top What is West Nile virus (WNV) and how does it spread?

  • WNV attacks the central nervous system, including the brain
  • It is transmitted by mosquitoes that feed on infected birds and it can infect both horses and people1
  • It is not contagious from horse to horse or horse to human1
  • It continues to be diagnosed throughout the country2
  • One in three WNV-infected horses may die3

WNV Transmission Cycle

top What are the signs and treatment for WNV?

  • Horses with WNV often show signs related to a nervous disorder, including:4,5
    • Lack of coordination
    • Stumbling
    • Excitability
    • Muscle trembling
  • WNV-infected horses may benefit from anti-inflammatory drugs and tranquilization5
  • Stumbling may result in wounds that require treatment with antibiotics5
  • Horses that cannot stand may require slinging5

top How can I help prevent my horse from being infected with WNV?

Preventing WNV starts with annual vaccination and good management. Since it is transmitted by mosquitoes, schedule annual vaccination prior to the mosquito season, and take steps to reduce mosquito populations.

top Why is it important to reduce mosquito populations?

Mosquitoes are the only way for your horse — or you — to contract WNV, Eastern equine encephalitis (EEE) or Western equine encephalitis (WEE). When you lower the number of mosquitoes, you lower the risk.

Here are a few tips to help reduce the number of mosquitoes around your horse:

  • Remove breeding areas, such as upside down trash can lids and discarded tires
  • Clean livestock watering troughs and birdbaths
  • Don’t let water gather in wheelbarrows, gutters or unused water buckets
  • Use approved insecticides

top What are EEE and WEE?

  • Both EEE and WEE are central nervous system infections6 that are transmitted by mosquitoes
  • Both pose a risk to horses and humans and are identified as core vaccinations that should be administered annually2
  • Because of the public health risk, both diseases also must be reported in the United States7

EEE has historically been restricted to all states east of the Mississippi River; WEE is usually restricted to Western and Midwestern states.8 Thanks partially to vaccination, outbreaks are becoming less common, but still occasionally occur — particularly in Florida and other southern states where the mosquito season is longer.9,10

top How are EEE and WEE spread?

  • EEE and WEE are transmitted by mosquitoes that feed on infected birds.9
  • Unlike WNV, birds do not develop EEE and WEE disease, but can act as a reservoir.9
  • Horse-to-horse or horse-to-human transmission is highly unlikely.10

There were 184 cases of EEE in the United States in 2008.11 This map shows which states have the greatest incidence.

2008 EEE Cases

top What are the signs of EEE and WEE and how can infected horses be treated?

EEE and WEE affect horses in very similar ways to other diseases of the nervous system, with signs including:9,12

Fever for 24 to 48 hours (WEE often does not progress beyond fever)

  • Depression
  • Hypersensitivity to stimuli
  • Involuntary muscle movements
  • Impaired vision
  • Aimless wandering or circling
  • Paralysis of the throat and tongue

There are no specific treatments for horses infected with EEE or WEE.9 Your veterinarian may recommend providing supportive care, including fluids and anti-inflammatory therapy.9 Even in horses that recover from either EEE or WEE, many can have permanent brain damage.12

top How can I help prevent my horse from being infected with EEE and WEE?

Preventing EEE and WEE starts with annual vaccination and good management. Since both diseases are transmitted by mosquitoes, schedule annual vaccination prior to the mosquito season, and take steps to reduce mosquito populations.

top What is tetanus and how does it spread?

Tetanus is a disease that affects the nervous system and brain, and can cause death. Almost all mammals are susceptible to the disease, but horses are the most sensitive of all species.13 The disease is the result of a powerful neurotoxin produced by the bacterium Clostridium tetani. 14,15

Tetanus is not contagious, but it can be easily picked up by horses through puncture wounds or cuts from rusty objects in pastures or stalls.14,16

top What are the signs of tetanus?

After an incubation of seven to 10 days,14 signs may include:

  • Colic16
  • Stiffness or difficulty moving16
  • Third eyelid protrusion14,16
  • Spasm14,16
  • Sweating14,16
  • Labored breathing16
  • Difficulty swallowing16
  • Jaw contractions and lockjaw14,16

top Is there a treatment for tetanus?

Veterinarians often recommend taking steps to provide a quiet environment with little light or noise. A large, well-bedded stall with low-hanging feed and water can help prevent further danger to the horse.14

Sedatives and muscle relaxants may be used to control spasms and stiffness, and an antitoxin may be administered to eliminate the toxins that cause the disease.14

top How can I help prevent my horse from being infected with tetanus?

The keys are good management and vaccination. For example, you can reduce the chance of exposure by keeping pastures and stalls clean and clear of rusty tools, wire and nails. Check and treat wounds — even small flesh wounds.16 Annual vaccination with RECOMBITEK rWNV-EWT may help prevent tetanus as well as West Nile virus, EEE and WEE.17

top What is equine influenza and how does it spread?

