EFFECTS OF AMMONIA INHALATION AND ACETIC-ACID PRETREATMENT ON COLONIZATION KINETICS OF TOXIGENIC PASTEURELLA-MULTOCIDA WITHIN UPPER RESPIRATORY TRACTS OF SWINE

Pigs reared in intensive production systems are continuously exposed t o ammonia released by the microbial degradation of their excrement. Ex posure to this gas has been shown to increase the severity of the dise ase progressive atrophic rhinitis by facilitating colonization of the pig's upper respiratory tract by Pasteurella multocida. The etiologica l mechanism responsible for this synergy was investigated by studying the colonization kinetics of P. multocida enhanced by ammonia and comp aring them with those evoked by an established disease model. Three-we ek-old Large White piglets were weaned and allocated to five experimen tal groups (groups A to E). Pigs in groups A and B were exposed contin uously to ammonia at 20 ppm for the first 2 weeks of the study. Pigs i n group C were pretreated with 0.5 ml of 1% acetic acid per nostril on days -2 and -1 of the study. On day 0 all the pigs in groups A, C, an d D were inoculated with 1.4 x 10(8) toxigenic P. multocida organisms given by the intranasal route. The kinetics of P. multocida colonizati on were established by testing samples obtained at weekly intervals th roughout the study. The study was terminated on day 37, and the extent of turbinate atrophy was determined by using a morphometric index. Th e results of the study showed that exposure to aerial ammonia for a li mited period had a marked effect on the colonization of toxigenic P. m ultocida in the nasal cavities of pigs, which resulted in the almost t otal exclusion of commensal flora. In contrast, ammonia had only a lim ited effect on P. multocida colonization at the tonsil. The exacerbati on of P. multocida colonization by ammonia was restricted to the perio d of ammonia exposure, and the number of P. multocida organisms coloni zing the upper respiratory tract declined rapidly upon the cessation o f exposure to ammonia. During the exposure period, the ammonia levels in mucus recovered from the nasal cavity and tonsil were found to be 7 - and 3.5-fold higher, respectively, than the levels in samples taken from unexposed controls. Acetic acid pretreatment also induced marked colonization of the nasal cavity which, in contrast to that induced by ammonia, persisted throughout the time course of the study. Furthermo re, acetic acid pretreatment induced marked but transient colonization of the tonsil. These findings suggest that the synergistic effect of ammonia acts through an etiological mechanism different from that evok ed by acetic acid pretreatment. A strong correlation was found between the numbers of P. multocida organisms isolated from the nasal cavity and the severity of clinical lesions, as determined by using a morphom etric index. The data presented in the paper highlight the potential i mportance of ammonia as an exacerbating factor in respiratory disease of intensively reared livestock.