# 8410
Readers with good memories will recall that back in 2010 I wrote a blog called Badgers? We Don’t Need No Stinkin’ Badgers!, that looked at a controversial plan to cull badgers in the UK in an attempt to reduce the wildlife reservoir of Mycobacterium bovis which is the cause of tuberculosis in cattle (known as bovine TB).
M. bovis is also capable of infecting humans (mainly through unpasteurized milk), although famers in contact with infected cattle are at risk as well.
In fact, of the three type of Tuberculosis bacteria (Mycobacterium bovis, M. avium, and M. tuberculosis – the most prevalent strain in humans), M. bovis has the largest host range – being capable of infecting just about all warm-blooded vertebrates.
Last year, a study appeared in the CDC’s EID Journal that attempted to estimate the global burden of M. bovis infection in humans (see Zoonotic Mycobacterium bovis–induced Tuberculosis in Humans), and found that while the number was small (roughly 1 per 100,000 pop.) - it was not insignificant – particularly in areas of the world where unpasteurized milk is still widely consumed.
DEFRA calls Bovine TB one of the biggest challenges facing UK cattle industry, and cites the following key facts:
- 5.5 Million – total number of TB tests on cattle in England in 2011.
- 28,000 – approximate number of cattle slaughtered for TB control in England in 2012.
- 3,900 – approximate number of new TB incidents in 2012 (herds where at least one animal tests positive for bovine TB, when the herd had previously been TB free).
- 11.5% of cattle herds in England were under cattle movement restrictions at some point in 2011 (the 2012 statistics will be published once additional quality assurance checks have been completed).
- 23.6% of cattle herds in the South-West were under cattle movement restrictions at some point in 2011 (the 2012 statistics will be published once additional quality assurance checks have been completed).
- £500 million – the amount it has cost the taxpayer to control the disease in England in the last 10 years.
- £1 billion – estimated cost of TB control in England over the next decade without taking further action.
- £34,000 – the average cost of a TB breakdown on a farm, of which around £12,000 falls to the farmer.
While many farmers see badgers as the primary source of their bovine TB woes, and blame them for reintroducing the disease into their herds each year, conservation and animal rights groups strongly disagree, and argue that the badger is the victim here.
They maintain that cow-to-cow transmission is the primary route of infection, and that badgers usually get the disease from cattle – not the other way around.
Four years, and a pilot culling program later, and the controversy still reigns. Recent media coverage has run the gamut from TB strategy about much more than badger culling – Paterson -Farmers Guardian to MPs vote overwhelmingly to halt badger cull in England – The Guardian.
Adding a new dynamic to an already complex and contentious debate, yesterday Public Health England released a report on two rare human infections with M. bovis – both associated with an outbreak in cats – which likely became infected via contact (directly or indirectly) with badger setts (dens).
The feline outbreak is described in a letter published in the BMJ’s Veterinary Journal called Mycobacterium bovis infection in cats by Nigel Gibbens, which prompted a full epidemiological investigation. A brief excerpt:
BETWEEN December 2012 and March 2013, a veterinary practice in Newbury (west Berkshire) diagnosed nine cases of Mycobacterium bovis infection in domestic cats. In seven of those cases the diagnosis was confirmed by bacteriological culture. The nine affected cats belonged to different households and six of them resided within a 250 metre radius. The animals presented with mycobacterial disease of variable severity including anorexia, non-healing or discharging infected wounds, evidence of pneumonia and different degrees of lymphadenopathy. The latest information is that six of the cats have been euthanased or have died. The three surviving animals are undergoing treatment and are reported to be responding. At the time of writing, no new cases had been detected in local cats since March 2013.
PHE published the following press release on their website yesterday regarding the epidemiological investigation that turned up two probable cases where humans contracted M. bovis from cats.
Cases of TB in domestic cats and cat-to-human transmission: risk to public very low
Published 27 March 2014
Two people in England have developed tuberculosis after contact with a domestic cat infected with ‘Mycobacterium bovis’ (‘M. bovis’), Public Health England (PHE) and the Animal Health and Veterinary Laboratories Agency (AHVLA) have announced.’‘M. bovis’ is the bacterium that causes tuberculosis (TB) in cattle (bovine TB) and in other species.
