Long-distance transportation of racehorses via horse trailer has become commonplace with the globalization of horse racing. Fever associated with transportation is a clinical sign that can disrupt training and racing schedules and is part of a syndrome (sometimes colloquially referred to as shipping fever in horses) that includes respiratory disease (eg, pneumonia or pleuropneumonia) caused by bacterial infection after long-distance transportation and can potentially increase mortality rate.1–3 Prevention of transportation-associated fever would potentially simplify planning of training and racing schedules and minimize the physical deconditioning associated with transportation. Various aspects of transportation-associated fever and respiratory disease have been investigated in earlier studies.1–5
Transportation-associated fever is influenced by transportation stress and deterioration of the environment inside the vehicle, and it typically develops ≥ 20 hours after the start of transportation.1–3 Bacterial infection develops in the bronchoalveolar regions of affected horses; the causative organism is usually Streptococcus equi subsp zooepidemicus, an opportunistic bacterium that is resident in the tonsillar tissues and trachea in healthy horses.2–4,6 After the start of transportation, infection of the lower respiratory tract with opportunistic bacteria such as S equi subsp zooepidemicus is considered to the primary cause of transportation-associated fever.2,6 During long-distance transportation, the horse's raised head position induces inflammation and increased bacterial abundance in the lower respiratory tract because of obstructed tracheal mucociliary clearance, and host immunity is greatly changed.7,8 Numbers of peripheral blood leukocytes, including neutrophils, increase, and the rate of phagocytosis by peripheral blood leukocytes, are substantially reduced.8 Circulating concentrations of SAA, a nonspecific marker of inflammation, can reflect the degree of the inflammation in horses with respiratory disease.9
On the basis of findings in previous studies,1–8 attempts have been made to prevent transportation-associated fever. Oral administration of interferon-α for 3 consecutive days (to stimulate the immune system) before transportation was associated with a decrease in the severity of transportation-associated fever and improvement in clinical condition after transportation.10,11 However, the protocol did not completely prevent fever in those horses, and further prophylactic measures are needed.
Results of the previous studies10,11 suggest that activation of immunity by interferon-α administration before transportation, together with suitable bronchoalveolar concentrations of an antimicrobial agent that is effective against the bacteria that exist in the tonsillar tissues and trachea, could potentially provide an effective means of preventing transportation-associated fever. However, most of the antimicrobial agents used in equine clinics have short terminal half-lives,12,13 and it is difficult to maintain effective circulating concentrations during long transportation periods after a single dose is administered. In contrast, enrofloxacin has been used clinically as a long-acting antimicrobial agent for infectious diseases, including bacterial pneumonia,14,15 and its efficacy has been acknowledged. Enrofloxacin is a broad-spectrum fluoroquinolone antimicrobial. When administered IV at a dose of 5 mg/kg every 24 hours to an adult horse, it is transported effectively to the bronchoalveolar region (unpublished data), and serum concentrations remain high 24 hours after administration.16 In an experimental study14 of horses with respiratory disease following long-distance transportation, the efficacy of enrofloxacin administration at various doses was investigated for the purpose of treatment. However, to our knowledge, no reports have been published regarding the administration of enrofloxacin for the prevention of transportation-associated fever. The purpose of the study reported here was to evaluate effects of a single dose of enrofloxacin (5 mg/kg, IV) on body temperature and tracheobronchial neutrophil count in healthy Thoroughbreds premedicated with interferon-α and undergoing long-distance transportation.
Serum amyloid A
Baytril 5% injection, Bayer Health Care Japan, Kita-ku, Osaka, Japan.
Bimuron, Biovet, Shibuya-ku, Tokyo, Japan.
VP-P100K, Terumo, Shibuya-ku, Tokyo, Japan.
VC-C50, Terumo, Shibuya-ku, Tokyo, Japan.
K-4500 automatic hemacytometer, Sysmex, Kobe-shi, Hyogo, Japan.
Diff-Quick 16920, Sysmex, Kobe-shi, Hyogo, Japan.
Latex agglomeration method LZ test, Eiken SAA, Eiken Chemical, Taito-ku, Tokyo, Japan.
VQ8303A videoendoscope, Olympus Corp, Shinjyuku-ku, Tokyo, Japan.
Shandon Cytospin 4 cytocentrifuge, Thermo Electron Corp, Marietta, Ohio.
Mycillin solution, Meiji Seika Ltd, Chuo-ku, Tokyo, Japan.
Coaxin, Chemix Co Ltd, Yokohama-shi, Japan.
StatView, version 5.0, SAS Institute Inc, Cary, NC.
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