Elsevier

Annals of Epidemiology

Volume 21, Issue 12, December 2011, Pages 922-929
Annals of Epidemiology

Yard Flooding by Irrigation Canals Increased the Risk of West Nile Disease in El Paso, Texas

https://doi.org/10.1016/j.annepidem.2011.08.001Get rights and content

Purpose

To investigate the effects of use of water from irrigation canals to flood residential yards on the risk of West Nile disease in El Paso, Texas.

Methods

West Nile disease confirmed cases in 2009 through 2010 were compared with a random sample of 50 residents of the county according to access to and use of water from irrigation canals by subjects or their neighbors, as well as geo-referenced closest distance between their home address and the nearest irrigation canal. A windshield survey of 600 m around the study subjects’ home address recorded the presence of irrigation canals. The distance from the residence of 182 confirmed cases of West Nile disease reported in 2003 through 2010 to canals was compared with that of the centroids of 182 blocks selected at random.

Results

Cases were more likely than controls to report their neighbors flooded their yards with water from canals. Irrigation canals were more often observed in neighborhoods of cases than of controls. Using the set of addresses of 182 confirmed cases and 182 hypothetical controls the authors found a significant, inverse relation with risk of West Nile disease.

Conclusions

Flooding of yards with water from canals increased the risk of West Nile disease.

Section snippets

Materials and Methods

We conducted a cumulative (“epidemic”) unmatched case-control study (12, p. 125) of 48 new WN virus (WNV) cases reported and investigated by the City of El Paso Department of Public Health from 2009 to 2010 among residents of El Paso County, Texas, who met the definition of confirmed cases (13). The authors conducted the study in October and November of 2010, as part of an ongoing field epidemic investigation. Controls were subjects randomly selected among El Paso County residents who

Results

In 2003 through 2010, 182 confirmed cases of WN disease were reported for an age-adjusted incidence rate of 3.5 cases per 100,000 person-years. Eighteen (or 9.9%) of the 182 confirmed WN disease cases were known to be deceased at the time of report. Cases of WN fever were reported since 2004, and 24 (16%) fell into this category, the rest were considered neuroinvasive. Incidence of WN disease increased 30 times between those in the 0 to 9 years of age and those in the 70 years or older age

Discussion

We tested the hypothesis that proximity to irrigation canals and use of water from canals to flood residential yards is a risk factor for WN disease in El Paso, Texas. The larger 2003 through 2010 case–hypothetical control study showed that persons living closer to irrigation canals were at increased risk of WN, whereas the smaller 2009 through 2010 case-control study, which obtained data through actual interviews, provided insights into one postulated mechanism by which the closeness to canals

Conclusions

We found evidence that living in close proximity to irrigation canals, the use of water from canals and the prevalent practice of flooding residential yards with water from canals in El Paso County, Texas, was a risk factor for WN disease. Furthermore, this study identified two other risk factors: Those at high risk are older individuals with chronic conditions who do not use mosquito repellent. Interventions should target these populations within areas close to irrigation canals.

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    This study received funding from UT School of Public Health Dr. Cardenas Faculty Incentive Plan and from an NIH National Centers for Research Resources grant (5G12RR008124) to UTEP.

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