  • Equine influenza is the leading cause of respiratory disease in horses.18
    • It is highly contagious and can spread quickly when a susceptible horse inhales secretions from an infected horse.
    • A coughing horse may spread infected droplets as far as 50 yards19 and a single horse infected with influenza can infect up to 10 others.20
  • Almost all unvaccinated horses that are exposed to equine influenza become infected and unprotected foals may develop a fatal viral pneumonia.19

top What are the signs of equine influenza and how is it treated?

  • Fever up to 106°F21
  • Nasal discharge21
  • Dry cough21
  • Poor appetite21
  • Depression or appear weak21
  • Many affected horses show no signs and still spread infection19
  • With fresh air and plenty of rest, a mildly affected horse will recover in two to three weeks21

top How can I help prevent my horse from being infected with equine influenza?

  • Because equine influenza is so easily spread between horses (see above), the practical way to prevent the spread of equine influenza is by vaccinating horses
  • RECOMBITEK Equine Influenza Virus vaccine works fast22 and significantly reduces viral shedding

top What is Potomac horse fever (PHF) and how is it transmitted?

PHF has been something of a mystery to researchers for nearly 30 years. It was named after the river where it was first discovered, but it has been detected in many parts of the United States and Canada.23 It is usually found near bodies of water, but also has been found in areas as remote and arid as northern Wyoming.24

PHF has a complex life cycle that is believed to involve bacteria, flukes, snails and aquatic insects, such as caddisflies and mayflies.23 This diagram provides an overview of the life cycle that can lead to infection in your horse.23,35

Potomavac Horse Fever Life Cycle

top What are the signs of PHF and how can infected horses be treated?

Signs can include:26

  • Depression
  • High fever (102 – 107°F [38.8 — 41.7°C])
  • Colic, watery diarrhea
  • Leukopenia
  • Abortion
  • Laminitis

Treatment usually consists of IV fluids and electrolytes and antibiotics.26

top How can I help prevent my horse from being infected with PHF?

  • Reduce populations of aquatic insects:
    • Use approved insecticides
    • Use netting
    • Reduce the use of nightlights, which attract insects
  • Annual vaccination just before peak insect hatch (mid- to late summer to early fall) with POTOMAVAC or POTOMAVAC + IMRAB can deliver reliable protection

top Why is it important to reduce aquatic insect populations?

  • Horses become infected with PHF after eating infected aquatic insects22 (see life cycle)
  • Reducing the number of caddisflies, mayflies and other aquatic insects — especially during peak hatch — can reduce contamination of your horse’s feed, bedding and water

top What is rabies and how is it transmitted? 27

  • Rabies is a deadly infection of the nervous system of horses and other mammals
  • Rabies is usually transmitted to horses through the bites of infected wildlife, including bats, skunks, foxes and raccoons
  • Once signs of rabies develop, the disease is invariably fatal

top What are the signs of rabies in horses? 28

  • Colic
  • Excessive salivation
  • Hyperexcitability
  • Aggressive behavior
  • Depression
  • Lack of coordination
  • Difficulty swallowing
  • Convulsions
  • Paralysis

top Can horses infected with rabies be treated?

There is no effective treatment for rabies infection. It is usually recommended that a rabid animal be humanely destroyed.28

If a horse is bitten, local regulations may require the horse to be quarantined for at least six months. If the horse is not vaccinated before the bite, it should not be vaccinated after potential exposure. Always contact your veterinarian if you suspect your horse has been exposed to rabies.

top How can I help reduce exposure to rabies?

Helpful tips include discouraging wild animals from coming near barns or pastures and not leaving pet food out as it may attract wildlife. Report wild animals or stray cats and dogs that are behaving strangely.

Finally, ask your veterinarian to vaccinate your horses every year with IMRAB Large Animal Rabies vaccine, and vaccinate your dogs, cats and/or ferrets with IMRAB 3.


top When and how often should I have my horse vaccinated against common equine diseases?

Your veterinarian knows your horse, local conditions and risks better than anyone, and is the best source for vaccination information and scheduling. Some diseases pose greater risks than others; some diseases are more prevalent at certain times of the year.

Your veterinarian also is familiar with the core vaccinations identified in the vaccination guidelines of the American Association of Equine Practitioners.2 These diseases (West Nile virus, EEE, WEE, tetanus and rabies) are singled out because they are widespread, present a public health risk, are highly infectious or pose a risk of severe disease, or are required by law. These core vaccinations are recommended for all horses — every year.

In addition, your horse’s individual program may include vaccinations for other diseases that pose special risks in your area.

top Do all horses require a similar vaccination schedule?

Aside from the core vaccinations recommended for all horses every year (see above), vaccination programs and schedules can be tailored to your horse’s specific needs. Consult your veterinarian for the program that will do the best job to help protect your horse.

top What should I expect following vaccination?

Horses often have reactions similar to the reactions people have after vaccinations. It’s fairly common for some horses to experience mild, temporary side effects beginning a few hours after vaccination. Clinical signs may include:29

  • Local muscle soreness or swelling
  • Fever
  • Loss of appetite
  • Lack of energy or alertness

All of these reactions are perfectly normal signs that an immune response has been stimulated. But if any of these clinical signs persist for more than 24 hours, contact your veterinarian.

Click here for a quick guide to what to expect when vaccinating.

top How are recombinant vaccines made?

