Laboratory diagnostics: Salmonella

Assays available:

Bacterial culture

• Isolation of live organism from tissues with lesions
• Sample types: feces, blood, spleen, lung, liver, lymph nodes
• 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 blood samples
  • Bacterial growth can be prevented in samples from pigs previously treated with antibiotics
  • Feces not reliable for diagnostics for systemic cases
  • Shedding is not consistent; there is intermittent shedding over time

Antimicrobial susceptibility

• Tests in vitro ability of live organism to grow under specific concentrations of different antimicrobials
• Sample types: feces, blood, spleen, lung, liver, lymph nodes
• 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

Bacterial serotyping

• Serogrouping occurs based on identification of different O and H antigens on Salmonella isolate
• Pros:
  • Serotyping can be done through:
    • Use of different antisera
    • A polymerase chain reaction (PCR) technique that detects presence of specific sequence of nucleic acids (DNA) associated with specific antigens
    • Using microarrays to detect specific DNA sequences for specific antigens
  • Helps identify pathogenicity of bacterial isolate based on serotyping associated with specific antigens
• Cons:
  • Requires a bacterial isolate
  • Moderate cost

Enzyme-linked immunosorbent assay (ELISA)

• Detects presence of antibodies
• Sample types: serum or oral fluids
• Pros:
  • Animals remain positive for several weeks
  • Can be used in chronic cases
  • Can use paired samples to support recent infection
• Cons:
  • Specific antibodies detected and timing of detection may vary slightly between the different commercial kits available.
  • Can vary from just 3 or 4 days to 3+ weeks for animals to become seropositive; variable time to seroconversion based on serotype and dose of exposure.
  • Unable to differentiate vaccine vs. wildtype infection.
  • There is no consistent serological response to infection; different serotypes and different doses of infection impact immune response.
  • Maternal antibodies can complicate interpretation of results in early nursery stages.

Result interpretation:

Bacterial culture

• Amount:
  • Moderate to high: highly suggestive of disease contributor
  • Low: questionable value (could be contaminant or nonpathogenic serotype)
  • No growth: Animal possibly previously treated with antibiotics, not shedding at that time (intermittent shedding), or not significant contributor

Antimicrobial susceptibility:

• Susceptible: Possible good choice for treatment if antimicrobial can reach target tissue
• Resistant: Select different antimicrobial
• MIC: If MICs are done ensure antimicrobial selected achieves the listed MIC value in the target organ

Serotyping:

• Serotyping allows grouping by serotypes which is very valuable in identifying significance of isolate

Table with the most common serotypes of Salmonella and their respective serogroups.

ELISA

• Positive:
  • Past exposure to vaccine or wildtype bacteria.
  • Can use paired serum samples two to three weeks apart to support recent infection.
• Negative:
  • Negative to vaccine or wildtype bacteria exposure.
  • Infection too early to detect.
  • Low exposure dose.
  • Infection with mild serotypes.
  • Antibiotic use may suppress bacterial growth resulting in lower exposure dose and lower immune response.

Scenarios:

Growing pigs with diarrhea (acute or chronic):

• Collect fecal samples from 10 or more untreated scouring pigs and pool in groups of 5 and submit for bacterial culture and sensitivity.
• Serotype any Salmonella positive samples.

Growing pigs with systemic disease (with or without diarrhea):

• Necropsy of 1-3 recently dead or euthanize scouring pigs. Collect tissues for bacterial culture (spleen, liver, gastro-hepatic lymph nodes).

Determining timing of exposure:

• Collect samples from 10-15 pigs at 8, 12, 16, and 20 weeks of age
  • Two approaches to collecting:
    • Cross-section – collect from different age groups at one time (quicker to obtain results).
    • Longitudinal – collect from same pigs over time (results are more accurate for serology interpretation).
• Serum samples test individually via ELISA.
• Fecal samples, pool in groups of 5 based on age group and submit for bacterial culture and sensitivity.
• Serotype any Salmonella positive samples.