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Applied and Environmental Microbiology, November 2007, p. 6972-6979, Vol. 73, No. 21
0099-2240/07/$08.00+0     doi:10.1128/AEM.00100-07
Copyright © 2007, American Society for Microbiology. All Rights Reserved.

Climate and On-Farm Risk Factors Associated with Giardia duodenalis Cysts in Storm Runoff from California Coastal Dairies

Department of Pathology, Microbiology, and Immunology, School of Veterinary Medicine, University of California, Davis, California 95616,¹ University of California Cooperative Extension, Santa Rosa, California 95403,² Veterinary Medicine Extension and Department of Population Health and Reproduction, School of Veterinary Medicine, University of California, Davis, California 95616,³ Department of Plant Sciences, University of California, Davis, California 95616⁴

Received 15 January 2007/ Accepted 7 September 2007

Climatic factors and on-farm management practices were evaluated for their association with the concentrations (cyst/liter) and instantaneous loads (cysts/second) of Giardia duodenalis in storm-based runoff from dairy lots and other high-cattle-use areas on five coastal California farms over two storm seasons. Direct fluorescent antibody analysis was used to quantitate cysts in 350 storm runoff samples. G. duodenalis was detected on all five dairy farms, with fluxes of 1 to 14,000 cysts/liter observed in 16% of samples. Cysts were detected in 41% of runoff samples collected near cattle less than 2 months old, compared to 10% of runoff samples collected near cattle over 6 months old. Furthermore, the concentrations and instantaneous loads of cysts were 65 and 79 times greater, respectively, in runoff from sites housing young calves than in sites housing other age classes of animals. Factors associated with environmental loading of G. duodenalis included cattle age, cattle stocking number, and precipitation but not lot area, land slope, or cattle density. Vegetated buffer strips were found to significantly reduce waterborne cysts in storm runoff: each additional meter of vegetated buffer placed below high-cattle-use areas was associated with reductions in the concentration and instantaneous load of cysts by factors of 0.86 and 0.79 (–0.07 and –0.10 log₁₀/m), respectively. Straw mulch, seed application, scraping of manure, and cattle exclusion did not significantly affect the concentration or load of G. duodenalis cysts. The study findings suggest that vegetated buffer strips, especially when placed near dairy calf areas, should help reduce the environmental loading of these fecal protozoa discharging from dairy farms.

Published ahead of print on 14 September 2007.