The horse industry is alive and well in Louisiana. Purses for racing Thoroughbreds and quarter horses are high, and quality show horses are found in nearly every barn. Knowledge of horse owners and their ability to care for their animals is also increasing. Neurologic diseases and how to best prevent them continue to present challenges to our horses as well as their humans.

Infectious Anemia (EIA or Swamp Fever)

EIA remains a viral disease of importance that affects horses in spite of required statewide testing. Signs include intermittent fever, weight loss, subcutaneous edema, anemia and neurological disorders.

Although the most common type of infection is relatively mild, infected horses become “unapparent carriers” and present a reservoir population of the virus that may be transmitted by blood from the infected animal to a susceptible host, which places them at risk.

Prevention is based on destroying infected horses, monitoring horses that congregate at sporting events and sales barns, and using individual needles for all vaccinations, blood sampling procedures and drug injections. Immersion in alcohol is not sufficient to disinfect instruments. Vaccination is not feasible because of the different varieties of virus in the population and the type of testing available for diagnosis of the disease. The testing of all horses in the state on a yearly basis is required by law. However, the equine census for 2003 revealed nearly 200,000 horses in Louisiana, yet only 31,000 EIA tests were performed. This deficiency in testing must be addressed if we are to eradicate this disease.

Encephalomyelitis (Sleeping Sickness)

The organisms that cause sleeping sickness are viruses transmitted by insects of the family Togaviridae that infect horses, birds and humans. These viruses produce inflammation of the brain and spinal cord (encephalomyelitis).

The most common encephalitis viruses include Eastern, Western and Venezuelan (EEE, WEE, VEE). Other groups that may infect the horse but prefer other hosts (such as humans) include the St. Louis and Japanese B encephalitis viruses. West Nile Virus (WNV) was introduced to this hemisphere in 1999. All of these viruses threaten horses and humans throughout the United States. All of the encephalitidies are considered reportable diseases in this country.

Clinical signs include nonspecific fever, anorexia and stiffness usually within five days of the initial infection. Other signs include hypersensitization of the skin (hyperesthesia), aggression and excitability with continuous chewing movements. Propulsive walking (the affected horse continually moves forward without going anywhere), depression and sleepiness may be seen as well as loss of orientation. Muscle tremors and twitching are common with WNV infections. Head pressing, circling with a head tilt and blindness commonly occur. The animals that die begin to lie down after three to five days. Survivors gradually improve over weeks to months. Complete recoveries from these diseases have been reported. Mortality rates for EEE range from 75 percent to 90 percent and for WEE range from 19 percent to 50 percent. VEE is similar, but animals with VEE may develop diarrhea, become moribund and die before they develop neurologic signs. Ulcers of the lips, tongue and gums, with nosebleed and abortion may be observed. The mortality rate of VEE infection ranges from 40 percent to 80 percent. The most recent figures regarding WNV put the mortality rate for infected horses between 40 percent and 60 percent.

These viruses persist in nature by infecting birds, small mammals and reptiles without causing disease in these hosts. Transmission of the viruses between the intermediate hosts and horses occurs through biting insects, most commonly the mosquito. Typical outbreaks of EEE, WEE or WNV occur when there is a high concentration of mosquitoes available to transmit the virus from infected birds to the horse. The horse is considered a dead-end host because the horse does not transmit the disease to others.

No known treatment exists for the viral encephalitidies. Nursing care to control seizures and prevent self-induced trauma are extremely important, and supportive nutritional care and grooming are necessary to prevent ulcers and urine scalding of the horse’s skin.

Prevention is based on the reduction of the insect vectors and intermediate hosts and bolstering the immune system of susceptible animals. Vaccination of the horse population is the most practical method for control of these diseases. There is a mutual enhancement of the specific antibody production to all antigens when trivalent (EEE, WEE and VEE) vaccines are administered. Two vaccines are available against West Nile Virus. Both appear to be efficacious in preventing disease, and some data suggest that the WNV vaccines may prevent infection for longer than the older trivalent vaccines. The frequency of vaccination against these diseases is debatable. Current recommendations
are to booster at six-month intervals.

It is important to get vaccines into horses, especially young horses, before the heavy mosquito season begins. It seems clear that vaccination of horses that are incubating the virus without external signs may cause enough immune suppression to allow the virus to overcome the horse and produce clinical signs.

Respiratory Disease

This is a common malady of horses that are either young and more susceptible or travel a great deal, which is often accompanied by an elevated level of stress. Two viruses, influenza and rhinopneumonitis, cause most of the respiratory diseases in horses and can be controlled with vaccines.

Rhinopneumonitis virus is important because of the other signs that may be associated with infection with this virus. Rhinopneumonitis is a herpes virus, which is ubiquitous in the equine population. Horses that become infected with rhinopneumonitis may demonstrate signs in one of three ways: respiratory, neurological or abortions.

Respiratory signs are usually seen in young animals only and are mild. The neurological form of the disease is not common, but when it occurs may produce death. The most common neurological signs are an unsteady gait and weakness in the limbs, sometimes with urinary incontinence and fecal retention. Often signs begin in the hind limbs and progress to the forelimbs. Many affected horses become lackluster and have difficulty standing. The disease may progress to quadriplegia and death. The important aspect to remember is that these signs mimic those of both rabies and the encephalitis viruses. Abortion storms can occur in breeding stock throughout the world that come in contact with the virus.

Control of this disease involves vaccination of healthy stock and isolation of infected animals. Breeding stock should not be in contact with horses that leave the premises for shows or exhibitions. Two different vaccines exist, Rhinomune and Pneumabort K. Frequent immunizations are necessary. It is important to remember that vaccination will not protect against the neurological form of this disease.

Rabies

Horses may exhibit a wide variety of neurological signs that make rabies difficult to diagnose. Extreme agitation, rolling or biting viciously is frequently seen. Affected horses may exhibit behavioral changes such as anorexia, nervousness, irritability or hyperexcitability. Death usually occurs in two to five days, rarely up to two weeks, once signs are observed.

Diagnosis is done by immunofluorescent antibody staining of the brain tissue or tactile hair follicles and can be completed within a few hours. The brain tissue must be preserved by refrigeration or kept cooled for testing.

Vaccination of horses is possible and is always done with killed virus products. No efficacy studies have been done to prove that protective antibodies are produced following vaccination. The important thing to remember is that any horse with neurologic disease should be considered a rabies suspect until proven otherwise.

Conclusions

Neurologic disease in a horse is frustrating and difficult for owners and veterinarians. The disease may affect the horse adversely, and it may also affect the handlers of the horse. The availability of quality vaccines allows horse owners the luxury of protecting their horses against most of these diseases. Because of the variable responses of horses to the stress of repeated transport, it is important to understand when and what to immunize horses against to provide maximum protection for our equine companions.