Cows catch E. Coli

Twenty years ago, there was evidence that only humans carried a strain of antibiotic-resistant E. coli. Today, an increasing number of dairy cows also carry a similar strain.

Margaret Davis, a veterinarian and molecular epidemiologist at Washington State University, explored the emergence of new antibiotic resistant E. coli genes called blaCTX-M.

After a retrospective study that compared fecal matter from dairy cows across Washington State, Davis found the gene appeared in an increasing volume of specimens after 2008. Prior to 2008, it had been found in humans and not food-producing animals in the U.S.

“This discovery is exciting because it is a new phenomenon and it was in Washington, which suggests a whole ecological change,” Davis said.

A majority of E. coli strains are harmless to humans and animals, but some strains cause bloody diarrhea and can lead to kidney failure and death, Davis said. E. coli can be treated with antibiotics, but when the bacteria develop resistance to antibiotics, scientists must research new treatments.

Davis’ study tested fecal matter samples from two previous studies conducted in 2008 and 2011, as well as samples the researchers collected in summer and fall of 2012. Researchers found that, prior to 2008, the antibiotic-resistant gene was not present in any fecal samples, but the gene emerged in the 2011 samples, and even more genes were in the 2012 samples.

Lisa Jones, a scientific assistant and laboratory manager, said finding the gene all over Washington dairies has been “one of the most fun things we have ever discovered.”

Davis speculated that the new resistance gene might have been passed from humans to animals, a phenomenon that is hard for some to accept. Generally, humans blame animals for passing diseases to the human population, like in the mad cow disease outbreak in 1992, but in this case humans may have carried this antibiotic-resistant gene long before cows, Davis said.

Because dairy calves are valuable to a farmer’s livelihood, they are treated heavily with antibiotics from a young age. Calves are highly susceptible to diarrhea and respiratory disease, Davis said, which means they receive doses of antibiotics when they are a few weeks old.

To the surprise of the researchers, the study did not find a relationship between antibiotic treatment and the resistance gene, but Davis speculates the relationship could still be there. The variety of methods for the different studies included in the retrospective study, as well as other confounding variables, might account for the lack of correlation.

The next step for Davis and her team is to look into how the gene was transmitted from humans to cows. Davis predicted wastewater could be the key.

Wastewater is treated with chemicals to kill bacteria before it is dumped into canals and rivers, but some bacteriophages survive the treatment. Bacteriophages are viruses that infect and replicate within bacteria.

Davis and her team will collect samples from rivers and wastewater treatment sites to look for traces of the antibiotic resistant gene within bacteriophages.