Fewer than 60% of horses in the UK are thought to be protected against tetanus — a remarkably low figure given the deadly nature of the disease.
It’s thought that vaccination is largely driven by owners adhering to competition regulations, rather than the desire to protect their animals. Furthermore, many simply don’t realise that their horse doesn’t have to leave his field to be at risk of contracting the infection.
Yet tetanus is ubiquitous and indiscriminate, meaning that unvaccinated horses, ponies and donkeys of all ages and types are vulnerable. Of all domesticated animals, horses are most susceptible to infection.
An ever-present risk
Tetanus is caused by the spore-forming bacteria Clostridium tetani, which is found in soil worldwide (although its prevalence may vary). It can also be found in intestinal tracts.
Soil contaminated with infected faeces is the most common method of transmission, with the bacteria able to survive for long periods of time — even for years — in most soil conditions.
Bacteria may also be ingested, and so can even affect horses with no external wounds. Unvaccinated horses are therefore at constant risk.
The bacteria C. tetani is anaerobic, meaning that it is unable to grow in the presence of oxygen but thrives in its absence. Its spores cannot survive in normal tissue, nor in wounds if the tissue remains well-oxygenated. Tissue death (necrosis) provides the perfect environment, however, for rapid multiplication of bacteria.
A deep puncture wound is by far the most common route by which the bacteria enter the horse’s body, punctures of the foot in particular. Sometimes the point of entry cannot be found because the wound itself may be minor or healed.
Rarely, infection can occur via the intestines if a horse eats contaminated soil or faeces.
In the case of newborn foals, bacteria can gain access via the umbilicus (navel).
Once inside the body, C. tetani produces a potent toxin that targets the nervous system and is absorbed by the motor nerves serving the infected area. From here it travels up the nerve tract to the spinal cord, from where it can affect the entire body and cause generalised “tetanic spasm” — sudden and involuntary contraction of a single muscle or muscle group.
The length of time from infection to clinical disease is in the region of 10-21 days. The disease is non-contagious, in that it cannot be transferred between horses or from horse to human, or vice versa.
The initial signs of tetanus are a consequence of the toxin attacking the nerves that control the skeletal (postural) muscles.
This is swiftly followed by a progressive deterioration.
Stiffness in the head and limbs is one of the first signs, accompanied by muscular spasms that progressively worsen. The horse becomes reluctant or unable to move, adopting a posture with extended limbs called the “sawhorse stance”. He may be unable to eat or chew, which is the reason why tetanus may be referred to as “lockjaw”.
Other signs include sweating, flaring of the nostrils and facial muscle spasms that cause a startled, wide-eyed expression.
The tail may be held out straight, the third eyelids may extend across the eyeballs and the horse may exhibit hypersensitivity to noise or light.
Exacerbated spasms and convulsions later cause collapse, followed quickly by death due to respiratory failure from paralysis of the breathing muscles.
Fatality rates are in the range of 75%. Once the horse has collapsed, the most humane course of action is euthanasia.
If a case is diagnosed early, however, large doses of tetanus antitoxin can be administered along with antibiotics, sedatives and muscle relaxants. Yet despite intensive treatment, most unprotected horses do not survive.
The simple solution
As ever, prevention is far better than cure. Vaccination is inexpensive and extremely effective, so there is really no excuse for any horse not to be protected.
In an emergency, an unvaccinated horse can be given an antitoxin to provide rapid but short-lived immunity of approximately four to six weeks to the toxin produced by C. tetani.
Vaccination involves the administration of tetanus toxoid — a dose of toxin that has been treated by heat or a chemical agent to destroy its deleterious properties without destroying its ability to combine with or stimulate the formation of antitoxin.
So whereas tetanus antitoxin provides fast but short-acting protection against the disease, vaccination stimulates the body to produce its own, long-lasting immunity.
The initial vaccination course consists of two primary injections of tetanus toxoid, four to six weeks apart, followed by a booster injection administered within 12 months. Subsequent boosters are normally administered every two years, but this can vary depending on the product used.
Tetanus vaccines can sometimes give rise to a reaction at the injection site. This usually comprises little more than a swelling of the muscle into which the injection was given, accompanied by local soreness and stiffness.
Short-term pain relief can be given if a stiff neck interferes with eating, for example, but this reaction will usually resolve itself within a few days.
Are they properly protected?
Mares and foals
Vaccinated pregnant mares should be given a tetanus booster four to six weeks before foaling.
This increases antibodies in the colostrum (first milk), providing the foal with a degree of maternal protection for approximately six to 12 weeks.
To supplement this, newborn foals can also be given a tetanus antitoxin soon after birth to provide temporary cover. A regular vaccination programme can then be initiated from approximately three to four months old.
Riders should check rules for their specific discipline as some require boosters as often as every six months.
Horses without a vaccination history, or those not previously vaccinated against tetanus, should receive a tetanus antitoxin in the event of a wound being sustained. The tetanus vaccination works on a different pathway to the antitoxin, so a vaccination course can be started at the same time.
Horses aged 20 years or more experience a reduced response to vaccines. It’s not the case that a horse that has been vaccinated for many years develops age-related immunity — all veterans need a regular tetanus booster.
Don’t forget donkeys — they should follow the same vaccination programme as horses.
Our knowledge of the effect of tetanus toxoid and antitoxin has been largely based on studies that are now 70 years old. New research undertaken by the Universities of Melbourne and Sydney in Australia has multiple aims: to identify the role of tetanus antitoxin in disease prevention; to determine correct dosage and the effectiveness of tetanus antitoxin, and to establish the optimum frequency of booster vaccination with tetanus toxoid.
According to Rachel Bradbury of MSD Animal Health, the national vaccination rate for tetanus among equines is estimated at 56%.
“The stats are worryingly low,” she says. “Based on the market research we have conducted, there is certainly a lack of awareness among owners of the risk of infectious disease. This is something we are working to address through our Keeping Britain’s Horses Healthy campaign.” www.healthyhorses.co.uk
Ref: Horse & Hound; 12 May 2016