The Definition of Equine Botulism
Botulism in horses occurs when the spores of the bacterium Clostridium botulinum contaminate a wound or a feed source. The ingestion of the spores or their introduction into a wound facilitates infection and releases the botulinum toxin.
Botulism poses a severe risk to horses as they are one of the most sensitive species to the effects of the bacteria’s toxin.
What Causes the Deadly Disease?
The spores of C. botulinum survive in the environment’s soils for several years and propagate well in moist conditions with an acidic pH and low oxygen saturation. The bacteria has a worldwide distribution, making it a risk to all domestic animals. Soils that contain decaying carcasses are high-risk sources of environmental contamination.
The bacteria produce a neurotoxin that blocks the signals between neuromuscular junctions resulting in progressive weakness and paralysis. The condition is not common, but it boasts a high mortality rate unless owners take drastic, intensive measures to administer anti-toxins and provide intensive nursing and supportive treatments.
The three main ways horses become infected with the bacteria include:
- Feed contamination – Decaying plant matter or moist haylage are ideal vectors for the bacteria as they are acidic and damp, which provides an anaerobic environment when stored.
- Ingestion of spores in foals under eight months – The bacteria proliferates in the intestines of the foals and releases the toxin, which results in a condition known as “Shaker Foal.” The shaking occurs due to muscle weakness from the toxin.
- Wound contamination – If a horse’s wound comes into contact with soil containing the botulism bacteria, the wound quickly becomes infected, and the toxin spreads throughout the horse’s body.
Clinical Signs and Symptoms of Botulism in Horses
The botulism bacteria toxin affects the nerves and the muscles by blocking the nerve impulse from reaching the muscle receptors. The signal does not conduct the muscle fibers, which prevents the contraction signal, thereby resulting in flaccid paralysis of the muscle.
Signs to watch for include the following:
- Horses develop a stiff, altered gait with shorter strides and awkward staggering or stumbling.
- Generalized weakness and decreased exercise tolerance.
- Muscle tremors.
- Increased salivation and a lack of control and strength of the tongue.
- Decreased tail tone.
- Bladder paralysis and colic.
- Recumbency due to weakness.
- The progression of flaccid paralysis leads to increased breathing difficulty and difficulty swallowing.
- Death results due to diaphragmatic paralysis that results in respiratory and heart failure.
Throughout the condition’s progression, the horse will remain bright and alert as their mentation is not affected. The horse will want to eat and be fully aware of their surroundings. The severity of symptoms depends on the concentration of the ingested toxin.
The Different Botulinum Toxin Serotypes
There are seven different serotypes of the toxin produced by C. botulinum. There is a link between serotypes, geographic distribution, and feed sources.
Listed below are the various serotypes of the botulinum toxins of most concern in horses:
- Type A toxicity is uncommon but has occurred in America’s North-West states.
- Type B is responsible for wound botulism and Shaker Foal Syndrome. The condition occurs in foals between two weeks and eight months. The geographic distribution of Shaker Foal Syndrome occurs in Kentucky and the mid-Atlantic states.
- Type C is known as carrion botulism due to the association of decomposing carcasses contaminating feed.
The Veterinary Diagnosis Process
A veterinarian will diagnose botulism based on clinical symptoms but must exclude neurological conditions such as viral diseases, tick paralysis, or toxicities.
If the vet identifies the infection source, they can confirm if the bacteria is the cause. Laboratory tests may not yield diagnostic results due to low serum toxin levels. A bio-assay mouse test requires a high volume of plasma or serum to detect the toxin.
The intestinal contents of foals with suspected Shaker Foal disease may contain C. botulinum spores.
The grain test is a basic test to determine if a horse has botulism. Horses suffering from botulism have problems grasping food with their tongue and lips. If a horse takes longer than two minutes to finish around 8 ounces (220 grams) of food, it indicates difficulty eating.
If a horse dies after showing signs of muscle weakness or paralysis, the vet may perform a post-mortem examination if botulism is suspected. Autopsies do not always reveal significant physical abnormalities. The vet can still send a sample from the gastrointestinal tract to test for the bacteria if botulism is suspected.
Available Treatment Options
Botulism requires intensive treatment, which is very costly. Horses with mild symptoms may recover with symptomatic treatment and supportive care, but horses with severe botulism symptoms require anti-toxin administration.
