3.3 Analysis of aggregate samples

The analysis of aggregate samples uses material widely available on the farm, rather than blends of specimens from a known number of subjects as in the case of pooled specimens.

Examples of aggregate samples that can be used for analysis are:

  • manure samples
  • feed samples
  • tank milk samples.

What is the main advantage of analysing aggregate samples?

Answer

Using aggregate samples enables a cost-effective screening of large number of farms for many different pathogens without the elevated costs of sampling.

An example of aggregate sample analysis is the milk ring test for Brucella antibody (Figure 4), which is used for a cost-effective surveillance of dairy herds for brucellosis in areas that have achieved a disease-free status. Often, calculations of the impact of dilution on the sensitivity of aggregate sample analysis are difficult to carry out if the number of animals contributing to the sample is not known.

Aggregate sample analysis of tank milk is currently in use by some farms in New Zealand to isolate S. aureus and generate farm antibiograms. This method assumes that the strains recovered from the tank milk represent the strains causing the intramammary infections.

Described image
Figure 4 Milk ring test showing Positive (left) and Negative (right) results for Brucellosis
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Two test tubes showing the milk ring test. The left test tube shows positive results for Brucellosis and the right test tubes shows negative results for Brucellosis.

Figure 4 Milk ring test showing Positive (left) and Negative (right) results for Brucellosis

Activity 5: Advantages and disadvantages of each approach

Timing: Allow about 10 minutes

Reflect on the relative strengths and weaknesses of individual animal specimen, multiple specimen, pooled specimen and aggregate sample analysis, and complete Table 2.

Table 2 Strengths and weaknesses of different sampling approaches
Type of specimen Relative strengths Relative weaknesses
Individual specimen
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Multiple specimen
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Pooled specimen
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Aggregate sample
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Answer

Table 2 Strengths and weaknesses of different sampling approaches
Type of specimen Relative strengths Relative weaknesses
Individual specimen Identifies the presence of a pathogen in an animal; agent not diluted Test result is negative if agent is not present in the specimen or is present below the LoD
Multiple specimen Always increases farm-level testing regime sensitivity High cost
Pooled specimen May increase farm level testing regime sensitivity. Enables cost effective analysis for multiple pathogens Pathogen may be diluted below culture LoD
Aggregate sample Avoids expensive sampling activities Agent’s dilution factor unknown

3.2 Analysis of pooled specimens

4 Bacterial isolation