By Stephanie Wescott BVSc MRCVS,
Ashbrook Equine Hospital
With all the recent hype about Equine Influenza following the UK-wide outbreak, equine vaccines are at the forefront of many of our minds. Although equestrian venues and governing bodies will focus on the influenza component of your vaccine record when you turn up to compete, it is equally important to the health of your horse that tetanus cover is kept up-to-date.
Any unvaccinated horse is at risk of developing tetanus and, compared to most other animal species, horses are particularly tetanus prone due to their unending ability to injure themselves. Tetanus occurs most commonly following penetrating injuries, skin wounds, foot abscesses or surgery (this can include surgeries performed in a sterile operating theatre as well as those performed ‘in the field’ such as colt castrates). Foaling is also a risky time for tetanus, as umbilical infections in the foal and foaling injuries in the mare can provide opportunity for tetanus to occur.
Tetanus is not contagious and so cannot spread from horse to horse. Clostridium tetani, the causative agent, survives in the form of spores in soil, dust and manure. Disease can occur when wounds are contaminated by these spores. In the correct conditions, the spores ‘germinate’ within the body to become bacteria which multiply and release tetanus toxin. Tetanus prefers low oxygen environments for germination and proliferation meaning small deep wounds and wounds containing necrotic (dead) tissue are a higher tetanus risk as less fresh air can get in. Tetanus toxins primarily target the Central Nervous System (CNS i.e. the brain and spinal cord) and can reach it by travelling along peripheral nerves that extend throughout the body. Toxins can take from a few days to several weeks to cause visible symptoms. The time taken depends on how far from the CNS the wound is and consequently the distance the toxins have to travel.
The range of symptoms of tetanus are wide and always progressive if untreated. Initial signs may include a stiff gait, partially raised tail and changes in the face including rigid/sticking up ears, partial prolapse of the third eyelid across the eye and flaring of the nostrils. These symptoms are all due to increased muscle activity which occurs due to toxins. Horses become abnormally sensitive to sound and touch. Eventually, they are unable to eat as the muscles that control chewing spasm and the mouth cannot properly open (hence the term ‘lockjaw’). Horses also begin to have difficulty swallowing food and saliva, causing them to regurgitate food, fluids and saliva. Eventually, extreme muscle spasms begin to occur all over the body. The horse will become severely dehydrated from its inability to drink and the respiratory muscles will fail leading to respiratory compromise and death. Colic, pneumonia and laminitis can also occur simultaneously.
If tetanus is caught early enough treatment can be attempted; however, the success rates are very low (<25%) and tetanus is all too frequently fatal despite the best efforts. Cases require prolonged courses of antitoxin and antibiotic medication alongside wound management, intensive nursing to meet fluid and nutrition needs, muscle relaxants to reduce muscle spasm and sedatives to reduce over-reaction to stimuli and risk of injury. The cost of such intensive treatment is very high. Horses that are recumbent (unable to stand) have the lowest chance of survival. Slings can be used to hold horses in a standing position but unfortunately in most recumbent cases euthanasia is the kindest option.
Happily, prevention of tetanus by vaccination is easily achieved. Tetanus vaccines contain tetanus ‘toxoid’, an inactive form of the tetanus toxin. The toxoid stimulates the horse’s system to develop a tetanus-specific immune response, so the body is ready to mount an immediate defence against tetanus should a true infection occur. Combined influenza and tetanus vaccines are frequently used for annual vaccine protocols. This means that if your horse is up to date with their influenza vaccines, they should be fully covered for tetanus too. Tetanus-only vaccines are an option for clients who do not wish to maintain full flu vaccine protection (e.g. if their horses never come into contact with others and do not compete) but still want to protect against tetanus, or for horses that are already up to date with flu cover but require additional tetanus protection (e.g. due to sustaining a high risk injury or being close to foaling). Horses that have never received tetanus vaccines, or become overdue, will require a primary course followed by regular boosters to maintain the required level of immune protection – the exact schedule will depend on the vaccine brand used.
Currently, our vaccine brand involves a primary course consisting of a first and second injection 4 weeks apart, with a third injection containing the tetanus toxoid component no more than 17 months later. Once this primary course is completed, tetanus boosters are then required every other year. In some cases, if a vaccinated horse sustains an injury more than six months after its last booster, immunity can be topped up with an additional tetanus vaccine to ensure the optimum response. Pregnant mares should receive a tetanus toxoid top-up 4-6 weeks before foaling to help protect the foal against tetanus by increasing the number of tetanus toxoid antibodies in her colostrum (first milk). Foals can receive their first injectable tetanus vaccine from 4 months old.
If your horse is not covered for tetanus at the time of an injury or surgery, injectable antitoxin is available which provides short term cover (3 weeks). However, antitoxin can be expensive and the protection it provides is less effective than the immune response of a well vaccinated horse. When wounds do occur in unvaccinated horses, it is recommended that horses receive a tetanus vaccine at the same time as antitoxin, though they must be injected at different sites of the body so as not to ‘cancel each other out’. Administering the vaccine will ensure the horses’ own immune response continues protection against tetanus toxins once the effect of the antitoxin has disappeared. The vaccine given at the time of injury should be followed up with further vaccines to ensure complete protection. Antitoxin may also be given to foals shortly after birth, providing essential protection if the mare has not been vaccinated 4-6 weeks before foaling or if the foal fails to drink sufficient colostrum within the first 8-12 hours of birth.