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Johne’s “hugely

on UK sheep farms

Johne’s disease is the most important of the iceberg diseases, infecting up to 79% of sheep ocks in the UK, research suggests. Yet only 2% of farmers are thought to be aware their ocks are a ected, with the greatest nancial losses coming from subclinical cases. Sarah Kidby spoke to Dr Peers Davies, about the latest research conducted by his research group at the University of Liverpool.

The prevalence of Johne’s in sheep ocks in England, Wales and Scotland, ranges from 59–79%, according to four separate studies carried out by Dr Davies and Dr Worsley, University of Liverpool, since 2018. The gure varies in di erent parts of the country and depending on which animals are being tested, with cull ewes being the most sensitive group, as expected. Prevalence in this context is the proportion of ocks and individuals that test positive for the bacteria that causes Johne’s, not just those that develop clinical disease.

“All of the studies report very high prevalence – vastly higher than any of the other iceberg diseases, and more than double the prevalence of the next most common iceberg disease: border disease. It really pulls Johne’s out from that group of ve diseases as being the one that we need to be most aware of,” he explains.

The proportion of ewes that test positive within the ock varied even more widely, from around 2% all the way up to 75%. It’s thought this is impacted by how long Johne’s has been in the ock, how intensively the ock is managed and how genetically susceptible the breeds are.

Impact of sub-clinical cases

Most animals su ering with the disease will likely be culled for productivity reasons long before they reach the nal visible stage of the disease, typically because they scan empty, so most farms may be unaware Johne’s is the cause. A survey of nearly 650 farmers in 2013, published in 2019, revealed less than 2% of respondents knew they had Johne’s in their ocks – whereas the true gure is likely to be closer to 60–70%.

A key part of the problem, Dr Davies believes, is that we don’t use production KPIs in sheep farming enough, meaning there is a lack of quantitative data on how ocks are performing.

To focus only on animals that show overt signs of disease is “missing the point” as the biggest nancial losses “are almost certainly associated with sub-clinical disease”, he says.

Researchers consistently found a signi cantly higher number of Johne’s-positive animals in groups that scanned empty compared to ewes who scanned in-lamb from the same ocks.

“What we think is happening is that Johne’s is causing in ammation in the gut, which is making sheep less e cient at absorbing nutrients, moving them into negative energy balance. When they’re struggling, the rst thing they’re going to sacri ce is their reproductive system and they become more likely to scan empty,” Dr Davies explains.

Ewes are therefore removed from the ock, which drives a much higher level of forced culling. “The Johne’spositive ocks are running a much younger ock, with signi cantly higher replacement costs – around 30% compared to around 22–24% typically for negative ocks.”

In infected ocks, the proportion of three-, four- and veyear-old animals drops to around half the number seen in uninfected ocks, ndings show.

“This has a massive impact because we're taking ewes out when they should be at their most productive in their third and fourth pregnancy, but those culls aren’t always being investigated.”

Testing and control

A blood sample is available but can only pick up Johne’s at a very late stage and cannot di erentiate between vaccinated animals and those with natural antibodies. Faecal PCR tests are more sensitive and for that reason should be used by vets for screening. Laboratories have done far more work on cattle PCR tests than sheep so there is a lot of variability between labs; Dr Davies says he has found the APHA lab to be particularly sensitive and appropriate for screening.

To detect whether Johne’s is present, Dr Davies recommends four pools of ve samples each from cull ewes. To understand how the presence of Johne’s might be causing losses in the ock, farmers and their vets can test two or more pools of cull ewes and an equal number of pools made up of healthy, in-lamb ewes to determine if there are more positive pools among the cull ewes.

Unlike other diseases such as Maedi Visna, the test and cull approach does not work with Johne’s, as the diagnostic tests cannot pick up infections early enough, Dr Davies advises. Management approaches to reducing transmission and infection are generally very labour intensive and “extremely di cult to the point of being impractical” for commercial lamb producers.

Vaccination is often the most straightforward method of control, he adds. A one-shot-for-life vaccine, Gudair, is available and costs around £3 per dose. The decision whether or not to vaccinate is one to take in consultation with your vet. Where it is deemed to be appropriate Dr Davies recommends only vaccinating replacements to minimise wasted doses and adverse reactions. The vaccine must be used in early life to be e ective – between 1–6 months old, but ideally around 4–5.5 months of age, he says. Farmers should be thinking about vaccinating after weaning.

Dr Davies and Dr Worsley are currently working to develop a calculator allowing farmers to input their replacement costs and ock fertility to determine what Johne’s is costing them, and therefore if it is cost e ective to vaccinate. FG