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Equine Herpesvirus (Rhinopneumonitis)

  • Tuesday, 14 October 2014 00:00

Equine herpesvirus type 1 (EHV-1) and equine herpesvirus type 4 (EHV-4) can each infect the respiratory tract, causing disease that varies in severity from sub-clinical to severe and is characterized by fever, lethargy, anorexia, nasal discharge, and cough. Infection of the respiratory tract with EHV-1 and EHV-4 typically first occurs in foals in the first weeks or months of life, but recurrent or recrudescent clinically apparent infections are seen in weanlings, yearlings, and young horses entering training, especially when horses from different sources are commingled. Equine herpesvirus type 1 causes epidemic abortion in mares, the birth of weak nonviable foals, or a sporadic paralytic neurologic disease (equine herpesvirus myeloencephalopathy-EHM) secondary to vasculitis of the spinal cord and brain.

Both EHV-1 and EHV-4 spread via aerosolized secretions from infected coughing horses, by direct and indirect (fomite) contact with nasal secretions, and, in the case of EHV-1, contact with aborted fetuses, fetal fluids, and placentae associated with abortions. Like herpesviruses in other species, these viruses establish latent infection in the majority of horses, which do not show clinical signs but may experience reactivation of infection and shedding of the virus when stressed. Those epidemiologic factors seriously compromise efforts to control these diseases and explain why outbreaks of EHV-1 or EHV-4 can occur in closed populations of horses.

Because both viruses are endemic in most equine populations, most mature horses have developed some immunity through repeated natural exposure; thus, most mature horses do not develop serious respiratory disease when they become infected but may be a source of exposure for other susceptible horses. In contrast, horses are not protected against the abortigenic or neurologic forms of the disease, even after repeated exposure, and mature horses are in fact more commonly affected by the neurologic form of the disease than are juvenile animals.

Recently, a genetic variant of EHV-1 has been described (defined by a single point mutation in the DNA polymerase [DNApol] gene) that is more commonly associated with neurologic disease. This mutation results in the presence of either aspartic acid (D) or an asparagine (N) residue at position 752.  Molecular diagnostic techniques can identify EHV-1 isolates carrying these genetic markers, although currently the implications of this finding for management of EHV-1 outbreaks, or individual horses actively or latently infected with these isolates, are uncertain. It is important to understand that both isolates can and do cause neurological disease, it is just more common for the D752 isolates to do so (it is estimated that 80-90% of neurological disease is caused by D752 isolates, and 10-20% by N752 isolates). Experts do not currently advise any specific management procedures for horses based on which isolate they are latently infected with, and it is possible that 5-10% of all horses normally carry the D752 form (this estimate is based on limited studies at this time). In the face of an active outbreak of EHV-1 disease, identification of a D752 isolate may be grounds for some increased concern about the risk of development of neurological disease.

Primary indications for use of equine herpesvirus vaccines include prevention of EHV-1-induced abortion in pregnant mares, and reduction of signs and spread of respiratory tract disease (rhinopneumonitis) in foals, weanlings, yearlings, young performance and show horses that are at high risk for exposure. Many horses do produce post-vaccinal antibodies against EHV, but the presence of those antibodies does not ensure complete protection. Consistent vaccination appears to reduce the frequency and severity of disease and limit the occurrence of abortion storms but unambiguously compelling evidence is lacking. Management of pregnant mares is of primary importance for control of abortion caused by EHV-1.


Inactivated vaccines

A variety of inactivated vaccines are available, including those licensed only for protection against respiratory disease, which currently all contain a low antigen load, and two that are licensed for protection against both respiratory disease and abortion which contain a high antigen load. Performance of the inactivated low antigen load respiratory vaccines is variable, with some vaccines outperforming others. Performance of the inactivated high antigen load respiratory/abortion vaccines is superior, resulting in higher antibody responses and some evidence of cellular responses to vaccination. This factor may provide good reason to choose the high antigen load respiratory/abortion vaccines when the slightly higher cost is not a decision factor.

Modified live vaccine

A single manufacturer provides a licensed modified live EHV-1 vaccine.  It is indicated for the vaccination of healthy horses 3 months of age or older as an aid in preventing respiratory disease caused by equine herpesvirus type 1 (EHV-1).


All available vaccines make no label claim to prevent the myeloencephalitic form of EHV-1 (EHM) infection. Vaccines may assist in limiting the spread of outbreaks of EHM by limiting nasal shedding EHV-1 and dissemination of infection. For this reason some experts hold the opinion that there may be an advantage to vaccinating in the face of an outbreak, but in advance of EHV-1 infection occurring in the group of horses to be vaccinated. The vaccines with the greatest ability to limit nasal shedding include the 2 high-antigen load, inactivated vaccines licensed for control of abortion (Pneumabort-K®: Pfizer; & Prodigy® Merck), a MLV vaccine (Rhinomune®, Boehringer Ingelheim Vetmedica) and an inactivated vaccine, (Calvenza®, Boehringer Ingelheim Vetmedica).

