Antibiotic resistance is a major public health threat that has been lurking for years. And things are not getting better as researchers have discovered that more and more European pigs are infected with a highly antibiotic-resistant strain of the superbug MRSA. This bacterium can transfer to humans.

From a recent study, which appeared in scientific magazine eLife, this MRSA strain, called CC398, has been shown to maintain resistance to most antibiotics for decades and has survived on pig farms for 50 years. It is now clear that CC398 can easily jump to humans and can cause life-threatening complications, especially in people with a weak constitution. Years of preventive administration of antibiotics is the cause of the resistance.

What is the MRSA bacteria?

The methicillin-resistant Staphylococcus aureus (MRSA) was first found in humans in 1960. The bacteria cannot be stopped with common antibiotics (containing methicillin), which can make bacterial infections dangerous. MRSA is also popularly referred to as the ‘hospital bacterium’ because there are many outbreaks in hospitals. The World Health Organization (WHO) considers the bacteria to be one of the greatest threats to public health. MRSA is much more common in other European countries and the rest of the world than in the Netherlands, RIVM writes. A research team from the University of Cambridge investigated a specific MRSA strain in pigs reared in intensive livestock farming. This CC398 mutation is the source of more and more dangerous infections in humans.

Shocking Danish figures
The infection rate is particularly alarming in Denmark. No less than 90 percent of the animals had the bacteria among their members in 2018. Ten years earlier this was only the case for 5 percent. The degree of antibiotic resistance of the bacteria has remained stable over all these years.

The MRSA bacterium is harmless to pigs, but can transfer to humans with all the associated consequences. Patients have sometimes come into direct contact with the pigs, but there are also cases where this has not been the case.

To blame for preventive antibiotics
“For decades, large amounts of antibiotics have been administered to the animals. This has led to the evolution of the superbug MRSA,” said Gemma Murray, study leader and expert on livestock and medicine. “The resistance of this MRSA strain to antibiotics has remained high for many years. Even after switching to another species, the bacterium retains these properties,” explains Murray.

Display of MRSA bacteria. Photo: Royaltystockphoto

In the past, much more antibiotics were used in factory farming. But stricter regulations seem to come too late for this particular superbug and its dangers to humans. CC398 has been found in several types of livestock, but is especially a problem in pigs. “It is extremely important to understand the rise and success of CC398. How are people infected? With this knowledge we can limit the risks to public health,” says co-author and professor of veterinary medicine Lucy Weinert.

Disturbing numbers
The success of the MRSA strain can be traced back to three genetic elements in its DNA. This makes the bacterium resistant to antibiotics, it easily jumps to other animal species and successfully evades the human immune system. “The number of victims of this specific MRSA strain is still relatively small, compared to the total number of MRSA patients. But we see that the percentage is increasing. These are disturbing numbers,” Weinert said.

Zinc

Due to intensive livestock farming and the preventive administration of antibiotics, the pigs have become a hotbed of bacteria that are dangerous for humans. For example, zinc oxide has been added to the food of suckling pigs for years to prevent diarrhoea. This drug will be banned in the EU from next month due to its impact on the environment and possible promotion of antibiotic resistance. But according to the researchers, this does not help to reduce the prevalence of MRSA strain CC398, because the genes that contribute to antibiotics are not always linked to the genes that contribute to resistance to the zinc treatment.