Irrigating lawns with canal water tied to West Nile

NEW YORK (Reuters Health) - El Paso residents infected with the mosquito-borne West Nile virus over the past eight years tended to live close to one of the county's irrigation canals, according to a new study.

They were also much more likely to say their neighbors used canal water to irrigate their lawns than other non-infected residents, researchers say.

That doesn't mean Texans should avoid canal water altogether, they added -- just that they should take the proper precautions to protect themselves against mosquitoes, which breed in water used for irrigation.

"I thought of not using the water, but of course that's not realistic, and people buy the land because it has cheap water," said Dr. Victor Cardenas, from the University of Texas-Houston School of Public Health.

"People whose neighbors flood their yards or they themselves flood their yards or live near areas that undergo flood irrigation should wear long sleeves during the summer and/or apply DEET," said Cardenas, who worked on the study.

This has been a mild year for West Nile, Cardenas said, with El Paso seeing far fewer cases than in 2009 or 2010, when there were a combined 48 confirmed.

West Nile symptoms include fever, body aches and headaches. In rare cases the virus can be fatal if it spreads to the brain and spinal cord.

Cardenas and his colleagues noticed that certain areas of El Paso County had higher-than-average reports of West Nile, including those that are served by canals, which some residents have access to for irrigation. So they designed two experiments to see if the canal water might in fact be to blame.

In the first, they surveyed people in the county who had been diagnosed with West Nile in 2009 and 2010, plus another 50 virus-free people who were scattered throughout El Paso. They asked each person whether or not they or their neighbors watered their lawns with canal water.

That kind of irrigation is worrisome because pools of canal water can sit for a long time in bits of clay when it sinks into the lawn, Cardenas said -- giving mosquitoes an ideal breeding area.

Sure enough, while only two or three people in each group watered their owns lawns with canal water, 11 out of 39 people the researchers tracked down in the West Nile group had seen their neighbors do it, compared to four out of 50 people who'd never had the virus.

Looking at data from 182 people with confirmed cases of West Nile from 2003 through 2010, those El Paso residents were more than twice as likely to live within 0.2 miles of an irrigation canal compared to another randomly selected group of Texans without the virus.

"I believe this may be at work in many places in the Western U.S. where people like to do flood irrigation," Cardenas told Reuters Health.

Laura Kramer, from the Wadsworth Center at the New York State Department of Health, said that while grassy, irrigated areas are concerns in the West, the type of virus-carrying mosquitoes found in cities including New York breed in water containers, drains and pools.

The researchers report in the Annals of Epidemiology that the majority of everyone they talked to -- people who had and hadn't gotten West Nile -- never wore mosquito repellant.

Using repellant or covering up is essential in places where canal water is used and mosquitoes may be breeding, Cardenas said. That's especially important for people who are at higher risk of West Nile to begin with, he added, which includes older men and people with chronic diseases.

Kramer, who was not involved in the study, agreed that it's important for people to use repellant, especially at times of day when mosquitoes are biting. But those who irrigate, she added, should take extra precautions, such as using anti-mosquito contraptions on their lawns.

"People should use repellant and it definitely does make a difference," she told Reuters Health. "But I think with Texas, if this holds up, where taking the water and irrigating your property creates a breeding site for mosquitoes, I think you have a community responsibility to know you're putting your neighbors at risk as well as yourself."

SOURCE: bit.ly/vjdD1Z Annals of Epidemiology, December 2011.