FieldDxM. haemolytica infectionClinical signs, differential diagnosis, treatment and prevention in horses.
Overview
A bacterial respiratory infection, a major component of the bovine/equine respiratory disease complex. Mannheimia haemolytica.
Clinical signs
- Listlessness and toxaemia.
- Fever of 40-41 C (104-106 F).
- Serous to mucopurulent nasal discharge.
- Moist cough with rapid, shallow breathing.
- Increased bronchial sounds, crackles and wheezes on auscultation.
- Severe cases develop pleurisy with irregular breathing and an expiratory grunt.
Predisposing factors
- The organism normally lives in the upper respiratory tract, in the tonsillar crypts.
- Environmental stress triggers it to proliferate.
- A viral respiratory infection usually precedes bacterial colonisation.
- Suppression of host immune defences.
Treatment
- Tulathromycin 2.5 mg/kg SC in the neck, once.
- Gamithromycin 6 mg/kg SC, once.
- Tilmicosin 10-20 mg/kg SC as a single dose, no more than 10 mL per site.
- Florfenicol 20 mg/kg IM in the neck repeated after 48 hours, or 40 mg/kg SC once, no more than 10 mL per site.
- Enrofloxacin 2.5-5 mg/kg SC every 24 hours for up to 5 days, or 7.5-12.5 mg/kg SC once, no more than 20 mL per site.
- NSAIDs alongside, to control fever.
Drugs mentioned: Ceftiofur Crystalline-Free Acid Tulathromycin Gamithromycin Enrofloxacin Florfenicol
Prevention
1. Vaccination cuts morbidity by roughly 25% in high-risk feedlot calves. 2. Vaccinate ideally 3 weeks before transport, and repeat on arrival. 3. Vaccinating the dam gives dairy calves useful passive immunity. 4. Preconditioning programmes. 5. Keep mixing of animals to a minimum. 6. Metaphylaxis in high-risk groups.
Source
MSD Veterinary Manual — Bacterial Pneumonia in Cattle with Bovine Respiratory Disease Complex (accessed 2026-07-31)
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