Laboratory diagnostics: Erysipelas

Assays available:

Gross pathology

• Visual inspection of lesions
• Pros:
  • Almost pathognomonic: Acute skin lesions (slightly raised), multifocal pink to purple diamond-shaped lesions

• Cons:
  • Other than skin lesions, all other lesions are common to many other diseases
  • Usually require other diagnostic confirmation

Bacterial culture

• Isolation of live organism from tissues with lesions
• Sample types: blood, spleen, lung, liver, kidney, lymph nodes, joint fluid
• Pros:
  • Bacteria is easy to grow in 1 or 2 days for acute cases
  • Easy to do in any lab (including in-house)
  • Relative low cost
• Cons:
  • Selective enrichment may be necessary for chronic cases
  • Bacterial growth can be prevented in samples from pigs previously treated with antibiotics

Antimicrobial susceptibility

• Tests in vitro ability of live organism to grow under specific concentrations of different antimicrobials
• Sample types: blood, spleen, lung, liver, kidney, lymph nodes, joint fluid
• Pros:
  • Identification of susceptibility or resistance of specific isolate to common antimicrobials
  • Identification of antimicrobial resistance trends
• Cons:
  • Requires a bacterial isolate
  • In vitro testing may be slightly different than in vivo results
  • Some specific antimicrobials may not be tested or require separate, special testing
  • Moderate cost

Immunohistochemistry (IHC)

• Detects presence of bacterial antigen
• Sample types: tissues
• Pros:
  • Detects bacteria at site of lesion (good proof of causation)
  • Can be useful in chronic cases or when animals have been treated with antibiotics and bacterial culture can be challenging.
• Cons:
  • Correct tissue sample must be submitted
  • Requires significantly more bacteria to be present than PCR
  • Only evaluating a small tissues sample

Polymerase chain reaction (PCR)

• Detects presence of specific sequence of bacterial nucleic acid (DNA)
• Sample types: tissues, whole blood, serum, oral fluids, etc.
• Pros:
  • Very high sensitivity (can detect small amounts of bacteria)
  • Many different sample types can be used
  • Moderate cost
• Cons:
  • Cannot differentiate live vaccine bacteria vs. wildtype infection

Enzyme-linked immunosorbent assay (ELISA)

• Detects presence of antibodies
• Sample types: serum
• Pros:
  • Animals can remain positive for many months
  • Antibody level in many assays correlate well with protection
  • Can be used in chronic cases
• Cons:
  • Specific antibodies detected and timing of detection may vary slightly between the different commercial kits available
  • Many healthy pigs can be carriers and test serologically positive
  • Unable to differentiate maternal antibodies vs. exposure
  • Unable to differentiate vaccine vs. wildtype infection

Result interpretation:

Gross pathology:

• Positive: Skin gross pathology can be used for presumptive diagnosis
• Negative: Chronic cases usually don’t manifest with skin lesions

Bacterial culture:

• Positive: Confirmatory of disease
• Negative: Negative, or may need enrichment especially in chronic cases or testing is done late after infection

Antimicrobial susceptibility:

• Susceptible: possible good choice for treatment if antimicrobial can reach target tissue
• Resistant: select different antimicrobial
• MIC: MICs are done to ensure that the antimicrobial selected achieves the listed MIC value in the target organ

IHC

• Positive: Bacteria is present at site of lesion
• Negative: Negative, or bacteria could have been missed if testing occurs late after infection or after antibiotic treatment

PCR

• Positive: Bacteria is present/circulating and confirms disease. Recent vaccination with a modified live vaccine can result in positive PCR results.
• Negative: Negative, or bacteria could have been missed if testing occurs late after infection or after antibiotic treatment

ELISA

• Positive: Maternal antibodies or past exposure (usually > 7-10 days post exposure) to vaccine or wildtype virus
• Negative: Negative, or infection too early to detect (usually takes 2 to 4 weeks post exposure)

Scenarios

• Pigs with skin lesions (usually > 20 kg):
  • Confirm gross lesions as true “diamond-shape” for a presumptive diagnosis
  • Collect spleen or liver from any dead pigs or collect blood sample from live pig for bacterial culture
  • If pigs have been treated with antibiotics, request culture via selective enrichment
• Chronic arthritis (usually > 20 kg):
  • Collect joint fluid from affected pig(s):
    • Individual PCRs
    • Culture via selective enrichment
  • From dead pig(s) also collect synovium and affected cartilage for Erysipelas IHC