Franklin Vets Blog
Is your horse at risk of tetanus?
The honest answer is I don’t know. To know the answer we need to understand a little about this terrifying disease which infects our horses.
When we think of infectious diseases we think about viruses like colds and flu that spread directly between individuals; those bugs that are put on door handles and shopping trolleys by people sneezing over them and then picked up by the next user who handles the item. In the vet world, we call it ‘nose-nose contact’ and this is how diseases such as Strangles or respiratory viruses in horses spread, often quickly between horses on the same yard or in contact at an event.
Tetanus does not act like this. An infected individual does not contaminate the items they touch or pass the infection directly to their paddock mates. Therefore it doesn’t spread quickly through populations and instead, it sits in the background, more precisely in soil, just waiting for an opportunity.
We don’t know where the tetanus bacteria lie dormant, however, if you know an animal has contracted tetanus on a property the chances are the clostridial bacteria is still present within the land. It survives in soil for many, many years.
The good news is that bodies are covered in a protective layer, mostly this is skin but the outer layer of our airway and gastrointestinal tract is also covered by protective epithelium. Tetanus bacteria cannot penetrate these areas and so we can come into contact with contaminated earth and not become infected. However, if our protective epithelial layer is damaged by a wound, ulcer, or other surface damage the bacteria can enter the body.
In horses, this includes penetrative injuries (that may lead little evidence of their entrance), ulcers in the mouth/gastrointestinal tract, or foot abscessed and even seedy toe. The tetanus bacteria thrives in tissue that has a poor oxygen supply and in such environments produces large quantities of toxin that eventually spreads through the body. This means dirty, extensive wounds with lots of dead tissue around them are a high risk for this severe disease that progresses very quickly.
When a case of tetanus is suspected we often look for a wound to see where the entry point is, however in one study on horses suffering signs of tetanus only 30% infected animals were found to have a wound. The other sites of entry were likely internal wounds, such as ulcers; small penetrative injuries that had sealed over straight away; or old healed wounds (AAEP give an average of 8days for signs of infection to show after infection and in some cases it may be 2-3 weeks before a tetanus infection leads to signs of the disease). It is a silent killer that can’t be easily seen or suspected until it is too late.
So how do we know if tetanus infection is present?
We diagnose it purely on some classic signs. Although samples of infected tissue around a wound can prove tetanus toxin is present, there is no test available that can definitively rule the infection out. Tetanus infection affects muscles making them rigid, in early stages horses usually walk stiffly with over-exaggerated joint movement; when the horse becomes excited or agitated the signs immediately get worse.
As toxin levels rise within the body, signs become more obvious. The face is often affected causing reduced movement of the mouth (which is why tetanus is often called ‘lockjaw’), the ears to be pulled back, and the lips to be pulled into a stereotypical grimace. In addition, the tail head can also be held high. As more muscles are affected and toxin levels increase, the horse is likely to become recumbent and unable to stand going through spasms where the legs are usually (but not always) stretched out rigid. As muscles are also involved in gastrointestinal and respiratory function, gut function usually halts and breathing becomes more difficult. Eventually, the effect on the respiratory muscles will lead to a distressing death. In severe cases where high levels of toxins rise quickly, sudden death may be the only sign seen and mistakenly owners may perceive the horse has had a heart attack.
Once tetanus has infected the body there is a 25-50% survival rate which decreases as signs become more severe. Of 4 horses I have known to have tetanus only one has survived; this horse was only mildly affected and responded quickly to intensive therapy at a high financial cost to the owner (the initial medication used to relieve signs costs several hundred dollars to give and only lasts a short period, most tetanus cases undergoing treatment also require hospitalization and fluid drips to maintain hydration, energy and electrolyte levels whilst the intestines are out of action). Horses with heavy infections are unlikely to respond even with the most intensive treatment and are usually euthanased on welfare grounds as signs become more severe.
Thankfully a very effective tetanus vaccine is available.
This requires 2 shots, 4 weeks apart followed by the final shot of the primary course followed by repeated boosters within every 5 years. Although an occasional horse who is fully vaccinated may show signs of tetanus if a large toxin burden occurs, this is much more likely to be treatable than in an unvaccinated horse. It is now considered that annual boosters of tetanus are more protective than waiting every 5 years and in most areas of the world, tetanus boosters are carried out at least every 2-3 years.
If a fully vaccinated horse goes over 5yrs since its last booster, the vaccination schedule will need re-starting. We recommend all horses are covered with a primary course and boosted ideally annually but at least every 2-3 years. If a horse is between 1-5 years since its last booster and suffers a traumatic injury, has a foot abscess, or is found to have ulcers of the mouth or stomach; we recommend a booster in these high-risk situations.
We estimate the cost of the first visit for a treatable tetanus case (with only 25-50% chance of treatment being successful) to be more than the value of 10 individual tetanus vaccine shots. If a horse is not vaccinated and suffers a lesion considered at risk of tetanus entry, an anti-toxin is available to give some protection for a short period. It acts like a sponge to mop up any tetanus toxin that might be floating around the body, however, it has a limit to what can be absorbed and does not stimulate the immune system to mop up the rest of the toxin as happens in vaccinated horses.