Acute and chronic exposure to ammonia and olfactory acuity for n-butanol in the pig

An associative learning method (using a food reward) was developed to measure pigs’ olfactory acuity for n-butanol, a standard odourant in human olfactometry. The pig could press two operant paddles but it only received a food reward when it pressed the one over which n-butanol was released. Once each pig had reached a training criterion (10 consecutive roots on the correct paddle on each of two consecutive sessions) this method was used to assess the impact of acute and chronic exposure to an atmosphere containing approximately 40 parts per million (ppm) ammonia gas, compared to fresh air, on its ability to perceive different concentrations of n-butanol. These were presented using a staircase pattern, i.e. if the pig gained or failed to gain a food reward on two consecutive occasions the concentration was reduced or increased, respectively. Acute exposure for approximately 45 min to about 40 ppm ammonia had no effect ( P>0.05) on the lowest detected concentration (LDC) of n-butanol in six pigs. The geometric mean LDC was 1.23 parts per trillion (ppt) in approximately 40 ppm ammonia and 2.09 ppt in fresh air. The LDC of three pigs increased, i.e. acuity fell, from 5.1 to 175.5 ppt over 24 days of exposure to ≅40 ppm ammonia. Ammonia had no effect on one of the other pigs and the high variability in the LDC for the remaining two pigs produced no meaningful assessment of its impact. Subsequent removal to fresh air for a further 24 days led to partial recovery of acuity in one of the three pigs that had shown evidence of olfactory impairment but not in the other two. Collectively our findings suggest that chronic, but not acute, exposure to ≅40 ppm ammonia can interfere with olfactory perception in some pigs (50% of our sample) and that this loss of acuity is not necessarily reversible.