Dog on leash2A few times a year I get questions about tuberculosis (TB) in dogs, typically after a person is diagnosed with active TB and people want to know what the risks are to, and from, their dog. It’s often tough to answer those questions, because there’s limited information available. Risks exist, but how much of a risk and how to assess that risk are difficult figure out.

A recent paper doesn’t answer all the questions but provides some more insight. The short communication in Transboundary and Emerging Diseases (Ribeiro et al 2016) describes a fatal case of tuberculosis in a dog. That’s interesting by itself, since TB infection in dogs is pretty rare, even in dogs with long-term contact with infected people. Another twist to the case is that the dog was infected with a strain of Mycobacterium tuberculosis that was resistant to some important anti-TB drugs.

Here are some of the highlights:

  • The two-year-old dog had a history of bloody stools and weight loss over the preceding eight months. The dog’s owner had been treated for over six months for TB.
  • Among other things, examination of the dog at the time of presentation identified thickening of the intestinal tract. An intestinal biopsy was taken and acid fast bacilli consistent with M. tuberculosis were identified. A fecal culture subsequently confirmed the identity of the bacterium.
  • The dog died shortly after the diagnosis (but probably would have been euthanized shortly thereafter regardless on the recommendation of public health officials).
  • Subsequent testing of the bacterial strain determined that it was resistant to a variety of drugs that are used to treat TB in people: isoniazid, ethambutol, ciprofloxacin, ofloxacin, streptomycin and amikacin. This places it in the “pre-multidrug resistant” category.

This case raises a few questions:

Where did the TB come from?

This one’s easy. It almost certainly came from close and prolonged contact with the owner. TB is not readily transmissible with transient contact, but living in a household with close contact is a high-risk factor for people, and that would probably apply at least to some degree to their pets too.

Why did this dog get TB?

That’s a tougher question. TB is rarely diagnosed in dogs owned by TB patients, even when they have had long-term, close contact. Whether this dog had some underlying problem that made it more susceptible to infection, the owner was shedding particularly large numbers of the bacterium, or it was simply bad luck (or a combination of these factors) isn’t known.

Does rare really mean rare?

Good question. We rarely identify TB in dogs, but does that mean it rarely occurs, or it’s rarely identified?  We don’t really know.

Why don’t we know much about TB in dogs?

There are few people who work in companion animal infectious diseases, so not a lot of study has been done in this area. TB isn’t a major health problem in dogs, so it’s hard to find money from canine groups to do that type of research. TB in dogs is a pretty oddball phenomenon for human healthcare funding to pay much attention to it. I’ve had a good study of TB in dogs in mind for a while, but it would be exceptionally difficult to get it funded.

Another problem is testing. Screening tests used in people like the tuberculin skin test and IGRA are not useful in dogs. So, we don’t have an easy, readily accessible test in this species. Diagnosing active TB including shedding of the bacterium is even more difficult. Unlike people, where sputum samples are easily collected and tested, getting a good respiratory sample from a dog is more of a challenge. Furthermore, TB may be more of a gastrointestinal issue in dogs (vs respiratory), so we don’t know the optimal testing approach.

Is euthanasia indicated if a dog has TB?

This dog died, and euthanasia in advance of that would probably have been justifiable based on its severe disease. The question that comes up in other cases is what to do with a dog that might have TB but isn’t very sick. TB is potentially treatable, but it requires long-term treatment with combinations of drugs, and we don’t have any evidence to to tell us what the optimal treatment regimen in dogs is. There’s also concern about human exposure during treatment and the (very unlikely but possible) emergence of resistance during treatment of the dog.

This case raises a similar issue, since the dog was infected with a strain that was already resistant to a variety of antibiotics, which is a very serious public health concern, and which would increase the support for euthanasia (based both on the inability to treat the dog effectively and concern about dissemination of this very concerning resistant strain).

TB is one of the most common and important infectious diseases in humans globally, and it’s a disease that is getting more problematic over time. Questions about what to do with exposed dogs are important to address, and more information is needed about the risks to and from pets to make sure we can handle these situations as effectively as possible.