Monday Micro – First documented cases of cat to human transmission of TB

By Siouxsie Wiles 31/03/2014

A paper just published in the Veterinary Journal documents the first cases of transmission of Mycobacterium bovis* (the bacterium responsible for TB in cattle and many other species) to humans (1). Before anyone get’s into a panic, it was a year ago.

Alas, the article is behind a paywall, but according to the free abstract a vet practice in Newbury, in the UK, diagnosed nine domestic moggies with M. bovis infection between December 2012 and March 2013. The nine cats ‘belonged’ to different households** and six of them resided within a 250 metre radius. The animals presented with varying symptoms and severity and 6 were euthanased or died, while the three surviving animals were responding well to treatment. At the time of the article, no new cases had been detected in local cats since March 2013.

As people can become infected with M. bovis, Public Health England offered to screen 39 people identified as having come into contact with the infected moggies. 24 people accepted their offer; 2 people were found to have active M. bovis TB (this means they would be infectious), while another 2 people were found to be latently infected (and therefore not infectious). All are reported to be responding to treatment.

Molecular analysis has shown that the M. bovis isolated from the infected cats and people with active TB infection were indistinguishable, indicating transmission of the bacterium from an infected. The same strain has also been found in cattle from nearby herds, although according to Professor Noel Smith, Head of the Bovine TB Genotyping Group at the UK Animal Health and Veterinary Laboratories Agency, the cats are unlikely to have caught if directly from the cattle but from infected wildlife. He probably means badgers.

If you are curious how often cats are diagnosed with bovine TB in the UK, the Department for Environment, Food and Rural Affairs (DEFRA) have put up a spreadsheet with data going back to 1997 (the data for 2013 is provisionally as it only covers up to September). As you can see from the graph below, 2013 was far from the highest year. That honour goes to 2009 with 26 cases.

TB cats

It’s not just cats that have been diagnosed with M. bovis though. Pet dogs, pigs and even a few ferrets have had TB.

TB domestic

To put all these numbers in perspective, according to DEFRA the number of cattle compulsorily slaughtered in 2013 alone because they tested positive for M. bovis, or are direct contacts of positive animals, was 32,620. So the cats are really just a drop in the ocean. What this case does show though is just how versatile M. bovis is when it comes to picking a host. It also shows that we share our microbes with our pets, for better and worse.

And finally, an update on last week’s Monday Micro, the current Ebola outbreak in west Africa. The strain of Ebola responsible is Zaire, the strain most frequently associated with outbreaks and the one with the highest mortality. So far the total number of suspected and confirmed cases in Guinea has increased to 103, with 66 deaths, a mortality rate of 64%. Both Sierra Leone and Liberia have now alerted the WHO of suspected cases and deaths consistent with Ebola infection among people who have recently traveled to Guinea. The numbers for Liberia are 8 suspected cases, including 6 deaths, while for Sierra Leone it is 6 suspected cases, including 5 deaths.

1. Gibbens N (2014). Mycobacterium bovis infection in cats. Veterinary Record 174:331-332 doi:10.1136/vr.g2344

*As an aside, the TB vaccine, commonly referred to as Bacille de Calmette et Guérin or BCG, is derived from a strain of M. bovis that was cultured in a laboratory for many years until it had lost some crucial genes that it needs to cause disease.

**Any cat owner will tell you its more like the other way around….

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