A polyvalent plasma anti-toxin that covers the neurotoxin serotypes A-E costs between $2,500 to $3,000 per unit, and the trivalent plasma, which covers only A to C, costs around $1,000 per unit. A single dose is necessary to treat a horse, and the anti-toxin offers protection for over 60 days after administration.
Secondary complications such as aspiration pneumonia require supportive management with antimicrobials or fluid therapy. Padded stables with minimal activity are best to reduce muscle activity.
What are the Outcomes of Equine Botulism?
The prognosis of botulism depends on the level of intoxication a patient experiences. Horses with subtle symptoms make a full recovery with symptomatic treatment.
Cases that result in several days of recumbency or acute symptoms often have a guarded prognosis and high mortality rate. The longer a horse suffers from botulism symptoms, the worse the muscle weakness becomes. Muscle loss associated with botulism may take weeks to recover.
If the symptoms do not receive treatment promptly – the risk of death becomes exponentially higher.
Preventing botulism is difficult, but through good husbandry practices, owners and stable yard managers can decrease the risk of exposure.
Botulism vaccines available provide insufficient protection for horses in endemic areas. The killed vaccine, also known as a toxoid, solely prevents botulism from C. botulinum serotype B. There is only one vaccine licensed for use in the United States, and it offers no cross-protection for serotypes A or C. Horse owners cannot entirely rely on vaccines for botulism in horses to keep their stable yard safe from the bacteria.
Foals usually receive colostrum antibodies from the vaccinated mare. All foals born to unvaccinated mares in C. botulinum endemic regions run a high risk of developing Shaker Foal Syndrome.
The vaccine protocol for equine botulism falls into the following categories:
- Naive foals or adult horses: Foals can receive vaccines from as early as two weeks old, and unvaccinated horses require three doses every four weeks over twelve weeks.
- Vaccinated adult horses: Annual booster vaccinations.
- Vaccinated pregnant mares: Single annual vaccination and a booster vaccination two to four weeks before foaling down.
To prevent botulism through good husbandry, stable managers must safely store and regularly inspect forage to detect any decaying vegetation or animal carcass contamination of feed. Owners should avoid any fermented forage, haylage, or silage as well. Strict bird and rodent deterrent protocols ensure that feed remains clean and uncontaminated.
Diligent equine wound care and vaccination schedule compliance add additional protection to vulnerable groups of horses. Read our article and find out What You Should Know About The Five-Way Horse Vaccine.
The Possibility of Inter-Horse Transmission
There is no possibility of botulism transmission between two horses unless the second horse is the cause of contamination after death. Botulism only spreads through ingesting feed contaminated by spores from infected carcasses or wound infections. The bacteria is not transferred directly between horses or people.
What to do if Your Horse Has Botulism
The early recognition of suspected botulism intoxication is crucial to treating the disease. Contact a veterinarian immediately if your horse shows neurological symptoms and muscle weakness.
The key is to minimize all activity and stimulation to a bare minimum to preserve energy reserves and muscle strength. Stable the horse in a cool, dimly lit, and quiet stable until a vet can attend to them.
If the vet diagnoses the horse with botulism, owners need to consider referral to a large hospital or prepare for long hours of intensive care and nursing in the stable yard. Reach out to other horse owners and staff to help delegate shifts to ease the workload.
The Role That Haylage Plays
Haylage is a hay-crop silage often used as a low-dust feed source for horses. There is a significant risk of botulism associated with feeding haylage to horses due to the high moisture content, pH, and risk of contamination with decaying plants or animal carcasses.
Feed managers must ensure haylage stays at a pH of 5.5. In cases where the haylage insulator becomes damaged, secondary fermentation alters the pH, providing the perfect environment for Clostridial multiplication.
To avoid contamination of haylage with C. botulinum spores, owners must not rake soil into the hay or apply any chicken or turkey manure on the area intended for horse hay usage. Drier haylage carries a lower risk of excess moisture that may increase bacteria proliferation. It is irresponsible to feed haylage to horses not vaccinated against botulism.
That’s All, Folks
Botulism in horses is rare but has severe consequences and should not be taken lightly in endemic areas.
Good equine husbandry is the best way to protect horses from botulism, including strict vaccine compliance and responsible feed sourcing.
Close monitoring and reporting suspected botulism cases lead to a much better prognosis. Early detection and rapid intervention could save your horse’s life.