Study sheds light on antimicrobial use in US wean-to-market pigs

Farms Pig, Veterinary Asians working in pig farms. Vaccinated pigs.


A recent study will help advance understanding of how antimicrobials are being used, particularly in integrated operations. Having more detailed information is a good way to educate producers and, more importantly,  veterinarians, said Peter Davies, PhD, principal of the study and a professor in the College of Veterinary Medicine at the University of Minnesota.

This was the first substantial description of antimicrobial use in US swine beyond annual sales and distribution data, Davies told Pig Health Today.

The study demonstrated the feasibility of obtaining data on antimicrobial use in the US swine industry through sharing of proprietary records. It also showed records could remain completely confidential in the process.

Nine large pork systems, collectively producing more than 20 million market pigs annually, voluntarily provided data to advance understanding of antimicrobial use in US swine herds. Conclusions gathered from the data will support future antimicrobial stewardship initiatives, Davies said, and is a significant step forward.

The Center for Veterinary Medicine of the Food and Drug Administration has published annual summaries of the amounts of antimicrobial drugs sold or distributed for food-producing species since 2009. The data are described by antimicrobial class, category of medical importance and route of administration, but to date, they have not included a defined denominator for population size.

Estimates of sales by species, based on estimates from distributors, have been included since 2016; however, sales data typically lack granularity, and few nations can accurately parse these data at species or sector (e.g., beef versus dairy) level.

The absence of a “gold standard” metric for antimicrobial use remains an obstacle to harmonization that would be necessary for any meaningful comparison of antimicrobial use in different geographical and industry settings, Davies said.

Study details

The project was guided by a preliminary assessment of options for measuring antimicrobial use in the US swine industry (Davies, 2017) and was developed and executed in consultation with an advisory group convened by the National Pork Board.

For the study, data on antimicrobial use were collected for the 2016 and 2017 calendar years from the nine systems.

Data from each system were processed to populate standardized fields (system, year, active ingredient, route, weight of active ingredient, live weight marketed) for aggregation and analysis. The aggregate data were summarized both qualitatively (relative amounts of antimicrobials used by drug and route) and quantitatively. Analysis was conducted for individual active ingredients, by antimicrobial class and by medical importance. Quantitative analysis used the weight of active ingredient(s) as the numerator and estimated live weight of pigs marketed as the denominator.

Administration was predominantly oral in feed and water, and injection comprised approximately 2% of use overall but around 12% for critically important antimicrobials, Davies explained.

The analysis was structured to report the patterns of use (number of systems using the ingredient) and median and mean use of individual active ingredients at an enterprise level across the aggregate population, Davies said.


Considerable variability was found among systems with respect to the distribution of antimicrobial classes used and also within systems between years, Davies said. Tetracyclines comprised around 60% of all antimicrobial use by weight. Eight classes of antimicrobials (tetracyclines, lincosamides, pleuromutilins, beta lactams, aminoglycosides, macrolides, quinolones and cephalosporins) were used in all nine collaborating systems. The six (2016) and seven (2017) most used classes comprised over 90% of total use.

Across all systems in both 2016 and 2017, the majority of antimicrobials (by weight) were administered in feed (70.5% and 67.8%, respectively). The bulk of the balance was administered in water and a relatively small proportion by injection.

Antimicrobial classes categorized as highly important comprised the majority of use (80.7% and 78.4% of total use in 2016 and 2017, respectively) and were predominantly tetracyclines.

Antimicrobials deemed critically important for human medicine were proportionally administered more by injection than were antimicrobials in other categories (less than 2% across both years).

At the level of individual antimicrobial active ingredients, chlortetracycline alone accounted for 49.3% of use in 2016, and 40.9% in 2017. The six most used antimicrobials (chlortetracycline, oxytetracycline, lincomycin, penicillin, bacitracin and tiamulin) accounted for 86.8% of total weight used in 2016 and 84.8% in 2017, the study reported. In contrast, the six least used antimicrobials (sulphadimethoxine, tetracycline, florfenicol, ampicillin, bambermycins and sulphamethoxazole) collectively accounted for only 0.09% of use across both years.

Important outcomes

Only about 5% of the drugs used by the systems in the study were in the critically important class, predominantly comprised of macrolide antimicrobials, Davies said. Of those, drugs that are viewed of greatest concern to human health (cephalosporin and fluoroquinolones) are used in a minor role in the pork industry.

“It’s important to note that most uses have been in young animals, many months before they go to market,” he said. “That is consistent with all the reports of antibiotic use we have. We don’t know what that means in terms of use and resistance, but the confirmation that most of the use is happening several months before marketing  is important to recognize.”

Different antimicrobials have different potencies (i.e., relationships between weight and dose required), and therefore combining weights of multiple and different active ingredients is uninformative, Davies said. For this reason, the quantitative analysis was reported for each individual antimicrobial and not in aggregate.

Meaningful interpretation of antimicrobial-use data demands, at a minimum, detailed comprehension of the metrics used to quantify antimicrobial amounts, estimate population denominators and define the period for collection, Davies pointed out.

“When talking about percentages of antimicrobials, they are ‘slices of a pie that are slices of a pie,’” Davies said.

“In the quest to minimize the human-health impact of antimicrobial use in food animals, yet meet the needs for animal health and welfare, the definition of success must lie in some measurable impact on human health,” the study said. However, “while reducing antimicrobial use per se (by any metric) in animal production is an intervention that has a logical foundation, it has yet to be clearly linked to any demonstrable benefit for human health.”

The most effective way to improve antibiotic use in the future is for producers to review their practices and make sure they’re using antimicrobials to obtain benefits to animal well-being, Davies explained.  Ultimately, veterinarians make health decisions for the herds for which they’re responsible. It’s important for them to carefully consider their use of antimicrobials, he added.