Viral respiratory and neurological diseases are leading contributors to economic loss within the horse industryAn infectious disease is defined as the presence of a foreign organism within or on the body which can then disturb normal body function. This disturbed body function results in an animal exhibiting abnormal behavior patterns, and clinical signs are classified and associated with a particular infectious agent. Bacteria and viruses are the two most common agents capable of causing infectious disease in the horse.
Economic losses to the horse industry as a result of equine viral respiratory disease have been substantial over the last 20 years. Some of the factors responsible include increases in the number of horses, greater concentrations of horses (at racetracks, training facilities, etc.), and the dramatic increase of international travel by horses. Also, horses being stabled under poor environmental conditions can allow these respiratory viruses to persist, grow, and continue to spread.
Improvements in diagnostic methods have helped us to better understand viruses and how they infect the body. Better tissue culture techniques to isolate these viruses, use of the fiber-optic endoscope, and more sophisticated serologic (blood) methods have enhanced our ability to make a rapid and accurate diagnosis of viral diseases. However, development of these novel diagnostic techniques has been offset by lack of equivalent progress in the actual treatment of viral conditions. To date, there are few effective antiviral agents available, and so these diseases continue to frustrate the horse owner, trainer, and veterinary clinician. The focus, therefore, must be on proper management, effective vaccines, and vaccination schedules to control these diseases.
The most common causes of viral respiratory disease in the horse are equine herpesvirus (rhinopneumonitis), influenza, and equine viral arteritis. Less common are the rhino-, adeno-, reo-, and parainfluenza viruses. Researchers consider these lesser pathogens as viruses looking for a disease.

Equine herpesvirus
For years the cause of rhinopneumonitis had been called equine herpesvirus type 1 (EHV-1). Still, no one could really explain how this single viral entity could cause so many different clinical syndromes in the horse.
As research progressed, it revealed that different isolates of this EHV-1 virus could be neutralized by different EHV-1 antibodiesindicating that the different EHV-1 infections were being caused by different EHV-1 viruses. This differing reactivity of EHV-1 became divided into two distinct groups, each of which became correlated with specific clinical syndromes. One group was identified with respiratory disease, the other with abortion and neurologic disease. Therefore, the nomenclature system was modified to include these two distinct groups as EHV-1, subtype-1 (abortion/neurological disease) and subtype-2 (respiratory disease).
Further research revealed even more differences between these viral subgroups. Thus, the classification system was further changed to EHV-1 as the cause of abortion and neurological disease and EHV-4 as the causative agent of respiratory disease. There are also two other equine herpes viruses: EHV-2, causing upper respiratory disease of questionable pathogenicity; and EHV-3, causing genital herpes. Epidemiological data from Kentucky has confirmed EHV-4 as the most common pathogen isolated from horses with clinical respiratory disease.
Viral studies have identified six major outer surface glycoproteins, and four of these are common to both EHV-1 and EHV-4. However, further studies of the internal makeup of these viruses have indicated that they are very different. Thus, even though these viruses are in some ways quite similar, they should be considered as distinct disease entities. Disease challenge from EHV-1 stimulates a completely different antibody response and disease syndrome than that of EHV-4.
The EHV-4 is normally restricted to a local infection of the respiratory tract and its associated lymph nodes. The inhalation of viral particles from aerosol droplets either directly or indirectly (via contaminated objects) are the most common sources of infection. These viral particles spread to the local lymph nodes and continue on to the respiratory tract.

Signs and treatment of rhinopneumonitis
Common clinical signs associated with rhinopneumonitis (EHV-4 [respiratory disease]) are depression, anorexia, and fever (102-107F), with a serous-to-mucopurulent (pus-type) nasal discharge. The incubation period of this disease is 2-to-10 days, and the course of the disease can vary according to the severity of the condition. Most uncomplicated cases last from 3-to-5 days.
Treatment is centered around stall rest, antibiotics for secondary infections, and supportive care. Control is the most important step to be taken to limit most of these infections. Isolate all new arrivals for at least 21 days and ensure accurate medical histories with current vaccinations. Isolate all infected animals and thoroughly disinfect the stalls and barn areas. Proper hygiene of all barn personnel is essential, especially in the course of an outbreak when they are closely associated with sick animals. Controlling the entrance of visitors is also essential.
The prognosis for a complete recovery from EHV-4 is generally excellent. Mortality is rare, but may occur in young, susceptible foals that develop severe bronchopneumonia.

Abortion/neurological disease
EHV-1 infection is much more complicated, since this virus is capable of infecting the circulating white blood cells. These viremic white blood cells can then enter the general circulation and travel to distant organ structures. As these infected cells enter the various organs of the body, they are capable of causing many different types of infection. The uterus and central nervous system are the two most common organ structures affected. Research has also revealed that there are actually two forms of this particular strain of EHV-1.
Herpes viruses tend to invade the body at privileged sites and set up latent infections within the regional lymph nodes. During times of stress, these latent infections can appear, causing severe disease. For example, an older, pregnant mare could develop a mild EHV-1 infection early in pregnancy and show few or no clinical signs of disease. Then, as the pregnancy develops, the mare may become heavily stressed. The weakened immune system of this stressed mare allows the EHV-1 infection to re-appear within the circulating white blood cells. The infection gradually overwhelms the pregnant mare, causing her to abort. Since these abortions generally occur in the last trimester of pregnancy when breeding back is no longer an option, they result in severe economic loss.

Mysterious infections
Disease of the nervous system associated with EHV-1 infection still remains something of a mystery. Reports have been received describing this disease among foals, geldings, stallions, and broodmares, at farms, racetracks, and at public stables. Clinical signs can vary from a mild ataxia (lack of coordination) to a complete recumbency and paralysis of the limbs. The incubation period is about seven days, and most horses gradually recover after 5-to-10 days. If the ataxia worsens and these horses become recumbent, they generally have a poor prognosis and must be euthanized.
Diagnosis of this paralytic form of EHV-1 infection can be difficult, as it is easily mistaken for other neurologic disorders, such as wobbler syndrome or trauma to the brain and/or spinal cord. There are no gross lesions of the spinal cord as a result of this infection, and histologic evidence indicates an inflammatory-type reaction, possibly of allergic nature. The paralysis resulting from this infection is due to the metabolic changes within the central nervous system and the death of functional nervous tissue. Some researchers have proposed that this neurologic form of EHV-1 infection may actually be a specific virus that has an affinity for the nervous system, such as rabies.

Summary
In summary, herpes viruses are amazingly complex structures, having multiple pathogenic syndromes that can affect the horse. The EHV-4 is associated with respiratory disease and EHV-1 with abortion and neurological disease. Immunity is very short-lived with these viruses, as they have the ability to co-circulate within the body in lieu of circulating antibodies. Efforts to control rhinopneumonitis infections must be directed toward proper management and the effective use of vaccines. Consult your veterinarian for his professional recommendations.