March 28, 2014

expert reaction to TB in cats and cat-to-human TB transmission

Nine cases of M. bovis infection, which causes bovine TB in cattle, were identified in domestic cats in Berkshire and Hampshire by the Animal Health and Veterinary Laboratories Agency (AHVLA) and Public Health England (PHE) during 2013. Two people were found to have developed TB after contact with a domestic cat while two cases of latent TB were also identified.

Dr Rachel Dean, Director of the Centre for Evidence-based Veterinary Medicine, Clinical Associate Professor in Feline Medicine, and Recognised RCVS Specialist in Feline Medicine, University of Nottingham, said:

“M bovis infection is very rare in both cats and in humans.  The absolute risk is still unclear but M bovis most commonly infects cows rather than humans and cats, so this cluster of cases is an unusual and probably isolated situation. As the PHE say the risk is VERY low, so cat owners in general don’t need to worry about contracting this disease from their cat.

“Whether infected cats should be treated or euthanased is a controversial area and opinions differ. The choice over whether to treat lies with the cat owner and their vet, and it is important to consult experts in this field when considering case management.

“We don’t know the prevalence of M bovis infection of cats, either now or in the past, so we can’t say whether this is an increase in number of cases or not. What has happened is that we have changed the way cases are reported so know the relevant authorities know about them.  It may be that in fact we see fewer cases now than in the past as we don’t give cats unpasteurised milk.  The number of cats reported with mycobacterial infection is small (60-130 cats per year) and approximately only 15% are due to M bovis. There is of course a big concern about the increase in M bovis infection in cattle, and M tuberculosis infection in humans – we must not let this minor concern over-ride the need to look at bovine and human TB seriously.  It is possible that the increase in M bovis in cattle may have an effect on M bovis in other species but we don’t know that yet.

“M bovis is an infection of cows, and M tuberculosis is an infection of people. It is possible that the different species infect other species – as in this case.  However, this is a rare event, but it does happen.  The epidemiology of this disease is poorly understood and needs more research.”

Dr Simon Clarke, a microbiologist at the University of Reading, said:

“The study by Public Health England has shown that a handful of cases of TB can be linked with infections in people’s cats by a bacterium called Mycobacterium bovis.  It’s important to understand that this is not proof that people caught the infection from their cats, but that is a possibility.

“While infection with this bacterium (Mycobacterium bovis) in cats is not unheard of, human infection with this bacterium is rare, indeed it’s more commonly associated with TB in cattle.  Human TB is usually caused by a different bug, called Mycobacterium tuberculosis.”

Prof Malcolm Bennett, Professor of Veterinary Pathology, University of Liverpool, said:

“Doubtless this report from Public Health England (PHE) and the Animal Health and Veterinary Laboratories Agency (AHVLA) will, through raising awareness, lead to further cases being diagnosed, and we will have to take care not to confuse increased diagnosis with increasing rates of infection.”

“The bacterial species that causes bovine TB, Mycobacterium bovis, is part of a closely related group of bacteria that, in Britain, includes the agents causing human TB and vole TB.  While mainly circulating in cattle, people and voles respectively, all can infect a wide range of mammals including humans.  In the 19th and early 20th Centuries, bovine TB was relatively common in people, who contracted it from drinking milk: nowadays it is relatively rare owing both to control of the disease in cattle and, particularly, the pasteurisation of milk, a heat treatment that kills the bacteria. Control in cattle was based on testing and culling. The re-emergence of bovine TB in cattle in recent decades is complicated by the involvement of wild badgers in the ecology of the infection: controlling the spread of the disease in cattle is hard enough, but control of any infection in wildlife is notoriously more difficult still and the means of doing so, certainly in the case of badgers and TB, politically contentious.

