By Izak Hofmeyr, Plaas Media
The results on a test report are not just a result – it contains a specific message. During a recent webinar presented by the Dairy Standard Agency (DSA), the organisation’s brucellosis laboratory manager, Deon Gallus, explained that, test results can contain ‘noise’ that can influence or distort the message.
Regarding the sampling process, he emphasised that incorrect sampling will influence the test report results. During the sampling process, bear in mind why the sample is being taken, what needs to be diagnosed or measured, whether the sample will reflect the whole picture, and what the sample will ultimately represent. “Also, it is never a good thing to take decisions based on one test result only,” he said.
Taking and transporting samples
The sampling process requires thorough planning and it is important to focus on good preparation the day before sampling. This relates to staff, equipment and material. There should be no interruptions during sampling, and the person taking the sample must be skilled in working systematically, taking the sample correctly, using the correct containers, and requesting the right tests. Labelling the sampling containers before sampling can assist in this process, as does an environment that is clean and clear of objects.
Samples must not be exposed to direct sunlight. Open and close the sampling container just before and immediately after sampling. If the cold chain is required, start at the sampling point and maintain the cold chain until the sample has been dispatched.
The sample must be ready for pick-up when the courier arrives and must have cooled down to the required temperature before packaging. Use ice packs that have been frozen for 24 hours and more, and ensure that the cooler box is not damaged (no patches) and is properly sealed with one layer of Sellotape. Do not stick Sellotape over the label.
Pack the sample upright and tightly. Preferably place an ice pack at the bottom and on top of the sample. The top one is important, as cold air will drop to the bottom. Prevent direct sample contact with the ice pack as it can freeze part of the sample.
Make sure that that the sample submission form has been completed in full and include it in the box. The sender details as well as delivery address and contact number must be clearly marked on the box. When transporting the sample box yourself, do not place it in direct sunlight.
Follow the principle of a three-layer system when packing bio-hazardous material – place the sample container in a plastic Ziplock bag and then in the cooler box, which must be marked as containing bio-hazardous material. Also check whether a Red Cross permit is needed.
At the laboratory
The laboratory can only test what is received and requested. “Use only an accredited and preferably established laboratory. Accreditation demonstrates operational and technical competency for a defined scope. Accredited laboratories are audited at least every 18 months to ensure compliance. Should the laboratory change anything regarding the test method, the South African National Accreditation System (SANAS) must be notified and the method re-audited. All SANAS accredited laboratories will have a customer complaint system as a benefit to the client.”
Ensure that the handwriting on the request form is legible, so that no time is wasted deciphering the details or incorrectly allocating samples. Provide adequate volumes of samples as spillage is possible. Following up on incomplete or illegible forms can be time consuming.
The test report
If the laboratory is SANAS accredited, the report will display a SANAS logo, laboratory number and accreditation type. The report will also display the company and client’s information. It is important to distinguish between a ‘preliminary test report’ and a ‘test report’.
Each test report has a unique reference number, the date on which the sample was received, tested and reported, as well as which test was conducted and the test method applied, along with the unit. Finally, it will display the signature of the individual that approved the test report.
“Carefully read the comments on the report, for it may contain valuable information on which to base critical decisions,” he said. Several factors can create ‘noise’, such as a lack of understanding, unexpected or upsetting results, or wrong information that led to the incorrect test being requested.
Laboratory tests used in South Africa fall into two broad categories: direct methods (tests to identify the pathogen itself through smears and culture cultivation, and indirect methods (tests to demonstrate the presence of specific antibodies in serum, milk, and semen).
The DSA is in the process of developing a test laboratory, the focus of which will be to implement and validate the milk ring test (MRT), followed by the Rose Bengal test (RBT), and the complement fixation test (CFT). The facility will also be used for research and development of alternative indirect test methods.
The milk ring test
This test is done by mixing a raw milk sample with a small quantity of stained B. abortus antigen and incubated at 37°C. If the milk contains Brucella antibodies the antigen will agglutinate and be carried to the surface by the cream, where it will form a dark blue/purple/pink ring. If no antibodies are present, the stained antigen will remain suspended in all the milk. Overnight incubation at 5°C increases the sensitivity of the test and allows for easier reading.
Possible ‘noise’ factors that could cause a positive reaction in the test include vaccinated S19 reactions when vaccinating after eight months of age, high volumes of colostrum in milk (early lactation), high cell concentrations in the milk (late lactation), mastitis, freezing of the milk (structural change), a variation in new antigens, and the Brucella species’ antigenic relationship with E. coli 0157, Salmonella serotype or Vibrio cholerae.
Factors that could yield a negative reaction include lack of infection in the herd, an infected cow that was not included in bulk tank sample, an infected cow not secreting antibodies, dilution of infected cows in large herds, milk that was shaken excessively, a milk sample that was not kept cool, and a variation in new antigens.
Steps for a positive result
Note that all test results are mailed to the relevant state veterinarian. If the results are confirmed positive, it is strongly advised that the MRT be repeated as quickly as possible and checks be done to ensure that the samples were taken and delivered correctly. All possible reasons for a positive reaction must be investigated.
All cattle in the herd will be bled by the state veterinarian and tested with the RBT to identify the individual infected animals. The RBT does not differentiate between actual infection and the S19 vaccine strain. All positive results must be confirmed using the CFT. This test provides a value in units per millilitre (U/mℓ). It will still pick up the S19 vaccine strain if animals were recently vaccinated or adult animals have been vaccinated.
The farm will be quarantined, and all positive animals will be branded with a C on the neck and will have to be culled. A herd is only brucellosis free after two full herd serum tests results prove negative, three months apart, and after 12 months of negative results on the MRT.
For more information, contact Deon Gallus at e-mail deon@dairystandard.co.za or 083 842 2005, or visit www.dairystandard.co.za
