With the Food and Drug Administration (FDA) issuing stricter guidelines for the usage of antibiotics, the search for alternatives is growing in North America. Trials around lysozyme, a naturally occurring enzyme, appear positive for nursery pigs.
By William Oliver and Jim Wells, USDA-ARS, US Meat Animal Research Center, Clay Center, NE, United States
Lysozyme, also known as muramidase, is a naturally occurring enzyme found in bodily secretions such as tears, saliva, and milk. It functions as an antimicrobial by cleaving the peptidoglycan (amino acid/ sugar) component of bacterial cell walls,
which leads to cell death. Antibiotics are also antimicrobials and have been fed at subtherapeutic levels to swine as growth promoters for more than 50 years, and the majority of swine produced in the US receive antibiotics in their feed at some point in their production cycle. These compounds benefit the producers by minimising production losses by increasing feed efficiency and decreasing susceptibility to bacterial infection and disease. It is known that increased pathogen shedding
in pigs is associated with reduced performance. Reducing this pathogen load ultimately leads to increased growth rates, likely due to a less active immune system, and increased profitability. However, swine producers are currently under tremendous pressure to eliminate subtherapeutic antibiotic use throughout the production cycle. Finding safe and effective alternatives to traditional antibiotics will allow swine producers to keep the animal health and growth aspects of antibiotics without the stigma associated with their use.
Lysozyme was first used experimentally in pigs in the mid-2000s. In a few studies, human lysozyme derived from transgenic goats' milk was shown to change metabolite profiles, intestinal microflora, and intestinal morphology. However, due to the nature of these experiments, improvements in growth performance due to lysozyme were not detectable. In research, the USDA-ARS research team in Clay Center used a different source of lysozyme (chicken eggs; Entegard, Neova Technologies, Abbotsford, BC, Canada) and designed experiments to determine any effect on growth performance. The first trial was a proof of concept experiment feeding milk replacer to tenday old pigs. In total 48 pigs were fed a non-medicated milk replacer, milk replacer plus antibiotics, or milk replacer plus lysozyme for two weeks. Pigs consuming lysozyme or antibiotics grew approximately 12% faster than pigs consuming the non-medicated milk replacer, which is, to our knowledge, the first demonstration of improved growth performance in response to lysozyme consumption in pigs. In addition, pigs consuming the lysozyme liquid diet had improved small intestinal morphology and decreased Campylobacter prevalence in the gastrointestinal tract. Due to the study design, it was impossible to determine the effects of lysozyme on feed intake or feed efficiency, which is of obvious importance to the swine industry. Nonetheless, it was concluded that lysozyme is a suitable alternative to antibiotics, at least in ten-day-old pigs consuming liquid diets.
It was obvious that lysozyme potentially was a good replacement for antibiotics used in swine diets, but needed to be tested in more practical diets. The second study was conducted in pigs consuming typical US nursery diets (corn/ soybean meal/ specialty protein). The study design was similar to the proof of concept experiment, but with more animals and the ability to determine the effect on feed intake and feed efficiency. Altogether 192 pigs were weaned at 24 days of age and fed control diets, control diets plus carbadox/ copper sulfate, or control diets plus lysozyme. Similarly to the milk study, lysozyme and carbadox/ copper sulfate improved growth rates of nursery pigs. In addition, to our knowledge, this study is the first to demonstrate improved feed efficiency in response to lysozyme consumption in pigs. Detecting differences in small intestinal morphology is difficult in pigs as they age due to the increased variation in villi height both between and within pigs. Because it was possible to sample a significant number of pigs, a 28% increase in villi height was detected, as well as a 23% decrease in crypt depth, in pigs consuming lysozyme.
The most recent study, partially funded by the National Pork Board, was designed to determine the efficacy of lysozyme in ameliorating the effect of an immune response in nursery pigs. In addition to regulating the immune response, cytokines have a profound effect on nutrient metabolism. Pro-inflammatory cytokines, such as IL-6 and TNF-α, redirect nutrients toward the immune response and away from growth processes. This is due to, among other factors, increases in muscle protein degradation and acute phase protein production. A total of 1,200 pigs were weaned from the sow at 26 days of age. To induce an indirect immune challenge, half the pigs were weaned into a nursery room left unclean since the previous group of pigs. The 'clean' pigs were weaned into a nursery room that had been fully cleaned and disinfected. Within each room, pigs were fed identical diets to our first nursery study, except that carbadox/ copper sulfate was replaced with chlortetracycline and Denagard (Novartis). Similarly to the previous work, antibiotics or lysozyme improved pig performance, and it did so both in pigs with a chronic immune stimulation and pigs in the clean nursery (Figure 1).
In spite of their long history of use in the industry, the mechanism by which subtherapeutic antibiotics improve the performance of pigs is not completely understood. Presumably, lysozyme would work similarly to subtherapeutic antibiotics. Many potential mechanisms have been hypothesised, including increased beneficial microbes, reduced pathogen load, and improved gastrointestinal function. This research has shown that Campylobacter shedding is reduced by lysozyme. It remains to be seen if beneficial bacteria in the gastro-intestinal tract are improved.
A major effect of both antibiotics and lysozyme is on the small intestine. Both antibiotics and lysozyme increase villi height and decrease crypt depth in the jejunum of the small intestine, as shown by an increase in the villi height/ crypt depth ratio (Figure 2). Villi height is a major determinant of absorptive ability and most absorption takes place in the jejunum of the small intestine in pigs. Thus, it is likely that much of the benefit from antibiotics and lysozyme is due to improvements in small intestinal morphology. Although the USDA-ARS research team in Clay Center demonstrated a correlated response between lysozyme feeding and improved growth performance and intestinal health, it remains to be seen if there is a direct causative effect. Ongoing work in the laboratories is designed to determine the rate of nutrient flux across the gut, as well as to determine the whole body use of nutrients in pigs fed lysozyme-supplemented diets. There is tremendous pressure to eliminate or reduce subtherapeutic antibiotic use in the swine industry. This is due to the perceived contribution of their use to increased levels of antibiotic resistant bacteria in the environment and increased human infections with antibiotic resistant bacteria. Lysozyme has proven to be an effective alternative to subtherapeutic antibiotics. In the event that antibiotics are regulated out of use, safe and effective alternatives, such as lysozyme, will allow swine producers to maintain current levels of animal health, efficiency, and profitability.
Source: Pig Progress magazine 30.5 (2014).