Tetanus is a serious infectious disease of horses that is highly preventable. Despite the availability of an affordable and highly effective vaccine (tetanus toxoid) the disease continues to cause sporadic infection and death in horses all over the world. In developed countries most cases occur because vaccination has been simply overlooked.
Horses, along with sheep and humans, are the species that are most sensitive to the toxins that cause tetanus. The disease is caused by Clostridium tetani, whose spores are readily found in soil, as well as in the intestinal tract and faeces of animals. The spores are very resistant to environmental factors and disinfectants, but can destroyed by very high heat (115 C for 20 minutes).
How do horses get infected? The most common way that horses get infected is contamination of wounds with bacterial spores. Any disruption to skin or a mucous membrane can expose a horse to tetanus. Puncture wounds are particularly dangerous as these create the ideal environment for spores to germinate, bacterial proliferation and elaboration of toxins. Infections have occurred after punctures to the sole, in mares with retained placenta or metritis, umbilical infections in foals, and secondary to castration wounds. Only around one half of cases have an obvious wound.
What is the incubation period? This period is highly variable, ranging from several days to several months. Spores can remain viable in wounds that have healed, waiting for the right environment for germination. Most cases occur 1 – 3 weeks after contamination.
Signs of tetanus
The signs of tetanus are due to the toxin tetanospasmin. It is produced at the site of infection and then travels through blood to reach nerve endings. It then travels along peripheral nerves to the central nervous system (CNS) at a rate of 1-2 cm/day. The toxin blocks the release of the neurotransmitter gamma aminobutyric acid (GABA). GABA is an important inhibitory neurotransmitter. Reflexes that are normally inhibited are instead facilitated causing tetanic muscle contractions after sensory stimulation.
The initial signs often include rigidity (stiffness) of the muscles of the head and neck. Increased muscle activity of the muscles of mastication is known as trismus. There is a fixed and rigid facial expression, known as risus sardonicus. Retraction of the eyeball causes the third eyelid (nictitating membrane) to prolapse across the eye. The eyelids are elevated, the commissure of the lips are pulled caudally, and the ears are erect and retracted caudally. The mouth is clamped shut and cannot be pried open (lockjaw).
In most horses the signs progress to involve limbs, neck, back and tail. Horses develop a stiff, slow gait with frequent toe dragging. They also develop a characteristic stance that resembles the legs of a saw horse. The tailhead is elevated. Tonic muscle contractions or spasms are initiated by external stimuli, such as touch, noise or smell or by voluntary movements to eat or drink, or posture to urinate or defaecate.
There are also disturbances to the autonomic nervous system, including wide fluctuations in heart rate and blood pressure, as well as profuse sweating.
Without intervention horse will further progress to recumbency, they may develop respiratory failure due to laryngeal spasm, spasm of the diaphragm and thoracic and abdominal walls. Horses often succumb to tetanic convulsions.
The bottom-line is that this is a serious and often fatal disease that can be prevented.
Treatment of tetanus
This is a difficult disease to treat. The principles of treatment are to kill vegetative bacteria, reduce the muscular rigidity and spasms, neutralize any circulating toxin, and to boost immunity.
Metronidazole is the antibiotic of choice. It is also important to debride and disinfect any wounds.
There are numerous drugs that may be helpful in reducing anxiety, muscle rigidity and spasm. These are used along with reducing environmental stimuli by placing in dark, secluded environment, and placing cotton in both ears. It is important to have a stall flooring that allows good footing.
Drugs commonly used include a combination of acepromazine and GABAergic drugs, like diazepam. Phenobarbital can provide muscle relaxation while allowing the horse to continue standing. Similarly glycerol guaiacolate can be administered by slow intravenous infusion, titrated to effect, to achieve muscle relaxation and standing. Methocarbamol is generally inexpensive and readily available, but is probably only effective in very mild cases. Other drugs that may help include dantrolene, midazolam, or magnesium sulphate.
Tetanus antitoxin and tetanus toxoid are very different. Tetanus antitoxin directly neutralizes circulating toxin. Tetanus toxoid is a vaccination that is used to induce and maintain protective antibodies against tetanus. Tetanus toxoid uses modified tetanospasmin, which has been made nontoxic, but still retains the ability to stimulate the formation of antibodies against the toxin. The toxoid does not result in antibodies that bind to the bacteria.
Tetanus antitoxin (TAT) should be administered, although the massive doses that were once recommended are no longer considered necessary. The antitoxin can be administered intravenously, intramuscularly, or intrathecally. The benefits of intrathecal delivery are likely positive, but marginal. TAT has become very expensive in Australia in recent years, for example 10000 IU could cost a client as much as $500.
Tetanus toxoid should also be administered intramuscularly (away from any IM administered TAT).
The difficulty is that many affected horses are unable to eat and drink. Repeated tubing is stressful for all, although xylazine may facilitate the procedure. Intravenous fluid therapy is ideal.
Prognosis
The estimated recovery rate with therapy is 25-50%.
Horses that are recumbent and unable to rise should be euthanised.
Partial immune protection, such as only receiving a single vaccine, may improve survival.
Prevention
An initial vaccine should be given to foals at 6 months of age (assuming that the mare is current on vaccination) with a second dose 3-6 weeks later, and then annually.
Boosters are recommended after lacerations or penetrating wounds (including foot abscesses).
Mares should be vaccinated with tetanus toxoid during the last month of pregnancy. Foals born to inadequately vaccinated mares, or foals that have partial or full failure of passive transfer of colostral antibodies, should receive 1500 IU of tetanus antitoxin (TAT) shortly after birth.
Unvaccinated horses that acquire lacerations or wounds should receive antitoxin and toxoid (at different sites) followed by a toxoid booster 3 weeks later, followed by annual boosters.
Colts should receive both initial doses of toxoid before castration.
Tetanus antitoxin has been associated with severe liver disease, one to three months after administration. This liver disease is an acute fatal hepatic necrosis, known as Theiler’s disease. The incidence of Theiler’s disease in Australia is not known, but appears to be extremely rare. Recently, a parvovirus was identified as the causative agent of most cases of Theiler’s disease.
Dosing
Dosing information is intended for use by registered veterinarians or veterinary nurses. Equiimed assumes no responsibility for the information detailed below. Equiimed shall not be liable for any damages resulting from reliance on any information provided below, or by reason of any misstatement or typographical errors. Ultimately veterinarians should consult information provided by the manufacturer prior to use.
Tags: Infectious Diseases; Neurology