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…