Photo: The Horse Staff

Rhodococcus equi, a normal soil organism, can be damaging for a foal–if not now, then later. Affected foals develop fever, nasal discharge, and cough, and they become lethargic. While some succumb to subsequent pneumonia and pulmonary abscesses, survivors can experience significant performance-limiting problems later in life. Ben Buchanan, DVM, Dipl. ACVIM, Dipl. ACVECC, of Brazos Valley Equine Hospital in Navasota, Texas, described an antimicrobial strategy designed to reduce the incidence of R. equi infection at the 2011 American Association of Equine Practitioners convention, held Nov. 18-22, in San Antonio.

Buchanan explained that foals contract this disease early in life, but clinical signs emerge weeks later. One technique that has been fairly successful in limiting the development of clinical signs is the administration of azithromycin within two weeks of birth. Resistance to this macrolide antibiotic is beginning to show in 5% of R. equi cases so it is important for the veterinary community to identify other control techniques. Buchanan reported that risk factors for infection include transient populations of mares (coming and going) on a farm as well as a high stocking density of pastures-these conditions lead to increased R. equi organisms in the soil.

However, Buchanan pointed out that Rhodococcus pneumonia is associated with exposure to a high concentration of airborne organisms rather than those in the soil. The stress of pregnancy causes periparturient mares (around the time of giving birth) to shed virulent R. equi into the environment (“In most studies up to 100% of mares will shed R. equi,” Buchanan noted). In one study, higher airborne concentrations of these bacteria were identified in stalls than in paddocks. A second study demonstrated higher concentrations of airborne bacteria where horses spend most of their time