Rodents on farms pose health risk to livestock, people

We all know intuitively that rats and mice on farms are a bad thing. With their continually erupting teeth, these rodents can chew and nibble their way through important infrastructure. They can destroy walls, pipes and equipment. Barn fires have often been attributed to damaged electrical wires, and rodent arsons are usually prime suspects.

Rodents can also consume and waste large quantities of feed, particularly grain and pelleted ration formulations.

Perhaps a less obvious negative consequence of farm rodents is disease.

Mice and rats are capable of carrying a wide variety of viruses, bacteria and parasitic worms that can be spread to other animals and people.

As well, rodents carry and spread these disease-causing agents with little or no effect on their own health. In many ways, they have evolved to be little, scurrying pathogen factories.

Despite this risk, relatively few studies have investigated rodents for their ability to carry and spread pathogens on farms.

While working on my PhD at the Ontario Veterinary College, I had the opportunity to investigate this problem as a side project. Together with technician Joyce Rousseau, Dr. Scott Weese and Dr. Claire Jardine, I collected swab samples from 22 rats from pig farms in Ontario. We then tested these swabs for two important bacteria.

The first was methicillin-resistant Staphylococcus aureus (MRSA), one of the so-called super bugs. In animals and people, infections with MRSA can lead to serious soft tissue infections that are difficult to treat because of resistance to antibiotics.

This bacterium can also be carried without causing clinical illness but still be spread between individuals. There is evidence to support the theory that certain MRSA strains favour different species. Horses and pigs in particular have their own strains that seem well-adapted to these species, yet can jump into people if given the opportunity. In our study, we found that one of the rats was carrying the pig-type MRSA.

The second bacteria we studied was Clostridium difficile, which is also known as C. diff. These bacteria are present in the feces of many animal species.

It can cause severe diarrhea in young pigs, and in people is a major issue in hospital patients who have received antibiotics. While it is less clear if the animal-type C. diff can jump species into humans, the possibility has not been ruled out. We found that one farm rat was carrying the pig-type C. diff in our study.

This study, which was published in the January issue of the Canadian Journal of Veterinary Research, affirms that rodents on farms can be problematic from a disease control point of view. We don’t exactly know how rats are picking up these pig-type pathogens. It could be that rats directly contact infected pigs or that they pick up the bacteria from contaminated environments.

This and other previous research has shown that rodents can carry livestock pathogens, which is problematic for infectious disease management.

Even aggressive control strategies such as all-in-all-out production systems that are often used for hog and poultry may be rendered ineffective if rodents remain infected between groups and transmit pathogens into the newly introduced animals.

Farmers may also be at risk of contracting these pathogens if they come into contact with infected rodents or their urine or feces.

More studies are necessary to better understand how mice and rats on farms pick up and spread pathogens. However, the research does support the need for good on-farm rodent control practices as an important way to reduce the risk. One of the most important aspects of this is limited access to feed and keeping them out of buildings whenever possible.

Dr. Jamie Rothenburger, DVM, MVetSc, PhD, DACVP, is a veterinarian who practices pathology and is an assistant professor at the University of Calgary’s Faculty of Veterinary Medicine. Twitter: @JRothenburger

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