Vaccinations in horses are very important tools at our disposal for building immunity against a host of serious viral and bacterial diseases. Many of the viruses and bacteria we worry about in horses have a variety of transmission routes, such as vector-borne (e.g. mosquito) transmission, direct contact (e.g. nose-to-nose contact), and environmental contact with the pathogen (e.g. tetanus).

The core vaccines we focus on for the equine population are Rabies, Tetanus, Eastern and Western Equine Encephalitis (EEE/WEE) virus, West Nile Virus (WNV), Equine Influenza, and Equine Rhinopneumonitis/Herpesvirus. Risk-based vaccines vary depending on the location, but the Potomac Horse Fever vaccination is recommended in Virginia due to the increased risk of this disease and is therefore a recommended vaccination that is becoming more frequently accepted as a core vaccine.  These diseases are all highly transmissible and/or highly pathogenic that can have significant implications on your horse’s health.

Rabies:

Rabies is a neurologic viral disease in mammals that is 100% fatal. It occurs when infected saliva enters the bloodstream (e.g. through a bite wound from an infected animal) and the viruses replicates in the musculature and travels via peripheral nerves to the spinal cord and brain. The process varies in how long it takes, and therefore the incubation period can range from days to up to 6 months. Clinical signs in horses are highly variable but specifically include dysphagia and choke, colic, fever, sudden blindness, behaviour changes (depression/stupor is more common, mania/rage is less common), incontinence, ataxia, etc.

Risk factors include lack of vaccination, 24/7 access to pasture (due to exposure to wildlife), and presence of endemic wildlife vectors (skunks, raccoons, foxes, bats, infected domestic species)

Rabies is a AAEP core vaccine that is recommended to be revaccinated annually. The first dose of rabies vaccination should be performed at 4-6 months of age, and should be boostered after 4-6 weeks. Rabies is a required vaccination due to the risk that Rabies poses to the general population in the event a horse contracts Rabies and exposes a human. The vaccination must be administered by a licensed veterinarian.

 

Tetanus:

Tetanus is a neurological disease that is caused by a neurotoxin produced by the bacteria, Clostridium tetani. This bacteria is present in the soil and in oxygen-rich environments as a spore-state. When it is introduced into deep wounds or other sites (e.g. umbilicus, post-partum uterus, etc.), it can germinate into an active disease process. Tetanus blocks inhibitory signals in the spinal cord, listening to rigidity and spasmodic movements.

Clinical signs include stiffness, difficulty moving, wide based “saw horse” stance, third eyelid prolapse, elevated tend, extended neck, lock jaw, and dysphagia. Mortality is high in these cases and mortality rate is over 50%.

Events that promote entry and growth of this anaerobic bacteria include wounds, hoof abscesses, castration sites, retained fetal membranes, and umbilical infections. Your veterinarian likely will recommend your horse be boostered for tetanus in these situations if they have not been vaccinated in the last 6 months. A previously unvaccinated horse should receive 2 doses 4-6 weeks apart, and annually following the booster dose.

Tetanus toxoid vaccines are potent and rapidly induce immunologic responses in horses. All vaccines are labelled for annual revaccination, but horses that sustain a wound or laceration, or undergo surgery over 6 months from the previous tetanus vaccination should be revaccinated at the time of the event. The severity of the wound does not increase the risk of tetanus, and superficial wounds have been seen to result in tetanus as well.

 

Eastern and Western Equine Encephalitis Viruses

These viruses are arboviruses (transmitted via arthropods/mosquitoes). In the US, EEE has occurred in all states east of the Mississippi River, and has more recently extended to Texas and eastern Canada. It is a reportable disease, and has been known to transmit and cause severe disease and even death in humans. Mortality can exceed 90% in naïve horses, and vaccination lowers this rate.

EEE and WEE are indirectly transmitted to horses via bites from infected mosquitos after they feed on infected avian horsts (natural reservoirs for the virus). Direct transmission between horses or horse-to-human does not occur, and requires a mosquito to carry the virus from infected to naïve host. The incubation period can range from 5-14 days, and death usually occurs 2-3 days from the onset of clinical signs.

Clinical signs of EEE/WEE include moderate to high fever (102.5-104.5 F), depression, lethargy, inappetence, and signs of encephalitis (dysphagia, head pressing, tremors, weakness, ataxia, blindness, seizures), and death. In some horses, rapid behavioural changes (hyperexcitability, mania, self-mutilation) and cranial neuropathy (nystagmus, facial nerve paralysis, and motor issues of the tongue and pharynx) can be seen.

Annual vaccination is recommended, with an initial dose followed by a booster 4-6 weeks after the initial dose for previously unvaccinated horses or horses with an unknown vaccination history. A 6 month revaccination interval can be considered for horses <5 years old, or horses in endemic regions with longer mosquito/vector seasons. A conversation about a more frequent revaccination interval can be discussed with your veterinarian.

