Porcine respiratory disease complex (PRDC) is one of the most ambiguous, poorly defined terms to describe causes of respiratory diseases in pigs. With changes in vaccines, discoveries of new microorganisms and significant advancements in diagnostics, avoiding a diagnosis of PRDC should be a goal of veterinarians and producers alike. The term should be utilized only to describe a clinical situation, awaiting proper diagnosis.

With good diagnostics, informed, targeted intervention can then be initiated. This approach should help avoid the temptation to treat pigs for PRDC, and instead determine the precise causes of chronic respiratory disease in grow-finish pigs.

Case Study

An 800-sow, farrow-to-finish herd in central Indiana, positive for porcine reproductive and respiratory syndrome (PRRS), mass-vaccinates the breeding herd with modified-live virus vaccine every 3-4 months. Historical testing has shown pigs are free of the PRRS virus at weaning and remain so until late in the nursery or early finisher stages.

All rooms on the farm are pressure washed and disinfected between groups. PRRS does not cause significant clinical signs when exposure occurs.

Since 2007, various protocols have been used to vaccinate pigs for porcine circovirus type 2 (PCV2) and Mycoplasma pneumonia, but pigs have always been vaccinated.

In the spring of 2009, pigs were given a combination dose of circovirus and mycoplasma vaccine at 1-2 weeks postweaning. By the fall of 2009, an increase in fall-behind pigs was noted. Diarrhea (rarely with evidence of blood) was frequently observed in the early stages of finishing; coughing occurred in the last half of the finisher period. A serological survey showed no evidence of H1N1 swine influenza virus, but there were some suspected cases of H3N2.

There was exposure to mycoplasma with positive serology in 26-week-old pigs, but not in 18-week-old pigs. There was also evidence of exposure to ileitis at 15 weeks of age.

Since pigs were being vaccinated for mycoplasma, we felt most of the health challenges were likely due to the ileitis infection. Some adjustments were made in the feed-grade medication for ileitis control.

This step helped in controlling the clinical diarrhea; however, by the spring of 2010, the cough was more prominent and finishing mortality often exceeded 3%.

About this time, a company performing some mycoplasma studies contacted our clinic in search of herds with active mycoplasma infection. Some preliminary nasal swabs collected from coughing pigs were positive for mycoplasma by polymerase chain reaction (PCR). The herd was selected for further sampling of nasal swabs and bronchial swabs. Pigs were euthanized for collection of bronchial swabs, so that gross lung lesions could also be observed.

In addition to significant gross lesions typical of mycoplasma, nasal (9/30) and bronchial swabs (30/30) were PCR-positive. In consultation with the owner, we decided to booster all nursery pigs that were in inventory with 1cc of mycoplasma vaccine; change protocol to vaccinate for circovirus at weaning, and give 2cc of a different, one-dose mycoplasma vaccine at 2-3 weeks postweaning.

The cough has been reduced in finishers receiving the new mycoplasma vaccination protocol. Closeout mortality rates returned to the 2-3% range, with some below 2%. The number of fall-behind pigs was reduced.

This was a case where we wouldn’t have expected mycoplasma to be a problem since the herd was vaccinated. However, protocols had changed several times due to PCV2 vaccination. The reason for lack of mycoplasma control may have been due to timing, mixing of products or even PRRS.

Even though the diagnostics used in this case were somewhat unusual, new tests will likely help pinpoint the exact causes of PRDC.


Porcine respiratory disease is not going away, and neither will the complex of microorganisms interacting in swine respiratory cases.

However, excellent vaccines for PCV2 have drastically reduced, or perhaps even eliminated, the role of circovirus in PRDC. Mycoplasma-free herds and PRRS-free finisher flows are changing the dynamics of respiratory disease in pigs. Reduced antibiotic use may result in more significant bacterial flareups of respiratory infections.

As this case shows, vaccinations and/or vaccination combinations may need adjustments to optimize control of respiratory disease. Whether you call it PRDC or not, it’s important that each case be addressed individually from a diagnostic and control standpoint.