Actinobacillus pleuropneumoniae is a non-motile, gram-negative coccobacillus respiratory pathogen. Two biovars of APP exist with biovar 1 containing 13 different serovars and biovar 2 containing 2 separate serovars. Serovars differ in the way they exert their pathogenic effects on the pig and the severity of infection that they induce. Although serovars 1,5 and 7 are reportedly the most common types of APP found in the United States, swine practitioners have noted all types of serovars infecting swine herds and some believe that there is not a single or predominant type of APP serovar seen most often in the field. Serovar 1 of biovar 1 is reported to be the most virulent type, and biovar 1 strains usually cause higher morbidity than biovar 2 strains, but any serovar of APP can cause significant damage to swine respiratory systems, especially if a secondary bacterial or viral disease exists.
APP outbreaks are generally seen from late fall to early spring when the environmental temperature greatly fluctuates or during harsh weather conditions. Growing pigs are most likely to be affected when they are 12-16 weeks old, but the disease can occur in all ages of swine. The stress of moving and mixing animals often causes the disease to break out among the subclinical pigs, and then the bacteria is passed directly from pig to pig or via aerosol over very short distances. Some vertical transmission from sow to piglet can occur, but it is not nearly as common as the direct contact from incoming infected animals. Actinobacillus pleuropneumoniae bacteria can survive in a mucus covered environment outside of the pig, but if it is exposed to the open, the bacteria are quickly killed by environmental conditions.
APP quickly colonizes the host animal, first by attaching to the epithelial cells of the tonsils and then moving to the lower respiratory tract where it creates the majority of damage. As the bacteria multiplies, it releases particles of an outer membrane that contain lipopolysaccharides (LPS) and cytotoxins. Neutrophils arriving as the beginning of an inflammatory response are attracted by the LPS and are then destroyed by the cytotoxins. As the neutrophils are destroyed, lysozymes are released, damaging nearby tissue. The tissue damage can progress rapidly, and some animals that are infected with APP will die within 4-12 hours of exposure. The LPS and cytotoxins not only help the bacteria damage the host cells, they also prevent the bacteria from being destroyed by impairing phagocytosis and complement activity.
The amount and severity of clinical signs shown by affected swine vary with dose of bacterial exposure during APP outbreaks, but all types of infection can be seen on one farm during any time of the outbreak. In general, lower doses of bacterial exposure lead to subclinical animals and higher doses of exposure cause clinical disease. Subclinical animals are often chronic carriers and most clinical cases are acutely affected, although some peracute infections occur. If an animal is peracutely affected, the most common sign is sudden death. However, some peracute infections manifest with septicemia-like signs, such as extreme hyperthermia, anorexia, mild diarrhea, cyanosis of extremities, and severe dyspnea. Acutely infected pigs also show hyperthermia, though not as extreme, congested extremities, depression, anorexia, dyspnea, coughing, open-mouth breathing, and condition loss. These pigs usually progress in disease severity and die a few days after the beginning of the clinical signs or resolve and become chronic carriers. The chronic carriers are the hardest to identify and are usually the cause of disease outbreaks. Some signs, such as decreased appetite, decreased weight gain, and exercise intolerance may be seen, but many chronic carriers show no obvious clinical signs and are only identified with testing during APP outbreaks.
Necropsies of acutely infected swine are the best way to diagnose an outbreak of Actinobacillus pleuropneumoniae. Pleuropneumonia that is fibrinohemorrhagic and necrotizing is usually seen bilaterally in the lungs. It is often focal and found in the caudal lung lobes, but it can be diffuse and may be seen in any or all parts of the lung lobes. The lungs may be darkened and consolidated, and pleural adhesions and abscesses are often found in chronic cases. Fibrin is commonly found throughout the surface of the lungs, and peracutely affected animals may have foamy, bloody exudate in the trachea and bronchi. When viewing the animal’s clinical signs and gross lesions, additional differential diagnoses tend to include diseases that affect the respiratory system and show signs of septicemia such as Actinobacillus suis, Haemophilus parasuis, and Salmonella cholerasuis. Definitive diagnosis is usually confirmed with the aid of histopathology and bacterial cultures.
