Antibiotic-Resistant Bacterial Transmission from Animal to Human Observed

Antibiotic-resistant bacterial infections pose a significant challenge to public health. In a study published recently in EMBO Molecular Medicine, researchers using whole genome sequencing observed that antibiotic-resistant bacteria were transmitted from animals to humans in two disease outbreaks on different farms in Denmark. However, the method of transmission was still unclear, according to the researchers. The confirmation of animal-to-human transmission of methilcillin-resistant Staphylococcus aureus (MRSA), a disease-causing bacterium, adds support to the arguments for discontinuing the use of antibiotics in food-producing animals, especially species intended for human consumption.

In response to the study, Representative Louise Slaughter of New York, the only microbiologist in Congress, sent a letter to Dr. Margaret Hamburg, the Commissioner of the U.S. Food and Drug Administration, calling for immediate action to reduce the use of antibiotics in healthy livestock. Slaughter noted that since 1977, the FDA has failed to take action against the increased usage of antibiotics in animals. She stated that the study’s findings confirm that the FDA’s voluntary guidance of “judicious use” of antibiotics in animals was “doomed to fail.”

Rep. Slaughter is also the author of HR 1150, the “Preservation of Antibiotics for Medical Treatment Act” (PAMTA),  introduced March 4, 2013.  The legislation is designed to stop the overuse of antibiotics on the farm.

Data Supports Transmission from Different Strains

The research counters food industry arguments that administering antibiotics to livestock is not a risk to humans. Based on the study, livestock intended for human consumption could be a reservoir of antibiotic-resistant bacteria. The antibiotic-resistance observed in MRSA is attributable to a gene known as mecC, which blocks the function of the penicillin-like antibiotic methicillin. MRSA can lead to debilitating skin and soft tissue infections, bacteremia, pneumonia and endocarditis. It is estimated that 80 percent of all antibiotics sold in the United States are sold for agricultural use; generally these antibiotics are distributed at sub-therapeutic dosage to healthy animals to address crowded and unsanitary living conditions or to promote growth in the livestock.

In the first patient case, a 53-year-old female tested positive for mecC-MRSA; this individual lived on a farm with two cows, two horses and a dog. In the second patient case, a 69-year-old female with a wound infected with mecC-MRSA lived on a farm with a flock of ten sheep. The two farms were located 45 km (28 miles) apart.

Initially, researchers suspected that the same pathogen was the source of the two outbreaks because of the proximity of the two farms. However, by analyzing the single differences in the pathogen’s DNA sequences, the researchers concluded that two different strains were responsible for the disease outbreaks and that the direction of transmission was from animal to human in both cases.