Lush Desert in Arizona—A Sign of Trouble Ahead?
Stan Deresinski, MD, FACP, Clinical Professor of Medicine, Stanford; Associate Chief of Infectious Diseases, Santa Clara Valley Medical Center, is Associate Editor for Infectious Disease Alert
The exuberant rainfall in the southwestern United States this year has led to the greening of the deserts, highlighted by spectacular displays of wildflowers. But it may not be just the plant life that has benefited. A number of human pathogens may find the conditions just right.
Sin Nombre virus
El Nino reared its disruptive head in 1991-1992, accounting for greatly increased amounts of precipitation in the Southwest. This led to increased growth of vegetation, including pinyon pines that deposited their nuts in abundance on the ground, providing a large and readily available supply of food to the local rodent population that "went forth and multiplied". The resultant rodent population explosion led to an increase in their direct and indirect contact with humans, a number of whom became infected with a previously unkown hantavirus, Sin Nombre, as the result of contact with the deer mouse (Peromyscus maniculatus), and developed the hantavirus pulmonary syndrome.1 An El Nino event in 1997-1998 was followed in the subsequent year by a 5-fold increase in human cases of hantavirus infection in the same Four Corners region, where the borders of Arizona, Utah, New Mexico, and Colorado meet. These observations suggest that there may be a resurgence of hantavirus pulmonary syndrome in 2005.
Coccidioidomycosis
Coccidioides immitis mycelia residing in the dry soil of the Lower Sonoran Life Zone require increased moisture for growth. A subsequent period of dryness allows fragmentation of arthroconidia which can become airborne when the soil is disturbed, as by wind. A very recent exploration of the relationship of weather patterns to the incidence of coccidioidomycosis in Pima County (Tucson), Arizona concluded that ". . . precipitation during the normally arid foresummer [April through June], 1.5-2 years prior to the season of exposure, is the dominant predictor ofthe disease. . ."2 Thus, the current rains may lead to a significant increase the incidence of coccidioidomycosis in 2006.
West Nile virus
The activity of Culex mosquitoes, which are competent vectors of WNV, is reported to be maximal when dry weather follows heavy rains. Heavy rains followed by drying leaves areas of standing water that may provide an ideal habitat for breeding of Culex mosquitoes, including C. tarsalis, a highly competent vector of West Nile virus inthe western United States. Thus, the incidence of West Nile virus infection in Arizona in 2005 may well surpass the 391 cases recorded in 2004.3
Others
Culex mosquitoes, including C. tarsalis, are also vectors of St. Louis encephalitis virus, as they are for West Nile virus, suggesting that this arboviral infection may also be encountered in 2005. Aedes aegypti, the vector of dengue, is present in many cities and towns throughout Arizona, and may also encounter favorable conditions this year.4 Leptospirosis, a disease which may emerge after flooding, is present in many animal species in Arizona, but human cases have been rare in the past. The incidence of plague in New Mexico is associated with increased winter-spring rainfall, presumably because of favorable reproductive conditions for rodent or flea populations.5 Some tick populations are reported to increase after heavy periods of rainfall, raising the possibility of an increased incidence of some infections, including tick-borne relapsing fever, and Rocky Mountain spotted fever, although these may be more likely to be acquired in non-desert areas. For instance, tick-borne relapsing fever has been acquired at the north rim of the Grand Canyon.6
Conclusion: The bottom line is, watch out for an increased incidence of arthropod-borne diseases this year and of coccidioidomycosis in 2006.
References
- Hjelle B, et al. Outbreak of Hantavirus Infection in the Four Corners Region of the United States in the Wake of the 1997-1998 El Nino-Southern Oscillation. J Infect Dis. 2000;181:1569-1573.
- Comrie AC. Climate Factors Influencing Coccidioidomycosis Seaonality and Outbreaks. Environmental Health Perspectives (available at www.dx.doi.org).
- www.azdhs.gov/phs/oids/epi/9601cases.htm
- Merrill SA, et al. Phylogeography and Population Strucure of Aedes aegypti in Arizona. Am J Trop Med Hyg. 2005;72:304-310.
- Parmenter RR, et al. Incidence of plague associated with increased winter-spring precipitation in New Mexico. AM J Trop Med Hyg. 1999;61:814-821.
- Boyer KM, et al. Tick-Borne Relapsing Fever: An Interstate Outbreak Originating at Grand Canyon National Park. Am J Epidemiol. 1977;105:469-479.
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