Microscopic examination of APP lesions usually shows severe fibrinohemorrhagic pneumonia that is often suppurative and necrotizing. Vasculitis, pleuritis, and bronchiolitis are often additional components of the disease. Neutrophils are seen more often in an early course of APP, while, later in the disease, macrophages are increased and fibrosis is more severe. APP is easily grown on blood agar with a Staph epidermidis nurse streak. NAD is needed for the growth of most of the serovars, but not for the biovar 2 types. The bacteria is most often cultured from affected lung samples, but it can be found in spleen and liver samples and liver samples as well.
Although a combination of clinical signs, gross observation of necropsy lesions, histopathology review, and bacterial culture is the most common way to diagnose Actinobacillus pleuropneumoniae, other tests can be performed, many of which are particularly useful at identifying subclinical carrier animals. ELISA, immuno-fluorescence, ring precipitation, coagglutination, latex agglutination, and counter immunoelectrophoresis are all tests that diagnostic labs perform to look for APP. PCR is frequently used to serotype the bacteria and is often required to determine which type or types of serovars are involved on each farm. The tests that are chosen for each farm are usually determined by the individual outbreak characteristics and the diagnostic lab’s capabilities and resources.
Many options are available for treatment of APP, but best results are seen when the animals are treated immediately after the onset of clinical signs. If treatment is delayed, the number of chronic cases that develop is often increased and it becomes harder to eradicate the disease from the farm. Injectable antibiotics are the best way to treat Actinobacillus pleuropneumoniae because the infected swine are often anorectic and would not be able to consume adequate amounts of feed or water medications. An appropriate treatment regimen would be to treat all clinically ill pigs and all pigs that are in contact with the clinical pigs on the first day signs are observed, then treat only clinically ill pigs on subsequent days. Even with treatment, producers should except high morbidity and mortality.
Outbreaks of Actinobacillus pleuropneumoniae cause severe economic losses to swine operations. Not only do the producers have to deal with the mortality and cost due to increased growth rates, they also must calculate the cost of vaccines, medications, and losses at slaughter from infection site abscesses or pleural adhesions. Prevention is the best way to avoid the effects of APP infection and many of the methods used to prevent this disease are also effective at preventing other disease outbreaks. Strict biosecurity measures are the best way to avoid introduction of APP, including isolation of incoming animals and purchase of stock from APP-free herds. Management and environmental conditions should be optimized to avoid stressing the pigs whenever possible. Mild APP infections can be controlled with medicated feed, but these treatments usually only help eliminate residual infections and cannot be used to treat in the face of an outbreak. Vaccines are available for APP, but many cause side effects such as infection site swelling and abscesses, pyrexia and lethargy. The best method for prevention is to employ biosecurity and management practices to control the introduction of the bacteria.
Actinobacillus pleuropneumoniae can cause severe disease in a swine herd and lead to significant financial difficulty of a swine operation if an outbreak occurs. The disease is readily diagnosed by veterinarians and diagnostic labs and can be treated. However, preventative measures should be put in place before an outbreak occurs so that financial losses are minimized.
-by Betsy Brownfield, Class of 2007
-edited by Dr. Pam Mouser, ADDL Graduate Student
References:
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Dr. Karen Lehe, Swine Veterinary Practitioner, personal communication
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Farruggio et al. Diagnosis and Treatment of Actinobacillus pleuropneumoniae in Swine: An Update. Article from swine resource folder compiled by Dr. Sandy Amass and Dr. Darryl Ragland, Purdue University Production Medicine
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Fedorka Cray P et al: 1993. Actinobacillus pleuropneumoniae Part 1—History, Epidemiology, Serotyping and Treatment. The Compendium 15 (10): 1447-1452.
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Gottschalk M and Taylor D: 2006.Actinobacillus pleuropneumoniae. Diseases of Swine. Blackwell Publishing, Ames, IA 563-575.
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Marstellar T and Fenwick B: 1999. Actinobacillus pleuropneumoniae. Disease and Serology. Swine Health Production 7(4): 161-165.
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Veterinary Clinical Sciences 510 notes. Swine Production Medicine, Fall 2005.
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