Equine Upper Respiratory Infections

Originally written by Drs. Elizabeth Carr, Carrie Jacobs, Catherine Febvay, Judy Marteniuk, and Paul Bartlett, Michigan State University.

This module provides a case study involving antibiotic use in equine practice, the management of patients diagnosed with antimicrobial drug resistant pathogens, and clinical approaches to enhance antimicrobial stewardship in veterinary medicine.

Learning Outcomes

This module aims to introduce equine upper respiratory infections. By the end of the module, you will be able to:

  1. identify the equine veterinarian’s role regarding the appropriate use of both prescription and over-the-counter antimicrobial agents.
  2. describe the consequences of inappropriate use of antimicrobial agents in horses.
  3. recognize clinical presentations in equine medicine where antimicrobial treatment is likely to be ineffective and when alternatives to antimicrobial treatment should be employed.
  4. discuss how an animal could have a medical condition that improved after treatment with an antimicrobial drug, but improvement was not necessarily due to the drug.

Talking with the Owner

Physical Exam

Physical Exam Finding from Cowboy

  • Heart Rate: 68 beats per minute, tachycardic
  • Temperature: 103.2 degrees Fahrenheit (35.5 degrees Celsius)
  • CRT: Capillary refill time is four seconds; gums are red/purple color
  • Ultrasound: Small intestinal motility is sluggish; increased wall thickness of 5−7 mm (normal is 2−3 mm); large colon motility is decreased and filled with fluid ingesta
  • Rectal Exam: Palpable fluid-filled large intestine
  • Other Observations: Mildly colicky; poor peripheral pulses; green, watery, foul-smelling diarrhea

Treatment Record for Cowboy



Additional Notes*

Thursday, the 2nd

1st vet  visit

Veterinarian’s findings: Cowboy is depressed and not eating, has a fever of 102.8 degrees Fahrenheit (39.3 degrees Celsius), mild mucoid nasal discharge and inducible cough. A dose of flunixin meglumine was given.

(Flunixin meglumine is a nonsteroidal anti-inflammatory drug (NSAID), analgesic, and antipyretic used in horses. One of its common uses is to alleviate fever and pain.)

Saturday, the 4th

2nd vet visit

Veterinarian’s findings: Cowboy still has nasal discharge and cough, but no fever. He seems bright, alert, and responsive. Increased tracheal noise and lung sounds apparent upon auscultation. Bacterial pneumonia suspected and mare was started on a five-day course of trimethoprim sulfamethoxazole orally twice daily.

(Trimethoprim sulfamethoxazole is a combination of a bacteriostatic antibiotic, trimethoprim, and a sulfonamide antibiotic. Trimethoprim in combination with either sulfamethoxazole (SMZ) or sulfadiazine (SDZ) has a wide spectrum of activity against Gram-negative and -positive organisms.)

Tuesday, the 7th

3rd vet visit

Veterinarian’s findings: Cowboy has improved, vet told owner to finish previous five-day course of antibiotics(Trimethoprim sulfamethoxazole).

(Trimethoprim sulfamethoxazole is a combination of a bacteriostatic antibiotic, trimethoprim, and a sulfonamide antibiotic. Trimethoprim in combination with either sulfamethoxazole (SMZ) or sulfadiazine (SDZ) has a wide spectrum of activity against Gram-negative and -positive organisms.)

Saturday, the 11th

4th vet visit

Veterinarian’s findings: Mild nasal discharge had returned and Cowboy is coughing occasionally shortly after being fed. He is bright and alert, has no fever and has a slight increase in tracheal noise with normal lung sounds. Started on a second course of antibiotics consisting of enrofloxacin orally once daily for five days.

(Enrofloxacin is a fluoroquinolone)

Friday, the 17th

Owner treatment

Owner Treatment: After finishing the last veterinarian-prescribed antibiotic course, Cowboy seemed to have improved for a while, but the nasal discharge and cough returned again. The owners administered Procaine penicillin, which is a beta-lactam antibiotic. This antibiotic was purchased at a local farm store and given as an intramuscular injection once a day for three days. He was also given a three-day course of gentamicin, which is an aminoglycoside. This antibiotic was left over from a previous treatment for another horse.

Tuesday, the 21st

5th vet visit

Veterinarian’s findings: Cowboy still has nasal discharge and was started on oral erythromycin three times daily.

(Erythromycin is a macrolide antibiotic.)

Wednesday, the 22nd

Admitted to hospital

On the second day of treatment with erythromycin, Cowboy was dull and not eating. Upon physical examination, a temperature of 103 degrees Fahrenheit (39.4 degrees Celsius), a heart rate of 60 beats per minute, and a capillary refill time of four seconds were found. There was an increase in gastrointestinal sounds and diarrhea in his tail. He was referred to the veterinary school for further treatment and diagnostics for suspected enterocolitis.


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Module Summary

  • Treatment records for Cowboy show that he had been given various antibiotics at varying intervals throughout his treatment prior to referral for hospitalization due to suspected enterocolitis.
  • The prescribed antibiotics switched between different classes of antibiotics with different bacterial spectrums throughout the course of treatment.
  • No diagnostic tests were performed prior to prescribing any of the antibiotics.
  • Treatment of multiple antibiotics over the course of weeks led to disruption of normal gut flora and growth of Clostridium difficile with subsequent enterocolitis.
  • Antibiotics may improve a patient’s condition if a secondary bacterial infection is present, but they will do nothing to treat the original viral infection.



There are no references used for this section.

*Note: There were areas in which “mare” was present in the medical notes.  Cowboy is a male horse and the terminology for a male horse is a stallion or a gelding. In Cowboy’s case, he should be termed as a gelding.