“Occasional cases of TB in pets have always been seen – often human TB in dogs, probably caught from their owners, and vole TB in cats, presumably caught through hunting wild rodents. There seems to be an increase in the number of cases of bovine TB diagnosed in cats in recent years, and the report from PHE and the AHVLA describing a cluster of feline cases and associated human infections emphasises both the wide host range of these bacteria and that sharing our lives with other animals, whatever the benefits, carries some small risk. However, human infection, feline infection and transmission between the two remain rare.

“This cluster of feline cases and spill-over into their owners may reflect the expanding epidemic of bovine TB in cattle and wildlife, but it is unclear precisely how the cats involved became infected in the first place. It is difficult to imagine direct contact between all these cats and either cattle or badgers – perhaps another, transient, wildlife host was involved, perhaps there was a shared contaminated environment, or perhaps there was some cat-to-cat transmission.  Whatever the initial route, however, these cases provide a reminder to vets that tuberculosis can occur in pets and may put their owners at risk, and to doctors that not all human TB is caught from other humans. Given that treatment of cats and dogs requires lengthy use of special antibiotics and is seldom successful, the risk of transmission to owners and other animals probably means that euthanasia is the sensible option once a diagnosis is made.”

Prof Mark Fielder, medical microbiologist Kingston University, and Honorary General Secretary of the Society for applied Microbiology (SfAM), said:

“The recent report of Mycobacterium bovis infection in cats is a highly unusual situation and we currently have no evidence that this occurrence has spread. With this is mind the risk to the public and pet owners remains very low. Having said that we should, as a precautionary measure, take the advice of the PHE in that people who have been in contact with an animal with a known infection should see their doctor just to ensure they have not been exposed.”

Prof Bertie Squire, Professor of Clinical Tropical Medicine, Liverpool School of Tropical Medicine, and Director, Liverpool School of Tropical Medicine Centre for Applied Health Research and Delivery, said:

“While it is important to draw attention to these two unusual cases of M. bovis transmission from cats to humans, it is also important to retain a sense of proportion about what this means in relation to the problem of tuberculosis (TB) in the UK.  The latest data available on the Public Health England website are from 2012.  In that year there were 8,130 cases of human tuberculosis in England.  In the same year there were only 26 notified cases of M. bovis in England.  So M. bovis accounts for less than 0.5% of all human TB cases in the UK.  The real problem of TB in the UK is caused by M. tuberculosis which arises in humans and is transmitted from person to person.  If we are to control human TB in the UK then we need to focus on identifying and curing the TB that occurs in humans, and we need to do this much better than we do at present.  It is important that our efforts in this direction are not diverted.  The real problem of human TB in the UK has nothing to do with M. bovis in cattle, badgers, or cats.”

Prof David Moore, Professor of Infectious Diseases & Tropical Medicine, TB Centre, London School of Hygiene and Tropical Medicine, said:

“The original paper in Veterinary Record yesterday clearly demonstrates a cluster of M bovis cat infections in and around Newbury. Though the vets subsequently demonstrated that local cattle were also infected, many of the cats seem to have been infected through scratches or bites, probably from infected rodents that had been in contact with the cattle.

“There is no detail, beyond the PHE press release, about the 2 human patients with active TB, which it is said share the same strain according to genotypic fingerprinting. It is suggested that this demonstrates cat to human transmission which is interesting but without further epidemiological detail seems rather speculative. For example, though the two new active cases have developed disease since initial screening (and therefore are unlikely to be the index source of infection) there may be other human contacts amongst those who were not initially screened who could have had active disease and infected the cats. That said, we undertook a study of pet dogs of TB patients in Peru and were unable to demonstrate any infections amongst household pets.

“Even if the cat to human story is true it still only means that 2 of over 8500 cases of TB in the UK each year have occurred by this route so it is vanishingly rare (and this might be the only cases ever reported in the UK).

“It would certainly be interesting and important to have more information about the genotyping and clinical features of the human cases (particularly whether they are pulmonary or extrapulmonary, e.g. lymph node) and whether they may have had potential occupational exposures because an alternative hypothesis is that they could have been infected from the same common source as the cats. No doubt the excellent zoonotic group at PHE are on top of this.”