Vaccination against either EHV-1 or EHV-4 can provide partial protection against the heterologous strain; vaccines containing EHV-1 may be superior in this regard.

Vaccination schedules:

Adult, non-breeding, horses previously vaccinated against EHV :  Frequent vaccination of non-pregnant mature horses with EHV vaccines is generally not indicated as clinical respiratory disease is infrequent in horses over 4 years of age. In younger/juvenile horses, immunity following vaccination appears to be short-lived. It is recommended that the following horses be revaccinated at 6-month intervals:

  • Horses less than 5 years of age.
  • Horses on breeding farms or in contact with pregnant mares.
  • Horses housed at facilities with frequent equine movement on and off the premises, thus resulting in an increased risk of exposure.
  • Performance or show horses in high-risk areas, such as racetracks. More frequent vaccination may be required as a criterion for entry to the facility.

Adult, non-breeding horses unvaccinated or having unknown vaccinal history:  Administer a primary series of 3 doses of inactivated EHV-1/EHV-4 vaccine or modified-live EHV-1 vaccine. A 4 to 6 week interval between doses is recommended.

Pregnant mares: Vaccinate during the fifth, seventh, and ninth months of gestation using an inactivated EHV-1 vaccine licensed for prevention of abortion. Many veterinarians also recommend a dose during the third month of gestation and some recommend a dose at the time of breeding.

Vaccination of mares with an inactivated EHV-1/EHV-4 vaccine 4 to 6 weeks before foaling is commonly practiced to enhance concentrations of colostral immunoglobulins for transfer to the foal. Maternal antibody passively transferred to foals from vaccinated mares may decrease the incidence of respiratory disease in foals, but disease can still occur in those foals and infection is common.

Barren mares at breeding facilities: Vaccinate before the start of the breeding season and thereafter based on risk of exposure.

Stallions and teasers: Vaccinate before the start of the breeding season and thereafter based on risk of exposure.

Foals: Administer a primary series of 3 doses of inactivated EHV-1/EHV-4 vaccine or modified-live EHV-1 vaccine, beginning at 4 to 6 months of age and with a 4 to 6 week interval between the first and second doses. Administer the third dose at 10 to 12 months of age.

Immunity following vaccination appears to be short-lived and it is recommended that foals and young horses be revaccinated at 6-month intervals.

The benefit of intensive vaccination programs directed against EHV-1 and EHV-4 in foals and young horses is not clearly defined because, despite frequent vaccination, infection and clinical disease continue to occur.

Outbreak mitigation: In the face of an outbreak, horses at high risk of exposure, and consequent transmission of infection, may be revaccinated. Administration of a booster vaccination is likely to be of some value if there is a history of vaccination. The simplest approach is to vaccinate all horses in the exposure area—independent of their vaccination history. If horses are known to be unvaccinated, the single dose may still produce some protection.

There remain concerns that heavily vaccinated horses may be more susceptible to developing neurological disease caused by EHV-1. This possibility is unsubstantiated and a subject of active investigation. To date, the use of a single vaccine immediately before exposure has not shown any association with an increased incidence of neurological disease.

Horses having been naturally infected and recovered: Horses with a history of EHV infection and disease, including neurological disease, are likely to have immunity consequent to the infection that can be expected to last for 3 to 6 months (longer in older horses). Booster vaccination can be resumed 6 months after the disease occurrence.

Colic and Equine Enteroliths

  • Friday, 10 October 2014 00:00

Colic, or abdominal pain, is a common ailment in horses. More than 70 causes can trigger colic, including gas distention, food impactions, intestinal tract spasms, and intestinal displacement or twists. One of the more exotic forms is colic caused by enteroliths, or stone-like formations that form in a horse's digestive tract.

Enterolith stones are made up of minerals, such as magnesium ammonium phosphate salts. These minerals can build up around an object that a horse eats but does not digest, such as a small chunk of wood, pebble, wire, twine, or other foreign object. These masses can become quite large, sometimes weighing in at 10 to 15 pounds or more.

colic surgerySometimes horses develop one large stone, while others chronically develop multiple stones. The stones form in the large colon of the horses and cause a problem once they move to areas of the intestine that have smaller diameters, such as the transverse colon or the small colon.

