My hometown is currently waging war against mosquitoes. My part of Fort Collins is due for its second round of fogging with a compound called Aqualuer 20-20, a permethrin-based insecticide.

I can’t say I’ve found this mosquito season to be especially severe, but the number of cases of West Nile virus (WNV) that have been diagnosed so far this year is unusual. As I write, Larimer County reports 28 human cases of WNV and that the human WNV risk is “extremely high.” By spraying, our city government is hoping to prevent another season like the one we went through in 2003 when 546 county residents were diagnosed with WNV and 9 people died or the outbreak that hit nearby states in 2012.

According to Dr. Lyle Peterson, Director of the Division of Vector-borne Infectious Diseases at the Centers for Disease Control and Prevention, “West Nile virus is the most significant exotic mosquito-borne disease that has come to the contiguous United States in the last century. Outbreaks have proven extremely difficult to predict and control and have been associated with considerable morbidity [illness]. The large outbreak in 2012, which caught many by surprise, indicates that West Nile virus will remain a formidable public health challenge for years to come.” For more information about WNV as it applies to people, see the American Academy of Microbiology’s new report entitled FAQ: West Nile Virus.

From a veterinarian’s point of view, West Nile is of greatest concern for horses. The Colorado Department of Agriculture just sent out an alert regarding the first reported equine case of West Nile Virus in Colorado this year. According to the press release:

The transmission of the disease varies from year to year and depends on a number of factors including mosquito numbers. The WNV can be carried by infected birds and then spread locally by mosquitoes that bite those birds. The mosquitoes can then pass the virus to humans and animals.

Infected horses may display symptoms including head tilt, muscle tremors, stumbling, lack of coordination, weakness of the limbs or partial paralysis. The clinical signs of WNV are consistent with other important neurological diseases such as equine encephalitis, rabies, and equine herpes virus; therefore it is important to work with your veterinarian to get an accurate diagnosis through laboratory testing. Horse owners should also consult their private practicing veterinarian to determine an appropriate disease prevention plan for their horses.

[Equine] vaccines have proven to be a very effective prevention tool. Horses that have been vaccinated in past years will need an annual booster shot. However, if an owner did not vaccinate their animal in previous years, the horse will need the two-shot vaccination series within a three to six week period.

Dogs and cats can also be infected with West Nile virus via mosquito bites, but they rarely become sick from the exposure. Most individuals have such mild, minor, and short-lived symptoms (e.g., fever and lethargy, if they have any clinical signs associated with infection at all) that their owners are never even aware that infection has occurred.

To reduce your own and your animals’ potential exposure to West Nile virus, limit outdoor activity between dusk and dawn, remove standing water from your property, keep windows and doors shut, and make sure that screens are in good repair. Mosquito repellants can be helpful too, but always use a product that is labeled for the species in question. Cats are especially sensitive to certain types of insecticides, which is why mine will be locked indoors behind closed windows when the fogging truck rolls by tonight.

Dr. Jennifer Coates

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