Colorado State Researchers and Veterinarians Offer Facts on West Nile Virus for Families, Horse Owners

Experts with Colorado State University’s College of Veterinary Medicine and Biomedical Sciences are offering current, comprehensive information to Coloradoans on the dangers of West Nile virus, especially owners of horses, mules, donkeys and other equids.

Although drought conditions in Colorado have reduced cause for concern about West Nile virus, a disease spread by mosquitoes, two cases of infected birds were reported in Houston, Tex., this month. This represents the closest the virus has come to Colorado’s borders.

"We’ve been monitoring the slow, westward progression of the virus, so the news from Texas this month isn’t surprising," said Dr. Lance Perryman, Dean of the College of Veterinary Medicine and Biomedical Sciences.

"Protecting public health is part of our responsibility, our mission as a school of veterinary medicine. So when we can do so, we provide clear, concise, useful information and make it widely accessible. An educated public is a safer public."

West Nile virus was first detected in North America in 1999 in New York. Subsequent surveillance has reported cases in approximately 27 states and the District of Columbia. Last year there were 738 cases of the virus reported in equids in the United States.

As with other arboviruses, West Nile virus is spread through a bird-mosquito-bird cycle and transmitted to mammals, including humans, through the bite of an infected mosquito.

According to the Center for Disease Control in Atlanta, there have been no documented cases of person-to-person, animal-to-person or animal-to-animal transmission of WNV.

Currently, there is no vaccine available for humans. There is a conditionally approved vaccine for equids, which is available only through a licensed veterinarian.


What is West Nile virus?
West Nile Virus is a mosquito-borne viral disease previously seen only in Africa, Asia, the Middle East and southern Europe. This virus can cause encephalitis, an inflammation of the brain, or meningitis, an inflammation of the membrane surface of the brain and spinal cord in humans, horses, cats, dogs and domestic and wild birds. It is closely related to St. Louis encephalitis, commonly found in the United States for years.

Where did West Nile virus originate?
According to the Center for Disease Control, West Nile virus was first isolated in an adult woman in the West Nile District of Uganda in 1937. The virus became recognized as a cause of severe human meningoencephalitis in elderly patients during an outbreak in Israel in 1957. Equine disease was first noted in Egypt and France in the early 1960s, in 1996 in Morocco, in1998 in Italy and first appeared in North America in 1999. Although it is not clear where the North American virus originated, the strain appears to be most closely related genetically to those found in the Middle East. It is also closely related to St. Louis encephalitis virus, which is commonly found the United States. Since 1999, the virus has spread throughout 27 states and the District of Columbia. In 2000, 21 human cases of the virus were reported in the United States with two deaths. Last year, 56 human cases of the disease were reported with seven deaths.

How is West Nile virus spread?
West Nile Virus is spread by the bite of an infected mosquito. A mosquito will become infected by feeding on a bird that is carrying the virus. West Nile virus is NOT spread by person-to-person contact such as touching, kissing or caring for an infected person. There is also no evidence that handling infected animals spreads the disease. The virus has recently been found in the more aggressive Asian tiger mosquito, which is found mostly in the Southern United States.

Do all mosquitoes carry the West Nile virus?
No. Mosquitoes are a major insect pest problem in Colorado, but not all types of mosquitoes carry the virus. The principal transmitter of the virus is the Culex pipiens, or the northern house mosquito. Once the mosquito is infected, it can transmit the virus to people or other animals. (See Mosquitoes in Colorado below)

Can you get WNV directly from birds?
Currently, there is no evidence that West Nile virus can be spread directly from birds to people. However, to be safe, never handle a dead bird with bare hands. Use gloves to place dead birds in a plastic bag and then place the bag in an outdoor trash bin. If you want to report a dead crow, raven or magpie, potential carriers of WNV, contact your local animal control agency or the Colorado Department of Public Health and Environment at (303) 692-2700 or visit the website at for instructions on how to submit a dead bird for testing.

Besides mosquitoes, can WNV be contracted this virus from other insects or ticks?
Currently, there is no evidence to suggest that ticks or other insects transmit the West Nile virus.

What Are Signs of WNV Infection in People?
People with mild infections either have no signs or display signs of a mild illness such as headache, body ache, swollen lymph glands, a mild rash and fever before fully recovering. Those with more severe infections may experience high fever, headache, neck stiffness, stupor, disorientation, coma, tremors, convulsions and paralysis.