Nine cases of ‘M. bovis’ infection in domestic cats in Berkshire and Hampshire were investigated by AHVLA and PHE during 2013. PHE offered TB screening to 39 people identified as having had contact with the infected cats as a precautionary measure. 24 contacts accepted screening. Following further investigations, a total of 2 cases of active TB and 2 cases of latent TB were identified. Latent TB means they had been exposed to TB at some point but they did not have active disease. Both cases of active TB disease have confirmed infection with ‘M. bovis’ and are responding to treatment.
There have been no further cases of TB in cats reported in Berkshire or Hampshire since March 2013. PHE has assessed the risk of transmission of ‘M. bovis’ from cats to humans as being very low.
Dr Dilys Morgan, head of gastrointestinal, emerging and zoonotic diseases department at PHE, said:
“It’s important to remember that this was a very unusual cluster of TB in domestic cats. ‘M. bovis’ is still uncommon in cats - it mainly affects livestock animals. These are the first documented cases of cat-to-human transmission, and so although PHE has assessed the risk of people catching this infection from infected cats as being very low, we are recommending that household and close contacts of cats with confirmed ‘M. bovis’ infection should be assessed and receive public health advice.”
The findings of the animal health aspects of this investigation are published in The Veterinary Record today, 27 March 2014.
Molecular analysis at AHVLA showed that ‘M. bovis’ isolated from the infected cats and the human cases with active TB infection were indistinguishable, which indicates transmission of the bacterium from an infected cat. In the other cases of latent TB infection, it is not possible to confirm whether these were caused by ‘M. bovis’ or the source of their exposure.
Transmission of ‘M. bovis’ from infected animals to humans can occur by inhaling or ingesting bacteria shed by the animal or through contamination of unprotected cuts in the skin while handling infected animals or their carcasses.
Professor Noel Smith, Head of the Bovine TB Genotyping Group at AHVLA, said:
“Testing of nearby herds revealed a small number of infected cattle with the same strain of ‘M. bovis’ as the cats. However, direct contact of the cats with these cattle was unlikely considering their roaming ranges. The most likely source of infection is infected wildlife, but cat-to-cat transmission cannot be ruled out.”
Cattle herds with confirmed cases of bovine TB in the area have all been placed under movement restrictions to prevent the spread of disease.
Local human and animal health professionals are remaining vigilant for the occurrence of any further cases of disease caused by ‘M. bovis’ in humans, cats or any other pet and livestock animal species.
The PHE also released a HAIRS Risk Assessment, where they characterized the risk to public health as:
A Very low risk of transmission of M. bovis from cats to humans.
Although the risk of acquiring TB from a domestic cat in the UK is exceedingly low, and even less likely here in the United States, this report illustrates how animals – both wild and domestic – can carry and transmit zoonotic infections to humans.
This intersection of man and other species, and their sharing of viruses (zoonotic transmission), has increasingly been recognized as a driving factor in emerging infectious diseases, and even the creation of pandemics.
The age of emerging infectious diseases in humans really began in earnest about 10,000 years ago when humans began to domesticate – and live in close proximity to – other animals (see The Third Epidemiological Transition).
Measles probably evolved from canine distemper and/or the Rinderpest virus of cattle. Tuberculosis, which now infects 1/3rd of humanity, likely jumped from domesticated goats and cattle. And influenza’s all seem to have an origin in waterfowl.
Other zoonotic nasties include Babesiosis, Borrelia (Lyme), Nipah, Hendra, Malaria, Hantavirus, Ebola, Leptospirosis, Q-Fever, bird flu . . . the list is long and growing.
Roughly 70% of the infectious diseases that afflict man today are believed to have begun in some other species, and new ones (think MERS-CoV, H7N9, H5N1, SFTS, etc. ) continue to show up each year. We live in an amazingly complex and interconnected world, where what happens in a live poultry market in China, a camel stable in Saudi Arabia, or a pig farm in Mexico can ultimately impact the health of people around the world.
So we watch these spillovers of diseases from animals to humans – no matter how rare, or small they may be – with considerable interest.
