Frequently Asked Questions about the HTJ test

BJD (Bovine Johne’s Disease) is a bacterial disease that infects the intestines of cattle. It causes diarrhoea, wasting and death. The disease can’t be treated and the bacteria persist for a long time (up to 1 year) in the environment. Diagnosis is difficult, due to the fact that infection may exist for months or years before the animal returns a positive test result.

HTJ stands for High Throughput Johne’s. It is a recently developed molecular test to detect DNA consistent with the causative agent of Johne’s disease (Mycobacterium avium subsp. paratuberculosis) that provides a rapid alternative to the faecal culture test.

The new test has been approved for herd tests in Australia and New Zealand by the Subcommittee on Animal Health Laboratory Standards (SCAHLS).

Other BJD tests available are:

• ELISA blood test - detects antibodies against the bacteria, takes a few days
• Faecal culture (individual and pooled) for bacterial growth, takes several months
• HEC test (Herd environmental culture test suitable for assessing dairy herds)
• Tissue culture for bacterial growth, takes several months
• Histology- microscopic examination of tissues from a dead animal, takes approximately one week

For more details of specific test applications and limitations, refer to JD testing and diagnosis on the AHA website

The HTJ test is based on polymerase chain reaction (PCR), which detects DNA of the bacteria in a faecal sample. The HTJ test can be performed on faecal samples from live animals, is very specific to the bacteria, and results can take 1-2 weeks.

In comparison, it is more sensitive than the ELISA blood test, substantially quicker than culture tests, and does not require slaughter as with histology and tissue culture.

No. The HTJ test currently has similar sensitivity (ability to identify a true positive sample) as faecal culture. Unlike faecal culture, the HTJ test is used to assess herd infection status and not for confirming infection status of individual animals.

No, the HTJ test is a herd-based screening test and needs to be used in conjunction with other tests for any positive result.

The main advantage of the HTJ test is that it gives similar information as faecal culture, but only takes a fraction of the time. This means that large numbers of samples can be processed in just 1-2 weeks. HTJ testing can give a good indication of herd status of Johne’s disease in both endemic and non-endemic areas. Also, the HTJ test has potential for further development in the future, which could add greater convenience and sensitivity.

Faecal samples from each animal are pooled in the laboratory into groups of five or less and tested. If a pool is positive, the individual samples in that pool can be re-tested to find the animal(s) that may have contributed to the positive sample(s).

No. Because the shedding of bacteria from the gut of cattle isn’t constant or predictable, a single test on one animal cannot be used to confirm that a particular animal is not infected.

A positive result means the herd is likely to be infected but reactors to the HTJ test are not deemed to be infected until confirmed by further testing, Confirmation testing may include faecal culture test or other investigations.

Animal Health Committee has determined that negative HTJ test results across the whole herd when sampling is done of high risk animals (e.g animals greater than 2 years of age) is a good indication that cattle in the herd are not shedding the bacteria and are a guide that BJD does not exist in a herd (subject to the age and exposure time of the herd). While the HTJ test gives a quick result, if there is a positive, it may take many months to resolve, so this needs to be factored into the timing of testing.

No. The HTJ test results are reported as positive (bacterial DNA detected) or negative (bacterial DNA not detected).

No. Currently an isolate from a faecal or tissue culture test is required to detect the strain of bacteria.

Yes. This may occur when infection exists in very low levels in a herd or if the positive animal is shedding at very low levels (e.g with environmental exposure to the bacterium). Bear in mind however that repeating the same test twice can also sometimes show up a positive then a negative (or vice versa). This reflects the fact that bacterial shedding in the faeces is not constant or uniform.

False positive results can also occur in an uninfected herd but are uncommon (<1%), which is why confirmation of positives with faecal culture is required.

Yes. This is possible, but because the HTJ test detects DNA of bacteria, it does not distinguish between live and dead bacteria. Where this is suspected, a culture test will show if animals are genuinely infected, as culture only detects live bacteria. However, if a positive result is due to an animal is shedding dead bacteria it means that there is contamination in the environment and the herd is probably infected.

Dirt in the faeces e.g. in drought conditions, can interfere with the test. Age of animals and whether they are showing clinical disease or not will affect the extent of bacterial shedding and therefore the chance of showing up positive. As with any herd test, testing low numbers of samples is more likely to show a negative result. Keeping samples cool during storage and transport is essential for the test to perform effectively.

Because HTJ is new, staff in each of the laboratories that conduct the test have been trained, so that all laboratories will perform the test and interpret its results consistently. Australian laboratories participate in ongoing quality assurance and monitoring.

Each laboratory sets its own charges for diagnostic tests.

No. The method for sampling and dispatch to the laboratory is the same as for faecal culture.

Information on sampling techniques is available from laboratories or the ANZSDPs. Use ice packs and eskies to keep samples chilled once collected. Collection of samples should be timed such that they can be delivered to the laboratory without delay (Do not transport over weekend and/or public holidays to prevent warming and take care to avoid samples sitting at the post office over the weekend).

The HTJ test itself will not be affected by vaccination for BJD or other conditions. Due to the vaccine reducing the shedding of bacteria in the faeces, vaccinated infected herds are more likely to have a lower proportion of animals/pools test positive.

Each state government and show society has their own regulations concerning the use of BJD diagnostic tests. For specific requirements consult local authorities.

Your district veterinarian, private veterinarian or state government animal health laboratory will be able to answer questions about the type of tests, prices, uses and interpretation.