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Actinobacillus suis: Sudden Killer of Finishing Pigs

Disease most commonly affects 150-pound pigs

My first encounter with Actinobacillus suis in pigs was soon after graduation from college. It presented as a severe brain infection in nursing pigs in several litters on a 1,000-sow farm in northern Indiana. Thinking this just might be an emerging disease, since it was unusual that it appeared in the nervous system, I gave a case report at a swine practitioner conference.

In the 30 years since that time, I’ve never again diagnosed the nervous system form of this bacterial infection!

However, the bacteria have not disappeared from pig populations. Today the clinical sign of A. suis is almost always a killer pneumonia. Producers report sudden death of one or more good pigs, usually when pigs are 150-plus lb. Even though a related bacteria, Actinobacillus pleuropneumonia (APP), has become less common (due to herd elimination and depopulations), A. suis has continued to cause sporadic mortality in today’s swine population.

This bacterium is different than the APP cousin as it spreads through the pig via the blood, resulting in infection throughout the body, often with visible lesions in the lung. Skin lesions similar to erysipelas have been identified in sows and pigs due to A. suis infection.

Case Study No. 1

This case presented in the finisher of a two-site system in central Indiana. Pigs are weaned into nursery-grower rooms and moved to finishers at 120-plus lb. All rooms are all-in, all-out, but the site is continuous-flow. The finisher manager reported nearly 1% acute mortality over two days. Lesions were observed in the lungs consisting of red, hemorrhagic foci, primarily in lower lung lobes. Small white spots were observed in the liver. A. suis was cultured from both the liver and the lung. These lesions were unusual in that spots on the liver are rare, and lung lesions are often consolidated and show some pleuritis (surface infection), similar to APP infections.

However, A. suis grew in pure culture, and the microscopic lesions were consistent with the bacteria. The clinical problem in this herd occurred within a few days after moving pigs to the finisher. Since this pattern repeated itself, and the A. suis was sensitive to chlortetracycline (CTC), a routine five-day water pulse of CTC was applied to subsequent groups after movement.

The acute loss post-movement has been controlled, but the farm still occasionally has some A. suis loss in later finishing. On the day of a recent herd visit, a pig was found dead and was posted. This pig exhibited “typical” A. suis lung lesions, and the bacteria were isolated.

Because several pigs were gaunt in appearance, CTC water medication was recommended, and no further death loss occurred.

Case Study No. 2

A 5,000-sow herd in Ohio was selling weaners to finishing operations in Indiana. The veterinarian for the finishers reported significant mortality problems due to A. suis. Practitioners have had success removing APP (evaluated without clinical problems or seronegative finishers) from pig populations with antibiotic treatments and weaning off-site.

A report by Jerome Geiger, DVM, and others at an International Pig Veterinary Congress piqued our interest in the possibility of reducing or eliminating A. suis finisher infections by using similar targeted antibiotic protocols in the breeding herd and the baby pigs.

We first demonstrated the presence of A. suis in the tonsils of pigs due to be weaned by collecting tonsil swabs. In all, 10 of 10 pigs swabbed were positive for A. suis.

Antibiotics were given to the lactating sows and the baby pigs and were collected several weeks post-weaning.

Unfortunately, we were not able to significantly reduce the level of positive samples from the tonsil. This may have been due to less-intensive antibiotic levels compared to the Geiger study that demonstrated significant reductions in positive samples at weaning.

Since the finishers were not owned by the sow herd, we were not able to follow the performance after the early antibiotic intervention. It is possible that mortality could be reduced, even though the percentage of positive swabs did not indicate an impact on the bacteria.


Actinobacillus suis continues to cause significant mortality in some finisher systems. Because of the systemic nature of the bacteria and the fact that clinical signs of the remaining population may not be evident, it is possible that these bacteria can be a major cause of sudden/sporadic loss of “good” pigs.

If pigs that die acutely are posted but tissues are not cultured for bacteria, the cause of death may be overlooked. Even though these bacteria have been present for years, they remain susceptible to many antibiotics.

As producers continue to reduce antibiotic use, it is possible that A. suis may become an even more common killer of finisher pigs.   

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