Actinobacillus suis Infection in Pigs: Sepsis, Pneumonia, and Sudden Death

Actinobacillus suis is a bacterial infection of pigs that can cause septicemia, pneumonia, polyserositis, arthritis, and sudden death. In severe cases, the first sign may be a pig found dead with little warning. In less peracute cases, pigs may show fever, breathing distress, coughing, weakness, or reduced growth before they crash.

This organism is considered common in swine populations and often silent in carrier pigs, especially in the tonsils and upper respiratory tract. That means a pig can carry the bacteria without looking sick, then develop disease later or help spread it within a group. Young pigs and growing pigs are often affected, but disease can occur in different ages depending on herd immunity, stress, and other infections.

A. suis can look a lot like Actinobacillus pleuropneumoniae (APP) at first, especially when pneumonia is severe. The difference is important: A. suis is more likely to spread through the bloodstream, which is why some pigs also develop joint swelling, lameness, skin lesions, or inflammation around the heart, lungs, and abdominal organs. Because the disease can move fast, early veterinary involvement is critical.

Actinobacillus suis infection is caused by the bacterium Actinobacillus suis, a gram-negative organism found in swine. Many pigs can carry it without obvious illness, especially in the tonsils and upper respiratory tract. Disease tends to happen when the bacteria gain access to deeper tissues such as the lungs or bloodstream.

Spread is thought to occur mainly by the oronasal route, especially through close contact with carrier pigs. Mixing groups, transport, crowding, ventilation problems, temperature swings, and other stressors may increase the chance that a carrier pig becomes sick or that infection spreads through a group. Concurrent respiratory disease can also weaken normal airway defenses and make severe pneumonia more likely.

Unlike APP, A. suis is notable for its septicemic behavior. Once in the bloodstream, it can seed multiple organs and cause fibrinous inflammation in the chest, abdomen, or around the heart, along with arthritis or skin lesions. That is why some pigs show more than lung disease alone.

Pet parents with miniature pigs should know that the same basic principles apply: a pig that is stressed, newly introduced, or already fighting another illness may be more vulnerable. Your vet may also look for management factors that allowed the bacteria to move from silent carriage to active disease.

Diagnosis starts with the pattern of disease: sudden death, acute pneumonia, septicemia, polyserositis, or arthritis in one or more pigs raises concern. Still, signs alone are not enough. A. suis can resemble APP, Glaesserella parasuis, Streptococcus suis, Salmonella Choleraesuis, and Mycoplasma hyorhinis, so your vet usually needs laboratory confirmation.

If a pig dies, rapid necropsy is often the most useful next step. Gross lesions may include severe hemorrhagic or necrotizing pneumonia, fibrin in the chest or abdomen, pericarditis, and swollen joints. Fresh samples from acutely affected lung tissue are especially important for bacterial culture. PCR can also help detect A. suis and may be run on lung, pleura, pericardium, peritoneum, joints, tonsil swabs, oral fluids, and other tissues depending on the case.

In live pigs, your vet may collect swabs or samples from affected sites, but results can be harder to interpret because some pigs carry the organism without disease. That is why your vet will combine history, exam findings, herd pattern, necropsy, culture, PCR, and sometimes susceptibility testing before making treatment and control recommendations.

Because this disease can move quickly through a group, diagnosis is not only about the sick pig in front of you. It also guides isolation, treatment timing, monitoring of penmates, and prevention planning for the rest of the herd.

Prevention focuses on reducing exposure, lowering stress, and catching outbreaks early. Because healthy carrier pigs can harbor A. suis, prevention is not always about keeping the bacterium completely off a property. It is often about keeping it from turning into active disease.

Work with your vet on practical herd measures such as all-in/all-out flow when possible, careful quarantine of new arrivals, avoiding unnecessary mixing, improving ventilation, reducing crowding, and keeping temperature changes to a minimum. Good sanitation matters too. Thorough cleaning and disinfection between groups can help reduce bacterial pressure in the environment.

Prompt attention to other respiratory disease is also important. Damaged airways are more vulnerable to secondary bacterial invasion, so controlling broader respiratory problems can lower the risk of severe A. suis pneumonia. If you have repeated sudden deaths or recurring respiratory disease, ask your vet whether a herd investigation is needed rather than treating cases one by one.

At this time, there is no widely used commercial vaccine specifically for A. suis. That makes management, early recognition, and fast veterinary response especially important. If a pig dies unexpectedly, preserving the body cool and arranging rapid necropsy may be one of the most valuable prevention steps for the rest of the group.