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Resolve to remove reservoirs of infection (Nov/Dec 23)

Taking a more systematic approach to tackling endemic bovine TB could mirror the progress achieved by UK herds enrolled in Johne’s disease and BVD control schemes.


TEXT RACHAEL PORTER



There are still many producers who believe there’s a limit to what they can do to mitigate the impact of bovine TB on their herds and businesses. “The disease’s insidious nature can make attempts to control it seem futile, but there are steps that producers can and should take to tackle it. And we know that they work,” says vet Keith Cutler, from Synergy Farm Health, who is a CHeCS board member and has been involved in developing bovine TB (bTB) control strategies and diagnostics for the past 20 years.


He’s set to share some of TB Advisory Service’s work with delegates attending November’s National TB conference. “Monitoring the impact of a combination of tools and approaches for the past 15 years has proved that it is possible to reduce the risk of new infection and help prevent herds from being closed by bTB,” he says. Some infection does come from outside vectors – badgers and/or bought-in or neighbouring cattle. But some is the result of endemic infection – the disease is present and circulating within the herd.


“Yet minimising the risk of spread within the herd is often overlooked,” he says. “It’s easy to believe it’s out of producers’ control. But there are steps producers can take, particularly when it comes to cattle that are resolved inconclusive reactors,” says Mr Cutler. An approach similar to that taken to control and eradicate Johne’s disease and bovine viral diarrhoea (BVD) also works well with bTB. “Bovine TB is, put simply, just another infectious disease of cattle. Like Johne’s disease, it is a mycobacterial disease, so it’s worth talking a closer look at how the industry is taking steps to control and manage Johne’s, and applying a similar approach to tackling bTB.”


Identifying and addressing reservoirs of infection and breaking routes of disease transmission are vital. Boundary biosecurity and quarantine are also important, as is ensuring that the herd is robust and resilient. If cattle are fit and healthy they are better able to fend off a disease challenge.


Skin-test results


Key is to look at bTB skin-test results in more detail and to use and act on that information. “Yes, the test finds reactors that have to be culled from the herd. But what about inconclusive reactors and ‘near misses’ with smaller bovine reactions than avian reactions? They pass the skin test and, according to DEFRA rules, can remain in the herd. But why have they got a bovine reaction at all? Are they infected and do they pose a threat to the health of the herd?”.


This ‘nuanced’ approach means these cattle have passed the test, but can be identified as high-risk. Data also shows that 75% of resolved inconclusive reactors (rIRs) will go on to react, or test positive for bTB, in the next five years. Producers are allowed, officially, to keep these cattle in their herds. But should they? Mr Cutler says no. “They pose a risk to the rest of the herd, so producers need to weigh that up. It’s similar to keeping a Johne’s disease red cow or a BVD PI in the herd. They are potential ‘shedders’ and reservoirs of infection and that puts other cattle at risk. So I suggest a similar ‘hard-line’ approach, where possible, and that these rIRs are culled at the first available opportunity.”


He says making such decisions, when producers really think about it, is usually easier than they realise. “Closer inspection of these rIRs often reveals other disease, fertility and production issues that make it easier to justify early culling. It’s surprising how many of these ‘high-risk’ cows also have persistently high SCCs, chronic lameness, test red or amber for Johne’s disease, or have poor fertility.


“So, fundamentally, we know these are not ‘healthy’ cows and they also pose a potential disease risk to the rest of the herd,” says Mr Cutler. “Some honest conversations have to be had about the true merits of keeping these cows in the herd and the wider herd health and economic risks that they pose.”


This approach is hampered by the misconception among some producers that the skin test is flawed. “If a postmortem report says a reactor has ‘no visible lesions’ some think that means that the animal was negative for bTB and the skin-test result was a false positive. The reality is that a positive skin test result is 99.98% accurate and ‘no visible lesions’ means just that. The animal still most definitely had bTB,” stresses Mr Cutler.


“Producers often say they can’t afford to cull rIRs, but yet most dairy herd replacement rates are running at 25% or 30%. So producers could be more discerning about voluntary culls and make some short-term decisions that will pay substantial long-term dividends to their herd’s health, welfare, productivity and overall efficiency.”


“These rIRs should be treated similarly to cattle classified as at high risk of Johne’s disease (red or orange) or BVD PIs if producers are to rid their herds of bTB.”


The bottom line is that it would be a good start to remove rIRs from the herd: “Ideally, as soon as possible after their clearing test. Producers should certainly avoid serving rIRs again, and cull them at the end of their lactation or when their calf is weaned. Just get them out and away from the herd. They are highly likely to be carrying the disease and have the potential to shed and spread it.”

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