A recent study conducted by scientists at North Carolina State University has demonstrated the presence of drug resistant Enterococcus spp. in flies and roaches gathered from hog farms in North Carolina and Kansas. This is not unexpected since the use of some antibiotics for either therapeutic purposes but more frequently growth promotion, especially over prolonged periods, will result in the emergence of drug-resistant strains of both Gram-positive and negative bacteria. Scientists and epidemiologists are rightfully concerned over direct inter-species infection and transmissible resistance through exchange of plasmids between similar and related species of bacteria.
The situation in medical facilities
It is known that Enterococcus fecalis strains, which have developed resistance to a number of drug classes, has become a significant problem in medical facilities. Enterococcus is responsible for nosocomial (hospital related) infections, especially in debilitated and immune suppressed patients in intensive care units.
The question arises as to whether there is a direct link between the use of antibiotics in livestock and the situation in medical facilities. Dr. Coby Schal, professor of entomology who conducted the survey stated “the farm is not as isolated as we used to think because the insects break the barrier between the farm and the human community, especially house flies which are highly mobile.” Studies conducted in the 1980s confirmed that flies and other insects can ingest and serve as reservoirs for both drug resistant and susceptible bacteria present in the environment of livestock facilities. The statement implying a direct connection between livestock facilities and hospitals is somewhat ingenuous since it will be unlikely that flies will transmit drug resistant enterococci from a hog farm to a medical facility, based on proximity. It is possible that workers on farms may carry drug resistant organisms in their intestinal tract, which may be indirectly introduced into hospitals. It would have been interesting for the North Carolina researchers to have investigated the drug-resistant status of bacteria isolated from workers on the farms concerned, in addition to the hogs.
Scientific literature on Campylobacter jejuni infection confirms that strains of the bacteria that were resistant to fluoroquinolines derived from chickens and hogs did not survive in the intestinal tract of human volunteers for longer than 7 days after ingestion. The link between intensive production units and hospitals has not been established. It is possible that there is no link whatsoever and that there are separate cycles of introduction, persistence and infection in both livestock housing and in medical facilities. In commenting on the news report Dr. David Weber, an infectious disease specialist at University of North Carolina School of Medicine in Chapel Hill, commented, “Doctors and patients also bear responsibility, and should only use or request antibiotics for confirmed bacterial infections.” This is a reality accepted by medical specialists under the dictum of “physician heal thyself.”
Drug resistant organisms
An important issue relating to the findings on the North Carolina and Kansas farms relates more specifically to the presence of drug resistant organisms which are potential pathogens. If in fact drugs resistance occurs on specific farms then the antibiotics to which the organisms are resistant are obviously ineffective and accordingly their continued use is a waste of money in addition to a potential health hazard. The FDA Prudent Use Principles call for veterinary health professionals to continually monitor for resistance and select therapeutic antibiotics on the basis of efficacy.
There is increasing opposition among consumers to the use of antibiotics for growth promotion. Europe has banned all but a few compounds for other than therapeutic purposes to be administered under strict veterinary supervision. In the U.S. the FDA allows inclusion of specific antibiotics at low dose in feeds for the purpose of enhancing production and suppressing Gram positive pathogens including Clostridium spp. Approval by the FDA of these drugs requires extensive trials to demonstrate efficacy and safety and approval can be revoked if any deleterious effect to human or animal health is shown.
Notwithstanding the scientific evidence supporting continued but restricted use of antibiotic growth promoters, it is inevitable that legislation will become increasing more restrictive. At the present time there are a number of non-antibiotic growth promoters including separate and combination probiotics and prebiotics which have demonstrated equivalent efficacy to antibiotics in flocks and herds subject to acceptable nutrition, vaccination, management and biosecurity. It is also evident that any minor loss in performance by abandoning performance enhancers which may increase cost can be compensated by a retail premium. A number of broiler companies are producing premium specialty products certified as raised without the use of antibiotics and fed diets composed of ingredients of vegetable-origin.
It would be in the interests of the livestock industries to actively investigate and adopt alternatives to antibiotics as growth promoters since legislators responding to consumer activists will increasingly place restrictions on the use of existing licensed compounds. From a public relations standpoint it would be more beneficial for the industry to transition from antibiotics as a voluntary initiative than for changes to be imposed by legislation after acrimonious scientific and public debates.