As an Associate Editor for CDC’s Emerging Infectious Diseases journal, I get an early look at some interesting articles (and have to read things that I wouldn’t necessarily get around to otherwise). The December edition just came out, and it had a variety of interesting articles. I’ll try to get to a few over the next couple of days, but this first one is an interesting case report of Mycobacterium bovis infection associated with cats (O’Connor et al 2019).

Mycobacterium bovis is a bacterium that’s closely related to the bug that causes most human tuberculosis, Mycobacterium tuberculosis. Mycobacterium bovis is linked primarily to cattle, but can be found in a variety of wild and domestic animal species, and it can infect people. Classically, human infections with M. bovis are associated with consumption of unpasteurized milk, but direct contact with infected animals can also result in transmission.

M. bovis is an issue in the UK, where it’s found in a variety of animal species, including cattle and wildlife (such as badgers).  This case report report from the UK involves two separate incidents of human M. bovis infection linked to cats. M. bovis is a rare cause of disease in cats, but it can happen in areas where the bacterium is present, through exposure to infected domestic animals or wildlife (or from eating contaminated raw meat).

The human cases were linked to a cluster of infections in cats in Dec 2012-Mar 2013. Seven confirmed feline cases were identified, with at least three more suspected. All of the cats were from households in the same area (less than 250 metres apart), and they had severe disease, consistent with tuberculosis. Presumably, they were exposed while tangling with infected badgers in the neighbourhood.

While sporadic cases in cats wouldn’t necessarily get much attention, the number of cases in a short period of time in close proximity to each other led to an investigation of human contacts. Close contacts were offered TB screening, and 24 people agreed to be tested. Three were positives for latent TB, meaning they were infected but didn’t have signs of active disease and were not shedding the bacterium. Essentially, these individuals had become infected and the bug had gone dormant in their body. That’s good, in that they’re not sick, but there’s always the potential for the bug to re-activate and cause active infection later on (and make the person a risk for transmitting the bacterium to others). Treatment was offered and one person accepted. One of the two that declined treatment then developed active TB six months later. A contact of that person (who also had an infected cat) had initially declined screening but then developed chest pain and fever…

What’s the take-home?

Beyond being an interesting story, the main thing to consider from a disease prevention standpoint is how to reduce human exposure to M. bovis in such a case. Here are some key points to take away from this report:

  • While rare, M. bovis is a risk to cats in areas where it’s present in wildlife. Outdoor cats have greater risk of exposure to a lot of things, including this bug in some parts of the world. Whether or not to allow your cat to go outdoors requires assessment of the risks (e.g. disease, trauma, wandering, environmental impacts) and benefits (e.g. cat’s behaviour and welfare).
  • All of the people who were infected had contact with cats with active disease, including discharge from visible lesions. Contact with infected sites logically increases the risk of exposure to whatever’s causing the disease. Avoiding contact with infected site, particularly pus, and good attention to hand washing and other basic hygiene practices probably reduce the risk of exposure and infection.
  • Diagnostic testing of your pet might help protect your health. There are instances where we can find something in an animal that poses a human health risk. Knowing that can be important (e.g. so you can tell your physician you were exposed to the bug if you get sick, speeding up your diagnosis / knowing what precautions to take when handling the pet). That’s the One Health concept that gets talked about a lot (but actually used much less commonly).

Encouragingly, the conclusion of the article points out some actual One Health actions:

Public Health England now advises that all close contacts of household companion animals with confirmed M. bovis infections should be assessed by a public health professional and receive guidance on how to best minimize zoonotic transmission. In addition, as part of an enhanced surveillance system in England and Wales, newly diagnosed human case-patients with M. bovis infection are now also asked explicitly about contact with pets with suspected or confirmed M. bovis disease.