INTRANASAL ADMINISTRATION OF PASTEURELLA-MULTOCIDA TOXIN IN A CHALLENGE-EXPOSURE MODEL USED TO INDUCE SUBCLINICAL SIGNS OF ATROPHIC RHINITIS IN PIGS

A challenge-exposure model was developed for dose-dependent induction of subclinical (moderate) atrophic rhinitis (AR) in conventionally rai sed Dutch Landrace and Large White pigs, about 4 weeks old. Under favo rable climatic and housing conditions, pigs were intranasally challeng e-exposed with Pasteurella multocida-derived toxin (Pm-T) 3 days after pretreatment by inoculation with 1% acetic acid. Pigs were challenge- exposed with 1 of the following Pm-T doses: 0 (control), 5, 13, 20, or 40 mug of Pm-T/ml of phosphate-buffered saline solution (PBSS), 0.5 m l/nostril/d on 3 consecutive days. Five weeks after challenge exposure , subclinical (moderate) AR status was defined as intermediate conchal atrophy (grade 2 for ventral conchae on a 0 to 4 scale and grade 1 or 2 for dorsal conchae on a 0 to 3 scale, respectively) and perceptible difference in change in brachygnathia superior (CBS) between control and challenge-exposed pigs between the beginning and end of the study. All Pm-T-exposed pigs had nasal damage that was dose-dependent. The h igher Pm-T doses resulted in higher ventral conchae atrophy and dorsal conchae atrophy scores. The CBS increased with applied Pm-T dose, res ulting in significant (P < 0.05) differences between controls (3.88 mm ) and the 13-, 20-, and 40-mug Pm-T-treated groups (7.77, 6.58, and 7. 98 mm, respectively). In response to the applied dose, weight gain per week for Pm-T-exposed pigs was lower than that of controls after week 3 (P < 0.01). Difference from controls was 32, 54, 52, and 96 g/d/pig for 5-, 13-, 20-, and 40-mug Pm-T-treated groups respectively, in the last 2 weeks, For Dutch Landrace and Large White pigs, intra. nasally administered Pm-T mimicked the pathogenic effect of in vivo infection with toxigenic Pm strains. The optimal model to induce subclinical AR appeared to be 13 mug of Pm-T/ml (0.5 ml/nostril/d) on 3 consecutive days. Our model should enable studies of exogenous and endogenous fact ors involved in development of AR, independent of the colonizing abili ty of the Pm strain used.