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.
Adult horses are susceptible to colitis (inflammation of the large bowel) when given macrolide antibiotics, so this is not a treatment option for broodmares to reduce fecal shedding. Gallium nitrate, a unique liquid metal, inhibits R. equi bacteria. Buchanan discussed a trial study in which he and colleagues examined the effects of once-daily low-dose oral treatment of 10 mares with gallium nitrate from Day 320 of gestation through Day 7 post-foaling. Another group of 11 nontreated mares served as controls. Forty-nine percent of fecal cultures from Day 320 of gestation, the week before foaling, and the week after foaling tested positive for R. equi. Over time, the treated mares shed less R. equi in their feces. Air samples taken within six hours post-foaling were tested, and there was no statistical difference between treatment and control groups-20% of treatment samples and 45% of control samples were positive for R. equi.
While there is not a direct association between mare fecal shedding and development of foal disease, Buchanan concluded, "Treatment of mares with oral gallium nitrate significantly reduced fecal concentrations of virulent R. equi but had no detectable impact on airborne concentrations." Buchanan noted that further research is needed, however, before gallium nitrate treatment becomes common place.
He suggests that there are likely other factors involved in airborne R. equi concentration in stalls, possibly related to previous stall occupants. "Reducing the environmental burden of R. equi is an important tool to decrease incidence of disease," urges Buchanan. "In this study half of the horses were not treated. Next up will be to treat all the horses on the farm and compare to untreated farms. I feel like treating all or the majority of the horses will be important to reducing the environmental burden."
Disclaimer: Seek the advice of a qualified veterinarian before proceeding with any diagnosis, treatment, or therapy.