Recently Imported Horses Have Stronger Reactions to Vaccination
Mild vaccines reactions—injection site swelling, fever, and colic signs—aren’t unusual in horses. Sometimes, however, these reactions can be difficult to differentiate from actual disease. Say you just imported a horse into the country, he received his core vaccinations upon arrival, and 48 hours later he’s lethargic and off his feed. Could it be a vaccine response or something more serious?

Meagan Smith, DVM, Dipl. ACVS, assistant professor of clinical equine field science at the University of Pennsylvania, compared blood parameters between recently vaccinated imported and U.S. native horses to find out. She presented her findings at the 65th Annual American Association of Equine Practitioners Convention, held Dec. 7-11 in Denver.

Specifically, Smith looked at the acute phase proteins (APP) fibrinogen and serum amyloid A (SAA), which rise in response to inflammation and infection. The body also produces them in response to any type of tissue damage.

She recruited 30 horses for her study: 21 routinely vaccinated native horses and nine recently imported horses on a quarantine farm that hadn’t been exposed to U.S. vaccination protocols. Researchers administered three different vaccines intramuscularly to all horses, drew blood, and assessed rectal temperature, attitude, appetite, and APP levels at 0, 24, 48, 72, 96, and 168 hours after vaccination. Smith found:

  • SAA increased significantly (with a mean of 627 ug/mL, all the way up to above 2,000 ug/mL) in all horses post-vaccination, with most horses returning to normal ranges by 168 hours.
  • Imported horses had greater increases in SAA at 72, 96, and 168 hours compared to native horses.
  • Fibrinogen levels increased but not significantly and with no significant difference between the two groups.
  • None of the horses had significant changes in appetite, attitude, or temperature.

Based on these findings, “timing of vaccination is important when measuring APPs,” said Smith, particularly if a horse has been recently imported. She hypothesized that imported horses’ higher SAA levels post-vaccination might be because their bodies are reacting to different antigens and adjuvants (substances used to stimulate a stronger immune response) than they would have been exposed to abroad.