Dr Morgan Morrow Swine Veterinary Specialist

The resolution specifically does not limit antibiotics during the nursery stage (50-pound pigs or lighter). It also allows antibiotics to be used to treat diagnosed disease. Sick pigs should always be treated with the best medicine available; to do otherwise (withhold treatment) would be a welfare issue.

However, given that about 50 percent of the antibiotics produced in the U.S. are used in food animals, decreasing the amount of sub-therapeutic use should be an achievable goal.

Information presented at the 2003 meeting of the American Association of Swine Veterinarians outlines some easy steps that producers can take to reduce antibiotic resistance on hog farms.

According to Dr. Alan Mathew, Department of Animal Science, University of Tennessee, Knoxville, "A number of strategies might be effective in reducing the prevalence of antibiotic-resistant bacteria without exclusion of antibiotics from swine-production systems. Our results indicate that the prevalence of E. coli (organisms) resistant to antibiotics is significantly increased upon exposure to various stressors, and these organisms may persist following withdrawal of antibiotics from feed." He also said, "It is not clear from our study how stressors applied to animals might affect resistance patterns of the fecal microflora."

In one 149-day study, Mathew's group exposed pigs to a variety of stressors and measured the resistance of E. coli fecal isolates to apramycin, which had been fed at 150g/ton for 14 days. Whereas control pigs (not fed apramycin) maintained the lowest resistance for the duration of the study, pigs exposed to cold stress (a temperature reduction of 6o C) had significantly higher minimum inhibitory concentrations (MIC) up to 64 days after the introduction of apramycin. Heat stress (a temperature increase of 6o C) had a similar effect at 14 days but not thereafter. This difference in response is good news for North Carolina producers, as it is harder to cool pigs in summer than it is to keep them warm in winter.

The next most adverse effect was from overcrowding. A 40 percent reduction in floor space resulted in a significantly higher MIC up to 64 days after the introduction of apramycin. This response is similar to the adverse effect of cold stress, as detailed above. Feeding Oxytetracycline at 100g/ton (plus the apramycin) for 14 days also resulted in a significantly higher MIC up to 64 days, indicating the problem of feeding low levels of antibiotics.

In another trial, the investigators allowed manure to accumulate on the floors of the pens, and this resulted in a significantly higher MIC at 14 days but not thereafter. I doubt that this effect was caused by the continual access to the manure and the resistant organisms therein, but may be serving as a surrogate for the increased exposure to noxious gases (ammonia, etc.) that are associated with dirty pens.

From these results, it is clear that apart from the improved production gained from minimizing temperature fluctuations and avoiding overcrowding, there are real opportunities for decreasing antibiotic resistance through some simple management steps.

In other studies, Mathew's group demonstrated that rotation of similar antibiotics (apramycin for 5 days, then gentamycin for 5 days, then neomycin for 4 days) produced the greatest resistance to all 3 antibiotics. By contrast, pulse feeding (cycling apramycin for 3 days, then 3 days without, for 14 days) produced the least resistance and at a level similar to the control group. This provides another opportunity for smaller pork producers to cut costs and antibiotic resistance problems on the farm.

In larger operations served by big in-house feed mills, this option would be harder to implement because of the difficulty of managing a variety of farm-specific rations. One option would be to "pulse" the whole system, regardless of the pigs' stages of growth in the finishing barns.

In a final study, Mathew's group was able to show that short-term exposure of sows to feed-based, sub-therapeutic antibiotics increases the prevalence of antibiotic-resistant bacteria in their piglets. Thus, by reducing the amount of antibiotics fed to sows (a doubtful practice in my mind anyway), the producer may be able to further limit overall resistance in pigs destined to be pork.

In summary, amid the growing trend by consumers to worry about how their food is produced, the sub-therapeutic use of antibiotics in animal production is bound to decrease as farmers respond to demands. An outright ban on any use would be inhumane to animals needing antibiotic treatment. Managing stocking density, controlling temperature, and pulsing antibiotics offer real opportunities to reduce these materials' use. More research is needed to understand completely how bacteria acquire resistance in the field and how to manage it on the farm, but Mathew and his group have made notable progress.
Source: North Carolina State University Swine Extension - February 2004