Smaller enteroliths might pass with feces, but if horses produce smaller stones, larger ones might remain. Mild chronic or intermittent colic can plague horses with a single stone that has not grown large enough to cause a total impaction or has not moved to a narrower area. These horses might also suffer from weight loss, diarrhea, or lethargy.

There are several risk factors for enterolith development. Geography is one. Certain areas like the southwestern U.S. produce many more cases than others, particularly California and Arizona. In California, enteroliths are the leading cause of surgical colic cases. The mineral composition of the soil and feed material grown there are suspected culprits. The risk does not seem as high in the Northwest, but approximately five cases are seen at the WSU Veterinary Teaching Hospital each year.

"Even horses that travel to Arizona or California for shows or other events can develop enteroliths while visiting," said Julie Cary, DVM, MS, Dipl. ACVS, clinical instructor of equine surgery and emergency care at WSU.

Enteroliths also seem more common in horses fed a diet of more than 50% alfalfa hay. They are more commonly found in adult horses between 10 and 15 years of age, in female horses, and in the Arabian horse breed, although the problem can occur in horses of any breed.

If a horse is suspected to have enteroliths, the best way to confirm a diagnosis, other than exploratory surgery, is to take abdominal radiographs or X rays. Large veterinary hospitals or clinics usually have a radiograph generator powerful enough to produce a picture of a horse's abdomen.

If surgery is required, most horses have a very good outcome unless a portion of the intestine was badly damaged or ruptured. After surgery, horses generally need three months to recover before returning to full exercise. Owners should continue to monitor them for colic. Stall rest is required for about a month before being placed back to pasture.

Because enteroliths are uncommon in the Pacific Northwest, the faculty at the Veterinary Teaching Hospital does not recommend specific preventive measures for most horses. However, if your horse has been diagnosed with an enterolith in the past, the following measures may help prevent recurrence:

  • Decrease alfalfa hay in the diet and feed good quality grass hay instead.
  • Don't feed bran because it contains very high levels of phosphorus.
  • Add a cup of vinegar a day to your horse's diet to help decrease the pH inside the intestine.
  • Provide your horse with frequent access to pasture. Continual grazing will help to encourage regular movement of feed material through the intestines.
  • Ask your veterinarian about including occasional doses of psyllium in your horse's diet to increase bulk movement through the intestines.
  • Provide regular, consistent exercise for your horse.

Vesicular Stomatitis In Horses

  • Wednesday, 22 October 2014 00:00

Vesicular Stomatitis (VS) is a contagious disease that afflicts horses, livestock, wildlife and even humans. The disease is caused by a virus, which although rarely life threatening, can have significant financial impact on the horse industry. Vesicular Stomatitis is a reportable disease; in a suspect case, state and federal animal health authorities will be contacted by your veterinarian. When a case of vesicular stomatitis is confirmed, your state veterinarian’s office will quarantine the affected farm or ranch. In an effort to minimize risk of spread of the disease, horses will be confined to that location for a specified period, usually for 30 days following resolution of the last case on that property. Equestrian event organizers may also choose to cancel horse shows, rodeos, and other equestrian activities in the surrounding area. Imports and exports of horses may also be restricted.

Clinical Signs & Diagnosis

When vesicular stomatitis occurs in horses, blister-like lesions develop on the tongue, mouth lining, nose and lips. In some cases, lesions also develop on the coronary bands, or on the udder or sheath. When VS is suspected, an exact diagnosis should be obtained by testing the blood for virus-specific antibodies. Testing is necessary to rule out the possibility that the lesions are caused by photosensitivity (sunburn), irritating feeds or weeds, or toxicity from non-steroidal anti-inflammatory medications like phenylbutazone.

VS should not be confused with foot and mouth disease(FMD), which does not affect horses, and was eradicated from the U.S.A. in 1929. The incubation period for vesicular stomatitis – meaning the time from exposure until the first signs appear – ranges from 2 to 8 days. A fever may develop initially as blisters form on the tongue, gums, or coronary bands. One of the most obvious clinical signs is drooling or frothing at the mouth. This occurs following rupture of the blisters that create painful ulcers in the mouth. The surface of the tongue may slough. Excessive salivation is often mistaken as a result of a dental problem just as a horse that is not eating well may be suspected as having colic. Weight loss may be a secondary effect, as a horse with mouth ulcers finds it too painful to eat. If lesions form around the coronary band, inflammation within the foot may result in lameness or laminitis. In severe (but rare) cases, the lesions on the coronary band may cause the hoof to slough.

The disease generally runs its course within two weeks, although it may take as long as two months for the sores to entirely heal. Until the ulcers are completely healed, the horse remains infective and the potential remains for disease to spread.


