Laboratory diagnosis of actinobacillus pleuropneumonia in pigs – Articles

Available tests:

Macroscopic pathology

• It evaluates the presence of tissue lesions, which can be practically diagnostic signs of the disease.
• Localization of the lesion: lungs.
• Areas of necrohemorrhagic compaction in the lung, accompanied by fibrinous pleurisy (Figure 1).

• Pros:
  • A presumptive diagnosis can be made after macroscopic examination of the lungs.

• Minuses:
  • Actinobacillus suis This may result in similar but less serious injuries.
  • It does not identify the serotype.

Bacterial culture

• Isolation of living organisms.
• Sample types: light.
• Pros:
  • It can be performed in any laboratory (even internal).
  • Biotype I requires NAD (usually provided by cultured colony Staphylococcus aureus).
  • Biotype II does not require NAD.
  • Relatively low cost.

• Minuses:
  • Not always easy to grow.
  • It does not identify the serotype.
  • If pigs have been previously treated with antibiotics, this may prevent bacterial growth.

Antimicrobial sensitivity (antibiogram)

• Test in vitro: the ability of a living organism to grow at certain concentrations of various antimicrobials.
• Sample types: light.
• Pros:
  • Determining the sensitivity or resistance of specific strains to conventional antimicrobials.
  • Identifying trends in antimicrobial resistance.

• Minuses:
  • A bacterial isolate is required.
  • Tests in vitro results may vary slightly in natural conditionsor.
  • Some specific antimicrobials may not be included or may require separate dedicated testing.
  • Moderate cost.

Polymerase chain reaction (PCR) General or serotyping

• Detects the presence of specific bacterial nucleic acid (DNA) sequences.
• Sample types: lungs and tonsils.
• Pros:
  • High sensitivity.
  • General
    • It targets the ApxIV toxin genes.
    • Present in all APP serotypes.

  • Serotyping
    • It targets different parts of the toxin genes ApxI, ApxII and ApxIII.
    • See Table 1 for serotyping and toxin production.

  • Moderate cost.
    • Tissue samples can often be combined to reduce cost and minimize loss of sensitivity.

• Minuses:
  • Requires several primers specific (primers) to determine the serotype.
  • Not all strains can be typed.
    • Currently, 19 serotypes have been identified, and this number will continue to grow.

  • Not all tests are validated for the analysis of tonsil samples.

Table 1: Toxin production for each serotype.

	Toxin production
Serotype	Apxi	ApxII	ApxIII	ApxIV
1	X	X		X
2		X	X	X
3		X	X	X
4		X	X	X
5	X	X		X
6		X	X	X
7		X		X
8		X	X	X
9	X	X		X
10	X			X
eleven	X	X		X
12		X		X
13		X		X
14	X			X
fifteen		X	X	X
16	X	X		X
17		X		X
18		X		X
19		X		X

Enzyme-linked immunosorbent assay (ELISA) Antigen LPS-O

• Detects the presence of antibodies.
• Sample types: serum.
• Target: LPS-O antigen.
• Pros:
  • Assistance with serotyping may be helpful.
  • High sensitivity.

• Minuses:
  • High degree of cross-reactivity between serotypes due to shared capsular antigen and LPS-O antigen epitopes.
    • The following serotypes cross-react because they share LPS-O antigens:
      • Cross-reaction between serotypes 1, 9 and 11.
      • Cross-reaction between serotypes 3, 6, 8, 15, 17 and 19.
      • Cross-reaction between serotypes 4, 7 and 18.

  • It does not identify a specific serotype with a single test.
  • It is very expensive and impractical to test for all serotypes.

Enzyme-linked immunosorbent assay for ApxIV antigen (ELISA)

• Detects the presence of antibodies.
• Sample types: serum.
• Target: ApxIV antigen.
• Pros:
  • High sensitivity.
  • Detects all APP serotypes.

• Minuses:
  • It does not identify a specific serotype.
  • It does not determine the virulence of strains.

Interpretation of results:

Macroscopic pathology

• Positive: A preliminary diagnosis is often sufficient.
• Negative result: there are no macroscopic lung lesions.

Bacterial culture

• Positive:
  • Biotype 1: NAD-dependent.
  • Biotype 2: NAD-independent.

• Negative: Negative or an animal that may have been previously treated with antibiotics.

Antimicrobial sensitivity (antibiogram)

• Susceptible: Possibly a good choice for treatment if the antimicrobial can reach the target tissue.
• Resistance: a different antimicrobial agent must be selected.
• MIC: If MIC (minimum inhibitory concentration) is performed, it is necessary to ensure that the selected antimicrobial agent achieves the specified MIC value in the target organ.

PCR

• Positive:
  • The organism is present.
  • It can reveal the presence of various genes that help in serotyping.

• Negative:
  • Negative result or infection too old.
  • The animal may have been previously treated with antibiotics.
  • Not all strains can be serotyped (untyped).

ELISA for LPS-O antigen

• Positive: helps determine the serotype or identify a group of serotypes.
• Negative: Negative, the infection is too old or the animal may have been previously treated with antibiotics.

ELISA for ApxIV antigen

• Positive: the organism is present.
• Negative: Negative, the infection is too old or the animal may have been previously treated with antibiotics.

Scenarios:

Fattening pigs with sudden death or (acute) respiratory disease

• Necropsy of 1-3 recently deceased pigs or euthanasia of coughing pigs. Macroscopically evaluate the lungs for areas of necrohemorrhagic consolidation accompanied by fibrinous pleurisy.
• Submit affected lungs for bacterial culture and general PCR test for APP and, if positive, perform PCR for serotyping.

Set farm status to negative

• Select the 30 largest pigs in the feedlot and test them 2-3 times per year using the ApxIV ELISA.