Who is most at risk?
Although anyone can get the virus, people over 50 years of age and those with compromised immune systems are in the highest risk group.

What is the treatment?
There is no specific treatment for West Nile viral infection. In more severe cases, hospitalization is recommended with intensive supportive therapy: intravenous fluids, airway management and ventilator support if required. This will also help to prevent secondary infections, such as pneumonia.

If bitten by an infected mosquito, how long before symptoms appear?
Being bitten by an infected mosquito will not necessarily make an individual sick. If illness were to occur, it would begin to appear in 3 to 15 days of being bitten.

How is WNV diagnosed?
To diagnose a West Nile Virus infection, doctors must test either blood or cerebrospinal fluid from a spinal tap for antibodies to the virus. A second blood test is required two to three weeks later to confirm the diagnosis.

Is there a vaccine?
Currently, there is no vaccine for humans since it is rarely deadly. There are no antibiotics or antiviral medications that can be prescribed for treatment. There is a vaccine for horses and horse owners should contact their veterinarian. (See West Nile Virus and Horse Owners)


Can dogs and cats contract West Nile virus?
Yes. Dogs and cats can become infected the same way humans become infected-by the bite of infectious mosquitoes. It is possible that eating dead, infected animals such as birds, could infect dogs and cats but this has not been proven.

Can infected dogs or cats transmit the virus to humans or to other animals?
No. There is not documented evidence that WNV can be transmitted by animal-to-animal or animal-to-human contact. Infected mosquitoes transmit West Nile virus.

Should a dog or cat infected with WNV be euthanized?
No. There is no reason to destroy an animal because it has been infected with West Nile virus. Full recovery is expected. Treatment should be supportive and consistent with standard veterinary practices for animals with a viral infection.


How do horses become infected with West Nile virus?
Horses become infected the same way people do-by the bite of infected mosquitoes. The virus is located in the mosquito’s salivary glands and when the mosquito "feeds" on the horse, the virus is injected into its blood system. In the bloodstream, the virus multiplies and may cause illness.

What are the signs of West Nile virus in horses?
Not all horses become clinically ill. In those that do, following transmission by an infected mosquito, the virus multiplies in the horse’s blood system, crosses the blood brain barrier and infects the brain. The virus interferes with normal central nervous system functioning and causes inflammation of the brain. Clinical signs include loss of appetite and depression, fever, weakness or paralysis of hind limbs, muscle fasciculations or muzzle twitching, impaired vision, lack of coordination (ataxia), head pressing, convulsions, difficulty in swallowing, circling, hyperexcitability or coma.

Do these signs always indicate West Nile virus?
No. Some other mosquito-borne viral encephalitic diseases of horses caused by Eastern, Western and Venezuelan encephalitis viruses can cause a horse do demonstrate signs similar to WNV. Additionally, diseases such as rabies, botulism and equine protozoal myeloencephalitis (EPM) also have signs similar to WNV. If you think your horse is exhibiting signs of encephalitis, contact your veterinarian. Only laboratory tests can confirm a diagnosis of West Nile encephalitis.

Can an infected horse transmit the virus to horses in neighboring stalls?
No. There is no documented evidence that WNV is transmitted from horse to horse. However, precaution should be taken by removing the horse from further potential mosquito-borne infection and tested for the virus.

What is the treatment for an infected horse?
There is nor specific treatment for West Nile virus in horses. Most horses recover from the infection. Treatment would be supportive and consistent with standard veterinary practices for animals infected with a viral agent. Caregivers should prevent the animal from injuring itself.

Can my horse be vaccinated against WNV?
Yes. A West Nile virus vaccine for horses has been conditionally approved and is available only though a licensed veterinarian. The vaccine consists of two intramuscular doses administered 3-4 weeks apart, and then revaccination is required annually. Check with your veterinarian for an explanation of the process and how the vaccine works.

Can I vaccinate my horse myself?
No. Since the vaccine has been released only under conditional licensure, the vaccine is distributed only to licensed veterinarians for administering.

My horse is already vaccinated against Eastern Equine Encephalitis, Western Equine Encephalitis and Venezuelan Equine Encephalitis. Will these vaccinations protect against WNV?
No. These viruses belong to another family of viruses for which there is no cross-protection.

