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California Department of Food and Agriculture

INFORMATION ABOUT EQUINE WEST NILE VIRUS


2015 Equine West Nile Virus (WNV) Update: Seven Confirmed Equine WNV Cases

On September 3, 2015, an unvaccinated yearling filly displaying neurologic signs in San Joaquin County was confirmed positive for WNV. The filly is recovering. For 2015, a total of eight (8) horses were confirmed positive for WNV. The positive horses were located in the following counties; Modoc, Riverside (2), San Joaquin, Shasta(2), Tehama and Ventura. Three of the eight horses were euthanized or died.

CDFA continually monitors and investigates equine neurologic cases for the presence of WNV in California. CDFA urges horse owners to consult their veterinarian concerning a WNV vaccination program to ensure maximum protection of their horses.

Confirmed Equine WNV Case Summary Table

Year

Confirmed Cases

Died or Euthanized

2015

8

3

2014

15

2

2013*

8

4

2012

16

8

2011

15

4

2010

19

5

2009

18

7

2008

32

17

2007

28

14

2006

58

24

2005

456

200

2004

540

229

2003

1

0

* Due to budget cuts equine WNV cases were not confirmed after August 30, 2013.

West Nile Virus

West Nile Virus (WNV) is a mosquito-borne virus first detected in the United States (US) in the New York City area in 1999. Since 1999, the virus has spread throughout the US and Canada, infecting birds, humans, horses and other mammals. As of 2012, more than 25,000 horses in the US have been infected since the disease was first identified. The virus is maintained in the wild bird population and is spread between birds by mosquitoes. Birds are considered the natural reservoir for WNV since high levels of virus circulate in their bloodstream. Mosquitoes acquire WNV in blood meals from infected birds and pass it on to other birds, animals, and people. Mosquitoes that feed on an infected horse or human have not demonstrated the ability to ingest enough of the virus to transmit it to other animals or humans; therefore, horses and humans are considered “dead end hosts.”

Clinical Signs

West Nile Virus may cause a wide range of clinical illness ranging from mild” flu-like” signs to encephalitis (inflammation of the brain) that may be fatal to both humans and horses. While horses are susceptible to WNV infection, many infected horses do not develop clinical illness and recover uneventfully.

Consult a veterinarian if your horse exhibits any of the following clinical signs:

  • Fever
  • Incoordination, especially in rear limbs, causing stumbling and falling
  • Generalized weakness, muscle twitching, seizures or coma
  • Drooping lips and lip smacking, head drooping, grinding teeth
  • Hypersensitivity to touch or sound
  • Recumbency (inability to rise)

Diagnosis

Any horse displaying abnormal behavior or neurologic signs should be examined by a veterinarian to rule out neurologic diseases, such as WNV, Equine Herpes Virus-1, Equine Protozoal Myeloencephalopathy, Eastern/Western Encephalomyelitis and Rabies.

Blood samples should be collected by a veterinarian and sent for diagnostic testing. The WNV IgM capture ELISA test is a specific test that detects acute WNV infection in animals, is usually positive within six (6) days post infection and can remain positive for up to two (2) months post infection. A positive WNV IgM Capture ELISA test is indicative of recent infection. Unexposed animals vaccinated for WNV will test negative on the WNV IgM capture ELISA test.

Treatment and Prognosis

Currently, there is no specific treatment for WNV. Supportive care includes administration of anti-inflammatory drugs and intravenous fluids. Recumbent WNV positive horses are at a higher risk of dying or requiring euthanasia. For horses exhibiting clinical signs of WNV, the case fatality rate is approximately 33%; however, many infected horses will fully recover following infection.

Prevention

Vaccination and mosquito control minimize the risk of WNV infection in horses. Although the number of WNV infected horses has declined over the last five years, WNV remains an important disease in unvaccinated horses. The American Association of Equine Practitioners recommends incorporation of WNV vaccine as an annual core vaccine in equine vaccination protocols. Horse owners should consult with their veterinary practitioner to ensure current WNV vaccination status of their horses.

Mosquito Control

Minimizing horse exposure to mosquitoes during the peak mosquito feeding periods at dawn and dusk decreases the risk of horse exposure to WNV. Application of mosquito repellant can also effectively reduce the number of mosquito bites on horses. Mosquito control efforts to eliminate mosquito breeding sites are also important. Methods to eliminate mosquito breeding sites 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.

California’s WNV Surveillance

WNV surveillance includes testing samples from dead birds, sentinel chickens, mosquito pools, horses and humans. The California Department of Food and Agriculture (CDFA) works with federal, state and local health and agricultural agencies to minimize the impact of WNV on the equine industry. CDFA confirms suspect equine WNV cases, maintains horse surveillance data and educates horse owners on WNV disease control and prevention. Collaborating agencies use the surveillance information to focus disease control efforts.