There are several different kinds of recombinant vaccines. The canarypox-vectored vaccine technology used in RECOMBITEK® Brand Equine Vaccines isolates the genes for specific proteins of the West Nile virus or equine influenza virus that stimulate protective immunity. This graphic shows a simple example of how this material is inserted into the genome of the canarypox virus, which acts as a vector to transfer this immunity-stimulating genetic material to the immune system of the horse.

Vaccine Technology Chart
Click to view full-size PDF


1West Nile Virus on the AAEP Web site. Available at: http://www.aaep.org/wnv.htm. Accessed August 17, 2009.
2West Nile Veterinary 2008. Available at: http://diseasemaps.usgs.gov/wnv_us_veterinary.html. Accessed August 17, 2009.
3Guidelines for Vaccination of Horses. American Association of Equine Practitioners. Available at: http://www.aaep.org/vaccination_guidelines.htm. Accessed August 3, 2009.
4Ostlund EN, et al. Equine West Nile encephalitis, United States. Emerging Infectious Diseases 2001;7(4):665-669.
5Nervous System. In: Kahn CM, ed. Merck Veterinary Manual, Whitehouse Station: Merck 2005;1077-1081.
6Merck Veterinary Manual. Ninth edition. 2005:1027-1031.
7American Veterinary Medical Association. Viral Encephalitis Backgrounder August 23, 2006.
8Gibbs EP, Long MT. Chapter 20 — Equine alphaviruses. In: Sellon DC, Long MT. Equine Infectious Diseases. St. Louis: Saunders Elsevier, 2007:191-197.
9Eastern equine encephalomyelitis, Western equine encephalomyelitis and Venezuelan equine encephalomyelitis. Available at: http://www.cfsph.iastate.edu/Factsheets/pdfs/easter_wester_venezuelan_equine_encephalomyelitis.pdf. Accessed August 3, 2009.
10Eastern/Western equine encephalomyelitis. American Association of Equine Practitioners. Available at: http://www.aaep.org/eee_wee.htm. Accessed August 3, 2009.
11Animal Health Monitoring and Surveillance: Eastern equine encephalitis cases, 2008 (updated March 11, 2009). Available at: http://www.aphis.usda.gov/vs/nahss/equine/ee/eee_distribution_maps.htm. Accessed August 3, 2009.
12Mosquito borne disease: Eastern and Western equine encephalomyelitis and West Nile virus — prevention is just a vaccine away! Available at: http://www.cag.uconn.edu/ansci/ext/mosquito.htm. Accessed August 18, 2009.
13Merck Veterinary Manual. Ninth edition. 2005:495-497.
14MacKay RJ. Chapter 47 — Tetanus. In: Sellon DC, Long MT. Equine Infectious Diseases. St. Louis: Saunders Elsevier, 2007:376-380.
15Tetanus. American Association of Equine Practitioners. Available at: http://www.aaep.org/tetanus.htm. Accessed August 1, 2009.
16Horses and tetanus. Horseman Magazine. Available at: http://www.horsemanmagazine.com/2009/02/horses-and-tetanus. Accessed August 3, 2009.
17RECOMBITEK rWNV-EWT product label.
18Paillot R, Hannant D, Kydd JH, Daly JM. Vaccination against equine influenza: Quid novi? Vaccine 2006;24:4047-4061.
19Influenza updates from the AVMA. Backgrounder: Equine influenza. Available at: http://www.avma.org/public_health/influenza/equine_bgnd.asp. Accessed March 5, 2008.
20Minke JM, Bublot M. Equine influenza: an update. Shedding is possible even when the infected horse shows no signs of sickness. Bull. Acad. Vét France 2004;157(4):43-49.
21Merck Veterinary Manual. Ninth edition. 2005:1206.
22Paillot R, et al. Antibody and IFN-g responses induced by a recombinant canarypox vaccine and challenge infection with equine influenza virus. Vet Immuno Immunopathol 2006;112:225-233.
23Madigan J and Pusterla N. Life cycle of Potomac horse fever – implications for diagnosis, treatment and control: a review. AAEP Proceedings; 2005;51:158-162.
24Hamende V. Potomac horse fever cases confirmed in northern Wyoming. University of Wyoming Cooperative Extension Service. Press Release, September 13, 2002. Available at: http://wyovet.uwyo.edu/Diseases/2002/PotomacConf.pdf. Accessed February 18, 2008.
25Wilson JH, Pusterla N, et al. Incrimination of mayflies as a vector of Potomac horse fever in an outbreak in Minnesota. AAEP Proceedings 2006;52:324-328.
26Merck Veterinary Manual. Ninth edition. 2005:236.
27Merck Veterinary Manual. Available at: http://www.merckvetmanual.com/mvm/index.jsp?cfile=htm/bc/102300.htm. Accessed August 3, 2009.
28Michigan State University College of Veterinary Medicine online publication. January 2004. Available at: http://old.cvm.msu.edu/extension/equine/RabiesinHorses.pdf. Accessed August 3, 2009.
29Merck Veterinary Manual. Ninth edition. 2005:2181.

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