Effects of inhaled amikacin in bacterial tr-IAD affected racehorses and comparison with intravenous administration

Purpose of the work. Inflammatory Airway Disease (IAD) is a syndrome characterized by inflammation of lower airways without systemic implication such as fever or dyspnea. The abbreviation “tr” indicates IAD diagnosed by tracheal wash (TW)[1]. IAD is defined as “bacterial” when more than 100 Colony Forming Units (CFU) are isolated from TW[2]; in this case antibiotic therapy is recommended. Efficacy of antibiotic therapy in respiratory disease is directly correlated with the minimal inhibitory concentration (MIC) reached in the respiratory tract[3]. Studies have shown that inhaled antibiotics, used at lower dosage, could reach higher concentration in horses respiratory fluids than systemic administration[3]. A recently published study reports the clinical efficacy of inhaled amikacin in human patients[4]. Aim of the present work is to evaluate the effect of inhaled amikacin, compared to systemic administration, on TW cytology and bacteriology and on clinical scoring in bacterial tr-IAD affected racehorses. Materials and used methods. Thoroughbred racehorses, aged between 3 to 4 years old, affected with bacterial tr-IAD with a sensitivity testing positive for amikacin, were selected. In all horses a clinical evaluation was performed and, using a modified respiratory clinical scoring system[5], a score was assigned to each horse at the beginning and the end of the trial. TW was endoscopically (Olympus) performed into the thoracic portion of trachea and 60 ml of saline at 37°C was flushed, through a sterile Teflon catheter passed into the biopsy channel, and then immediately collected. The sample was cytocentrifuged (Rotofix 32, Hettich Zentrifugen) and stained with May Grumwald Giemsa for differential cell count and also cultured on blood agar plate for CFU/ml count and agar diffusion amikacin sensitivity test. Horses were then randomly divided in 2 groups; the horses of group A were treated with 3,3 mg/Kg of amikacin (Amikavet® Merial), diluted in 0,45% NaCl solution at 50 mg/ml, administered by a jet nebulizer (Fiac F3000) and Equine Aeromask®, q24h for 10 days, while horses of group B received 10 mg/Kg of amikacin (Amikavet® Merial) intravenously q24h for 10 days. Clinical scoring and TW cytology and bacteriology were performed in all horses at the end of the treatment. Values of clinical score, TW neutrophils count and CFU/ml before and after the treatment in both groups were compared by Student-T test. Rate of recurrence of bacteria isolation after therapy between the two groups was compared with chi-squared test. Outcomes. In group A, a statistically significant decrease in clinical score (p<0.01), TW neutrophils count (p<0.05) and CFU (p<0.01) was found after inhaled amikacin treatment. In group B a statistically significant decrease in clinical score (p<0,05) and CFU (p<0.01) was recognized after intravenous amikacin administration. Although, no significant difference was found in TW neutrophils count for group B, a decrease in their count was evident. No statistically significant difference in bacteria isolation after the treatment was found between the two groups. 358 Conclusions. Inhaled amikacin at dosage of 3,3 mg/Kg q24h for 10 days administered by jet nebulizer and Equine Aeromask® seems to be effective in the reduction of clinical signs, TW neutrophils count and bacterial CFU. The absence of a significant difference between the two groups in the isolation of bacteria after treatment suggests that, despite a lower inhaled dose, amikacin may be equally effective for both routes of administration. However, inhaled amikacin in group A, seems to be able to induce a lower chemotactic action on neutrophils, less damage of the lower respiratory tract mucosa, and consequently a reduction of clinical signs. In contrast, the absence of a statistically significant decrease in TW neutrophils count of group B, after intravenous administration of amikacin, may be related to the achievement of a lower drug concentration in the lung or simply due to the smaller number of cases of group B. In addition, the use of jet nebulizer and Equine Aeromask® was well tolerated by the horses and during or after the inhalation therapy none of the horses showed any side effects; therefore, amikacin at dilution of 50 mg/ml seems to be safe for aerosol administration in equine patients.