Pneumonia, Viral
Basic Information 
Epidemiology
Species, Age, Sex
• Equine influenza virus: Affects horses, donkeys, and mules; less common in young foals. Outbreaks are usually associated with horses gathered and housed in close proximity with one another. The incubation period is about 2 days with viral shedding lasting 6 to 7 days (experimentally).
• EHV-1 and EHV-4: Latent infections are believed to occur early in life with stress or immunosuppression leading to disease recrudescence. The virus is present in the respiratory tract within 12 to 24 hours of recrudescence, with viral shedding typically lasting 4 to 7 days (possible for >14 days) and viremia possibly persisting for 21 days. Infection occurs through inhalation secondary to close contact. Respiratory outbreaks are uncommon in patients older than 2 years.
• EHV-2: Infection is often associated with foals and young horses and has been implicated in chronic lymphoid hyperplasia. A humoral immune response is thought to clear the virus with age.
• EAV: Stallions are persistent carriers (testosterone dependent) and transmit the virus via the venereal route, both through live-cover and infected semen. Respiratory viral shedding in exposed mares can then lead to horizontal spread of the virus for 7 to 16 days.
• Equine adenovirus: Adult horses may act as a reservoir for infection, although it plays an uncertain role in adult respiratory disease. Adenovirus may cause pneumonia in foals, with fatalities seen in foals with immunodeficiency syndromes. Transmission can occur through direct contact or via fomites. Adenoviruses may persist in the environment for 1 year at 4° C.
• ERV: Equine rhinitis A virus (ERAV) has been shown to infect multiple species, including humans. Horses are believed to be predominately infected as 2-year-olds. Contact with other horses or entrance into large groups of horses increases the risk of exposure. The incidence of ERAV infection increases in the late winter and spring. Persistent shedding has been shown 12 months after an acute infection.
• Hendra virus: Several Equine outbreaks have occurred in Australia since 1994. HeV can cause naturally occurring clinical disease in both humans and horses. Close contact with infected animals appears to be necessary for the transmission of infection. In experimental cases, the incubation period is 6 to 12 days with animals succumbing to the illness within 36 hours of the onset of clinical signs.
Clinical Presentation
Physical Exam Findings
Clinical signs: Pyrexia, cough, nasal discharge, conjunctivitis, submandibular lymphadenopathy, and vasculitis or edema.
Clinical signs: Poorly defined; possible keratoconjunctivitis, pharyngitis, and lymphoid hyperplasia
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