Deposition of differently sized airborne microspheres in the respiratory tract of chickens

As a part of the development of an efficient dry powder aerosol vaccine for poultry, the objective of this study was to accurately determine the deposition pattern of nebulized microspheres in the airways of unanaesthetized chickens of different ages (1 day, 2 weeks and 4 weeks old). In the first part of the study, the aerosol administration method was characterized: the influence of different nebulizers and nebulizing protocols on the relative humidity in the exposure chamber, the particle size distributions, the microsphere output and single microsphere percentage were determined. In the second part, birds were exposed to nebulized fluorescently labelled polystyrene microspheres (1 to 20 microm). Respiratory and gastro-intestinal tract tissue samples were collected and the number of fluorescent microspheres per sample was determined. In 2-week-old and 4-week-old chickens, microspheres of 5 and 10 microm, respectively, were too large for deposition in the lungs and air sacs as less than 5% of these microspheres penetrated into the lower airways. The larger size of microspheres reaching the lower airways of 4-week-old birds was explained by increasing airway dimensions with age. For 1-day-old chickens, deposition in the lungs decreased from 17 to 3% with increasing particle size (1 to 20 microm), but increased in the air sacs from 6 to 20%. Consequently, the total deposition percentage in the lower airways was independent of microsphere size and even 20 microm particles were able to penetrate into the lower airways, which was attributed to mouth breathing of the 1-day-old chickens.