While a horse is suffering from vesicular stomatitis, feeding soft feeds may reduce mouth discomfort. Anti-inflammatory medications as supportive care help to minimize swelling and pain so a horse will continue to eat and drink. Secondary bacterial infection of ulcerated areas is another concern. If fever, swelling, inflammation or pus develops around the sores, treatment with antibiotics may be required. However, there is little an owner or veterinarian can do but wait for healing to occur and take appropriate precautions to minimize the risk of spread of the disease to other horses and livestock.

Disease Transmission

There are still many questions regarding how vesicular stomatitis is transmitted. The disease is distributed only in North and South America, with a greater incidence in warmer regions. Due to the seasonal occurrence of VS during summer through early fall, it is believed that insects such as biting flies and gnats transmit the virus. Stable and houseflies are other possible but unlikely vectors. VS also seems to be passed from horse to horse by contact with saliva or fluid from ruptured blisters. Physical contact between animals, or contact with buckets, equipment, housing, trailers, feed, bedding or other items used by an infected horse can provide a ready means of spread.


By observing the following guidelines you can help prevent the occurrence of VS:

· Healthy horses are more disease resistant so provide good nutrition, regular exercise, deworming and routine vaccinations.

· Isolate new horses for at least 21 days before introducing them into the herd or stable.

· Observe your horse closely. Immediately isolate any horse that shows signs of infection.

· Implement an effective insect control program. Keep stabling areas clean and dry. Remove waste and eliminate potential breeding grounds (standing water, muddy areas) for insect vectors.

· Use individual rather than communal feeders and equipment.

· Clean and disinfect feed bunks, waterers, horse trailers and other equipment regularly.

· Be sure that your farrier and other equine professionals who come into direct contact with your animals exercise due care so as not to spread the disease from one horse or facility to the next.

· On farms where VS has been confirmed, handle healthy animals first, ill animals last. Handlers should then shower, change clothing and disinfect equipment to prevent exposing others.

· Anyone handling infected horses should implement proper biosafety methods, including wearing latex gloves.

· If you are sponsoring an event during an outbreak, require a current health certificate on every horse entering the venue.

Vesicular Stomatitis In Humans

Humans can contract vesicular stomatitis from infected horses. Therefore, it is important to follow proper biosafety measures. Precautions should include wearing latex gloves and avoiding direct contact with the horse’s saliva or blister fluids. Special care should be taken to keep mouth, eyes and any open wounds from being exposed to infection.

Vesicular stomatitis in humans tends to cause severe flu-like symptoms such as headache, fever, muscle aches, and extreme fatigue. People rarely develop blisters in their mouths. However, if you experience influenza-like symptoms after working with a VS infected horse, contact your physician immediately.

Vaccines And Disinfectants

Vaccines have been developed to help combat vesicular stomatitis. However, there is considerable debate over their efficacy in preventing or reducing the severity of an outbreak. The period of protection is thought to be fairly limited and once vaccinated, a horse will test positive thereby incurring travel restrictions. VS may recur once the antibodies within the horse’s system wane. Contact your state veterinarian for information on the availability of vaccines and any permits that may be required.

Sunlight and heat are known to quickly destroy the virus that causes vesicular stomatitis. Commercial disinfectants such as chlorine bleach (0.645%), Wescodyne (4 %), Roccal (1:200), Septisol (1:50), and cresylic acids (1 %) are also effective.

Working With Your Veterinarian

By working closely with your equine veterinarian, you can develop strategies to reduce the likelihood of a vesicular stomatitis outbreak, or to minimize the effects should one occur. Veterinarians and owners who suspect that an animal has vesicular stomatitis should immediately contact state or federal animal health authorities. For more information, contact:

U.S. Dept. of Agriculture

Animal and Plant Heath Inspection Services

Veterinary Services, Emergency Programs

4700 River Road, Unit 41

Riverdale, MD 20737-1231

(301) 734-8073

This information is provided courtesy of the American Association of Equine Practitioners.

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Breaking News

  • Colic and Equine Enteroliths +

    Colic and Equine Enteroliths Colic, or abdominal pain, is a common ailment in horses. More than 70 causes can trigger colic, including gas distention, Read More
  • Vesicular Stomatitis In Horses +

    Vesicular Stomatitis In Horses Vesicular Stomatitis (VS) is a contagious disease that afflicts horses, livestock, wildlife and even humans. The disease is caused by Read More
  • Equine Herpesvirus (Rhinopneumonitis) +

    Equine Herpesvirus (Rhinopneumonitis) Equine herpesvirus type 1 (EHV-1) and equine herpesvirus type 4 (EHV-4) can each infect the respiratory tract, causing disease that Read More
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