Porcine Respiratory Disease Complex (PRDC), of which swine influenza virus (SIV) is an integral part, has been very costly to U.S. hog producers, especially this past year.

When SIV combines with porcine reproductive and respiratory syndrome and Mycoplasmal pneumonia, the disease complex can create a synergistic effect that can cause economic devastation.

These organisms suppress the immune system, but in different ways. Secondary bacterial pathogens invade, which usually cause the death losses.

Although there is still a flu season, SIV outbreaks can occur any time, especially when stressed.

Case Study No. 1

The first case study involves a multiple-site production system, with a 2,400-sow herd and 2-year-old facilities. The buildings are top of the line with tunnel-ventilated gestation and farrowing rooms with woven wire flooring over a pull-plug pit. The pigs are weaned at 16-18 days of age and delivered to an offsite nursery about five miles away. The pigs are then moved to 1,000-head finishers, 50-100 miles from the nursery.

One of the finishers broke with a barking cough. Several pigs had fevers; many pigs went off feed. The outside temperature had dropped from 50° F. to the upper 20s overnight, just prior to the outbreak. The finisher is a typical 1,000-head unit with double curtains and total slats over an 8-ft. pit. The pig flow is all-in, all-out. The management is excellent and biosecurity is very strict.

During this outbreak, overall feed consumption dropped dramatically and the pigs seemed very lethargic. Water medication was used to control fever and feed grade antibiotics were used to control any secondary bacterial infections. A viral infection was suspected.

Diagnostic tests revealed both H1N1 and H3N2 SIV strains as the primary culprits. A medication regimen continued for 5-7 days and the flu cycle subsided with minimal death loss.

A strategic vaccination protocol was implemented after all sites were evaluated.

The outbreak was linked to aerosol spread from an adjacent finisher one mile away. The sow herd and nursery tested negative for SIV.

Due to the closeness of the continuous-flow finisher, the vaccination protocol continues at this site.

Reproductive problems can also be encountered with herds infected with SIV, especially with the H3N2 strain. Clinical signs include abortions, resorbed fetuses, erratic returns to estrus, farrowing rates lowered by as much as 10% and overall losses in production. A lot of these clinical signs are due to high fevers in the sows during the acute outbreak.

Case Study No. 2

I was called to a 1,200-sow unit reporting sows off feed and abortions. Many of the sows were very lethargic and exhibited very little interest in getting up and eating. They were feverish but did get up to drink water.

Aborted fetuses revealed very little from a diagnostic standpoint. A serological workup was done in addition to nasal swabs on the acutely ill pigs. Both revealed SIV strain H3N2 as the primary pathogen. Appropriate water and feed medications were administered. The sow herd recovered and improvements were also seen in the health of incoming gilts and boars in the isolation/acclimation unit.

This outbreak affected sows in all stages of gestation, eliciting varied sow immune responses and production performance based on whether they were naturally infected or vaccinated.

The end result was a nightmare in the breeding barn, with oddball recycles, nonbreeders and other problems with breeding females.

It turned out the breeding barn became overcrowded, adding to frustration and the ability to do a good job with constant heat detection and breeding.

This is just one of many cases illustrating the complications of trying to handle SIV infections.


We all realize there is severe immune pressure especially in high-lean genetics. Also, there are antigenic variations between strains of SIV.

The heat stress period last summer (July-September) literally “burned” out some of our sows. We still have problems getting sows to breed and conceive.

SIV infections have plagued our swine industry and there appear to be mutant strains on the horizon. Diagnostics that identify these strains continue to be developed with diligence. We as veterinarians continue to study the research and advise on development of new diagnostics and vaccines to help control this agonizing virus.

It's important for your veterinarian to develop a database of diagnostics for PRDC. He/she should monitor this information at least quarterly.

Development of PRDC control programs, not only for viral agents, but for secondary bacterial agents as well, is equally important. Strict biosecurity measures are also essential.