Disease Characteristics (1, 2)

Equine viral rhinopneumonitis is a widespread disease caused by equine herpesvirus 1 and 4. These herpesviruses are associated with several different disease syndromes, namely:

Upper respiratory tract disease
- the symptom that is most commonly associated with this disease
- most common in young horses or in groups of horses housed together
- difficult to distinguish the symptoms of cough, nasal discharge, and discharge from the eyes that are seen here from the similar signs seen in other respiratory infections
- fever is common, with the temperature varying from 39 to 40.5 degrees Celsius (102.5 to 105.5 degrees Fahrenheit)
- inapparent infection is common (The horse is infected by the virus, but shows no symptoms of the disease.)
- incubation period (time between when the horse was infected and when symptoms are first seen) can be anywhere between 2-20 days
- the illness usually last between 2-5 days, but the cough and nasal discharge may last for 1-3 weeks
- extremely rare additional symptoms may include slight enlargement of the lymph nodes of the throat, swelling of the legs, or diarrhea.

- usually occurs in the 8-10 month period, but can happen as early as 5 months
- the foal may be stillborn or may die soon after birth
- the abortion is sudden and the placenta is passed normally
- affected mares will not abort in subsequent years due to this infection

Neurologic disease (Myeloencephalopathy)
- most common in adult horses
- includes varying degrees of paralysis, especially of the hind legs (seen as stumbling, toe dragging, pivoting, inability to lift the tail), and fecal and urinary incontinence
- may progress so that the horse is unable to rise (most horses with this severe a disease are euthanized)
- there is rarely a fever, and affected horses will still have a normal appetite
- the neurologic signs will appear quickly, within 1-2 days
- the neurologic signs are often preceded by cases of respiratory disease, fever, or abortion in other animals on the same property (although they may not be)

Neonatal viremia and septicemia
- this includes foals that are infected in utero with the disease and die within 3-7 days of birth with respiratory distress, and very young foals infected after birth that show fever and nasal discharge and die from secondary bacterial infection or from the virus itself