Updated research about tackling respiratory disease was reported at the NAVC Conference in February 2017 by Erin Groover, DVM, DACVIM, from Auburn University.
Rhodococcus equi Pneumonia
Because Rhodococcus equi pneumonia in foals is associated with a high mortality rate, it has a significant financial impact on a breeding farm. A key objective is to be able to detect the presence of this infection as early as possible. To this end, Madrigal et al (https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4913589/) looked at quantitative PCR testing of foal feces for the virulence-associated protein A (vapA). At 3, 5 and 7 weeks of age, fecal PCR testing was compared to the screening technique of thoracic ultrasonography. While higher concentrations of the vapA gene were present in feces of affected foals, the test was unable to “differentiate foals that would develop clinical signs of pneumonia from those that would remain free of clinical signs.” Based on this inaccuracy, this form of fecal testing is not reliable as a screening test for R. equi.
Treatment for Rhodococcus pneumonia can be expensive and difficult to administer as well as the potential for the development of bacterial resistance. Currently, the recommended treatment pairs a macrolide antimicrobial (like clarithromycin) with rifampin. Cohen et al (https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4913631/) evaluated use of intravenous liposomal gentamicin as an alternative treatment. While all foals treated with this form of gentamycin successfully resolved R. equi pneumonia, two out of five foals developed nephrotoxicity within a week of treatment. No foals in the clarithromycin-rifampin treated group developed azotemia. Follow-up research is looking into various modifications for dosage, route (possible nebulization) and frequency of administration to determine if there is potential to safely use liposomal gentamycin against R. equi pneumonia.
Lower Airway Inflammatory Disease
Treatment of recurrent airway obstruction (RAO) targets inflammation and related bronchoconstriction, often using inhaled albuterol. This drug contains two molecules, each a mirror image of one another – the S-enantiomer and the R-enantiomer. It is thought that the S-enantiomer might, in fact, amplify inflammation, whereas the R-enantiomer, levalbuterol, does not. In examining levalbuterol as a treatment for RAO, Arroyo et al (http://onlinelibrary.wiley.com/doi/10.1111/jvim.14320/full) found that it effectively improved bronchodilation similarly to albuterol. While the levalbuterol effect lasts twice as long as albuterol – two hours instead of one – neither is considered useful for long-term management of RAO.
Two different syndromes can impact a horse’s lower airways: Inflammatory airway disease (IAD) or recurrent airway obstruction (RAO). The American College of Veterinary Internal Medicine’s updated consensus statement (http://onlinelibrary.wiley.com/doi/10.1111/jvim.13824/full) now describes “the constellation of signs that incorporate IAD and RAO as equine asthma syndrome.” The consensus statement outlines practical ways to differentiate IAD from RAO, along with other differential diagnoses. Pharmacological and environmental management protocols for lower airway disease are further discussed in the new consensus statement.
The Bottom Line
As equine medicine advances with newfound knowledge, some assumptions previously made do not always bear out when placed under careful scrutiny and the scientific method of evaluation. It is important to take a look periodically at new research that corroborates or refutes long-held beliefs.
The topic of EMS is an important one to practitioners who, on a daily basis, deal with obese or soon-to-be obese horses. More focus is being directed to this syndrome with the result that more accurate diagnostic tools may be available in the coming future. In addition, different treatment options may be discovered that are adjunctive to addressing obesity with dietary restrictions and exercise.
The field of respiratory disease is also ever changing as more treatment options are tried and evaluated in efforts to give relief to what is now being termed “equine asthma.” Effective treatment helps horses remain comfortable in their breathing and optimizes their abilities as athletes.