Since a Marin County chicken tested positive for antibodies to West Nile Virus last week, the Marin/Sonoma Mosquito and Vector Control District is urging horse owners to vaccinate their animals, which generally are most vulnerable to the virus from July through October. “With the mortality rate in unvaccinated horses as high as 40 percent, we can’t stress enough the importance of taking preventative measures against the virus,” the district’s scientific programs director, Piper Kimball, said in a press release. The sentinel chicken was part of a Novato flock that district employees test biweekly; three dead birds found in Santa Rosa this summer have also tested positive. Mosquitoes feeding on infected birds spread the deadly virus to other animals and people. Horse owners should be on the lookout for the following signs: fever, stumbling, muscle weakness and twitching, drooping lips or head, teeth grinding and hypersensitivity to touch or sound. Fans can be placed inside barns and stalls to deter mosquitoes, troughs and ponds can be stocked with mosquitofish, and standing water should be drained. Fly sheets, masks and leg wraps can help, as can equine-approved repellent. — T.E.