Infections in Lower (Inflammatory) Airway Disease

Several epidemiological studies performed in recent decades have demonstrated that isolation of pathogens in tracheal wash (TW) may be associated with lower airway inflammation in racehorses. However, very limited data are available to date on the underlying immune mechanisms of mild equine asthma (mEA), and most of Koch postulates are still to be fulfilled for defining infectious causality.

A wide range of bacterial species may be isolated from TWs¹ but across many studies associations between bacterial infections and mEA are consistently limited to certain species². The risk of observing airway inflammation is significantly increased with number of CFU/ml of Streptococcus zooepidemicus, Actinobacillus spp. and Mycoplasma equirhinisin TW^3,4. In addition, cough was significantly associated with isolation of Streptococcus zooepidemicus or Pasteurella spp., although numbers < 103 CFU/ml of TW was found in approximately 60% of horses with such symptom5. The reduction in incidence and prevalence of both S. zooepidemicus infections and IAD with age, and the absence of a reduction in the frequency of other streptococcal species, has also been used to support the suggested role of S. zooepidemicus as a cause of mEA4. Alternatively, S. zooepidemicus has very barely been isolated in another longitudinal study6. However, cut-off values of 104 CFU/ml have been used for discriminating negative and positive samples; similarly, identification and quantification have also been performed only on samples with up to 3 different strains. Large epidemiological studies have also been performed, based either on serological analyses or more recently on direct detection of viral genome by qPCR in various airway samples. Apart from γ-herpesviruses (EHV-2, EHV-5) which detection is ubiquitous in horse population; the somehow high (sero)prevalence of equine rhinitis - A virus (ERAV) and ERBV in athletic horses, and their significant association between viral detection and (clinical signs of) MEA are worthy of importance. Horses with mEA were indeed significantly more likely to have a positive titre as well as higher log transformed titres to ERAV when compared to control horses; while the detection of EHV-2 by qPCR in nasal secretions was also significantly associated with mEA7. […]