By late June, West Nile virus (WNV) cases had popped up in areas of the United States, Canada, and Central America. Twenty-six of the United States had WNV detected in at least one species (birds, horses, humans, or mosquitoes). Six states had recorded equine cases in 2004, and while horse owners are more educated than ever on WNV prevention, officials expect to see more cases in late summer and fall, when equine and other case counts typically soar.

In the United States, as of June 23:

• Six states had discovered equine cases this year–Alabama, Arizona, Missouri, Oklahoma, Texas, and Virginia;
  
• Human cases had appeared in seven states–in Arizona, California, Florida, Nebraska, New Mexico, South Dakota, and Wyoming;
  
• Avian (bird) WNV had been detected in 20 states: Arizona, Arkansas, California, Florida, Georgia, Illinois, Iowa, Kentucky, Louisiana, Michigan, Mississippi, Missouri, New Jersey, New York, Ohio, Oklahoma, South Dakota, Texas, Virginia, and Wisconsin; and
  
• Mosquito pools with WNV had been found in eight states: Arizona, California, Illinois, Indiana, Louisiana, Missouri, Pennsylvania, and Texas.

In Canada, avian WNV has turned up in Ontario, Quebec, and Saskatchewan. No Canadian horse cases have been detected in 2004 so far.

According to a June 20 ProMED-mail report, three equine cases of WNV had been confirmed in Fajardo, Puerto Rico (in the northeastern corner of the island). On June 9, WNV had been detected in two birds in Ceiba (a city in the eastern part of the country), so according to the ProMED moderator, “Evidence of West Nile virus infection of horses represents a significant extension of host range, confirming that WNV is now circulating in Puerto Rico.”

By May 27 in Mexico, surveillance testing revealed 18 asymptomatic birds that were seropositive for WNV and 287 asymptomatic seropositive horses.

In the coming months of WNV activity, two elements of prevention continue to be strongly encouraged by veterinarians and public health officials: Source reduction of mosquitoes and vaccination. By reducing the amount of standing water around your facility, and therefore reducing areas in which mosquitoes love to breed, you lesssen the risk of WNV to you or your horses. Additionally, veterinarians continue to remind horse owners to make sure their equids are protected against WNV with one of the available equine WNV vaccines on the market.