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Lessons in TB control from home and abroad

At the Official Vet Conference 2018, discussions around bovine TB ranged from badger ecology in the UK to white-tailed deer hunting in the US

21 November 2018, at 9:11am

It may not have occurred to many veterinary surgeons that tins of beans, spaghetti and other comestibles could help to explain issues with cattle TB. A full-size tin represents a high level of control and a smaller tin partial control.

In comparing the control of BVD and TB, both have full tins of surveillance, with BVD showing good biosecurity and good resilience, whereas TB has inadequate biosecurity and resilience. BVD has better biocontainment than TB, but both need an upgrade. As James Russell, Director of VetCo, piled up the tins, delegates of the Official Vet Conference were predicting whether a full tin or half tin was to appear out of the bag.

The title of the talk was “TB – Does it deserve its pedestal?” Recognising that TB is given special status internationally, whole strategies are demanded of the UK to control and eradicate the disease. The speaker discussed the need to question whether the high level of activity taking place is matched by the disease occurrence. Further sessions explored the importance of understanding badger ecology, learnings from Cymorth TB in Wales, the test options for TB detection and the impact of deer hunting on bovine TB in Michigan.

Lessons in badger ecology

More farms need to engage with biocontainment, and one way of reducing the disease risk is to make farms less attractive to badgers. Testing and surveillance are expected to increase, particularly within the High Risk Area, and it is anticipated that more cases of cattle disease will be diagnosed. The involvement of veterinary practices with practical measures to assist on-farm control may also mean that more vets will take advantage of the TB Advisory Service.

Andy Robertson gave delegates a crash course in badger ecology
Andy Robertson gave delegates a crash course in badger ecology

Andy Robertson, a scientist at Exeter University, explained how a better understanding of badger ecology can help vets advise on TB biosecurity. The speaker pointed out that there was considerable misunderstanding at farm level about the activity of badgers. Badger activity and movements dictate how the animals interact with the farm and the cattle, and the ability of vets to relay aspects of badger ecology to farmers could help to reduce disease risk.

A primary consideration is the availability of food and delegates were shown various examples of means that could be used to prevent badgers entering yards and buildings. Sheeting on gates and raised feeding troughs are expected to limit the availability of food for the badgers, for example, potentially reducing contact with cattle.

TB in badgers can remain latent for many years. As with humans, the triggers for clinical disease remain unclear, but many badgers carrying the disease will show no outward signs of the disease. Mycobacterium bovis can survive in cattle and badger faeces for up to six months and in soil for up to three months. Keeping cattle away from badger latrines is therefore another recommended exclusion measure. It is accepted that cattle are at risk from infection during the grazing season, where badgers and cattle are cohabiting, but only a small proportion of the badgers on a farm are expected to be infected.

Extensive measures to keep badgers out of buildings are based on ecology and common sense; it should be noted that there is no study that shows how effective the measures are, particularly if the cattle herd is already infected. Understanding the local bTB situation is important and to assist awareness, the current disease status of neighbouring herds is available on the TB Hub website.

An update from Wales

A review of Cymorth TB in Wales is taking place, three years after the project was rolled out across the country, with the aim of identifying the lessons learned, progress in achieving the aims of the project and how best to take the programme forward. Katie Rose, who leads the Cymorth TB programme from the APHA, explained that the basis of the programme initially was for the farmer to take responsibility for farm animal disease, the private vet to provide good quality advice and for APHA to share current validated research and best practice disease control measures.

With a current bovine TB breakdown, a three-hour visit assessment by the farmer’s own vet, who has been trained, is available and paid for by the Welsh government. The findings will be reported to APHA. At the end of the breakdown, a one-hour visit is provided. To date, 135 OVs have been trained, and 76 are currently being trained.

There have been 497 completed farm visits and 2,279 recommendations made to farmers, with multiple measures for each farm. Sixty-seven veterinary practices are participating, with 500 vets carrying out TB testing in Wales; many testing vets have not yet engaged with the training. Vouchers are issued for the free visits – a total of 2,932 have been given to farmers, but only 22 percent of the new breakdown herds have taken advantage of the scheme. A farm-level report is available to support the farm visit that includes the results of past tests, the genotypes, details of reactors, cattle purchase history and whether cattle have been bought in from herds with a history of bTB.

Road casualty badgers have been collected and cultures indicate that it is common to find a badger from outside the home range of local badgers. Feedback from all concerned makes up an important part of the review and the results will be integrated to guide future activity. Improved communication between the practice vets and Cymorth is already being enacted through the provision of OV workshops.

Shelley Rhodes described testing options for TB in camelids
Shelley Rhodes described testing options for TB in camelids

Blood testing of cattle and camelids

Experiences with TB blood testing of cattle and camelids was discussed by Shelley Rhodes, test consultant for APHA’s TB Research Group. Gamma interferon testing improves the sensitivity in cattle to 90 percent, with the blood test detecting infected animals earlier. Skin test negatives can be shown to be positive with the blood test, but 3.5 percent of false positives are recorded with gamma interferon, compared to 0.2 percent with the skin test, Shelley explained.

In a study, 936 inconclusive reactors to the skin test were shown to be positive with gamma, and 3 to 20 percent of cattle slaughtered with no visible lesions at post-mortem are thought to be gamma positive. Antibody testing has shown that 3.6 percent of gamma negative samples are antibody positive.

Camelids are considered to be “spill-over” hosts for bovine TB. The skin test in camelids has a low sensitivity of 0 to 15 percent, whereas antibody testing shows a high specificity and sensitivity when used as serial and parallel tests. Antibody testing in camelids can identify TB-positive animals. In 2014, antibody testing for camelids was made available, providing test options for confirmed infected herds and TB-free camelid herds in a contiguous situation. The speaker concluded that an increasing use of blood testing is assisting TB control, helping on the road to disease eradication.

Paul Bartlett has found that deer hunting is a greater priority than TB control in Michigan
Paul Bartlett has found that deer hunting is a greater priority than TB control in Michigan

The impact of deer on TB transmission abroad

Paul Bartlett, Professor at Michigan State University, explained that endemic bovine TB in white-tailed deer is an important source of infection for cattle. Cattle herd-to-herd transmission is rare in Michigan, whereas contact with infected deer is commonplace. To increase deer numbers for hunting, deer have been fed and there have been feed station deer in large groups that stay close to the food, with wild deer in the woods.

Deer hunting is of greater economic benefit than cattle farming in the area studied. The concentration of deer in an unnatural environment leads to transfer of infection. Feeding deer is now illegal, but baiting (putting out feed prior to shooting) is legal and it is difficult for the authorities to enforce the law.

In 2016, 586,000 deer were shot. There is a strong local hunting tradition for meat as well as trophies. There is a low actual clinical incidence of human TB. The deer have TB lesions but appear healthy. Haemorrhagic disease in deer is an ongoing problem and the incidence of chronic wasting disease is increasing.

Rules for the control of TB in cattle are the same for each state but it appears to be a waste of time doing cattle tracing in Michigan as a bovine TB control activity. In conclusion, the speaker indicated that, for the control of bovine TB in Michigan, the social and political situation dwarfs the biological problems

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