By: Emily Nietrzeba, DVM, MPH

The first 2020 case of equine West Nile Virus (WNV) in California has been confirmed in Amador County. A 20-year-old Quarter Horse gelding displaying acute severe neurological signs with unknown vaccination history was confirmed positive for WNV on June 26, 2020. The gelding is currently alive.

West Nile Virus is the leading cause of arthropod-borne encephalitis (brain inflammation) in horses and humans in the United States, with horses representing 96.9% of all non-human mammalian cases. The virus is present in all 48 continental states, Mexico, and Canada, and is transmitted from avian reservoir hosts to mammals by a variety of mosquito species.

It is important to remember that both humans and horses are considered dead-end hosts for WNV and the virus is not directly contagious from horse to horse or horse to human. WNV vaccination is considered a core vaccination by the American Association of Equine Practitioners and an essential standard of care for all horses in North America. 

This is an excellent opportunity to remind friends and colleagues of the importance of developing and adhering to a protective vaccination program with their veterinarian, and to discuss vector control measures at home and on farm. In addition to minimizing equine (and human) exposure to mosquitos during peak feeding periods of dawn and dusk and appropriate application of mosquito repellant, effective mosquito abatement recommendations include:

• Draining unnecessary standing water found in wheelbarrows, tires, etc.

• Cleaning water containers at least weekly (i.e., bird baths, plant saucers)

• Scheduling pasture irrigation to minimize standing water

• Keeping swimming pools optimally chlorinated and draining water from pool covers

• Stocking of water tanks with fish that consume mosquito larvae (Contact local mosquito control for assistance) or use mosquito “dunk” available at hardware stores