West Nile Virus:

West Nile Virus is another virus spread by infected mosquitos that have fed on reservoir avian species. The virus penetrates the blood-brain barrier in horses and enters the central nervous system. It is known to cause neurological signs and fever in horses. Clinical signs include low grade fever often accompanied by lethargic behaviour and inappetence, and neurologic signs are often sudden onset and rapidly worsen. The neurological signs include tremors/fasciculations of the face and neck muscles, alternating hyperexcitability with stupor, cranial nerve deficits (head tilt, muzzle deviation, tongue weakness), ataxia and recumbency, and death.

Annual vaccination is recommended, and can be increased to every 6 months in areas that have year-round mosquito seasons or are endemic for West Nile Virus.

 

Equine Influenza Virus

Equine Influenza virus (EIV) is spread through inhalation of infected aerosolized droplets from infected horses coughing and sneezing. These droplets can travel as far as 45 meters. Indirect transmission via contaminated surfaces, clothing, and equipment can also spread the virus. Shedding of the contagious virus lasts 7-10 days after infection in naïve horses, and is much shorter in previously vaccinated horses.

Clinical signs of EIV include fever (up to 106 F), lethargy, anorexia, muscle pain, dry cough, mild lymphadenopathy, serous nasal discharge (can progress to mucopurulent due to secondary bacterial pneumonia), and rarely distal limb edema and cardiomyopathy.

Vaccination reduces the risk of clinical disease but subclinical shedding of the virus from vaccinated, infected horses can occur. Annual vaccination is recommended, and more frequent (i.e. every 6 months) is recommended in horses that frequently travel, show, and in younger horses.

 

Rhinopneumonitis/Equine Herpesvirus

 

The most significant alphaherpesviruses in horses are EHV-1, and EHV-4. EHV-1 causes respiratory disease, abortion, and neurological disease in horses, and EHV-1 primarily causes respiratory disease and more rarely abortion and neurologic disease. Rhinopneumonitis is the respiratory disease caused by EHV-1 or -4, often in weanlings and yearlings. Older horses are more likely to transmit the disease asymptomatically.  The most common routes of transmission include respiratory transmission (inhalation of droplets, direct contact with respiratory secretions), but can also be spread through contact with aborted fetuses and fetal membranes, and fomites on contaminated surfaces, clothing, and equipment.

EHV-1 myeloencephalopathy (EHM) occurs when viremia from infection leads to vasculitis, thrombosis, and focal infarction (acute lack of blood flow) within the vasculature supplying the central nervous system. This can occur sporadically in individuals or as outbreaks in multiple horses.

Clinical signs of the respiratory form are fever, coughing, nasal discharge, lymphadenopathy, lethargy/anorexia, and can have distal limb swelling.

Clinical signs of EHV-1 abortion occur 2 weeks to months following exposure, and mares show no outward signs prior to impending abortion. This typically occurs in late gestation (7+ months), but can occur earlier in rarer cases.

Clinical signs of EHV-1 EHM include ataxia, dribbling urine, recumbency with inability to stand, and can show seizures, cranial nerve deficits. These signs are often preceded by fever and respiratory signs, but not always.

Foals infected in utero are usually very sick at birth (fever, lethargy, jaundice, respiratory distress, etc) and often die within 3 days of being born.

Vaccination is recommended annually for the EHV-1 and -4. Vaccination is only protective against the respiratory and abortive aspects of the disease. Currently, there are no licensed vaccine products for the neurologic form.

 

Potomac Horse Fever (PHF):

PHF is a non-contagious, infectious disease caused by the bacteria, Neorickettsia risticii and Neorickettsia findlayensis. The bacteria is consumed by trematode worms in freshwater sources, which are either subsequently consumed by mayflies that then land in water sources or hay/feed sources, or consumed by horses directly when drinking from freshwater sources. The most common time for this disease to occur is in summer and fall.

Risk factors can include housing horses within 5 miles of a freshwater stream, or river, and nighttime use of barn lights that attract these insects into the barn.

Clinical signs include fever (up to 107 F reported), colitis and diarrhea, laminitis (with or without diarrhea), toxic mucous membranes, anorexia, lethargy, and colic.

Virginia is considered an endemic area for PHF, and therefore maintaining a regular vaccine schedule for this virus is recommended. Annual revaccination is the baseline recommendation for this disease, but yearly revaccination with two doses 3-4 months apart should be considered in endemic areas. The first dose should be given prior to the spring when risk of exposure to the bacterium begins to peak.

 

What to Expect from Woodside Equine Clinic for Vaccinations:

At Woodside Equine Clinic, we offer vaccinations as either outside of, or within a wellness package. When we come out to your farm, a full physical examination will be performed to assess your horse’s general health prior to vaccination. This includes a heart rate, respiratory rate, and temperature check, as well as assessment of multiple other parameters (e.g. mucous membranes, gut sounds). After this, the vaccines are administered intramuscularly in the neck or hindquarters.

Your veterinarian will discuss with you what to look for in terms of any adverse vaccine reactions, and what to do if such vaccine reactions were to occur. Vaccination reactions are rare in horses, and are most often associated with the adjuvant that a vaccine is made with. If your horse has a history of vaccination reactions, an individualized plan to split up the vaccinations and reduce the degree of immune stimulation at a given time will be made to suit your horse.