This bacterial organism causes unpredictable outbreaks, especially in multi-site production systems.

Today, young growing pigs become exposed to disease-causing bacteria, but their immune systems ward off the spread of the organisms in the body. These bacteria are harbored in the tonsil or lungs, without causing clinical disease.

When naive pigs encounter other pigs shedding the organisms, they may become infected and develop clinical disease, depending on bacterial load and various stress factors.

One such bacterial organism causing unpredictable outbreaks is Actinobacillus suis (A. suis). This bacteria is more common in pigs reared in multi-site production systems, where a few carrier animals pose a risk of exposure to the rest of the population. These carrier pigs can shed the A. suis organism from the tonsil, where it resides in these clinically normal animals. Susceptible animals develop a bacterial septicemia causing acute respiratory disease or sudden death.

A. suis is sensitive to many antibiotics, and animals respond well to individual, prompt treatment.

Case Study No. 1

A farm purchasing 2,400 weaner pigs every two weeks was facing persistent challenges with respiratory disease two to eight weeks after delivery. The pigs were negative for porcine reproductive and respiratory syndrome (PRRS). The producer normally placed the pigs on water medication for five days, one week after arrival at the wean-to-finish barns. The pigs developed a mild, chronic cough.

One morning the producer called to report that 35 “good-looking” pigs died in three days.

At the wean-to-finish barn, 15% of the pigs had a mild cough; 2% of the pigs demonstrated labored breathing with a cyanotic (red-purple) appearance of the ears. Eleven more good quality pigs were found dead.

Upon postmortem examination of several pigs, the thoracic and abdominal cavities contained a moderate amount of light yellow fluid. All of the lungs had lesions. Six pigs showing signs of labored breathing were euthanized. Tissues were sent to the lab.

However, due to the severity of this respiratory outbreak, a treatment plan had to be developed immediately.

Lung appearance and the acute nature of the outbreak suggested either Actinobacillus pleuropneumonia (APP) infection or A. suis. The entire population of pigs was injected with an antibiotic and given water medication.

The lab results confirmed A. suis on the lungs, liver and spleen. A test for porcine reproductive and respiratory syndrome (PRRS) virus was negative; however, lung tissue was positive for swine influenza virus (SIV). Streptococcus suis was also isolated.

Based on the isolation of the two bacterial organisms, I revised the feed and water medication programs to address the antibiotic sensitivities of both organisms. The lab confirmed SIV, and the sow unit agreed to vaccinate sows. Also, the producer started treating all incoming pigs with a long-acting injectable antibiotic. Water medication was to be a last resort.

Case Study No. 2

A farrow-to-finish producer, who expanded his sow herd three years ago to 1,200 sows, was having some unexplained deaths at a finishing site. Semi-annual blood testing of the sow unit and nursery were negative for PRRS and SIV. Nursery mortality remained under 2%. The pigs were vaccinated for Mycoplasmal pneumonia at 5 and 7 weeks of age.

A walkthrough of the nursery revealed no signs of clinical disease. In walkthroughs of finishers, a mild cough was heard in a barn with pigs 11 and 12 weeks old, as well as in a barn with pigs 13 and 14 weeks old. Several pigs also had labored breathing, gaunted with cyanotic ears and underlines. Three pigs were euthanized, and postmortem exams demonstrated extremely enlarged lymph nodes between the two lung lobes. The problem rapidly dissipated in the rest of the finishing sites as the pigs got older; fall-behind pigs were observed.

The lab had cultured A. suis from the lung and also found PCV2 (porcine circovirus type 2). The lymph nodes were also stained positive for PCV2.

A. suis was the likely cause of the unexplained finishing deaths; however, the presence of PCV2 posed a challenge to the immune system.

I revised the farm's vaccination program to include PCV2 vaccination of all piglets, and the owner elected to inject all piglets at weaning with a long-acting antibiotic for A. suis.


Actinobacillus suis is not easy to routinely diagnose. Many times the animals are carriers and the infection goes unrecognized until there is a problem. Recently, a PCR (polymerase chain reaction) test was developed by tonsilar swabs. A diagnosis is usually made by tissue analysis. Commercial vaccines are not available. Therefore, control and prevention are customized for each farm. Consult with your veterinarian to confirm and implement a control program for A. suis on your farm.