An epizootic of equine viral arteritis at a Thoroughbred racetrack in Illinois: 223 cases.

An outbreak of equine viral arteritis occurred over eight weeks at a Thoroughbred racetrack in Illinois in 1993, affecting 223 horses with symptoms such as fever and edema. The study tracked the disease spread, risk factors, and control measures used to stop the outbreak while allowing racing to continue.

Overview of the Epizootic

• An epizootic (animal epidemic) of equine viral arteritis (EVA) occurred in summer 1993 at a Thoroughbred racetrack in Illinois, lasting eight weeks.
• A total of 223 horses were affected, including 213 Thoroughbreds and 10 lead ponies.
• The racetrack had a midwestern setting and ceased operations in 2021.

Clinical Signs and Diagnosis

• Common symptoms observed were:
  • Fever ranging from 38.3–40.6°C (101–105°F)
  • Depression and inappetence (loss of appetite)
  • Diffuse urticaria (hives), including swelling (edema) of distal limbs and prepuce (the sheath covering the penis)
• Virus detection:
  • Equine arteritis virus (EAV) was successfully cultured from buffy coat samples of 24 out of 28 acutely ill horses.
  • Virus neutralization testing showed 17 of 18 recovered (convalescent) horses were EAV-positive, indicating prior infection.
  • Among non-affected horses, only 1 out of 92 tested positive, confirming localized spread.

Risk Factors

• Statistical analysis identified key risk factors:
  • Geldings (castrated males) had an odds ratio (OR) of 2.08, showing they were about twice as likely to be infected compared to intact males (p < 0.001).
  • Two-year-old horses had an OR of 2.33, indicating greater susceptibility compared to older horses (p < 0.001).

Infection Source and Spread

• The initial source of infection was traced to two horses previously hospitalized for unrelated elective procedures at a nearby veterinary referral clinic.
• The virus initially affected two of 33 barns at the racetrack.
• Movement of horses facilitated a secondary spread to seven additional barns, all sharing a similar design.
• Notably, barns with an older, better-ventilated design did not have any cases, even when horses from infected barns were moved there, suggesting ventilation differences influenced transmission.

Control Measures and Outcomes

• Control strategies combined:
  • Isolation of acutely ill horses to prevent further spread.
  • Restriction of movement of all horses on the premises.
  • Vaccination against EVA.
• These measures effectively halted the epizootic while allowing racing events to continue.
• Despite vaccination efforts, 39 horses still developed EVA, indicating possible vaccine breakthrough or timing issues.
• Performance outcomes:
  • Out of 213 affected Thoroughbreds, 58% were able to return to racing within 60 days after the last case was recorded on August 21, 1993.

Significance of Findings

• The study highlights the importance of:
  • Identifying and managing sources of infection in veterinary hospital settings.
  • Facility design, notably ventilation, as a factor in disease transmission among stabled horses.
  • Implementing combined preventative measures—quarantine, movement control, and vaccination—to control viral outbreaks.
  • Understanding risk factors like age and sex to target biosecurity and monitoring efforts effectively.
• Despite vaccination, some cases occurred, pointing to the need for ongoing surveillance and possibly vaccine improvement.
• The ability to maintain racing activities during an outbreak suggests that rapid response and biosecurity can mitigate economic impacts.