Bacteria occurs everywhere, and while most of these microorganisms do not cause clinical disease in horse, others most certainly can. Horses are exposed to potentially dangerous bacteria daily; in most of the time their immune system is able to fight it off without the development of clinical disease. Bacterial disease occurs when a horse’s immune system is weakened and bacteria are able to replicate and spread throughout a horse’s body.

When a horse breaks with bacterial disease, the source of bacteria is usually contaminated water, food or feces. Horses often become infected with bacteria after their immune system has already 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. 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 that are sick with a bacterial infection usually have digestive upset (diarrhea) or an upper respiratory infection (coughing, runny nose). Bacterial infections typically cause fever, lethargy and loss of appetite.

Bacterial Diseases of the Horse

There are many common bacterial organisms that have the potential to cause disease in horses. Rhodococcus equi is a bacterium that inhabits the soil and can cause pneumonia in foals aged 1 to 6 months. R. equi also can cause disease in other sites in the body, including the eyes, bone, joints, and the gastrointestinal tract and intestinal lymph nodes. Signs of disease in these other organs may occur in foals with or without concurrent pneumonia.

In horses and humans, Haemophilus influenzae type b (Hib) can cause pneumonia, bacteremia, meningitis, epiglottitis, septic arthritis, cellulitis, otitis media, purulent pericarditis, and less commonly, endocarditis, endophthalmitis, osteomyelitis, peritonitis, and gangrene. Non-type b–encapsulated strains present in a similar manner to type b infections. H.influenzae is abacterium that is of particular concern globally, as it was the leading cause of non-epidemic bacterial meningitis worldwide in children prior to the introduction of the Hib vaccine.2 

Gastric ulcers (equine gastric ulcer syndrome or EGUS) are common in horses and foals. This syndrome is most closely associated with horses involved in performance disciplines, although they can occur in any horse. Gastric ulcers in equines are usually caused by Helicobacter spp infection. Most horses with gastric ulcers do not initially exhibit clinical signs, but these become apparent when the ulceration is widespread or severe. The classic clinical signs for gastric ulcers in foals include diarrhea, bruxism, poor nursing and dorsal recumbency.3

Strangles is another highly-contagious disease of equids, including horses, donkeys and ponies.  It is caused by the bacteria Streptococcus equi. This syndrome is called "strangles" because of the strangled breathing sounds the affected horse makes as a result of profuse nasal discharge and swellings that form in the head and neck region. This disease is spread via nasal secretions (snorting, coughing, physical nose-to-nose contact) and pus from draining abscesses. It is possible for humans to spread the disease through contaminated clothing, hands or equipment, etc. The bacteria can contaminated water or feed sources and be transmitted orally as well as via the respiratory tract.1

Other common types of pathogenic bacteria affecting horses include:

• Tetanus

• Potomac Horse Fever

• Salmonella

• E Coli

• Mycobacterium avium complex (MAC)

Cleanliness practices around facilities in which horses are kept or frequent are critical in keeping bacteria from spreading. It is prudent to ensure that each horse has their own supplies and grooming tools to prevent spreading germs in stables where many horses are living together.

Clarithromycin for the Treatment of Bacterial Infections

The use of macrolide antibiotics began with the discovery of erythromycin, which was isolated from soil bacteria in the early 1950's.  Newer semi-synthethic relatives of erythromycin include claithromycin and azithromycin. Working via the inhibition of the production of protein by susceptible bacteria, they may be used alone or in combination with the antibiotic, rifampin.4 Macrolide antibiotics are often used in the treatment of disease caused by the bacteria R. equi, which causes severe pneumonia in foals. Clarithromycin is considered to be the most effective drug of the three macrolide antibiotics, but an increased incidence of diarrhea is sometimes noted with its use.

In foals with R equi pneumonia, the combination of clarithromycin (7.5 mg/kg, PO, bid) and rifampin is superior to erythromycin-rifampin and azithromycin-rifampin. Foals treated with clarithromycin-rifampin have improved survival rates and shorter febrile periods than foals treated with erythromycin-rifampin and azithromycin-rifampin.1 

Where to buy Clarithromycin 

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

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

1aaep.org.

2who.int.

3Merck Veterinary Manual.

4equimed.com.