I was once called by a client who explained over the telephone that her horse had an eye problem. The appointment was made and I went to the farm site with three intrepid veterinary students. Upon arrival, we were greeted by a flock of humans, all related in some way to the lady who made the original call. We were led to the pasture area where the affected horse was housed. We observed an 8-year-old, palomino quarter horse mare with good overall body condition. No eye lesions were readily apparent. Facilities were not suitable for an eye examination in this pasture, so I elected to have the horse walked up to the barn. Observation of the horse as she walked revealed that her gait was slightly stilted, or mechanical, and the mare appeared apprehensive about her surroundings.

The mare was placed in a stall and the students began their examination. They obtained a rectal body temperature, auscultated (listen to) the heart and lungs, and obtained rates for both the pulse and respiration. It was during their examination that I finally saw the "eye problem" that was the presenting complaint. Upon insertion of the thermometer, the third eyelid of both eyes prolapsed, or covered, both eyes for just a second.

Results of the physical examination were an elevated body temperature of 1020F, pulse rate of 48 beats per minute with a normal rhythm, and a respiratory rate of 24 breaths per minute with normal sounds and character. Close examination of the eye did not reveal any specific lesions.

The students and the crowd of handlers all left the stall, and we began a discussion of their findings. The mare, once left alone, dropped her head and gave the appearance of being very fatigued and depressed. We all agreed that the physical examination findings were nonspecific for a particular disease entity, and when I asked if anyone had observed the prolapsing third eyelid, none had. This presented a perfect opportunity to demonstrate what I had seen. With the crowd quiet, and the mare standing head down in her stall, I slammed my hands together in a loud clap! Immediately, the horse's head shot up and both of her third eyelids covered her eyes completely. Her body position went from one of a depressed, almost sulking, horse to a rigid, erect sawhorse. This was a dramatic sign! The mare took about a minute to overcome this erect, extensor rigidity and to become more relaxed in her stall.

The discussion that followed between the students and me revolved around the problem list that we had developed. Elevated body temperature, elevation of heart rate, slight prolapse of the third eyelid upon minor stimulation, and extensor rigidity accompanied by complete prolapse of the third eyelid when a loud stimulus is given. The one disease that "fits" all of those signs is tetanus, and unfortunately that is what this mare had. She became progressively worse over the next two days and was euthanatized due to unrelenting clinical signs and lack of response to therapy.

This real case scenario is presented to emphasize how important prevention is for the horses we care for with regard to tetanus. Tetanus, or lockjaw, is caused by Clostridium tetani, a bacteria that likes anaerobic conditions such as puncture wounds, sole abscesses, or deep muscle bruises. The organism is very common around horses, and the horse is one of the most susceptible animals to the disease. Once a horse develops signs of disease such as described, they nearly always progress to death due to asphyxiation caused by paralysis of the diaphragm. We were not able to isolate the site of initial insult for the mare described in the case above.

The great news for all horses and their owners is that tetanus is easily preventable with immunization. The vaccine is the tetanus toxoid, which is the inactivated form of the toxin. It produces very good antibody response when administered to immunologically competent individuals. Tetanus vaccine should be given each year to every horse. If a horse gets injured and the last tetanus vaccination has been given more than 6 months before, the horse should receive a tetanus toxoid booster promptly.

 Another disease that wreaks havoc on susceptible horses is encephalomyelitis. The causative agents are viruses carried by the wild bird population, without causing problems for the infected birds. Known as "sleeping sickness," the encephalomyelitis viruses can cause serious disease in horses and humans. Mosquitoes are responsible for transmission of the viruses from birds to susceptible animals. The signs of disease are often acute, (rapid onset) starting with fever and depression, followed by signs related to brain and spinal cord inflammation. Most affected animals become very depressed, appear dazed early in the course of the disease and progress to circling, head pressing, blindness and somnolence. The affected horse appears to be "sleepwalking" thereby giving the signs of this viral infection a very descriptive name.

Three strains of the virus are of concern to horse owners in the United States: Eastern, Western and Venezuelan. The Eastern and Western forms of encephalomyelitis are not transmitted from horse to horse, meaning that the horse is a dead-end host for these viruses. Venezuelan encephalomyelitis is contagious, which means it can be transmitted from one infected horse to another; therefore, it has a far greater risk to the overall horse population. The good news is that only two areas of the United States currently have the Venezuelan equine encephalomyelitis virus: southern Texas, and southern Florida. The horse population also acts as a sentinel for human health because the horse is a dead-end host and the virus is transmitted by mosquitoes. When horses in an area develop signs of the disease, public health officials will issue warnings to the human population because those same mosquitoes might bite us instead of the horses.

Vaccination is possible and effective in preventing encephalomyelitis. The vaccination produces protective antibodies for about nine months following immunization. In areas that have high mosquito populations, it is wise to consider twice yearly vaccination to provide the highest level of immunity. Most of the currently available vaccines are composed of only the Eastern and Western viral antigens. However, the trivalent vaccines, those that have all three strains of the virus, produce the best antibody response. These trivalent vaccines are especially recommended if the horse will be taken to areas that are known to be infected with the Venezuelan strain of the virus. Regardless of the type of vaccine, all horses should be immunized against encephalomyelitis each year.

 The social diseases of the horse are related to the respiratory system. Influenza, rhinopneumonitis and strangles all occur when horses gather into a group. Influenza and rhinopneumonitis are highly contagious respiratory diseases caused by viruses; therefore, they are extremely difficult to control. The racehorse and showhorse populations are most commonly affected, although horses in boarding stables also have increased exposure potential because of movement of horses. These respiratory viruses will make affected horses sick and may predispose them to secondary bacterial infection of the lungs. Influenza and rhinopneumonitis usually are not life threatening diseases. Vaccines are available and provide protection, but the protection is not absolute. Multiple doses may benefit those horses that are travelling an extensive show circuit, or have high levels of stress in their lives.

Recomendation of vaccination with the strangles bacterin has been debatable. The bacterin is made from the cell wall extract of Streptococcus equi and has caused some ugly, injection site bumps on vaccinated horses. Recently, a study was performed at LSU by the Veterinary Field Service on the reactivity of the vaccine when injected in horses of all ages, breeds, and sex. Very few reactions were observed in horses that received the vaccination. These findings indicate that the rate of complications due to strangles vaccine is not as high as previously thought. The protection against disease provided by the bacterin is not complete, but the severity of signs is reduced. Strangles vaccination is recommended primarily for horses residing in, or going to, farms in which the disease is endemic. Since strangles is highly contagious, isolation of sick horses is essential for control of disease outbreaks.

 Rabies vaccination is recommended for horses kept in environments where rabies-infected wildlife pose a potential risk. Although rabies is not common in horses, it is a fatal disease and it represents a threat to humans. A yearly booster of rabies vaccine is recommended in endemic areas. 

Brood mares should receive their vaccination before breeding, and they should not be vaccinated during the first 3 months of pregnancy due to the risk of damaging the fetus. Pregnant mares should be vaccinated with killed equine herpesvirus (this is the same virus that causes rhinopneumonitis) at 5, 7 and 9 months of pregnancy, to prevent late pregnancy abortion. Pregnant mares should also be vaccinated with tetanus and encephalomyelitis vaccines 4 to 6 weeks before foaling to assure good levels of antibodies in their colostrum.