Statistical modeling of valley fever data in Kern County, California

Jorge Talamantes,Sam Behseta,Charles S Zender

doi:10.1007/s00484-006-0065-4

Abstract

Coccidioidomycosis (valley fever) is a fungal infection found in the southwestern US, northern Mexico, and some places in Central and South America. The fungus that causes it (Coccidioides immitis) is normally soil-dwelling but, if disturbed, becomes air-borne and infects the host when its spores are inhaled. It is thus natural to surmise that weather conditions that foster the growth and dispersal of the fungus must have an effect on the number of cases in the endemic areas. We present here an attempt at the modeling of valley fever incidence in Kern County, California, by the implementation of a generalized auto regressive moving average (GARMA) model. We show that the number of valley fever cases can be predicted mainly by considering only the previous history of incidence rates in the county. The inclusion of weather-related time sequences improves the model only to a relatively minor extent. This suggests that fluctuations of incidence rates (about a seasonally varying background value) are related to biological and/or anthropogenic reasons, and not so much to weather anomalies.

Highlights

Much is known about the biological, medical, and the epidemiologic aspects of Coccidioides immitis, the fungus which causes valley fever (see Pappagianis (1988), and references therein)
We aim to study the effect of three possible environmental predictors, namely, precipitation Pt, wind speed Ut, and nearsurface air temperature Tt on the response variable Nt - the reported incidence of valley fever cases in Kern county (t is the time in weeks)
We have presented a model that allows us to predict the incidence Nt of valley fever at some t, given the incidence history for earlier times

Summary

Introduction

Much is known about the biological, medical, and the epidemiologic aspects of Coccidioides immitis, the fungus which causes valley fever (coccidioidomycosis) (see Pappagianis (1988), and references therein). C. immitis has a complete life cycle as a soil-dwelling organism, but if it is disturbed and becomes air-borne, it is able to infect a host via the respiratory tract when it inhales the fungus spores. About 60% of infected patients report no symptoms (Lee 1944); about 25% exhibit severe flu-like symptoms such as cough, sputum, fever, and muscle aches; the remaining 15% become very ill with pneumonia-like symptoms (e.g. pleurisy and heavier sputum) requiring medication and bed rest. Endemic areas include the southern part of the San Joaquin Valley in California, southern California, the southern part of Arizona, New Mexico and Texas, most of northern Mexico, and some areas in Guatemala, Honduras, Venezuela, northeastern Brazil, Argentina and Paraguay (Pappagianis 1994).

Objectives

Methods

Results

Conclusion