number of peak cases in Pima Co. usually occurred in the dry months right before the summer monsoon and again in November and December.
The researchers developed a model to predict valley fever outbreaks for Pima Co. using climate forecasts and weather data to project future risks for the disease.
Each year, an estimated 100,000 people contract valley fever (medical name coccidioidomycosis)—a lung infection caused by a soil-dwelling fungus found primarily in the desert regions of Arizona and California. Most never show any symptoms, while others experience mild cold or flu-like ailments.
Less than 10% of infections progress to more severe illnesses and in rare cases the fungus moves outside the lungs to the muscles, bones, or skin. At its worst, the disease can cause a form of meningitis—leading to between 50 and 100 deaths per year. Annual healthcare costs related to valley fever are upwards of $60 million.
Researchers in the 1950s discovered the microscopic culprit for the disease, the fungus Coccidioides immitis. They also noted links between outbreaks of valley fever and climatic conditions, with peak cases following heavy rainy seasons.
While researchers are still learning about the disease's link to climate, it is believed that the fungus grows during wet periods and then forms tiny spores as the soil dries out. The spores release into the air when windy conditions or human activities, such as construction, disturb the soil.
Hot weather may also be an important factor in the mix. It is believed that extremely high temperatures can sterilize the soil, possibly killing other microorganisms while C. immitis remains dormant in deeper layers of the soil. When conditions change, the fungus may return to the surface layer flourish in the soil with little competition.
As the population of the Soutwestern United States continues to swell, thousands of more people are exposed to the disease each year. The highest incidences of valley fever occur near Tucson and Phoenix in Arizona and near Bakersfield, Calif., but the fungus is also found in parts of Texas, Nevada, and New Mexico. If climate change makes conditions more favorable for the fungus in these areas, large cities such as Los Angeles, El Paso, and Las Vegas could be at risk.
An exploratory data analysis was conducted during the early part of the project to determine the basic relationship between climate and incidence. This portion of the study involved a bivariate analysis of individual climate variables, including temperature (minimum, maximum, dew point, and average), precipitation, wind speed, and the Palmer Drought Severity Index.
A composite analysis was also conducted, which examined the climate conditions leading up to months with especially high and especially low incidence.
The results of the exploratory analysis informed the development of monthly multivariate predictive models. For each month, temperature and precipitation variables at varying time scales up to four years prior to the month being predicted were identified from the exploratory analysis. These variables were entered into a backward stepwise regression. The resulting models were tested on independent data to evaluate their ability to predict incidence.
Comrie, A. 2005. Climate factors influencing coccidioidomycosis seasonality and outbreaks. Environmental Health Perspectives 113:688-692.
Kolivras, K. and A. Comrie. 2003. Modeling valley fever (coccidioidomycosis) incidence on the basis of climate conditions, International Journal of Biometeorology 47(2): 87. DOI 10.1007/s00484-002-0155-x
Kolivras, K. and A. Comrie. 2000. Climate and Valley Fever. In Chapter 8 - Human Health, by Southwest Regional Assessment Group, Preparing for a Changing Climate: The Potential Consequences of Climate Variability and Change. Companion Volume to the National Assessment of Climate Change. Institute for the Study of Planet Earth, University of Arizona.
Kolivras, K., P. Johnson, A. Comrie, and S. Yool. 2001. Environmental Variability and Coccidioidomycosis (Valley Fever). Aerobiologia 17:31-42.
Park, B., K. Sigel, V. Vaz, K. Komatsu, C. McRill, M. Phelan, T. Colman, A. Comrie, D Warnock, J. Galgiani, and R. Hajjeh. 2005. An epidemic of coccidioidomycosis in Arizona associated with climate changes, 1998-2001. Journal of Infectious Diseases 191:1981–1987.