Respiratory disease in growing pigs frequently involves co-infections with more than one virus or bacteria working together. Synergistically with environmental stressors, these disease agents do more harm than they would independently. This phenomena is so prevalent, the term porcine respiratory disease complex (PRDC) was coined to describe it.

How much damage is done when co-infections occur? In a 2012 study, the productivity and economic impact of Mycoplasma hyopneumoniae (M hyo), porcine reproductive and respiratory syndrome virus (PRRSV), and influenza A virus in swine (IAV-S, formerly swine influenza virus [SIV]), was estimated using information from a single large production system (Dykhuis, Haden C, 2012). Diagnostic reports were used to determine which of these pathogens, or combinations of pathogens, were diagnosed for each group of growing pigs. Information from closeout summaries for the same groups of pigs was then used to estimate productivity and economic impacts.

Relative to a high-health baseline, the impact of M hyo, PRRSV and IAV-S alone was $0.63, $5.57 and $3.23 per pig placed. However, when more than one of these pathogens was diagnosed together for a group of pigs, the economic impact was much larger: $9.69 for PRRSV and M hyo; $10.41 for PRRSV and IAV-S; and $10.12 for IAV-S and M hyo.

Higher Control Costs
The severity of disease caused by co-infections also frequently results in higher medication and diagnostic costs. Uncomplicated infections of single pathogens are often manageable with individual pig treatments. However, the more severe disease caused by co-infections frequently requires mass medication of groups in the water or feed raising greater concerns about development of antimicrobial resistance.

Another impact of severe disease caused by co-infections that is frequently underappreciated is variation. The increased variation of individual pig weights at marketing can lead to either an extended barn tie-up to allow pigs more time to grow, marketing more pigs at suboptimal weights or both. Of particular concern with groups of pigs that experience severe disease due to co-infections is the increased number of pigs that end up in the bottom tail of the weight distribution at marketing and are sold with substantial sort loss discounts.
All groups of pigs will have some pigs sold at suboptimal weights. However, because both average daily gain is lower and variation in gain is substantially higher in groups severely affected by disease, these groups frequently have enough pigs sold at suboptimal weights to greatly reduce the overall average price received and profit generated for a group of pigs.  

Savings Through Management
The good news is that large improvements in health and productivity of growing pigs can be made by managing the key viral or bacterial agents involved. The problems often arise further up the chain, for example, in the breeding herd or with acclimation of gilts. Diagnostics are important for figuring out how and where to manage immunity and exposure to pathogens through vaccination, biosecurity, and other management practices.

For farms, flows or systems burdened with co-infections, and the severe disease and productivity losses that follow, the return on investment made in diagnostics, vaccinations, biosecurity and other management practices to break the problems with co-infections in growing pigs is typically very large.

Editor’s Note: Dr. Derald Holtkamp, MS, DVM, is an Associate Professor in the Iowa State University College of Veterinary Medicine and he provided this article. To hear more of his comments about co-infections, watch this video on