Effect of benzoic acid in the feed on Salmonella typhimurium in weaned pigs

Abstract

The effect of benzoic acid on Salmonella Typhimurium was compared to formic acid + lactic acid in naturally infected weaners. Addition of 1% benzoic acid or 0.5% lactic acid + 0.5% formic acid did not result in a significant reduction of Salmonella prevalence, neither bacteriologi- cal nor serological, after 5 weeks of administration, compared to a control diet consisting of pel- leted feed without organic acid. In contrast, adding organic acid to the diet improved average daily gain and other production parameters, irrespective of the type of acid. Introduction Reduction of Salmonella infection pressure in pig herds can be achieved in different ways, e.g. through feed intervention strategies. Many studies have shown a decrease in Salmonella prevalence in pigs following the use of meal feed instead of pelleted feed, inclusion of barley in the diet, coarse grinding, and addition of organic acids (e.g. Jorgensen et al, 2003). Especially addition of organic acids such as formic acid and lactic acid have proven very fast and effective means to reduce Salmonella prevalence rapidly in problem herds. Recent experimental work at the Danish Institute of Agricultural Science (DIAS) has shown very promising effects of benzoic acid on Salmonella. In vitro studies showed that benzoic acid was much more potent in the reduction of Salmonella compared to the documented effects of formic acid and lactic acid. Therefore, it was decided to carry out an intervention study with the aim to measure the effect of benzoic acid on natural Salmonella infections in weaned pigs and to compare its effect with an often used and well documented combined feed intervention of formic and lactic acid. Materials and Methods The study was performed at an experimental farm in Jutland, with facili- ties for raising pigs from 7 to 30kg. Pigs were purchased weighing 6-8kg (one week after wean- ing) from a farm that had been experiencing recurring problems with Salmonella Typhimurium dur- ing a one-year period. Repeated fecal sampling and bacteriological analysis using pen floor sam- ples had revealed a persistent high prevalence in weaner pigs of 40-50%. Upon arrival at the experimental farm, the pigs were randomly assigned to one of three experimental groups: 1) Pelleted feed without organic acids (control) 2) Pelleted feed with addition of 0.5% lactic acid + 0.5% formic acid 3) Pelleted feed with addition of 1% benzoic acid Each group contained 226 pigs, which were housed pen-wise in groups of 6 pigs, i.e. 37 pens per treatment group. All pigs were fed a standard weaner diet during the first two weeks after arrival. Subsequently, from week 3 to week 7 the pigs were fed the experimental diets as described above. Excretion of Salmonella was monitored at pen level by examining rectal fecal samples from 5 pigs in each pen as 1 pooled sample five weeks after the start of the experimental diets. To improve the probability of detection, this procedure was carried out in duplicate in each pen, and at the laboratory all samples were incubated for 48 h instead of 24 h. In addition, blood samples were collected from all pigs 5 weeks after the start of the experimental diets and examined for antibodies against Salmonella enterica using the Danish mix-ELISA. Finally, average daily gain and feed units per pigs (FUp) were recorded to assess the effect organic acids on produc- tion parameters. Disease, medical treatment and mortality were also recorded at pen level.