A new test that is able to produce quick and accurate results for the presence of Johne’s Disease in cattle is a potential “game-changer” for control on farms, local scientists claim.
The test, developed by researchers at Queen’s University Belfast (QUB), can be used on blood and faeces samples, as well as milk, and has a turnaround time of 24 hours.
Initial trials of the new test on-farm found the disease in a significant number of NI herds, but suggest it is endemic on only a small number of these farms.
The QUB researchers report that 26.5% of bulk tank milk samples supplied to an NI-based dairy processor contained the bacteria which causes Johne’s.
Infection usually takes place in the first 30 days of calf’s life
That compares with the more commonly used Milk-ELISA test, which only picked out 2.6% of the herds as being positive.
Johne’s is an infectious intestinal disease, but it takes years for cattle to show clinical signs after becoming infected. Symptoms include reduced milk yields and poor fertility, followed by weight loss, chronic diarrhoea, and eventually death.
“Infection usually takes place in the first 30 days of calf’s life. The problem is that the disease doesn’t become obvious to the farmer until two to seven years later,” explained Professor Irene Grant from QUB.
Speaking in an online event last week, Grant described Johne’s as an “iceberg disease” because many animals within a herd will be infected by the time the first animal displays symptoms.
The other factor that leads to Johne’s being hidden within herds surrounds the lack of accurate diagnostic tests.
For example, the Milk-ELISA gives a significant number of false negatives, so it misses animals that are truly infected with Johne’s, and it only detects animals in advanced stages of infection after they have had the opportunity to spread the disease.
The new test has greater sensitivity than the current Milk-ELISA
Also, the Milk-ELISA screening test works by detecting the presence of specific antibodies. These are cells that an animal produces after being exposed to Johne’s-causing bacteria.
An animal that tests positive under Milk-ELISA might only have been temporarily exposed to Johne’s at some point in the past and is therefore neither persistently infected nor actively spreading the disease.
“The new test has greater sensitivity than the current Milk-ELISA which means more infected animals are detected,” Grant explained.
“You are also detecting the animals that are actually shedding the bacteria, rather than animals with just antibodies (which indicates past exposure),” she added.
She said that, of the 392 herds that were tested with the new QUB test, only four had very high levels of bacteria which would indicate widespread Johne’s infection within the herd.
Each cow within these four herds was later tested individually and 21.5% of samples showed viable Johne’s bacteria under the QUB test. However, when the Milk-ELISA test was used, only 11.9% of the cows in the highly infected herds tested positive.
Another test, known as faecal qPCR, is available, although it detects both living and dead Johne’s bacteria
Other tests are already available for Johne’s, but they have issues too.
For example, growing a culture of the Johne’s bacteria in a laboratory is seen as the gold standard test. However, results take at least eight weeks and it is therefore too slow for on-farm use.
Another test, known as faecal qPCR, is available, although it detects both living and dead Johne’s bacteria, so animals that are not truly infected may throw up a positive result.
Dairy farmers in the Red Tractor quality assurance scheme are required to have a Johne’s management plan in place. This can include taking steps to not feed pooled milk to calves and regular bulk tank milk screening.
If you have Johne’s control measures in place in your herd, these will be unsuccessful, simply because you are not detecting all infected animals
However, Grant was adamant that without accurate diagnostic testing, the disease has room to persist within herds.
“If you have Johne’s control measures in place in your herd, these will be unsuccessful, simply because you are not detecting all infected animals,” she maintained.
“While the new test still needs to undergo some further validation, it has the potential to become a commercially available test in the future and that’s our goal,” Grant concluded.