Antimicrobial resistance of Salmonella: How much do we know?

Gaps in knowledge need to be filled on the prevalence of antibiotic-resistant Salmonella in poultry.

The White House document on “National Strategy for Combating Antibiotic-Resistant Bacteria” was recently released along with several steps critical for controlling antibiotic resistance. The document also suggested that antibiotic resistance is increasing at an alarming rate and efforts to counter this cannot keep pace. However, the relevant question is whether antibiotic resistance is new or something that has always been there but that we now have better methods and our attention is focused on identification of antibiotic-resistant pathogens in animals and humans.

The topic of antibiotic resistance, and, more specifically, its relation to animal agriculture, is making headlines and generating a need to provide the right information and educate people to increase awareness. The lack of programs that monitor antibiotic use in human health and in agriculture, coupled with no organized surveillance of antibiotic resistance challenges, have created large gaps in the understanding of antibiotic resistance and its prevalence among common foodborne pathogens.

Scope of the antibiotic resistance problem

The Centers for Disease Control and Prevention (CDC) estimate that each year more than 2 million people in the United States are affected by antibiotic-resistant infections, out of which approximately 23,000 of these infections result in death. These often multi-drug resistant infections result in approximately $35 billion in lost productivity and an additional $20 billion cost to the U.S. health-care system as a result of prolonged or costlier treatment and extended hospital stays (CDC, 2013). Salmonella is the most common bacterial foodborne pathogen in the U.S., causing over 1 million infections in the U.S. each year. Additionally, the CDC reports that infections with Salmonella are responsible for the highest number of both hospitalizations and deaths compared to other foodborne illnesses.

While many foods have been linked to salmonellosis, the bacterium is readily isolated from both poultry and poultry products, and outbreak investigations continue to identify poultry consumption as a high risk for the illness. As such, to reduce human illnesses due to antibiotic-resistant Salmonella an efficient and cost-effective control program needs to be developed to reduce its horizontal spread at various stages of the poultry production chain and to reduce the contamination of finished products.

Efforts to curb antibiotic resistance

The latest strategies proposed provide a framework to combat antibiotic resistance by preventing the spread of resistant bacteria, focused and strengthened efforts to identify antibiotic resistance and developing new vaccines and antibiotics (FDA; http://1.usa.gov/1ByVwjK). Aarestrup et al. (2008) suggested that over the past two decades, a great deal of energy has focused on curbing emergence of antibiotic resistance with a focus on the impact of antibiotic use in both agriculture and veterinary medicine.

Antibiotics in livestock and poultry production

In livestock and poultry production, drugs are used in two basic manners: prophylaxis (sub-therapeutic use) and treatment of established infections (therapeutic use). However, recently the FDA has initiated guidance for voluntary phase out of using medically important antibiotics for production purposes (FDA Guidance for Industry #213). This guidance is intended to facilitate the voluntary process by providing useful information for sponsors intending to revise their approved labeling through a supplemental new animal drug application. This could possibly be due to the reports and documented studies that have suggested use of antimicrobial agents in animal production and human health environments create cross-protection that favors survival of antibiotic-resistant pathogens. According to the World Health Organization (WHO; 2000) prevalence of these antibiotic-resistant pathogens had been increasing worldwide. Research shows that both uses of antibiotics often lead to higher concentrations of antibiotic-resistant bacteria in the animal, and these bacteria are often transferred to meat products, posing a clear public health risk (Aarestrup et al., 2001).

In U.S. livestock and poultry production, the majority of antibiotics (by volume) are used in swine and poultry production. As salmonellosis is often associated with poultry consumption, there has been considerable attention focused on contaminated poultry products and specifically prevalence of antibiotic-resistant Salmonella and its potential impact on public health (Gyles, 2008 and CDC 1997). Prevalence and antimicrobial resistance of Campylobacter and Salmonella serovars in organic chicken from Maryland retail stores was evaluated, and it was reported that 76 percent of organic and 74 percent of conventional chicken products were contaminated with Campylobacter. Salmonella was recovered from 61 percent of the organic and 44 percent of the conventional chicken, respectively (Cui et al., 2005). They further reported that all Salmonella isolates from conventional products were resistant to at least five antimicrobials, which is a reason for concern when treating foodborne illnesses.

Gaps in knowledge need to be filled

The ease with which pathogens can spread from animals to foods via food is a critical factor that can be attributed to the contraction of foodborne illnesses with antibiotic-resistant pathogens. Gaps in the knowledge of prevalence of antimicrobial resistance of Salmonella specifically in poultry has been limited due to lack of capacity to detect and respond to emerging antibiotic-resistant bacteria. In addition to this, there is a need for developing a systematic approach for the use of antibiotics in agriculture and human health care.

The FDA has specific concerns regarding the use of drugs and the development of antimicrobial resistance in pathogens and humans. To this extent, recommendations on the judicious use (avoiding misuse or overuse) of such antimicrobial drugs are provided by the FDA and specifically limit the use of such drugs to ensure animal health and administration of drugs with veterinary oversight.

Research studies and judicious use

There is hope that with new reports and mandates and press releases about antibiotic resistance of pathogens, studies to determine effective antibiotics for animal health and improved prevalence detection (and tracking pathogens in humans) will gain momentum and provide useful insight.

Current farm management practices -- monitoring water quality, feed, vectors (insects, flies and humans) and air quality of houses -- to avoid lapses in biosecurity measures and prevent foodborne outbreaks could also be useful in preventing antibiotic-resistant pathogens from entering the food supply. Farmers' judicious use of antibiotics, veterinarian oversight and implementation of strict biosecurity measures can be other critical factors in controlling the spread of these antibiotic-resistant pathogens.

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