Prevention of West Nile mainly consists of controlling the mosquito population (Photo: Achkin / Shutterstock)

By Audrey Hill, Feature Contributor, Valley Ag Voice

West Nile has been endemic in the U.S. since 2002, and most recent data shows that California, Colorado, and Florida have since faced the brunt of this virus’ affects. Most common reports of West Nile across the U.S. are in humans and horses; however, it can also infect other animals such as cats, dogs, squirrels, and domestic rabbits. 

In nature, West Nile virus cycles between mosquitoes (especially Culex species) and birds. Some infected birds, can develop high levels of the virus in their bloodstream and mosquitoes can become infected by biting these infected birds. After about a week, infected mosquitoes can pass the virus to more birds when they bite. Mosquitoes with West Nile virus also bite and infect people, horses and other mammals. However, humans, horses and other mammals are ‘dead end’ hosts. This means that they do not develop high levels of virus in their bloodstream, and cannot pass the virus on to other biting mosquitoes. (CDC)

The virus is transmitted from birds, reservoir hosts of the virus, to people and animals through mosquitoes. So, prevention of West Nile mainly consists of mosquito control. This virus is not airborne and can only be transmitted via blood and “mother to baby during pregnancy, delivery or breastfeeding,” according to the CDC. This makes mosquitoes a likely mode of transmission but also makes hosts like humans and horses “dead end” hosts, which can only transmit the virus via sufficient blood/organ transfusion–meaning neither you nor your horses can infect others. 

Most people infected with the virus will not feel any symptoms. The CDC states that roughly 1 in 5 people infected with West Nile will develop symptoms including fever, headache, body aches, joint pains, vomiting, diarrhea, or rash, and about 1 in 150 people infected will develop more serious symptoms such as convulsions, vision loss, muscle weakness, numbness, coma, and paralysis. However, these symptoms are uncommon: a 1:50 ratio. Currently there is no vaccine available for humans, “as we have a relatively minimal reaction to the virus,” according to the CDC’s West Nile online index. 

Horses, however, can have a very different reaction to the virus. Horses make up 96.9% of reported non-human cases of West Nile, says UC Davis School of Veterinary Medicine. Their clinical signs can be as minor as the flu to as major as encephalitis (inflammation of the brain), as stated by the California Dept. of Food and Agriculture. Common symptoms of an infected horse are “fever, incoordination, especially in rear limbs, twitching, seizures, drooping lip, drooling, hypersensitivity to touch or sound and recumbency,” again stated by the CDFA. Two equine West Nile vaccines exist and have been proven effective, however it does not fully prevent contraction – meaning a vaccinated horse could still contract West Nile. 

John Tolley, D.M.V and Hector Gonzalez, D.M.V, both resident veterinarians at Bakersfield Large Animal Clinic state that the best way to prevent contraction and/or an elevated response to the virus is to vaccinate your horses, bring them in at night when mosquito activity is the highest, and remove standing water. Dr. Gonzalez commented, “Like COVID, I would vax.” 

So far, 6 cases of equine West Nile have been reported for California in 2021, and unfortunately all of these cases were found in the Central Valley according to the CDFA: 2 in Fresno County, 2 in Sacramento County, 1 in Merced county and 1 in Kings County.

Veterinarian Thomas Willis, D.M.V and owner of San Joaquin Veterinary Hospital states that he has only seen two cases of equine West Nile in his practice and disclaims he has not been in large animal practice in 5 years. Willis has mainly seen small animals for the past five years at his hospital in west Bakersfield and even though cats and dogs can contract West Nile, Willis states, “Whether or not I was right, I have not suspected [West Nile] in dogs or cats.”

The two horses he diagnosed with West Nile contracted the virus within a year of each other, roughly a decade ago. This was before a vaccine had come out, and unfortunately both horses had to be put down due to the severity of their symptoms. Willis states that the case he remembers most vividly experienced high fever, inability to stand, muscle tremors, and later that they both experienced “jerky motion” and paralyzed hind limbs. West Nile causes encephalitis in the central nervous system in horses, which these symptoms suggest. 

West Nile is not 100% preventable, however there are recommended ways to keep your horses and yourselves protected. Mainly, have your vet vaccinate your horses with the West Nile vaccine or a combination vaccine including West Nile prevention. Remove standing water from your property and strive to prevent mosquito bites on everyone. 

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