The tally of states with 2004 U.S. West Nile virus (WNV) equine cases has risen to five with the addition of Arizona and Colorado to the group this week. Alabama, Florida, and Texas had already logged equine cases this year.

Arizona’s first equine WNV case was detected in mid-August of last year, and a total of 138 equine cases were reported in the state in 2003. Last year was Colorado’s second year for WNV in horses, and a total of 604 equine WNV cases were reported beginning in early July. Clinical signs of WNV in horses include signs of neurologic disease such as incoordination, circling, muscle tremors, and head pressing. Additional signs are inappetence, depression, and recumbency (the horse is down and unable to rise). If a horse displays these symptoms, a veterinarian should be contacted.

Arizona public health officials announced May 6 that two mosquito pools in Maricopa County tested positive for WNV at the Arizona State Health Laboratory, marking the arrival of the virus in the state. The Arizona Republic reported on May 13 that the Arizona Department of Agriculture (ADA) had confirmed Arizona’s first 2004 equine case at a Phoenix farm. Officials from the ADA said the horse contracted the virus locally, since the sick horse had spent several years at this stable without being transported.

“We knew it was a matter of when, not if, we would see West Nile this year,” said Catherine Eden, Director of the Arizona Department of Health Services. “(The discovery of the positive mosquitoes) tells us that our surveillance is working and makes it more important than ever that people take precautions to prevent mosquito breeding around their homes