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Bacterial Pathogens Kill Quickly

Article-Bacterial Pathogens Kill Quickly

Respiratory diseases – pneumonia that is – still account for two-thirds of the 12,000 swine cases submitted to the Iowa State University Veterinary Diagnostic Laboratory (ISU-VDL). Viruses (porcine reproductive and respiratory syndrome [PRRS]), swine influenza virus [SIV]) and Mycoplasma pneumonia seem to get the most attention. But it is usually the bacteria that end up causing the mortality in pneumonia outbreaks. Pasteurella multocida and Streptococcus suis are the most frequently isolated bacteria from pneumonic lungs, but Actinobacillus pleuropneumonia (APP) and Actinobacillus suis (A.suis) are the most feared respiratory bacterial pathogens and remain all too common (Figure 1).

APP and A. suis are particularly frustrating because they often kill quickly – pigs are found dead or are severely affected when noticed. The disease is very acute and is sometimes called “quick pneumonia.” Necropsy reveals very severe lesions in lungs with necrosis, hemorrhage and pleuritis all quite prominent. In addition, A.suis can cause septicemia, swollen joints, polyserositis, and skin lesions that may resemble erysipelas or Haemophilus parasuis.

Actinobacillus pleuropneumonia (APP) has 15 different serotypes of which types 1, 3, 5 and 7 are most common; however, other serotypes are also present in U.S. swine herds. There is some variation in virulence between serotypes, with serotypes 1, 5 and 7 usually associated with more severe outbreaks. There is little cross-protection between serotypes and more than one serotype may be present in a population of pigs. Actinobacillus suis (A.suis) can cause lesions virtually identical to APP and cannot be differentiated without culture and histopathology (tissue examination).

APP Fears
The producer’s fear of APP is because in severe cases, dozens of pigs can be found dead virtually overnight. The disease will respond to antimicrobials, but groups of pigs will often continue to develop disease throughout the grow-finish period. Chronic pleuritis and condemnations are increased in survivors. A. suis also kills pigs, but usually less in a given outbreak.

Figure 1 suggests that the frequency of cases of pneumonia diagnosed with these agents has changed very little over recent years.

Pneumonia Cases Peak
Cases of pneumonia submitted to the ISU-VDL peak in the fall months (November data not shown) and are lowest in the summer months, seemingly following the pattern set by SIV. For cases of pneumonia that have Actinobacillus as a component, the peak frequency is in January and February (Figure 2). Interestingly, APP and A. suis follow virtually identical seasonal patterns.

APP and A.suis tend to be more severe in the later grower period (e.g. 12-16 weeks of age), with most cases between 9 and 20 weeks of age. The age distribution (based on the percent of cases per age group) is virtually identical for both APP and A. suis (Figure 3).

Control Strategies
Prevention of outbreaks of APP by use of elimination, vaccination or strategic medication is high priority for veterinarians and producers. There is legitimate concern for losses induced by A. suis, but this agent usually causes less morbidity and mortality than APP. Effective control relies on an accurate diagnosis. It is essential to identify whether the specific cause is APP or A. suis. If it’s APP, determine which serotype of APP is present.

Disease Diagnosis
Definitive diagnosis of acute disease is by isolation of the bacteria, combined with pathologic examination for compatible lesions. Actinobacillus organisms, particularly APP, are considered fastidious, thus, they require special media and growth factors to cultivate. They can be easily overgrown when other organisms are present. Steps to increase the accuracy of laboratory testing are:

1. Sample only from non-medicated, acutely affected pigs. Medication will prevent growth of the organism, and chronic cases often have other bacteria that “outgrow” Actinobacillus. Samples from euthanized animals are preferable to samples from dead pigs; pigs that are dead more than two hours have little value for isolation of the organism.

2. Collect fresh lung samples (no less than 2-in. cubes) using sterile or aseptic technique. Isolation of APP from nasal or pharyngeal swabs is largely unrewarding due to the heavy background growth of other organisms.

3. Collect ½- in. slices of affected lung – each part that looks or feels different — into 10% formalin for pathologic examination.

4. Immediately refrigerate fresh lung samples and ship both fresh and fixed specimens on ice to the laboratory for culture.

5. Accurately communicate via the submission form, and send any additional tissues or information that may be useful to rule out other suspected disease processes. Take a picture (camera phones) of the opened chest cavity and send to the laboratory.

Once an accurate diagnosis is established, the appropriate intervention can be implemented. This may include antimicrobial therapy. An example of antimicrobial sensitivity data can be found at

PCR (polymerase chain reaction) assays are available in some laboratories to detect the presence of APP and/or A. suis. Consult your laboratory for preferred specimens and appropriate sampling strategies. The detection of antibodies in serum can be useful to monitor herds for exposure, and in some cases, can be used to match populations for similar infection status when commingling positive flows or adding seedstock to a known positive herd. Consult your veterinarian or diagnostic laboratory for availability of tests, suggested sampling strategies and possible interpretations of results.

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Kent Schwartz, DVM; Tim Frana, DVM; John Johnson, DVM; and Erin Strait, DVM
Iowa State University Veterinary Diagnostic Laboratory
[email protected]