Ready to be swallowed up. Salmonella organisms (in green) are engulfed by larger protozoan organisms inside the cow’s rumen.

In like Woody Allen, out like Arnold Swartzenegger.”

That analogy made by a USDA research scientist pretty well sums up the situation. Salmonella bacteria actually become stronger and tougher once they are ingested by protozoa inside a cow’s rumen. And, when the protozoa are broken apart later in the digestive process, they release the salmonella back into the cow’s system -— more virulent than ever.

If the cow receives a large enough infective dose, it can result in a disease condition known as salmonellosis.

Occasional defaunation — removal of protozoa from the rumen — may be a possible solution to this problem.

Get rid of freeloaders
Does removal of the protozoa from the rumen cause any harm? 

Some rumen protozoa have the capability to digest cellulose, says Mark Rasmussen, research leader in pre-harvest food safety and enteric diseases at the National Animal Disease Center in Ames, Iowa. But if you take those protozoa away, other organisms like anaerobic fungi and cellulotyic bacteria fill in and do the job. 


They continue to thrive. Salmonella organisms continue to live inside the protozoan, getting tougher and more virulent.

A protozoan can be a freeloader, Rasmussen says. “It takes from the system, but doesn’t necessarily contribute back.” It certainly ingests its share of bacteria. One research study in the 1970s showed that a single protozoan can engulf up to 21,000 bacterial organisms per hour.

Marshall Stern, ruminant nutritionist at the University of Minnesota, says the exact role of protozoa in the rumen is not well-defined.

“The animal can live without protozoa,” he says. “The rumen still functions.”

However, there is a tendency for volatile fatty acids to decrease with defaunation, Stern adds, along with a decrease in acetate and butyrate concentrations in the rumen. Protein degradation and cellulose digestion may decrease. Lactic acid production, meanwhile, may increase.

“These are general observations,” he adds.

It’s fairly obvious that the protozoa can strike up a symbiotic relationship with bacteria in the rumen. 

But one of these relationships -— the ability of salmonella to live inside the protozoa — may be a negative. By interacting with other organisms inside the protozoa — through gene-swapping with various other mechanisms — salmonella can become more virulent and antibiotic-resistant over time.


But it all comes to an end. The protozoan breaks apart in the abomasum, releasing the salmonella back into the cow’s system.

Rasmussen likens the toughening process that salmonella go through to a “boot camp for terrorists.”   

Get rid of sanctuary
With defaunation, the goal is to keep the protozoa suppressed so they cannot serve as a sanctuary for antibiotic-resistant salmonella. It’s a temporary measure of sorts, since protozoa can be re-introduced through various means, such as animal-to-animal contact or the cow ingesting feed that another animal has chewed on. 

“We look at this defaunation process as kind of a deworming — you get temporary accomplishment,” Rasmussen says.

So far, scientists have used different compounds to kill protozoa inside the rumen. A chemical known as DSS accomplishes this task by obliterating protozoa, but DSS also is toxic to animals. Therefore, its role is somewhat limited. Other possibilities include yucca-extract supplements that appear to be non-toxic to animals.   

Steve Carlson, veterinary medical officer at NADC, says cilantro oil (from the coriander plant) may offer a dual benefit. Not only does it kill protozoa, it has potent anti-salmonella properties as well.  


A single protozoan can ingest up to 21,000 bacterial organisms an hour, according to a research report. The bacterial organisms often bunch up and appear as a flock near the protozoan’s mouth.

Still a theory
All of this remains a theory. If you can get rid of a sanctuary for pathogenic salmonella -— and possibly other bacterial pathogens as well — you might gain some health benefits. But those benefits have not been quantified yet by researchers.

“I wouldn’t call it a done deal at this point,” says Rasmussen. But he and others at the National Animal Disease Center agree that it is a concept worth pursuing.