Can a human become infected by caring for an infected horse?
No. There is no documented evidence of person-to-person or animal-to-human transmission of WNV.

How can I protect my horses from mosquitoes?
There are several easy steps to help protect your horses from potentially infectious mosquitoes.

  • Move horses indoors during dusk and dawn, peak periods of mosquito activity.
  • Check the barn for cracks and crevices where mosquitoes may rest during the day and use an appropriately safe insecticide to kill adult mosquitoes. Always read and follow directions carefully.
  • Use topical preparations approved for horses containing repellents specifically identified for mosquitoes. Always read the product label carefully and follow directions.
  • Fog stable premises in the evening with a safe pesticide to reduce mosquitoes
  • Use fans on horses while in the stable to help deter mosquitoes.
  • Avoid turning on lights in the stable during evening hours and overnight.
  • Monitor watering troughs, small ponds, irrigation ditches, rain barrels and manure lagoons where mosquitoes may breed.
  • Eliminate areas where shallow standing water can accumulate and offer breeding ground, such as ruts where ranch equipment frequently travels, wheelbarrows, seepage areas from air conditioners, used tires, drainage areas and well-worn horse paths where old hoof prints can provide a receptacle for water.
    • Trim weeds around barns and paddocks where mosquitoes may rest.
    • Use face masks, ear covers, leg covers or sheets to protect your horse. Consider spraying the covering lightly with a fly repellent to increase effectiveness.
  • Look into microbial larvacides and insect growth regulators at your local hardware store. There are a number of inexpensive brands that are safe for animals and the environment, including mosquito "dunks." A number of products can be found on these websites:,,
  • Talk to your local pest control professional about a regular plan for keeping your property safe from mosquitoes and other insects that might be dangerous for your horses.
  • Consider screens over doors and windows to further keep out any flying pests, including WNV carrying mosquitoes.
  • Monitor your property for dead birds such as crows, ravens and magpies (corvids species). Notify your local animal protection and control agency to report a dead bird and to have it removed; or, contact the Colorado Department of Public Health and Environment at (303) 692-2700 for instructions on how to submit a dead bird (corvids only) for testing.

Other websites for information on West Nile virus:


Quick Facts

  • Mosquitoes can transmit certain human diseases, deter tourism and cause economic losses in cattle and other livestock.
  • Immature mosquitoes, or wrigglers, live in shallow water and feed on microorganisms.
  • The Aedes and Culex/Culiseta are two important types of mosquitoes in Colorado.
  • Larval management is the key to mosquito control
  • Adult mosquitoes can be controlled with insecticide applications, but a community should agree on how to decide when treatments are necessary.

Life of a Mosquito

Immature mosquitoes are worm-like larvae known as "wrigglers," that live in shallow water and feed mostly on microorganisms. They prefer areas protected from wave action by vegetation and floating objects. Irrigated pastures, watering troughs, ponds, irrigation ditches and rain barrels are common mosquito habitat in Colorado. Mosquitoes rarely breed in moving water.

Only female adult mosquitoes feed on blood. Male mosquitoes feed on nectar and are not considered pests. Two important types of mosquitoes in Colorado are the Aedes and the Culex/Culiseta.

Aedes Mosqitoes
Adult Aedes feed day and night. They can be extremely annoying but do not transmit any important human diseases in Colorado. Aedes vexans is a fierce biter commonly associated with irrigated areas in Colorado but is known to travel many miles to feed. Snowpool Aedes, which develop in temporary ponds that result from snowmelt, often are serious pests in the high country.

Culex/Culiseta Mosquitoes
Mosquitoes in the Culex/Culiseta group feed only at night. The Culex mosquito makes the typical whining noise for which mosquitoes are known. The Culex tarsalis mosquito transmits Western equine encephalitis virus. These mosquitoes pass the winter and other unfavorable periods as adults inside houses and other sheltered areas. In spring and summer, adults lay floating egg "rafts" on water. Wrigglers and pupae are found in permanent or semi permanent standing water, which includes wading pools, birdbaths and clogged gutters. Development from egg to adult usually takes 10 to 14 days. Single and multigenerational species occur in Colorado. For detailed information on mosquito management around the home, family protection, insecticides and mosquito habitat modification, visit