From the Editor

Zoonotic diseases - what is the "best" measure of risk?

Cate Dewey

I know you will be interested to read "Prevalence and serovars of Salmonella enterica isolated from ileocolic lymph nodes of market pigs reared in selected midwest US swine herds" by Bahnson et al.1 The authors cultured Salmonella from ileocolic lymph nodes taken from pigs at the abattoir. Tissues from five pigs were pooled prior to culture. From these results, the authors estimated the individual pig-level prevalence. In addition, the authors compared the results from frozen and fresh samples. I will leave you to read the authors' results.

I have been involved with measuring the prevalence of zoonotic disease agents in pig research over the past few years. Prevalence of zoonotic diseases can be measured in many ways. Specifically, if we look at Salmonella, we can determine the presence of antibodies in pigs' sera or we can culture lymph nodes, fresh manure from individual pigs, composite manure samples from the pen floor, frozen manure samples, swabs of the carcass hanging in the abattoir, or swabs of pork chops in the grocery store. How do we compare the results from studies sampling different things, using different culture methods and different sample handling techniques?

Not only are there multiple ways to determine positive versus negative status in pigs, there are multiple levels at which the measurements can occur. Do we need to know if individual pigs are positive, pens are positive, herds are positive, or groups of pigs at an abattoir are positive? If a pen fecal sample is positive, are the pigs in the pen positive? Certainly it is more expensive to culture individual animals than the pen. Finally, if the culture or serological samples are positive, does that indicate that the pig is carrying a Salmonella organism of potential risk to humans? It may be a serovar common in pigs and rare or nonexistent in cases of human illness.

If you purchase a pork chop, do you worry about getting Salmonella, Yersinia, or Campylobacter infections from handling the meat? Do you worry about becoming infected with a zoonotic disease as a direct result of consuming the pork chop? I don't. I believe that my pork is extremely safe. Having said that, I also take kitchen hygiene seriously. I do know that food carries bacteria - using one set of cutting boards and knives for the meat and another for the salad is important to me.

If the key question is whether or not a farm is Salmonella-positive, then we need to do everything in our power to find the organism. This might be best accomplished by a combination of culture and serological testing. We might also want to culture fresh fecal samples from individual pigs and also composite fecal samples from the pen floor. We need to include some very sophisticated laboratory techniques to be sure that we identify each and every positive sample. This type of study is typically used to determine the association between management factors and the prevalence of Salmonella on the farm. Further, it can be used to determine the impact of an intervention strategy. For example, does prevalence decrease after you switch from pelleted feed to liquid feed?

We may also want to know pig and herd prevalence for pork export purposes. Certainly, we want to understand these quantities when we are establishing farm-to-fork disease control programs. To do this, we must also monitor the Salmonella carried and shed by pigs at the abattoir. At the abattoir, we are most interested in the Salmonella serovars that typically cause disease in humans rather than those that occur in pigs but are not found in cases of human disease.

If we are interested in the potential risk to human health, perhaps we are most interested in the final product that goes to the consumer. What is the prevalence of Salmonella-positive pork chops in your local grocery store? If we can culture Salmonella from this meat, are they the serovars of Salmonella most likely to cause human health problems?

I believe on-going monitoring of pigs, farms, abattoirs, and grocery stores for potential zoonotic disease agents is extremely important. If we do not measure the prevalence of these organisms, we will not reduce it. We can only change something that we are measuring. We learned that fact years ago when we began keeping records of pig productivity. Perhaps the key to zoonotic disease research is what we do with the results and how we interpret the findings. Similarly, we need to be cognizant of how others are interpreting our results.


1. Bahnson PB, Damman DJ, Isaacson RE, Miller GY, Weigel RM, Troutt HF. Prevalence and serovars of Salmonella enterica isolated from ileocolic lymph nodes of market pigs reared in selected midwest US swine herds. J Swine Health Prod. 2006;14:182-188.

--Cate Dewey