While there are numerous bacterial infections that can affect horses, there are around twenty that equine veterinarians see with regularity, and under a dozen that occur less frequently, but which can give rise to serious disease. The bacteria in the environments most horses occupy can number in the billions per square inch.1 Most bacteria with which horses come into contact are harmless, some are beneficial, but some are harmful bacteria known as pathogens. These can cause disease under the right circumstances, such as when a horse’s body or immune defenses are compromised. Some normally harmless bacteria can also become infectious under the right circumstances.

Stress can be a factor in the onset of disease in horses, and this includes bacterial disease. While physical stressors such as overwork can contribute to this, environmental conditions and psychological stressors can also tax a horse’s immune system, increasing the likelihood of disease taking hold. When a horse develops bacterial disease, the source of bacteria is usually contaminated water, food or feces. Horses can also fall prey to bacterial disease after their immune system has been compromised by a virus. Bacterial infections are more common in horses that that travel or spend a lot of time at training stables where lots of different horses are coming and going.2 Foals and geriatric horses are much more at risk of becoming sick because their immune systems are not as strong as adult horses in their prime.   

Horses have a unique set of internal microorganisms as well. There are both benign and beneficial bacteria that live in the horse’s body and on its skin. The complex and multi-levelled interactions between these resident microorganisms with respect to disease risks, health preservation, immunity and therapeutic possibilities are currently expanding research fields in veterinary medicine. The intestinal tract of the horse contains a diverse community of microorganisms that consists of fungi, parasites, protozoa, archaea, viruses and bacteria.3 While this range of microorganisms includes essential nutrition suppliers and immune response supporters, it also contains organisms capable of causing disease.

Horses are herbivorous mammals that possess a certain hindgut (caecum and colon) microbiota, enabling forage utilization for optimal nutrition.3 These microbes provide a substantial proportion of the horses’ daily energy needs through the fermentation of plant material to short chain fatty acids such as acetate, propionate, and butyrate.4 Thus, gastrointestinal disturbance in the equine microbiota can result in alteration of fermentation patterns and stressors that can compromise the immune system.

Antibiotics

Topical (or local) antibiotics are widely used in equine veterinary medicine for the treatment of skin disorders, lesions and wounds. These can be used prophylactically in fresh wounds to prevent infection. In the case of systemic infections, systemic antibiotics (or combinations thereof) must be used. These antibiotics must reach therapeutic levels in the target tissue or organ in order to be effective. The target organ depends on the nature of the infection.3 For example, the target organ for pneumonia is the lung tissue, whereas the target organ for bacterial encephalitis is the brain. Oral antibiotics must first be absorbed into the bloodstream from the GI tract. An IM antibiotic is absorbed at a particular rate into the blood from the muscle tissue. An IV antibiotic goes directly into the bloodstream. Once the drug reaches an adequate level in blood, it is then distributed from the blood to particular tissues within the body, at different levels, again dependent on the chemical structure of the antibiotic and its biochemical interaction with different tissue types.3 Systemic antibiotics are typically delivered via oral or injectable (IV or IM) routes.

Macrolide Antibiotics

Macrolide antibiotics are active against:

• Aerobic and anaerobic gram-positive cocci, except for most enterococci, many Staphylococcus aureus strains (especially methicillin-resistant strains), and some Streptococcus pneumoniae and S. pyogenes strains

• Rhodococcus equi

• Streptococcus equi

• Mycoplasma pneumoniae

• Chlamydia trachomatis

• Chlamydophila pneumoniae

• Legionella species

• Corynebacterium diphtheriae

• Campylobacter species

• Treponema pallidum

• Borrelia burgdorferi

• Haemophilus influenza

• Mycobacterium avium complex (MAC)

Macrolide antibiotics have been considered the drug of choice for group A streptococcal and pneumococcal infections when penicillin cannot be used.2Macrolide antibiotics are often used in the treatment of disease caused by the bacteria R. equi, which causes severe pneumonia in foals.

Where to buy Erythromycin + Rifampin 

Erythromycin + Rifampin is available in the U.S. through several pharmaceutical manufacturers and through veterinary custom compounding companies. 

Erythromycin carries numerous potential drug interactions. Please consult your veterinarian prior to beginning any treatment regimen.

FOR RX ONLY: A valid prescription from a licensed veterinarian is required for dispensing this medication.

1equusmagazine.com.

2Muscatello G, Leadon D, Klay M, Meijer W & Gilkerson J (2007) Rhodococcus equi infection in foals: the science of 'Rattles'. Equine Vet J 39 (5), 470-478.

3Kauter, A., et al. The gut microbiome of horses: current research on equine enteral microbiota and future perspectives. anim microbiome 1, 14 (2019).

4Argenzio R., et. al. Sites of organic acid production and absorption in the equine gastrointestinal tract. Am J Phys. 1974; 226:1043–50.