Thursday, March 28, 2024

Finding and managing Johne’s disease

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Dairy cows can now be tested for Johne’s disease (JD) before they show clinical signs of infection, potentially saving the industry hundreds of thousands of dollars in lost production.
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The newly developed test is a spin-off from research by the University of Otago-based Disease Research Laboratory (DRL) and an extension of tests they designed earlier to identify TB and JD in deer.

Johne’s disease is believed to be draining at least $100 million from the back pockets of the cattle, sheep and deer sectors through production losses, reduced fertility rates and animal deaths.

Several of the dairy farmers involved in the onfarm studies were suffering annual capital losses directly attributable to JD of $100,000-plus.

Direct costs included culling infected cows before the end of their productive life, reduced milk solids production, poorer reproductive performance and increased susceptibility to other diseases.

Vetlife Temuka veterinarian Andrew Bates said infection levels in some herds in his area were so high the number of cows being culled ran into double digits each season. Until now dairy farmers unlucky enough to experience ongoing production losses because of clinical disease had few control options.

“Control has been traditionally challenging. Diagnosis is straightforward once the clinical signs show, [but] by this time it is too late, the damage has been done,” Bates said.

While JD might only present as clinical disease or result in significant productivity losses in a small proportion of herds, evidence suggested about 600 New Zealand dairy farms could benefit from JD management plans.

A research team of Frank Griffin, Rory O’Brien and Simon Liggett first began working with several “JD-problem” dairy herds in 2006, showing the wasting disease could be controlled relatively quickly and cost-effectively within one to two years.

However, rejection by industry advisors of any real relevance of JD to NZ dairying meant gaining funding was difficult so the DRL team funded the research themselves.

Onfarm studies began in 2008 involving two South Island herds and 2400 cows. The breakthrough came in 2010 when the DRL team applied two unique tests in combination to dairy cows, the first an immunological test (ELISA), the other a microbiological test (qPCR); each targeting a different aspect of JD diagnosis.

The immunological ELISA test looks at the cow’s immune response to infection (a measure of her exposure to Johne’s-causing bacteria) and the qPCR test determines the number of Johne’s bacteria excreted in her dung (directly proportional to disease severity).

“Seldom does one simple test provide a complete solution to a complex problem but two, or more, used in combination may get the job done,” Griffin said.

Since 2010 more than 10,000 cows, 5000 sampled in detail from four herds and the rest spread over 30-40 herds, have been involved in validating the technologies by matching blood serums and faecal samples of individual cows.

With a 2014 Agmardt grant the DRL researchers translated the technology from an expensive, low-throughput research tool into a practical and usable frontline in-herd diagnostic test for everyday use.

The result is a high-tech and high-throughput, yet practical and inexpensive diagnostic tool with real commercial applicability, Griffin said, which has subsequently gained international validation through USDA testing and accreditation.

The growth in dairying over past 10-15 years, bringing together the animals from many different sources, has been the ideal scenario epidemiologically to ensure establishment and spread of disease between herds.

The ability to back up serology with qPCR testing in a two-tiered combo format is a major breakthrough in controlling JD – the results prioritise the elective culling of only the most seriously affected cows, which also contribute the greatest infection risk for the rest of the herd, and provide the farmer with confidence and reassurance they are making the best culling decisions. This strategy allows less-affected cows to be flagged for appropriate risk management and targeted monitoring before any downstream production losses.

The stratification of the affected cows allows investment in JD management programmes to be tailored according to available resources as well as the scale of the problem.

Initial screening with the lower-cost blood test keeps to a minimum the number of animals assigned for dung testing with the more expensive qPCR test, keeping costs down. As a significant proportion of blood test-positive animals (>80%) will be exposed-infected but not necessarily affected or diseased, the follow-up qPCR test can be used to rank the diseased cows to minimise culling.

While the most severely affected cows are usually only a very small proportion of the herd, they also represent the bulk of the infection risk to the rest of the cohort and their early removal allows overall management gains to be made quickly.

Johne’s disease in a dairy herd is often compared to an iceberg, with only a small proportion of the problem immediately obvious but the greater part lurking beneath the surface.

