Bovine tuberculosis (bTB) is an infectious disease of cattle (Bos primigenius) caused by the bacterium Mycobacterium bovis. It develops slowly but it ends up very nasty. You might choose to follow the ‘tuberculosis’ tag on my blog.
M. bovis can also infect people – that is, bTB is zoonotic. So it’s a notifiable disease. If you find it, you’ve to report it. DEFRA’s veterinary wing, the Animal Health and Veterinary Laboratories Agency (AHVLA) provides a list of notifiable diseases affecting livestock in Britain.
M. bovis can also infect other animals. For example it can infect the European badger (Meles meles) which is native here, and the brushtail possum (Trichosurus vulpecula) in New Zealand where it’s been introduced from its native Australia. Badgers, possums and various other animals can be wildlife ‘reservoirs’ carrying bTB.
But there’s a vaccine against bTB! It’s called the Bacillus Calmette-Guerin (BCG) vaccine. It can be used against all kinds of tuberculosis.
Animal Research Info (ARI) tells us how BCG was developed in the early 20th century. It was developed from a strain of M. bovis that had become ‘attenuated’ in the lab, making it unable to cause disease. BCG was developed as a treatment but, says ARI, ‘[This drug] can be used to prevent infection rather than being limited to restricting the extent of the disease.’ Sounds good doesn’t it? A drug that works as a vaccine.
So why not just vaccinate? If it were that simple, we’d simply do it. But it’s not that simple.
The downside is that BCG vaccination makes the vaccinated animal or person ‘seroconvert’. Antibodies against M. bovis appear in the blood and stay there. That is, vaccinated animals and people test positive for bTB as if they actually had the disease. They become ‘reactors’. If we’re talking about cattle, under current UK law, reactors are shot.
DEFRA requires everybody who keeps cattle in Britain to get the herd tested. ‘The primary screening test for TB in cattle in Great Britain is the single intradermal comparative cervical tuberculin (SICCT) test, commonly known as the tuberculin “skin test”… As of 1 January 2013, England is divided into two cattle TB testing frequency areas. Farms in counties in the south-west, west of the country and East Sussex where the majority of TB cases are found are to be tested for TB annually. The rest of England is on four-yearly testing, although higher risk herds in these areas will be tested annually.’
AHVLA tells us what happens if bTB is detected in your herd. ‘Movement restrictions, preventing cattle from moving off your premises, will be imposed… Reactor cattle… must be immediately isolated until AHVLA can arrange for their removal to slaughter. Milk from reactor animals must not be used for human consumption.’
DEFRA continues, ‘Cattle that react to the skin test (or any other diagnostic TB test) are removed for slaughter and the cattle owner compensated. The herd is placed under movement restrictions (its Officially Tuberculosis-Free (OTF) status is suspended, or withdrawn if disease is confirmed) until all remaining cattle in the herd have cleared two further short interval tests (at 60 days apart). Movement restrictions are then lifted but the herd has further tests 6 and 12 months later. If the herd remains clear, testing reverts to the routine test frequency set for the area.’
Some people think that these rules aren’t strong enough. David Heath at DEFRA tells us that the rules will become tighter next month. He says that DEFRA aims at ‘stamping out infection in areas where the disease is spreading, known as the “edge” area… [which] includes Nottinghamshire, Leicestershire, Northamptonshire, Berkshire, Hampshire, and parts of Cheshire, Derbyshire, Warwickshire, Oxfordshire and East Sussex…
‘The new edge area measures include:
• Immediate skin testing of any herds in Cheshire and Derbyshire within a 3km radius of a farm with a new TB outbreak, and another test after 6 months.
• Herds that have their TB Free status suspended following skin testing will need to show two further clean tests.
• Herds that have their TB Free Status withdrawn will all require gamma-interferon blood testing, which is a more sensitive test for spotting infection.
• Breaking of Cattle Tracing System ‘links’ between the edge area and high risk areas, which allow farmers to move cattle between two areas without reporting the movement.’
Mr Heath’s announcement discusses badger screening and vaccination too. I’ll write about those topics. Also, I’ve more to say about BCG and about the other tests that can be done. Just not today.