Imported Horses: Prevalence of Fevers Examined (AAEP 2011)

Thousands of horses enter the United States each year for a variety of reasons, and those arriving via air or ocean must go through a USDA Animal and Plant Health Inspection Service (APHIS) quarantine center to reduce the risk of spreading infectious diseases.

A veterinarian examines each horse when it arrives and throughout the quarantine period. Before a horse can be cleared for release from the quarantine center, he must be held there for a specified period of time, be clinically healthy, officially test negative for several diseases, and have three nonelevated temperatures (less than 101.5°F) recorded for the 24 hours immediately prior to release. USDA:APHIS:Veterinary Services officials completed a study in which they summarized the prevalence of elevated temperature among imported horses and determined risk factors for its occurrence.

"Above-normal body temperature can be the result of a true fever—most often caused by an infection—inflammation, hyperthermia because of heat stress, drug reactions, allergies, tumors, or other causes," explained Josie Traub-Dargatz, DVM, MS, Dipl. ACVIM, a professor of equine medicine and epidemiology at Colorado State University, during a presentation at the 2011 American Association of Equine Practitioners convention, held Nov. 18-22 in San Antonio, Texas.

"Other risk factors, such as age, breed, previous experience with air travel, disposition, and location in the cargo area might explain an increased risk for occurrence of elevated body temperature," she added.

To analyze elevated temperature incidence among imported horses, Traub-Dargatz and co-author Barbara Bischoff, MA, DVM, an APHIS veterinary analyst based in Fort Collins, Colo., examined the records of 4,720 horses imported to the United States through one of the three USDA equine quarantine facilities during one calendar year:

  • The New York Animal Import Center (NYAIC), in Newburgh, N.Y., in 2008;
  • The Miami Animal Import Center (MAIC), in Miami, Fla., in 2008; and
  • The Los Angeles Animal Import Center (LA-AIC), in Los Angeles, Calif., in 2009.

Because many factors varied by location, the team used separate data analysis models for each import center, Traub-Dargatz noted.

New York

The majority of the 2,062 horses that arrived at the NYAIC in 2008 were imported from Europe, with the most common countries of origin being Germany (32.4%); the Netherlands (22.2%); and England (17.8%).

Key findings from the New York data included:

  • Of the 2,062 imported horses, 236 horses (11.4%) had at least one elevated temperature during their stay at the NYAIC;
  • Veterinarians treated 139 (6.7%) of the imports with a non-steroidal anti-inflammatory drug (NSAID) at least one time and 22 horses (1.1%) with an antimicrobial drug at least once; most of the treated horses had an elevated body temperature;
  • The highest prevalence of elevated temperature (19.6%) was among Friesians, while Thoroughbreds and other hot-blooded horses (such as Arabians and Akhal-Tekes) had the lowest prevalence (6.7%);
  • Horses 1 year old or younger had the highest prevalence of elevated temperature (26.2%), while the prevalence among horses of all other age groups was between 9% and 11%; and
  • Gender and the time of year horses were imported did not have a statistical association with elevated body temperature.


In 2008 the MAIC was a destination for 1,600 horses from South American nations, the U.S. Virgin Islands, Europe, and Australasia, with 75% of the horses originating from Argentina or the Netherlands.

Key findings from the Miami review included:

  • Of the imports, 106 horses (6.6%) had at least one elevated body temperature recorded during their stay at the MAIC;
  • Of those 106, 13.3% had a recorded temperature of 102.5°F (39°C) or higher at least once  from the time of arrival to the time of release;
  • Slightly more than 88% of horses with elevated temperatures recorded had only one occurrence of an elevated body temperature;
  • Elevated temperature prevalence was significantly lower among Warmbloods (3.6%) when compared to Thoroughbreds and other hot-blooded horses and the "other" breed category;
  • Of the horses with elevated temperature records, 10 horses were treated with an NSAID at least once and six were treated with an antimicrobial drug at least once;
  • Intact male horses were significantly more likely to have an elevated temperature than females or geldings; and
  • There was no significant difference in prevalence of elevated temperature and season of arrival at MAIC.

Los Angeles

The majority of the horses arriving in LA in 2009 originated from Europe, followed by Australia and New Zealand.

Key findings from the Los Angeles review included:

  • Twelve percent of imported horses (127) had at least one elevated temperature recorded during their stay at the LA-AIC;
  • Of those, 15% had a temperature of 102.5°F (39°C) or higher at least once;
  • The highest prevalence of elevated temperature was among horses originating in Europe (13.9%), while the lowest prevalence was among horses originating from Australia or New Zealand (4%);
  • Veterinarians noticed the highest prevalence of elevated body temperature records from October to December (17.1%), while they saw the lowest prevalence from July to September (8%);
  • The prevalence of elevated temperature decreased with increasing age (older horses were less likely to have a fever);
  • The prevalence of elevated temperature in intact males and in females (16.1% and 17.7%, respectively) was significantly higher than in geldings (6%); and
  • Veterinarians treated 43 of the horses with elevated temperature records with an antimicrobial and 35 with an NSAID.

What Does it All Mean?

"This U.S. study showed some clear associations within each import center between the occurrence of elevation in body temperature and risk factors, such as age and breed," Traub-Dargatz explained.

One association the veterinarians found was more elevated temperature records among younger horses than older horses at all three import centers.

Traub-Dargatz suggested it's possible that younger horses could be at higher risk for developing elevated temperatures due to lack of experience with air transport and/or susceptibility to transport stress.

"It's also possible that the normal body temperature for some of these younger animals is about the 101.5°F (38.6°F) defined as elevated for this report," she said.

Also, Traub-Dargatz noted that "Friesians were at greater risk than other breeds for elevated temperatures at both LA-AIC and NYAIC; no Friesians were imported through MAIC." She added that further details related to why certain breeds seem more likely to develop an elevated body temperature were outside the scope of this particular study.

Additionally, "The prevalence of elevated temperatures among horses in quarantine varied by center, and there seemed to be an influence of center not described by the available data," Traub-Dargatz noted, adding that there were likely additional factors not reviewed in the current study that influenced the elevated temperatures in some of the horses.

Finally, Traub-Dargatz briefly discussed a condition called psychological stress-induced rise in core temperature (PSRCT) that has been described in the literature: "It seems possible that some of the elevated body temperatures experienced by imported horses on arrival at the import center, particularly those that resolved without treatment or with a single dose of NSAID, represent a form of PRSCT."

Although the current study yielded a host of information about the prevalence of elevated body temperature in imported horses, there is much left to learn: "Analysis of the existing data identified several factors associated with risk of an elevation in temperature," Traub-Dargatz concluded, noting that additional research is needed to precisely pinpoint the cause of and treatments for these elevated temperatures among imported horses.

Disclaimer: Seek the advice of a qualified veterinarian before proceeding with any diagnosis, treatment, or therapy.