Jumping at the chance to make their herd available, Gareth and Michelle Hale from South Canterbury were among the first dairy farmers to join the onfarm research. Andrew McLaughlin also of Vetlife Temuka, had been involved in early testing of the Hales’ herd in 2009. At this time, only the Paralisa test was available and 25% of the herd showed up as exposed to infection. Gareth and Michelle couldn’t afford to cull, there were simply too many, so they decided to do nothing.

Four years later, having suffered the financial losses from ongoing wastage from culling large numbers of clinically affected cows each year, but with the new testing technology available, Bates carried out a second whole-herd test and racked up similar numbers of JD-infected cows, again about 25% testing positive to the Paralisa test.

Johne’s disease researcher Frank Griffin.

These blood-positive cows were faecal-sampled and the microbiological qPCR test used to identify how many of the infected cows were shedding significant numbers of JD “bugs” in their dung, a key indicator of disease progression, and to use this information to stratify the cows based on their shedding status.

About 5% were deemed to be high-risk (‘supershedders’) and the advice was to immediately cull them from the herd. As a further control measure, the farm operates as a closed herd policy so no replacement animals are brought into the farm. Animals shedding smaller numbers of bacteria were identified but retained in the herd. No heifer replacement calves were kept from those animals and their milk and colostrum were excluded from replacement calves. All heifer replacement calves are reared in paddocks inaccessible to the milking herd. At weaning, calves were taken to a dedicated runoff where they will remain until two years old.

These steps help to reduce the risk of fresh infections in younger animals.

“Using the tests in combination dropped the percentage of animals that needed to be culled urgently five-fold, a more manageable and economically viable option for the Hales,” O’Brien said.

Gareth and Michelle culled the highest-risk animals, those responsible for most of the spread in their herd. Few were showing any of the typical clinical JD signs such as scouring or obvious weight loss. The alternative down the track was they would perform poorly, spread the disease further and eventually just be fit for onfarm slaughter and disposal.

The outcome has been win-win, with maximum disease control and salvage, lesser financial costs involved with culling and greatly reducing the ongoing JD spread within their herd, especially from cows to their offspring at calving.

Complete eradication of JD is generally accepted to be impractical and unfeasible; rather, efforts are directed towards effective management informed by diagnostic testing to minimise the economic and welfare impact. While eradication is rarely a realistic option, clinical disease may be managed out of the herd over time.

What is the JD test capable of?

• Selectively identifying diseased animals that are unlikely to resolve their infection and if left in the herd will cause significant economic losses and spread of disease, especially to the offspring.

• Identifying and salvaging uninfected animals or animals that have been exposed to infection but have developed protective immunity and will not progress to develop disease.

• Faecal test: Count the number of Johne’s bacteria shed in the dung of affected animals. Used with the Paralisa test in herds where culling of all ELISA-positives is neither feasible nor sensible. A standalone test to quickly confirm suspected cases of JD. Might not detect early-stage JD, when shedding might be intermittent – no test can. Similarly, the antibody immune response measured by serological tests like ELISA, develops after local cellular responses have failed to control the infection.

• Used serially as a secondary test, the faecal test is used to confirm the JD status of ELISA-positive individuals. Active shedders are ranked in order of immediate risk to prioritise which cows to cull. Test results are broken down into shedding categories ranging from Very High, High, Moderate, Low, Background and Not Detected.

• Faecal qPCR lends itself to pooled sampling because high-shedding individuals can be easily identified among low or non-shedding cows. Pooled sampling is an option to initially screen the herd. ELISA is preferred because of cost and convenience.

• Animals less than two years old are unlikely to be identified by diagnostic tests because Johne’s disease normally first appears in cows from three years old.

• Ideally all cows should be tested in the autumn and all shedders culled before calving because their offspring have a high risk of becoming infected.

 

Colostrum from cows testing positive for JD should never be fed to calves – it’s a major transmission source.

Feeding pooled colostrum means total exposure to the disease. Without testing and selective use of colostrum all calves are put at risk.

More? www.dairynz.co.nz and www.drl.otago.ac.nz.

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