PRO/AH/EDR> West Nile virus (25): USA (OR, SC, WA, UT) livestock, horse


A ProMED-mail post
ProMED-mail is a program of the
International Society for Infectious Diseases

In this post:
[1] Oregon (Union County): livestock
[2] South Carolina (Dillon County): horse
[3] Washington (Grant and Benton counties): horses
[4] Utah (Weber and Cache counties): horses

[1] Oregon (Union County): livestock
Date: Fri 15 Sep 2023
Source: Elkhorn Media Group [edited]

Though West Nile virus (WNv) is common every year, the summer of 2023
proved to be an especially active time for it. Numerous mosquito pools
were confirmed to be carrying the virus throughout August [2023], with
county vector control offices working hard to track and, if necessary,
contain the nasty little bugs. Sadly, WNv may have caused the recent
deaths of several livestock in Union County.

Information provided by Union County Vector Control Manager Chris Law,
following unofficial reports of a sick mule in Summerville, indicates
4 livestock animals were put down following possible WNv infection.
According to Law, a veterinarian at the Animal Health Center in Island
City examined 2 sick horses and a mule from the Summerville/Elgin area
(the names of the persons the animals belonged to were not provided
due to confidentiality concerns), which showed signs of a neurological
disorder. Given the heavy presence of WNv in northeast Oregon this
year [2023], the vet in question felt it very probable the animals had
been infected with WNv.

Note: Definitive testing was not carried out by the vet before the
animals had to be put down. The Animal Health Center put down 2 of the
animals (which ones specifically were not specified) while the 3rd was
put down by its respective owner. Unofficial reports also indicate
that a horse belonging to a City of Union resident may have also been
infected. This horse is allegedly recovering after being treated at a
veterinary clinic in Idaho. An exact timeline of when the animals
became sick is unknown. Unofficial reports of a sick mule in
Summerville were initially noted around 9 Sep [2023], while other
reports indicate the horse in Union may have shown signs of WNv
infection around 5 Sep [2023].

Of the 4 animals believed to be infected, only the mule had received a
prior vaccination. According to details from the Animal Health Center,
this vaccination was provided in a dose alongside 5 other
vaccinations, making it less effective than a dedicated WNv
vaccination. Dr Terrence McCoy at the Animal Health Center also
commented that many livestock owners in Union County are not actually
vaccinating their horses against WNv anymore. It’s speculated this is
due to the relatively limited number of cases of WNv in Union County
over the past few years and the unpredictability of the virus’s

Going forward, Law recommends the following precautions for livestock
owners to avoid further WNv deaths:

  • Make sure all the animals that can be vaccinated are fully
  • Weekly, search for and empty any containers holding water.
  • Ensure water troughs are emptied and cleaned weekly or have us treat
    them. [As an example, dunk donuts are designed to be put into water
    tanks to control mosquito larvae. – Mod.TG]
  • Properly irrigate to prevent standing water that could breed
  • If a water storage system for sprinklers exists, ensure it’s
    accessible for treatment.
  • Use mosquito [repellants] that are specifically approved for horses
    and other animals.
  • Livestock owners can contact Union County Vector Control for further

Union County residents can visit for
more information or reach out to Union County Vector Control via
telephone or email []. For more details
on West Nile virus specifically, see our previous interview with Chris
Law at

[Byline: Garrett Christensen]

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