Exploring Enteric Disease Trends

Transmissible gastroenteritis (TGE) incidence is seasonal, with the winter of 2010-11 proving to be similar to past years (Figure 1). TGE is a cold-hardy virus that penetrates compromised biosecurity efforts during cold weather, which probably explains a very consistent seasonal occurrence dating back at least 50 years.

Ileitis (porcine proliferative enteritis caused by Lawsonia intracellularis) cases this year increased compared to previous years (Figure 2). A rise in the number of cases in the fall has been typical for the past eight years (Figure 3).

Salmonella isolation from enteric disease cases at the Iowa State University Veterinary Diagnostic Laboratory (ISU VDL) is common (Figure 4). Although fewer cases were reported as positive for salmonella in 2009 and 2010, this is an artifact of case reporting by the ISU VDL. Salmonella remains very common in pigs as well as in virtually all vertebrates and their droppings.

In addition, salmonella is hardy in the environment. Consequently, the source of salmonella infections to pigs are innumerable, including: other pigs; contaminated environment; rodents, birds and other farm varmints; contaminated feed; or tracked in by vehicles, machines or people. Taken together, these factors make elimination unfeasible.

Some serotypes of salmonella are more virulent for pigs than others. For example, Salmonella typhimurium or Salmonella cholerasuis and perhaps Salmonella heidelberg can cause disease when infecting healthy pigs. However, many of the other serotypes simply take advantage of pigs with compromised immune resistance due to other diseases, poor nutrition or stressful environments. Indeed, many of the salmonella isolates since 2000 are from pigs with other primary disease processes or from pigs that are “failing to thrive.”

The disease caused by salmonella is called salmonellosis, meaning the presence of the agent is causing detriment to the host. It is important to differentiate whether the salmonella detected is the single cause of disease (salmonellosis), whether it is significantly contributing to a disease complex (something else and salmonellosis), or whether it is present but not important (salmonella infection/pigs are simply carrying salmonella). When salmonella isolation is reported from a laboratory, determining whether it is a primary pathogen should be confirmed by demonstrating lesions (gross and/or microscopic) and by ruling out the presence of other primary pathogens, such as ileitis, TGE, rotavirus, Brachyspira or porcine reproductive and respiratory syndrome.

All Salmonella serotypes are viewed as potentially pathogenic to humans. For this reason, considerable effort and research are placed on minimizing the contamination of pork and pork products by this organism. Most of the types of salmonella detected in pork products do not cause disease in pigs, but efforts will continue to find ways to decrease the rate and magnitude of salmonella carriage in market swine.

Age segregation, all in/all-out production, disinfection, pest control, and continuing hygiene are important in maintaining health and breaking disease cycles. Antimicrobials are considered a crutch that will eventually fail. Vaccination with modified-live, avirulent salmonella is useful in challenging situations by stimulating immunity in vaccinated pigs, and perhaps may help to decrease overall buildup of virulent salmonella in the production system.

Brachyspira are common bacteria in the large intestine of pigs; not all cause disease in pigs. Figure 5 shows increasing numbers of Brachyspira isolation attempts from pigs as veterinarians try to determine the causes of diarrhea and poor grow-finish performance. One problem, however, is determining if a particular isolate is pathogenic – that is, if the Brachyspira detected is having a detrimental effect. Polymerase chain reaction (PCR) testing does confirm that some of the Brachyspira isolated do have characteristics of pathogens, such as Brachyspira hyodysenteriae (the cause of swine dysentery) and Brachyspira pilosicoli (one cause of spirochaetal colitis). However, many of the organisms recently isolated from pigs with large intestinal lesions have not been confirmed as either one of these pathogens, yet are still associated with colitis. Currently, researchers are re-focusing on this organism after 20 years of assuming it was no longer relevant. Stay tuned.

All data in this article are compiled from cases submitted to the Iowa State University Veterinary Diagnostic Laboratory (VDL).

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Kent Schwartz, DVM
Iowa State University Veterinary Diagnostic Laboratory